THE FARGO F5 OF 20 JUNE 1957: HISTORICAL RE-ANALYSIS AND OVERVIEW OF THE ENVIRONMENTAL CONDITIONS

Chauncy J. Schultz National Weather Service

NorthDavid Platte, J. Kellenbenz Nebraska National Weather Service Grand Forks, North Dakota

Jonathan Finch National Weather Service

Dodge City, Kansas

Abstract

On the evening of 20 June 1957, an F5 tornado struck the city of Fargo, North Dakota killing 13 people and injuring over 100. This tornado was researched extensively by Dr. Tetsuya Fujita, and became a basis for his creation of the F scale (later developed in 1971) and coining of the phrases , collar cloud and tail cloud cyclic . Observational data from 1957 were used to examine the synoptic and mesoscale environment (Fujita 1960).The Fargo tornado was one in a family of at least five tornadoes produced by a single, long-lived, from the perspective of present-day tornado research, with a focus on the possibility of an outflow boundary enhancing the tornado potential near Fargo. Strong instability, strong vertical wind shear, high storm-relative tohelicity be enhanced (SRH), favorable via evapotranspiration storm-relative (ET)flow (SRF)and moisture and lowered convergence. lifted condensation Approximately levels 200 (LCLs) photographs seemed toof play a pivotal role in the strength and longevity of the tornado. In addition, boundary-layer moisture appeared understood in 1957. the supercell and tornado also provided better insight to the storm-scale environment, which was not well

NOAA/National Weather Service Corresponding Author: Chauncy J. Schultz

5250 E. Lee Bird Drive, North Platte, NE 69101-2473 email: [email protected] Schultz, et al.

1. Introduction b. Meteorological reanalysis

On the evening of 20 June 1957, an F5 intensity The reanalysis of the synoptic environment that tornado on the Fujita scale (Table 1; Fujita 1971) struck produced the Fargo tornado was completed with Fargo, North Dakota. The tornado killed 13 people, archived North American mandatory level rawinsonde injured over 100, and badly damaged or destroyed over 1,300 homes as it moved through the city beginning observations and official surface observation forms (U.S. EF5 damage are extremely rare; only 52 Department of Commerce Weather Bureau Form WBAN havearound been 0027 documented UTC 21 June in 1957.the United Tornadoes States causing F5/ 10A and 10B). Both were obtained from the National between 1950 and 2009, and only one other was recorded in North Dakota’s history, at

Fort Rice on 29 May 1953 (Storm Prediction (0.8Center km) 2009). wide. TheThis Fargotornado tornado was one traveled in a 9 miles (14.5 km) and was about 0.5 miles intermittent damage track of nearly 70 miles (113family km) of atfrom least Wheatland, five tornadoes North that Dakota had anto and cyclic supercell lasted at least 6 hours, and Dale, Minnesota (Fig. 1). The long-tracked to eyewitness reports and Fujita’s detailed analysiswas tornadic (Fujita for 1960). almost 4 hours, according The synoptic and mesoscale the limited data available. Environmental Fig. 1. kinematicenvironment and has thermodynamic been re-created properties using southeastern Tornado North tracks Dakota of the and Fargo west tornado central family Minnesota. on 20-21 June 1957. Five distinct tornadoes were produced from a long-lived and cyclic supercell in stormwill be motion explored of the and supercell related and to present-day its possible interaction http://www. research. The long-lived, cyclic nature and ncdc.noaa.gov/oa/climate/stationlocator.html. The data the development of high surface dewpoints in the area wereClimatic plotted Data using Center the available Digital onlineAtmosphere at software impactedwith an outflow by the tornadoesboundary will be investigated.discussed. In addition, package (version 2.07; Weather Graphics 2010), and then subjective analyses were done by hand after exporting the 2. Data and Methodology images out of Digital Atmosphere. a. Observations of the tornadic event 1957 Fargo sounding was also constructed by interpolating A representative approximated 0000 UTC 21 June The Fargo tornado was one of the most widely documented tornadic events of its period. Fujita mandatory and significant pressure level data from the (1960) collected around 200 photographs of the event, soundingobserved 0000values UTC were soundings greatly atsupplemented Bismarck, North using Dakota, data obtained many eyewitness accounts, and provided a and International Falls and St. Cloud, Minnesota. The detailed analysis in his manuscript. In addition, personal communication with Dr. Ray Jensen (2007), who was available from the National Centers for Environmental surfacePrediction data (NCEP) for the and sounding National was Center taken directlyfor Atmospheric from the of the tornado and who issued public warnings for the observationsResearch (NCAR) at Fargo. reanalysis (Kistler et al. 2001). The on duty at the U.S. Weather Bureau in Fargo at the time meteorological conditions associated with the Fargo event.event, providedJensen’s fascinating an additional account first-hand of the event account and of 1957 the A vertical wind profile was approximated for Fargo warning operations is available online at http://www. at 0000 UTC 21 June 1957, utilizing the same methods crh.noaa.gov/images/fgf/pdf/Jensen_Fargo_Warning_ described above. However, the vertical wind profile also . onbenefited 20 June from 1957. actual The observed pilot balloon pilot databalloon contained data taken wind at Fargo’s Weather Bureau Office during the late afternoon Paper_11_19_09.pdf28 National Weather Digest The Fargo F5 Tornado of 20 June 1957

