AN EXAMINATION OF A SEVERE DOWNSLOPE WINDSTORM WEST OFTHE COLORADO PARK RANGE

2 2 Christopher N. Jones\ Jeffrey D. Colton , Ray L. McAnelly and Michael P. Meyers

1NOANNationai Weather Service, Riverton, Wyoming 2NOAAlNationai Weather Service, Grand Junction, Colorado 3Colorado State University, Fort Collins, Colorado

Abstract ing the windstorm was atypically east-to-west. This event showed similarities to the 25 October 1997 Routt During the overnight hours of 22-23 April 1999 National Forest blowdown event where 13,000 acres of (23 April case) a damaging downslope windstorm old growth forest were devastated by wind gusts in excess occurred west of the Park Range (3000-3700 m MSL) in of 50 m S-1. Meyers et al. (2002) detailed the nature and northern Colorado. Although downslope windstorms are possible mechanisms responsible for this destructive known to be common in the eastern foothills of the storm. The research presented here will detail the synop­ Colorado Rockies, only recently has this phenomenon been tic and mesoscale environment of the 22-23 April 1999 documented on the West Slope of the northern Colorado windstorm through analyses of various NWS National mountains. The strong east winds toppled numerous pine Centers for Environmental Prediction (NCEP) models, trees and damaged at least six homes in the vicinity of Regional Atmospheric Modeling System (RAMS) fore­ Steamboat Springs (SBS; 2100 m MSL). The lower and casts, and observed conditions. Comparisons will be made middle tropospheric wind flow during the windstorm was to the 25 October 1997 Routt National Forest devastation atypically east-to-west, similar to the 25 October 1997 and post-storm research in an attempt to aid operational Routt National Forest blowdown. The NWS National - meteorologists in forecasting these rare events. Centers for Environmental Prediction (NCEP) guidance predicted strong synoptic winds over northern Colorado, 2. Background but did not indicate a downslope windstorm. To supple­ ment the NCEP guidance, an operational version of the Although the effects of mountain waves are commonly Regional Atmospheric Modeling System (RAMS) was observed east of mountain barriers, such as the Colorado employed to explore the mesoscale processes of the downs­ Front Range, due to the prevailing westerly flow at mid­ lope windstorm. Forecasts of cross-barrier sea-level pres­ latitudes, for the same reason mountain windstorms are sure gradient and near-surface winds nearly equaled the observed far less frequently west of mountain barriers observed values of these parameters. Cross-sections of (e.g., Santa Ana (Svejkovsky 1985) and Croatian Bora wind and potential temperature indicated strong cross­ (Smith 1987)). In fact, the bulk of the theoretical and barrier easterly flow and a mountain wave pattern. The observational research pertaining to mountain wave for­ potential temperature pattern also depicted a favorable mation has focused on westerly lower and middle tropos­ stability profile, with a layer of instability in the middle pheric wind flow. Lilly and Zipser (1972) conducted a troposphere overlaying a stable crest-level layer. thorough survey of a mountain wave event near Boulder, Comparisons to the 25 October 1997 forest blowdown Colorado on 11 January 1972, which has been the focus were conducted in order to find common characteristics of of several subsequent studies and model comparisons this rare phenomenon. In both cases, strong cross-barrier (e.g., Klemp and Lilly 1975; Clark and Peltier 1977; easterly winds, favorable stability profile, and a critical Peltier and Clark 1979; Clark and Farley 1984; Durran level had occurred. The 25 October 1997 event had and Klemp 1987; Doyle et al. 2000). Other research stud­ stronger cross-barrier flow, and much colder temperatures ies have also examined other Colorado Front Range than the 23 April case. A strong cross-barrier pressure windstorms (Lee et al. 1989; Brown et al. 1992; Cotton et gradient was observed in the 23 April case but not the al. 1995). More recently, Meyers et al. (2002) described a 25 October 1997 event. high wind event resulting from easterly winds traversing the Continental Divide. Colle and Mass (1998) investi­ 1. Introduction gated 55 western Washington windstorms and describe several common characteristics found in these and other During the overnight hours of 22-23 April 1999 wind studies on downslope windstorms: gusts to 40 m S-1 occurred west of the Park Range (3000- 1) Strong cross barrier flow with weak vertical shear 3700 m MSL) in north central Colorado (Fig. 1) toppling above it, where the upper-tropospheric winds are not numerous pine trees and damaging at least six homes in excessively strong. the vicinity of Steamboat Springs (SBS; 2100 m MSL; 2) Magnitude of cross-barrier sea-level pressure Fig. 2). The lower and middle tropospheric wind flow dur- gradient.