miniature hurricane,” known today as the rotating wall cloud. The storm structure was visible from greater than observations below 3 km, which increased confidence in 20 miles (32 km; Jensen 2007, personal communication). 1957the low-level hodograph shear using calculations the surface presented observations herein. at Fargo. The This contributed to the large number of photographs Hourlyestimated hodographs winds were discussed also plotted in section as a 0000 5 were UTC developed 21 June available for study by Fujita, which helped him coin the phrases wall cloud, collar cloud, and tail cloud. only the surface wind vector observed at Fargo for the The Fargo supercell produced intermittent timefrom ofthe interest. 0000 UTC 21 June 1957 hodograph by altering core of the Weather Research and Forecasting model tornadoes from the first touchdown in Wheatland, Finally, a high-resolution Advanced Research North Dakota around 2230 UTC 20 June 1957 until the was conducted for the case with the numerical model occurredlast tornado for about ended 3.75 around hours 0215 with UTC a total 21 Junedamage 1957 path in (WRF-ARW; Skamarock and Klemp 2008) simulation Dale, Minnesota. Therefore, non-continuous tornadoes (NWS) in Marquette, Michigan (Hulquist et al. 2007). The Storm Data [available online at http://www.ncdc.noaa. construction utilized at the National Weather Service gov/oa/climate/sd/length of approximately] shows 47 miles that (76 seven km). However,buildings NOAAwere grid spacing on the outer nest and 5 km spacing in the simulation utilized a nested grid with 80 km horizontal destroyed by high winds around 2000 UTC in Stutsman inner nest, where no convective parameterization was County, North Dakota, which is upstream of and directly in ofused. the The surface model boundaries was initialized in the at model1200 UTC simulation 20 June 1957were reportedline with inthe the later area. reports If in associatedfact this damage with the was long-lived also the muchusing NCEP/NCARdifferent from reanalysis reality due data. to the However, development the position of an resultsupercell. of one Baseball or more size tornadoes, hail and funnel the time clouds during were which also as a result, the model results will only be used to verify the longer. At the very least, this suggests that the parent instabilityoutflow boundary present thatin this will case. be discussed in section 6, and supercellnon-continuous responsible tornadoes for thisoccurred event may likely be significantlyhad a path length of at least 110 miles (177 km) over a period of at applied to the creation of soundings and hodographs for least 6 hours. Fargo. The The authors values recognize interpolated the limitations to Fargo and were subjectivity derived waterspout,” developed near Wheatland, North Dakota of both the soundings and the hodographs. The resulting The first tornado, described as a “long and ropelike diagramstwice, and are an averagethus smoothed was then and used certainly in the final do not production capture km). No property damage was reported, although some all details of the vertical tropospheric structure during haystacksand moved were east-northeast picked up and for aroundthere was 11 some miles crop (18 the event, especially where boundary layer processes damage. This tornado was rate as F0 (Fujita 1960). The are important. Several severe weather indices were also second tornado, an F2 with a path length of approximately calculated from the soundings and hodographs upon their completion. It follows that there is an inherent degree of uncertainty in these values, as well. Where applicable, dissipated.5 miles (8 km),The developedinitial stages near of Casselton, the tornado North are Dakota seen ranges of results will be presented. around 2310 UTC, shortly after the Wheatland tornado

3. Overview of the Fargo Tornadoes on 20 June distinctlyin Figs. 2-3 different (Fujita from 1960). one Asanother, Fig. 1and shows, from thethe later path 1957 threedirections tornadoes. of the WheatlandThe latter three and Casselton tornadoes tornadoes demonstrate are

Research indicates that associated with supercell motion, and cyclic paths similar most violent tornadoes tend to produce tornado families nearly identical paths indicative of relatively steady-state possible that the comparably different paths of the to those shown in Burgess et al. (1982). It is therefore tornado.(Dowell andIn addition Bluestein to 2002), the F5 as at was Fargo, the caseFujita with (1960) the an immature supercell, or cell mergers which may have long-lived, cyclic supercell responsible for the Fargo takenWheatland place upstream and Casselton of Fargo. tornadoes are indicative of The third tornado of the series was the Fargo F5 Northdocumented Dakota, an and F4 antornado F0 tornado in Glyndon, near Wheatland, Minnesota, Northan F3 Dakota.tornado Each in Dale, tornado Minnesota, is shown an inF2 Fig. tornado 1, with near distances Casselton, and times indicated between each successive tornado. Fujita’s tornado. The supercell moved eastward for 15 miles (24 km) from Casselton, North Dakota before producing a were produced by a rotating cloud something like a 1960),tornado which 2.5 miles included (4.0 km) a distinct west of left Fargo (northward) near 0027 curve UTC. (1960) investigation “confirmed that these tornadoes The tornado had a path length of 9 miles (14 km; Fujita Volume 35 Number 1 ~ August 2011 29 Schultz, et al.

Fig. 2. Fig. 3. the tornado Image was of Casselton, approximately North 10 Dakota miles tornado(16 km) aroundfrom the 2310 Image of Casselton, North Dakota tornado around UTC on 20 June 1957. View was to the east-southeast and 2315 UTC on 20 June 1957; view was to the east-southeast. Distance to tornado was approximately 2.5 miles (4.0 km). thatphotographer. occurred Photograph during the taken occlusion by Faught process (Fujita of 1960). the cyclic layerPhotograph relative taken humidity by Madsen (RH), (Fujita and 1960).low cloud base may be supercell. The fourth tornado produced by the cyclic supercell with relatively high buoyancy than environments more conducive to rear flank downdrafts (RFD) associated thenwas annortheastward F4 that occurred causing nearsevere Glyndon, tree damage Minnesota as it characterized by low boundary layer RH.” The absence around 0110 UTC. The tornado traveled eastward, of significant precipitation near the updraft implied that mayevaporationally-cooled have been greater potential air was likelyfor a relatively not being warm recycled RFD tornadocrossed theveered Buffalo sharply River to and the destroyed north. The one damage family farm.path justinto priorthe updraft. to the Fargo Both tornadocharacteristics than earlier suggest in the that storm’s there Significant damage was done to a second farm before the life cycle. was 10 miles (16 km) long (Fujita 1960). The fifth and destroyedfinal tornado, a family an F3, farmwas the as cone-shapedit turned northward Dale, Minnesota during The tornado was first observed 3.5 miles (5.6 km) tornado, which began around 0200 UTC. The tornado west of Fargo at 0027 UTC (Fig. 7). Three minutes later, the fact that a clock at the farm stopped at that time. The the tornado rapidly intensified and approached the damagethe occlusion path ofprocess the Dale at tornado0205 UTC, was a timearound known 7 miles due (11 to km; Fujita 1960).

4. Photographic Overview of the Fargo Tornado

The wall cloud preceding the Fargo tornado was first photographed 10 miles (16 km) west of the city at 0005 UTC, approximately 25 minutes before entering TheFargo next (Fig picture 4). This of wall the cloud wall cloud,was already shown well-developed in Fig. 5, was approximatelywith a precipitation-free 7 miles (11 updraftkm) west and of Fargo. rain-free The base. tail cloud extended to the north with a classic appearance of a Fig. 4. First image of Fargo wall cloud located about 10 In Fig. 6, the tail cloud and base of the wall cloud arelarge nearly tornadic on supercellthe ground (Figs. indicating 5-6). a very low lifted approximatelymiles (16 km) west5 miles of Fargo(8 km) around from the 0005 photographer. UTC on 20 June 1957. View was to the northeast and the wall cloud was condensation level (LCL). As Markowski et al. (2002) have noted, “environments characterized by high boundary Photograph taken by Stensrud (Fujita 1960). 30 National Weather Digest The Fargo F5 Tornado of 20 June 1957

Fig. 5.

Classic image of Fargo wall cloud located approximately approximately7 miles (11 km) 6.5 west miles of Fargo(10.5 aroundkm) from 0015 the UTCphotographer. on 20 June Fig. 6. Image of Fargo wall cloud located about 5 miles (8 km) 1957. View was to the west-southwest and the wall cloud was the northwest and the wall cloud was approximately 5 miles (8 Photograph taken by Bergquist (Fujita 1960). west of Fargo around 0025 UTC on 20 June 1957. View was to western parts of the city (Fig. 8) (Fujita 1960). According 1960). to Fujita, the Fargo tornado went through the entire life km) from the photographer. Photograph taken by Olsen (Fujita cycle of a typical large and destructive tornado, which he referred to as the “dropping stage, the rounded bottom stage, shrinking stage and the rope stage.” The tornado’s life cycle from its F5 stage to “roping out” stage can be

area around the Fargo tornado. The highly visible nature ofseen the in tornado Figs. 9-13. and early Note warning the extensive allowed precipitation-free many people to evacuate Fargo before the tornado struck the city, which saved countless lives (Jensen, personal communication, 2007).

Fig. 7.

The first image of the Fargo tornado located about was3.5 miles approximately (5.6 km) west 3 miles of Fargo (5 km) around from the0027 photographer. UTC on 20 June 1957. View was to the southwest and the tornado

Photograph taken by Frahm (Fujita 1960).

Fig. 8. The Fargo tornado was located about 3 miles (5 km) the southwest and the tornado was approximately 3.5 miles west of Fargo around 0030 UTC on 20 June 1957. View was to (Fujita 1960). (5.6 km) from the photographer. Photograph taken by Gebert

Volume 35 Number 1 ~ August 2011 31 Schultz, et al.

Fig. 9. The Fargo tornado was located about 1 mile (1.6 km) Fig. 10. The Fargo tornado was located about 0.5 miles west of downtown Fargo around 0034 UTC on 20 June 1957. approximately(0.8 km) west of 1 miledowntown (1.6 km) Fargo from around the photographer. 0036 UTC on The nearingView was Golden to the Ridge,southwest where and F5 the damage tornado was was produced. approximately tornado20 June 1957.was in View or very was close to the to southGolden and Ridge, the tornadowhere F5 was 1.5 miles (2.4 km) from the photographer. The tornado was (Fujita 1960). Photograph was taken by Kittelsrud (Fujita 1960). damage was produced. Photograph was taken by Hagen

Fig 11. The Fargo tornado was located about 0.3 miles Fig 12. The Fargo tornado was in downtown Fargo around

tornado was approximately 0.7 miles (1.1 km) from the approximately(0.5 km) west of 1 miledowntown (1.6 km) Fargo from around the photographer. 0039 UTC on photographer.0041 UTC on 20 The June tornado 1957. hadView weakened was to the to south an F3 and at this the The20 June tornado 1957. was View in Golden was to theRidge, south producing and the F5tornado damage. was

point. Photograph was taken by Gebert (Fujita 1960). Photograph was taken by Hagen (Fujita 1960).

32 National Weather Digest The Fargo F5 Tornado of 20 June 1957 5. Synoptic Environment

chart revealed a shortwave ridge centered over North Inspection of the 1200 UTC 20 June 1957 500-hPa

Dakota (Fig. 14a). This ridge translated eastward through 0000 UTC 21 June 1957, which allowed significant moisture advection and increasing mid-level flow to take W,place with over an the embedded Northern shortwavePlains. At 0000 trough UTC over 21 June Wyoming 1957, a 500-hPa longwave trough axis was located near 115º

and Utah (Fig 14b). Flow aloft increased-1 downwind of the trough, with a 40-50 kt (~20-25 m s ) 500-hPa jet) extending northeastward from Lander, Wyoming (KLND)-1 to Bismarck, North Dakota (KBIS). An 80 kt (~40 m s 250-hPa speed max also extended from northeastern Montana into southern Ontario at 0000 UTC (Fig. 15). ascentThe location directly of associated the upper-level with that trough feature with was respect likely to Fig 13. The Fargo tornado had crossed the Red River into the eventual tornado event suggests that the large-scale trough shown in the observed data was only as close as weak at best in that area. Even the 700-hPa short wave Minnesota, and was 4 miles (6 km) northeast of downtown Fargo around 0110 UTC on 21 June 1957. The tornado survived until 0130 UTC, before dissipating. View was to the western North Dakota by 0000 UTC (Fig. 16). Also, the 1960).north and the tornado was approximately 4 miles (6 km) from 1200 UTC) 850-hPaor less across chart the did Dakotas not reveal (Fig. 17a). a notable The 1200 low- the photographer. Photograph was taken by Mickelson (Fujita level jet across-1 the region with southerly flow of only 20 kt (~10 m s UTC 850-hPa analysis showed maximum dewpoints in the Dakotas around 10° C, but also revealed a significant dry layer upstream across the central Plains where 850-hPa dewpoints were at or below 0° C. At 0000 UTC, the 850- hPa chart showed a slight increase in dewpoints over the

Fig 14a. Fig 14b. conventional form (temperature and dewpoint depressions 500-hPa observations at 1200 UTC 20 June 1957 in As in Fig. 14a except at 0000 UTC on 21 June 1957. in °C and wind speed in kt) with height contours (solid black contours, contour interval [CI] = 30 m). Volume 35 Number 1 ~ August 2011 33 Schultz, et al.

Fig 15. Fig 16. conventional form (temperature and dewpoint depressions in conventional form (temperature and dewpoint depressions in 250-hPa observations at 0000 UTC 21 June 1957 in 700-hPa observations at 0000 UTC 21 June 1957 in

°C and wind speed in kt) with isotachs for wind speeds at or °C and wind speed in kt) with isotherms (dashed red contours, above 50 kt (colored lines and solid fill, CI = 20 kt). CI = 3° C) and heights (solid black contours, CI = 60 m).