73 74 National Weather Digest

development of larger amplitude mountain waves creating an envi­ ronment where breaking waves (Clark and Peltier 1977) are more likely.

3. Observations and Storm Overview

During the overnight hours of 22-23 April 1999 devastating winds affected Steamboat Springs (SBS) and vicinity damaging trees and personal property. This storm also produced up to 1.0 m of snowfall along the Front Range of Colorado east of SBS. The predominate wind direction in the lower and middle troposphere was from the east. Although damage was not as extensive, this event showed simi­ larities to the 25 October 1997 Routt National Forest blowdown event (Meyers et al. 2002) where 13,000 acres of old growth forest Fig. 1. State of Colorado topography map. Approximate location of selected cities, mountain were devastated by wind gusts in ranges, and automated weather instruments are shown. Symbols: Storm Peak Laboratory excess of 50 m S-I. In both cases, a (SPL). devastating snowstorm occurred on the Front Range of Colorado. In Colorado the 1997 storm up to 1.5 m of snowfall accumulated along the Front Range of Colorado (Poulos et al. 2002).

a. Local observations

Reports of downed trees and power lines in the vicini­ ty of SBS were received from the Colorado State Patrol around 0900 UTC 23 April 1999. Wind gusts were esti­ mated as high as 40 m S-1 by local law enforcement, and downed trees fell in an east-to-west pattern. The Dry Lake Remote Automated Weather Station (RAWS), locat­ ed 8 km northeast of SBS at an elevation of 2430 m MSL, recorded wind gusts from the east at 30 m S-1 at both the 1100 UTC and 1200 UTC 23 April observations. Local damage reports from county officials placed the Dry Lake Routt RAWS in an area of enhanced damage stretching from COUllty SBS to 16 km northeast of SBS (Fig. 2). Within this dam­ age swath, 6 homes were damaged, 2 sheds were destroyed, 12 power lines were downed and numerous Fig.2. State of Colorado county map with blowout of Routt County trees were toppled. in northwest Colorado. Shaded area northeast of SBS denotes Additional wind and temperature data were provided approximate location of wind damage. Highway 40 is depicted by Storm Peak Laboratory (SPL), a facility operated by through central Routt County. Symbols: Steamboat Springs (SBS), the Division of Atmospheric Sciences of the Desert and Hayden (HDN). Research Institute (Borys and Wetzel 1997). The SPL provided continual monitoring of the event from Mt. 3) An inversion or layer of strong stability near the Werner (7 km southeast of SBS - elevation 3210 m MSL). mountain crest with lower stability above. The 5-minute average wind plot in Fig. 3a showed that 4) Presence of critical layer (layer of zero wind or flow sustained winds ranged from 15 to 19 m S-1 between 1000 reversal), which reflects gravity wave energy downward. UTC and 1600 UTC 23 April. The 5-minute average wind Colle and Mass (1998) also found that 82% of variance gusts (Fig. 3b) during this 6-h period routinely topped of event severity was due to the magnitude of cross-bar­ 22 m S·I, with a peak of nearly 24 m S·1 at 1550 UTC rier flow and cross-barrier pressure gradient. Durran 23 April. The corresponding wind direction ranged from (1990) further suggests that these conditions promote the 085 to 105 degrees (not shown). Volume 26 Numbers 3, 4 December 2002 75

22-23 April 1999 Windstonn at Stonn Peak Lab 20 8. 8 it ••.., ..~: !.,., "I. 6 :, •. . ~...... ~.';+ 4 .-. . ." . . ."n. ~ 1 , ~ , . , ,.. • .." 0 . , .. :'1 :)1:":"'; : .. ~~ 8 . . ;:',";'r ~(>~.. ,,: 6 .