Fig 17a. Fig 17b. conventional form (temperature and dewpoint depressions in 850-hPa observations at 1200 UTC 20 June 1957 in As in Fig. 17a except at 0000 UTC 21 June 1957, and with green shading for dewpoint temperatures at or above 14° °C and wind speed in kt) with dewpoint temperatures (solid C. Some uncertainty exists in the depiction of dewpoints due to colored contours with CI = 2° C, solid fill with brown shading the absence of a 0000 UTC 21 June 1957 sounding from KHON. for dewpoint temperatures at or below 4° C, and solid fill with green shading for dewpoint temperatures at or above 12° C).

34 National Weather Digest The Fargo F5 Tornado of 20 June 1957 eastern Dakotas, as well as a shift of the winds to the west foster an environment suitable for severe convection, level (Fig. 17b). at Bismarck, implying the passage of a weak front at that lapseboth byrates strengthening aloft atop a warming the mid- and and moistening upper-level boundary winds layerand advecting in advance an of elevated the surface mixed cyclone. layer Although (EML) with the steep0000 upstream Despite trough the did displacement result in the of formation the strongest of a surface mid- andlow pressureupper-level system. forcing This for low ascent pressure from system the Fargo moved area, from the shown in Fig. 19 was taken well away from Fargo and did UTC 21 June 1957 St. Cloud, Minnesota (KSTC) sounding shown to exist in the tornado area, it does exemplify the southwestern North Dakota at 1200 UTC to the central not indicate the rich low-level moisture that will later be part of the state, just west of Bismarck, by 1500 UTC (Figs. 18a-b). By 1800 UTC, the surface low was located just synoptic-scale, deep-layer wind shear that was in place

Fig 18b. Fig 18a. Standard surface observations and subjectively As in Fig. 18a except at 1500 UTC on 20 June 1957. analyzed pressure (CI = 2 hPa) and fronts at 1200 UTC 20 June 1957. FAA station identifiers are shown for locations mentioned in the text. Outflow boundary is labeled. periodsouthwest the ofsurface Bismarck, cyclone and haddisplayed deepened a classic 6-hPa structure to ~998- withhPa during a surface the warm preceding front six extending hours (Fig. northeastward 18c). During from that the low center to near Bemidji, Minnesota (KBJI), while a cold front extended southward from just west of Pierre, South Dakota (KPIR) and Valentine, Nebraska (KVTN). hadNotice lifted in Fig.northward 18c that past Fargo Grand (KFAR) Forks, was North well intoDakota the warm sector of the system as by 1800 UTC the warm front dewpoints(KGFK), which of 62 is toapproximately 66° F and was 75 locatedmiles (120 to the km) west north of Fargo,of Fargo. over The northeastern surface moist South axis Dakota at 1800 and UTC southeastern contained North Dakota. driven by mesoscale processes, the synoptic setting did Fig 18c. Although the significant tornado event was likely As in Fig. 18aVolume except 35 at Number1800 UTC 1 ~on August 20 June 2011 1957. 35 Schultz, et al. across the region, as well as the presence of steep lapse 6. Mesoscale Environment rates aloft. The sounding contained bulk shear vectors a. General mesoscale characteristics ). These values suggested from 0-6 km above ground level-1 (AGL) in the range of Observations across northern North Dakota shown 30 to 40 kt (~15 to 20 m s in the series of surface charts in Fig. 18 suggest that a band msupercells s were possible (Rasmussen and Blanchard of precipitation progressed across that portion of the 1998).-1 The 0–8-km AGL bulk shear was 40-50 kt (~20-25 state during the day. In Fig.18a, light rain was reported at ),The which synoptic is slightly setting below seen the in threshold this case atis whicha relatively long- lived supercells are favored (Bunkers et al. 2006b). and in Fig. 18b, the same weather was reported at Minot the warm season. Similar situations occur many times Williston (KISN), in the northwestern corner of the state, duringcommon the occurrence year and do across not necessarily the Northern result Plains in violent during tornado outbreaks such as that which occurred on 20 June (KMOT), in the north central part of the state. The 1800 1957. As a result, the mesoscale evolution which occurred alongUTC surface a similar chart latitude in Fig. with 18c a showsspacing a of thunderstorm approximately at Devils Lake, North Dakota (KDVL). These sites are aligned Fargo tornado. This is not an uncommon characteristic, surface observations at these cities likely captured a asafter Edwards 1800 UTC and appears Thompson to have (2000)played a havepivotal noted role in that the complex130 miles of (209convection km). Giventhat had the an westerly eastward flow movement aloft, the of outbreaks of strong and violent tornadoes are not approximately 35 kt (18 m s necessarily associated with synoptically evident patterns, temperature and dewpoint -1at Grand Forks were 80° F and Rasmussen et al. (2000) noted that mesoscale and 62° F, respectively. Grand). Forks At 1800 is approximately UTC, the surface 90 enhancements to similar synoptic environments may be mentioned, the warm front appeared to move just through miles (145 km) east of Devils Lake, and as previously whenone key combinations to the formation of ofconvective significant available tornadoes. potential Finally, Grand Forks indicated that a thunderstorm also passed Rasmussen and Blanchard (1998) suggested that even the station by 1800 UTC. Successive observations from synopticenergy (CAPE) environments and storm-relative featuring those helicity conditions (SRH) values only northeastthrough that and stationthe temperature between had 1900 dropped and 2000 substantially UTC. By rarelyappear yield supportive such events. of significant (F2 or greater) tornadoes, to2000 68° UTC F. (Fig.A similar 20a), thedrop surface in surface wind hadtemperature shifted to was the

Fig 19.

36 National Weather Skew-T Digest for the observed sounding from St. Cloud, Minnesota at 0000 UTC 21 June 1957. The Fargo F5 Tornado of 20 June 1957 observed in the following hours at downstream sites in northwestern Minnesota. Given the earlier advancement pressurejust north falls. of the Surface city prior pressure to 0000 falls UTC, could and beisallobaric due to abrupt change associated with passing convection eitherageostrophic the deepening adjustment synoptic of low-level surface low, winds which due was to of the surface warm front, it is hypothesized that this

‘effectivesignaled thewarm development front’ demarking of a surface the northern outflow edge boundary of the located southwest of Jamestown, North Dakota (KJMS) by (Fig. 20a). The surface outflow boundary became the 0000 UTC (Fig. 20c), or to the possibility of a mesolow forming near the outflow boundary, coincident with

Fig 20a. Fig 20b. As in Fig. 18a except at 2300 UTC on 20 June 1957. warm sector, As in southeast Fig. 18a except of the at developing 2000 UTC on surface 20 June low. 1957. The elevated convection north of the warm front, combined with strong heating within the warm sector, allowed the boundary to sharpen during the day. The absence of surface observations between Grand Forks and Fargo makes it hourlyimpossible temperature to determine observations with certainty taken where at that the timeoutflow by boundary may have been situated by 0000 UTC, though coolingNational associated Weather with Service its passage Cooperative and sharp Observers change does in help refine its position. However, given the magnitude of wind direction at both Grand Forks and Bemidji, the latter of which is at a latitude that is approximately 40 miles (64 km) north of Fargo, it is very possible that the outflow theboundary evolution was of just the north tornadic of Fargo supercell and thatBrainerd impacted (KBRD) the area.by 2300 UTC (Fig. 20b), and thus may have been pivotal in backing in surface winds to a southeasterly direction Fig 20c. Surface observations at Fargo indicate a significant winds may be attributed to the close proximity of the As in Fig. 18a except at 0000 UTC on 21 June 1957, just before 0000 UTC. This distinct change in surface stationand with plot. the addition of the three-hour surface pressure change (in hPa) shown in red in the lower-right corner of each surface outflow boundary, which is believed to have been Volume 35 Number 1 ~ August 2011 37 Schultz, et al.

near 3000 J kg inthe speed, tornadic which supercell. is likely Just indicative after 0000of either UTC, the the passage winds contributed to -1an estimated surface-based CAPE (SBCAPE) shifted to an east-northeasterly direction and increased temperature and and dewpoint a surface-based of 80° F and convective 69° F, respectively inhibition (Fig.(SBCIN) 21). ofThe almost magnitude zero, of usingthe approximated the observed instability surface itof isthe impossible outflow boundary to know through for sure the what city orincreased the forward the easterlyflank downdraft component of the of the approaching surface winds supercell. at Fargo Although prior to the violent tornado given the limited data available, that is supported by the 12-hour forecast from the WRF-ARW change corresponded to an increase in surface dewpoint southeasternsimulation valid North at 0000 Dakota UTC with(not-1 shown).surface Thattemperature forecast to 69° F at the station, and thus suggested an enhancement andshows dewpoint SBCAPE values over 3000similar J kgto those and minimalobserved, SBCIN despite in

This change was instrumental in modifying the ambient match the observed winds since the simulated primary thermodynamicin the low-level convergence and kinematic zone environment in the vicinity in a of manner Fargo. surfacethe fact thatcyclone the surfaceand warm wind front fields were in the predicted model domuch not farther north than the observations indicated. Also, given The mesoscale environment which developed near the observed moisture convergence that was occurring that favored significant . deeper than that shown by the approximated sounding. Fargo shortly after 0000 UTC 21 June 1957 resemble the in the area, it is likely that the low-level moist layer was centralpatterns United which States. Johns This et al. work (2000) suggested identified that asmost being of thesefavorable events for werestrong closely and violent tied to tornadoesthe cool side in theof a north-warm The 0000 UTC Fargo sounding suggested an LCL Edwardsfor a surface-based and Thompson parcel (2000), was near and 700Johns m etAGL. al. (2000)This is suggestedin line with are the typically LCL heights associated Craven with and strong Brooks and (2004), violent frontal or quasi-stationary boundary with the surface tornadoes, and further supports the hypothesis that b.moisture Mesoscale axis thermodynamic coincident with characteristics the frontal zone. the local increase of dewpoints in the vicinity of Fargo for Fargo suggested that the enhanced boundary layer observationsled to a much ofmore photographs favorable low-leveltaken during thermodynamic the event moistening The approximated that took place 0000 in the UTC vicinity sounding of the developed station environment for significant tornadoes. In fact, careful

strongly suggest that the LCL with the parent updraft

Fig 21. 1957. Approximated Fargo, North Dakota Skew-T proximity sounding at 0000 UTC 21 June 38 National Weather Digest The Fargo F5 Tornado of 20 June 1957

in tornadic events (Markowski et al. 1998). As the surface was actually much lower than 700 m AGL. This could be a windsfound tobacked vary significantlyand increased both in temporallymagnitude, and the spatially0–1 km reflection of the supercell ingesting not only air from the and 0–3 km bulk shear vectors lengthened considerably warm sector, but also rain-cooled air from the north side c.of Mesoscalethe outflow vertical boundary. wind shear characteristics estimated to be over 20 kt (10 m s ) at Fargo using the 0023(Fig. 23). surface Both observation the 0-1 km takenand 0-3 immediately-1 km bulk shear prior were the The backing surface winds at Fargo had a pronounced tornado, which displayed a decidedly easterly direction. effect on the local vertical wind shear. Hourly hodographs

al. 1990).Storm-relative The SRH was helicity derived (SRH) for eachhas been hodograph shown into the be 1957,were constructedthe time of the for last Fargo surface from observation the period before 1800 UTC the an important tool in tornado forecasting (Davies-Jones et tornado20 June (Fig. 1957 22b). (Fig. The 22a) hodographs through were 0023 constructed UTC 21 June by altering only the surface wind vector from the mandatory timethe informationleading up to providedthe-1 tornado on utilizing the path a storm of the motion storm of pressure level winds developed for Fargo using the 270°documented at 15 kt by(~8 Fujita m s ).(1960). This motion The estimated was derived SRH utilizing further supports the hypothesis that the backing surface winds created a more favorable tornadic environment with time methods described in Section 2. Significant temporal kmchanges bulk shearin the was deep-layer in the 60 shear to 70 cankt (30 be to attributed 35 m s ) range,to the around 325 m2 s 2 s changewhich wasin surface very windssupportive in the of vicinity supercells of Fargo. (Rasmussen -1 The 0-6 at Fargo (Fig. 24).-2 By 0023 UTC, the SRH was estimated-2 in the 0-1 km layer and over 350 m in the 0-3 km layer. theand parentBlanchard supercell 1998). responsible This extremely for the large tornado shear eventvalue Storm-relative flow (SRF) was strong, and also likely was likely a significant contributor to the longevity of played a role in the maintenance of a strong, long-lived, low-level mesocyclone by providing a favorable balance substantially(Bunkers et al. in 2006b). the hours leading up to the tornado. This between the inflow and outflow of the supercell for an The low-level vertical shear at Fargo also increased importantextended period. role in the According maintenance to Brooks and longevity et al. (1993), of strong “the intensity of storm-relative winds” appears to play an is not surprising since low-level vertical shear has been