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Fig. 3. a) Storm Peak Laboratory 5-min average wind speed Fig. 4. a) Eta model analysis of 500-mb level height contours and (m s·') from 1500 UTe 22 April through 2100 UTe 23 April 1999. winds at 0000 UTe 23 April 1999. Heights are contoured every 60 b) Storm Peak Laboratory 5-min average wind gust speed (m s·') m. Wind barbs are in m s·'. b) Eta model analYSis of 700-mb level from 1500 UTe 22 April through 2100 UTe 23 April 1999. Shaded height contours and winds at 0000 UTe 23 April. Heights ~re con­ area denotes time of observed wind damage. (Data provided by toured every 30 m. Wind barbs are in m s·'. Drs. Randy Borys and Melanie Wetzel) b. Synoptic and mesoscale environment er SLP difference. CYS and CAG lie 169 km east and 72 km west ofthe Park Range crest, respectively. The SLP at Upper-air analyses of 0000 UTC 23 April 1999 (Fig. CYS increased from 1009.0 to 1026.8 mb, while measure­ 4) depicted a nearly vertically stacked closed low near ments at CAG showed an increase from 1004.1 mb to Las Vegas, Nevada. (LAS). The cut-off low moved closer to 1010.0 mb. Thus, the cross-barrier SLP difference LAS over the next 12 hours (Fig. 5) became more verti­ increased from 0.020 mb km·1 to 0.070 mb km·1 in 20 cally stacked and deepened significantly (30 m at both hours across a distance of241 km. Colle and Mass (1998) 500 and 700 mb). As a result of the tightened gradient, found a common factor among the strongest west 700-mb height rises of 50-70 m at Bismarck, North Cascade wind events to be a SLP difference of> 0.047 mb Dakota (BIS), Denver, Colorado (DEN), and Riverton, km-I between Seattle and Yakima (169 km). Similarly, a Wyoming (RIW) occurred during the 12-h period. Winds cross-barrier SLP difference between RIW and SLC (320 (Fig. 6) at both the Medicine Bow wind profiler (MBW) in km) of > 0.025 mb km-! is typically enough to generate south central Wyoming and the Platteville wind profiler gap winds on the west slope of the Wasatch (Dunn 1999). (PI'L) in north central Colorado showed easterly winds during this 12-h period. Slightly stronger winds 4. Numerical Guidance (20 m S·I) occurred at MBW than at PI'L. The presence of a critical level was evident at -5 km MSL at PI'L where The synoptic conditions predicted by various models of the wind became southerly, a direction parallel to the the NCEP suite available to the operational forecaster were Park Range. all quite similar for this event. The 1200 UTC The cross-barrier, sea-level pressure (SLP) gradient 22 April 1999 and the 0000 UTC 23 April 1999 model out­ increased dramatically between 1800 UTC 22 April and put also showed excellent run-to-run consistency. Therefore, 1400 UTC 23 April (not shown). Surface METAR obser­ only the 1200 UTC 22 April Eta model (Black 1994) prod­ vations from Cheyenne, Wyoming (CYS) and Craig, ucts are shown. To complement the NCEP guidance, the Colorado (CAG) were chosen to measure the cross-barri- 0000 UTC 23 April RAMS forecasts are also presented. 76 National Weather Digest

------JlIIJ ------,.. -

JlIU

Fig. 5. Same as in Fig. 4, except analyses valid at 1200 UTC Fig. 6. a) The Medicine Bow, Wyoming wind profiler data from 23 April 1999. 1500 UTC 22 April to 0900 UTC 23 April 1999. Medicine Bow is located approximately 158 km north northeast of Steamboat Springs, Colorado. Winds are in knots. Ordinate is height (MSL) in a. NCEP guidance meters. b) The Platteville, Colorado wind profiler data from 1500 UTC 22 April to 0900 UTC 23 April 1999. Platteville is located The 1200 UTC 22 April 1999 NCEP forecasts gave approximately 50 km north of Denver, Colorado. Winds are in strong indications to the possibility of strong winds in knots. Ordinate is height (MSL) in meters. northern Colorado and adjacent areas. A plan view of both the 500-mb level and 700-mb level height contours and grid-point winds (Fig. 7) indicated that the forecasts real atmosphere (Durran 2002). However using the same compared quite well to the observations shown earlier. caveats and assumptions used by Meyers et al. (2002), U The forecast cross-section of potential temperature and is approximated by 12 m S·l and N is approximated by wind (Fig. 8) showed crest-level winds of 20 m S·l and a 0.015 S·l. Assuming an effective barrier height of 2000 m, critical layer around 500 mb near SBS. The potential a Froude number of 0.4 was measured for the north cen­ temperature profile also showed a stable lower tropos­ tral portion of Colorado during this time. This value cor­ phere and a decrease in stability aloft. The cross-section responds to blocked flow, and non-linear flow behavior, also shows a strong stable layer up to 500 mb with a less including acceleration to the lee of the barrier, and possi­ stable layer up to the tropopause. NCEP guidance from ble wave breaking (e.g., Klemp and Lilly 1975; Poulos et the 0000 UTC 23 April forecasts (not shown) showed lit­ al.2000). tle change from the 1200 UTC 22 April forecast indicat­ The wave-like structure of the e and w fields (not ing good continuity between runs. shown) observed in previous research was not seen in the A typical measure of the upstream blocking and the NCEP forecasts. This was most likely due to the coarse potential for non-linear behavior is expressed by the resolution and smoothed terrain surfaces of the model. dimensionless quantity called the Froude number Thus, we used the RAMS forecast to complement the Eta model output and hopefully provide even more evidence Fr=U/NH (1) of possible mountain wave formation. where U is the upstream wind speed, N is the square root b. Local-area model of the Brunt-Vaisrua frequency representative over the mountain depth H (Carruthers and Hunt 1990). There RAMS is a local mesoscale model which was run once are some concerns based on the applicability of Fr in the a day at 0000 UTC during the 1998-99 winter season at Volume 26 Numbers 3, 4 December 2002 77