Fig. 22b North Dakota, which uses the last surface wind observation Fig. 22a. . 0023 UTC (“tornado time”) hodograph at Fargo, increments.1800 UTC hodograph at Fargo, North Dakota. before the F5 tornado passed the weather office. Wind speeds Wind speeds are in kt and heights AGL are labeled in 1 km are in kt and heights AGL are labeled in 1 km increments. Volume 35 Number 1 ~ August 2011 39 Schultz, et al.

low-level . Furthermore, “some storms with high inflow speeds playedproduce a key significant role in long-lived,its maintenance low-level and longevityrotation” (Brooksof what photographs et al. 1993). infer SRF waslikely a

very strong low-level mesocyclone), with in thisthe case. The inflow, or low-level -10–1 km SRF was around 30 kt (~15). Fig. m 25 s shows that 8–12the SRF km increased SRF outflow markedly-1 winds in also the loweststrong near 40 kt (~20 m s

kilometer AGL) before as surface the tornado winds became struck Fig 23. Fargo.east-northeasterly -1 and increased to 27 kt (14 m s 0–1 km and 0–3 km bulk shear values at Fargo from 1800 UTC until 7. Supercell Longevity and Storm 0023 UTC. Motion Considerations

a. Supercell longevity

The supercell that produced the Fargo tornado lasted for approximately 6 hours according to Fujita (1960),

and was therefore classified as a long- lived supercellsupercell, (Bunkers“then the et al.probability 2006a). If a tornado is produced by a long-

longevityof significant of the tornadoes Fargo event is was enhanced” similar (Bunkers et al. 2006a). The supercell including the supercell that produced Fig 24 anto otherF5 tornado significant during tornado the 3 outbreaks,May 1999 . As in Fig 23 except for storm-relative helicity (SRH). Oklahoma outbreak (Thompson and Edwards 2000). Recent research indicates that

long-lived supercells are favored in environments characterized by strong, deep-layered shear and SRF. The). SRF Fargo in tornado had 0–8-km bulk shear-1 values mof s70 to 80 kt (~35 to 40 m s the -1 0–8-km layer was around 40 kt (20 appeared), conducive to have toplayed long-lived a pivotal supercells role in lofting(Bunkers hydrometeors et al. 2006b). downstream The SRFand

layered SRF acted to keep hydrometeors loftedsustaining downstream the supercell. from Strongthe updraft, deep-

air from being ingested into the updraft. and prohibited evaporationally-cooled Fig40 25.National As in Fig. Weather 23 except Digest for 0-1 and 0-3 km storm-relative flow (SRF). The Fargo F5 Tornado of 20 June 1957

This allowed prolonged ingestion of moist, unstable North Dakota, northeastern South Dakota, and west (Markowski et al. 2002). The Fargo supercell had little centralin low-level Minnesota moisture by early that evening. took place At Fargo, in southeastern dewpoint orair, no making precipitation formation near of the a warm-type updraft, which RFD was more evident likely from the photographs of the tornado development stages temperatures climbed from 59° F at 1800 UTC 20 June enhances the hypothesis that a near perfect balance 1957 to 69° F at 0000 UTC 21 June 1957. Persistent, occurred(Figs 8-10.). between This lackthe updraft of precipitation and downdraft near theinterface updraft in moderate surface flow continued through that time across relation to hydrometeors. remainedthe Central lower Plains, than but those dewpoint observed temperatures in the area upstreamimpacted byin thethe well-mixed tornado event.air mass Thus, across it Kansasappears and that Nebraska simple b. Storm motion in relation to the surface boundary advection of moisture did not alone explain the increase in surface moisture in the Fargo area. Model simulations by Atkins et al. (1999) indicated that mesocyclones tracking along surface boundaries were stronger than storms moving across or perpendicular to (1998) The demonstrated growing season the correlation in the north-central between vegetative United boundaries. There is a strong possibility that the Fargo developmentStates is well underwayto the seasonal by the patternlatter half of ofconvection June. Raddatz and supercell either interacted with or tracked along the late afternoon and early evening. The environmental the period of greatest ET. Raddatz (2000) showed ET settingoutflow in boundary the vicinity that of the sagged boundary southward likely duringcaused the linkedas a secondary, moisture butfrom significant ET to the moisture agroecosystems source forof the mesocyclone to become stronger at times, and more Canadian Prairie Provinces. Kellenbenz et al. (2007) persistent than if no boundary was present. That is, the (2000) also suggested that ET plays an important role inNorthern the occurrence Plains and of stronga violent and tornado violent event. tornadoes Johns in et this al. enhancement of boundary layer convergence and backed region. The Fargo F5 tornado event appears to fall into easterlycombination winds of near increased and just surface north of moisture, the front provided localized this category. an ideal setting for tornado development. Storm motion along this boundary would have provided the supercell summer of 1957 shows a surplus of precipitation in an opportunity to interact with increased values of storm Climatological data for the spring and early amounts were from 2.5 to 7.5 centimeters (1 to 3 inches) abovethe area normal immediately from far upstream southeastern of Fargo. North PrecipitationDakota into observedscale instability in their and simulations shear. These that localized “the strongest enhancements storm, determinedmay have maximized from vertical near velocities, Fargo. Atkins was etthe al. one (1999) that propagated along, parallel to the boundary.” It appears that northeastern South Dakota during May and the first half the surface boundary played a key role in the maintenance indexof June. indicated The Palmer extremely Z index moist (Palmer conditions 1965) incan the be region used and longevity of the Fargo supercell, in addition to the duringas a measure May 1957 of short-termand moderate moisture to extreme adequacy. moisture This

surface dewpoints in the region occurred locally on 20 8.production Potential of multiple Effects significant of Evapotranspiration tornadoes. (ET) Juneconditions 1957 during for June the 1957period (Fig. of maximum 26a-b). Theheating increase without in on Low-Level Moisture Characteristics series of dewpoints at Fargo showed a similar rise in It has been shown that a direct contributor to the evidence of significant surface moisture advection. A time the days immediately preceding the tornado (Fig. 27). The repeateddewpoints signal during of moisturethe 1800 increase to 2200 during UTC time the afternoonframe for favorable thermodynamic profile associated with the and evening hours under differing synoptic regimes in the theFargo surface tornado dewpoint was the increases presence for of this significant, case. low-level days prior to the violent tornado provides some evidence moisture.The ETmaximum appeared surface to have dewpoints played a significant shown inrole the in of a possible local moisture source enhancement. Thus, the increase in ambient surface moisture may be over 60° F in an area extending from northeastern South Dakotaregion on into the west1200 centralUTC surface Minnesota. chart in AtFig. that 18a time,were justthe precipitation during the active vegetative growth phase ofpartly crops attributed in the area to enhancedduring the ET early due summer to above-normal of 1957. across the central plains were only between 55 and 59° F maximum dewpoints within the southerly flow regime not result in the local increase in surface moisture. Strong moistureHowever, convergenceit must be emphasized likely existed that in ET the alone vicinity likely of anddid as far south as southern Kansas. Comparing this with the 0000 UTC chart in Fig. 20c reveals the marked increase Volume 35 Number 1 ~ August 2011 41 Schultz, et al.

Palmer Z-Index May 1957

Fig. 26a.

Observed Palmer Z Index for May 1957. Palmer Z-Index June 1957

Fig. 26b. As in Fig. 26a except for June 1957.

42 National Weather Digest The Fargo F5 Tornado of 20 June 1957

Fig. 27. Hourly dewpoint trace for Fargo on consecutive days leading up to the tornado. For clarity, the hourly dewpoint traces from the 16th and 17th of June 1957 are not shown, but they displayed similar tendencies. south of the surface fronts from eastern North Dakota two F5/EF5 events in North Dakota’s history. into eastern South Dakota. This convergence likely played A complicating factor in examining the 1957 F5 a critical role in elevating moisture levels, or at the least, Fargo tornado is the limited data that existed decades ago, enabled ET processes to contribute to increased surface forcing the authors to rely on observed surface and upper air data. However, the ample photographs of the Fargo inhibiting mixing in the boundary layer. Finally, the 1200 dewpoints beneath the EML, which was likely important in research, allowed for additional insights to be gleamed maximum in dewpoints over northeastern South Dakota. fromstorm, the along observed with adata. present-day Indeed, perspectivethe analyses of from tornado this ItUTC is possible900-hPa thatdewpoint downward analysis mixing shown of thisin Fig. moisture 28 shows may a study are comparable with those from more recent events have also slightly increased surface dewpoints during the and contribute to our understanding of relatively rare and afternoon. violent tornadic events. Johns et al. (2000) found that 63% of strong In the case of the Fargo tornado, it appears that and violent tornado cases in the region exhibited enhanced wind shear and instability in the presence of higher dewpoints along or north of a warm frontal or quasistationary boundary. This appears to be the case in the 20 June 1957 event. That is, the enhanced moisture (as suggest.a well-defined It also surface appears boundary that ET, greatly in combination influenced with the well as vertical wind shear) in the vicinity of the boundary focusedtornado potential,moisture muchconvergence, like many played present-day a key role studies in

leading up to and during the event. Rain that had fallen allowed the supercell to maintain an inflow of adequate inaugmenting previous months local boundary-layer likely provided moisture needed moisture in the days to 9.boundary-layer Conclusions moisture during its prolonged life cycle. layer moisture, coupled with both high bulk shear and The Fargo F5 tornado of 20 June 1957 was a growing crops into June of 1957. This enhanced boundary-

SRF parameters, aided in the development of a long- andsignificant propelled historic Fujita’s event. interest It is arguably in quantifying one of thetornado most boundary.lived supercell. This mayIt is havehypothesized contributed that to the the supercell occurrence may of damage.well-documented It is in part and due photographed to his extensive tornadoes research of all of time, the have either interacted with or tracked along an outflow Fargo tornado that the original F scale was developed. devastation to North Dakota’s largest city – but not before Fujita’s intense study and photogrammetric work of the manyseveral residents significant heeded tornadoes, a successful including early an F5 warning that brought and supercell’s structure enabled him to coin the phrases left. collar cloud, tail cloud, and wall cloud that are widely used today. The Fargo tornado remains to this day one of only

Volume 35 Number 1 ~ August 2011 43 Schultz, et al.

Fig 28.

900-hPa dewpoint observations at 1200 UTC 20 June 1957 (solid green contours and fill, CI = 3° C) and subjective frontal analysis.

Acknowledgments

thoroughThe authors reviews, would comments, like to thank and Bradley suggestions Bramer, on John this Stoppkotte,study. The authors Jeffrey Manion,are also Chris Buonanno, Jeffry Evans, Dan Valle, Gary Ellrod, and Kermit Keeter for their for performing the WRF simulation. Special thanks are extended to Dr. Ray grateful to Mathew Bunkers for providing data and images, and to Tom Hulquist his invaluable insights. Jensen, who was working at the Weather Bureau in Fargo during this event, for

44 National Weather Digest The Fargo F5 Tornado of 20 June 1957

Authors 12th Conf. Chauncy J. Schultz is a Meteorologist Intern at the Na Burgess,on Severe D. W.,Local V. Storms, T. Wood, San Antonio, and R. A.TX, Brown, Amer. Meteor. 1982: Mesocyclone evolution statistics. Preprints, - tional Weather Service in North Platte, Nebraska. Previ- Soc., 422–424. ously, he was a Student Intern at the NWS offices in both of sounding derived parameters associated with deep, UniversityBismarck and of North Grand Dakota. Forks, North His primary Dakota. interests He graduated are the in Craven,moist J. P.,convection. and H. E. Nat.Brooks, Wea. 2004: Digest Baseline climatology application2009 with a of B.S. research degree to in the Atmospheric operational Sciences prediction from of these , 28, 13-24. and hydrology. - Test of helicity as a tornado forecast parameter. vere convection, warning decision-making, fire weather, Davies-Jones, R.16 P.,th ConfD. W. onBurgess, Severe andLocal M. StormsP. Foster, 1990: David J. Kellenbenz is a Senior Meteorologist at the Na tional Weather Service in Grand Forks, North Dakota. He Preprints, , Boston, - MA, Amer. Meteor. Soc., 588-592. mesoscaleobtained his meteorology, B.S. degree inincluding Atmospheric all forms Science of severe from Lyn and- Dowell,tornadogenesis. D.C., and Bluestein, Mon. Wea. H.B., Rev. 2002: 130, The June 8, 1995 winterdon State weather. College He in enjoysVermont. taking His primaryinformation interests gleaned are McLean, Texas storm. Part I: Observations of cyclic from case study events and bringing them into operation 2626-2648. al forecasting in an effort to better understand the com plexities of the atmosphere. - Edwards,assessment R., and R.L.of Thompson,supercell 2000:forecast RUC-2 parameters. supercell - proximity 20th soundings, Conf. on Severe Part Local II: An Storms independent, Orlando, Jonathan Finch is a Forecaster at the National Weather Preprints, Fujita,FL, Amer.T., T., Meteor.1960: Soc.,A detailed 435-438. analysis of the Fargo M.S.Service degree in Dodge in meteorology City, Kansas. at theHe receivedUniversity a B.A. of Oklahoma. degree in tornadoes of June 20, 1957, U. S. Department of HeEnvironmental enjoys studying Science historical at the Universityweather events of Virginia and apply and a 67 pp. - Commerce, Weather Bureau, Research Paper No. 42, Referencesing findings to current weather situations.

______, 1971: Proposed characterization of tornadoes and hurricanes by area and intensity. SMRP Res. Atkins,evolution. Nolan Mon. T., M.L. Wea. Weisman Rev., and L.J. Wicker, 1999: The influence of preexisting boundaries on supercell Paper 91, Univ. of Chicago, 42 pp. 127, 2910-2927. Hulquist, T. R., J. L. Lee, and M. Zika, 2007: Use of high Tornado detection and warning by radar. The Tornado: resolution WRF-ARW output at a National Weather, 8th Brooks,Its Structure,H. E., Doswell, Dynamics, C. A., andPrediction, Davies-Jones, and R.,Hazards, 1993: WRFService Users’ Forecast Workshop Office: The 21 July 2002 and 9 August Geophys. Monogr., No. 79, Amer. Geophys. Union, 97- 2005 Upper Michigan bow echo events. Preprints 104. , National Center for Atmospheric Research, Boulder, CO, paper 9-1. Harding, 2000: The role of synoptic patterns and Johns,moisture R. H., C.distribution Broyles, D. in Eastlack, determining H. Guerrero the location and K. Bunkers, M.J., M.R. Hjelmfelt, and P.L. Smith, 2006a:Wea. An of strong and violent tornado episodes in the north Forecastingobservational, 21, examination 673–688. of long-lived supercells. central United States: A preliminary examination. Part I: Characteristics, evolution, and demise. 20th Conference on Severe Local Storms,

Preprints, ______, J.S. Johnson, L.J. Czepyha, J.M. Grzywacz, Orlando, FL, Amer. Meteor. Soc., 489-492. B.A. Klimowski, and M.R. Hjelmfelt, 2006b: An 2007: The North Dakota tornadic supercells of 18 observational examination of long-lived supercells. Wea. Forecasting Kellenbenz, D. J., T. J. Grafenauer, and J. M. Davies, Part II: Environmental conditions and forecasting. evapotranspiration. Wea. Forecasting, 22, 1200–1213. , 21, 689–714. July 2004: Issues concerning high LCL heights and Volume 35 Number 1 ~ August 2011 45 Schultz, et al.

Kistler,H. van R. E., den Kalnay, Dool, W. R. Collins, Jenne, S.and Saha, M. G.Fiorino, White, 2001:J. Woollen, The Rasmussen,tornado E.forecast N., and parameters. D. O. Blanchard, Wea. 1998:Forecasting A baseline, 13, M. Chelliah, W. Ebisuzaki, M. Kanamitsu, V. Kousky, climatology of sounding-derived supercell and ROM and documentation. Bull. Amer. Meteor Soc., 82, NCEP-NCAR 50-year reanalysis: Monthly means CD- 1148-1164.

247-267. ______,tornadoes S. Richardson, with a baroclinic J.M. Straka, boundary P.M. Markowski, on 2 June 1995. and Mon.D.O. Blanchard,Wea. Rev. 2000: The association of significant Markowski P. M., J. M.Mon. Straka, Wea. E. Rev N. Rasmussen, and D. O. Blanchard, 1998: Variability of storm-relative helicity , 128, 174–191. ______,during ______, VORTEX. ______, and ______,., 126, 2002: 2959-2971. Direct surface hydrostatic atmospheric model, J. Comput. Phys., 227, Skamarock, W.C., and J.B. Klemp, 2008: A time-split non- downdrafts of nontornadic and tornadic supercells. Mon.thermodynamic Wea. Rev., 130 measurements, 1692–1721. within the rear-flank 3465–3485. of the United States. [Available online at http://www. Stormspc.noaa.gov/faq/tornado/f5torns.html Prediction Center, cited 2009: F5 and EF5.] Tornadoes

Palmer, W. M., 1965: Meteorological drought. Research Paper 45, U.S. Weather Bureau, 58 pp. [Available from environmental conditions and forecast implications NOAA Library and Information Services Division, Thompson,of the R.L.,3 May and R.1999 Edwards, tornado 2000: outbreak. An overview Wea. of Washington, DC 20852.] Forecasting transformation and the potential for deep convection 2010: Digital Atmosphere. [Available online at www. Raddatz, R. L., 1998: AnthropogenicCan. J. Soil. Sci. vegetation weathergraphics.com/da, 15, 682-699..] Weather Graphics, cited

______,on the 2000: Canadian Summer prairies. rainfall recycling for an, 78, agricultural 656-666. Can. J. Soil. Sci., 80,

region of the Canadian prairies. 367-373.

46 National Weather Digest