'50

Fig. 8. 1200 UTC 22 April 1999 Eta 18-h forecast valid 0600 UTC 23 April of cross-section of potential temperature (contour interval is 5 K) and wind barbs (m S·'). Symbols: Grand Junction (GJT), Steamboat Springs (SBS), Fort Collins (FNL) and Scottsbluff (BFF).

Fig.7. a) 1200 UTC 22 April 1999 Eta 18-h forecast valid at 0600 UTC 23 April for 500-mb level height contours (60 m intervals) and wind barbs (m s·'). b) 1200 UTC 22 April 1999 Eta 18-h forecast valid at 0600 UTC for 700-mb height contours (30 m intervals) and wind barbs (m S·' ). 1 .",-"".'1"-'-' ...... 9-'''-' - ~oo. - 400. - 300. - 100. o. 100. 200. x (km) both Colorado State University (CSU) and the NOAA Fig. 9. RAMS 8-h forecast plan view of mean sea-level pressure Forecasts Systems Laboratory (FSL). The operational isobars (4 mb interval; solid bold) valid at 0800 UTC 23 April 1999. RAMS forecasts were supported through funding provid­ RAMS topography is shown in thin solid contours at 300 m inter­ ed by the Cooperative Program for Operational vals; darkest shade is highest elevations. Symbols: Evanston Meteorology, Education, and Training (COMET), operat­ (EVW), Rock Springs (RKS), Rawlins (RWL), Laramie (LAR), ed jointly in cooperation with the National Weather Cheyenne (CYS), Scottsbluff (BFF), Sidney (SNY), Vernal (VEL), Service (NWS), CSU, and FSL. A nested-grid version of Craig (CAG), Steamboat Springs (SBS), Fort Collins (FCL), Price RAMS (Pielke et al. 1992) provided by CSU was utilized (PUC), Eagle (EGE), Denver (DNR), Akron (AKa), Canyonlands for this research. The CSU model utilizes parallel pro­ (CNY), Grand Junction (GJT), Aspen (ASE), Leadville (LXV), cessing for the interactive nested-grid version of the Limon (LlC), Hanksville (4HV), Montrose (MTJ), Gunnison (GUC), Colorado Springs (COS), Pueblo (PUB), La Junta (LHX), Lamar model. The model includes two interactive, nested grids. (LAA), Blanding (4BL), Cortez (CEZ), Durango (ORO), Alamosa The coarse grid covers the western United States with 48 (ALS), Trinidad (TAD) and Farmington (FMN). km horizontal grid spacing; and the fine grid encompass­ es all of Colorado and portions of Wyoming, Utah, South Dakota, Nebraska, and Kansas at 12-km grid spacing. 1999 were employed for this research. All forecast fields Both grids utilize 26 vertical levels beginning at 250-m discussed here are valid at 0800 UTC 23 April, which grid spacing near the surface, gradually stretching to a approximates the start of the severe wind event. The maximum of 1000 m. One distinct advantage of RAMS is forecast ofMSL pressure is shown in (Fig. 9). The RAMS the more advanced physics employed compared to that of forecast shows a tightening pressure gradient between the NCEP model suite. RAMS includes a mixed-phased CYS and CAG with a cross-barrier difference of 0.063 mb 1 microphysical scheme described by Walko et al. (1995). km· • This value is similar to the observed cross-barrier 1 Another distinct advantage of this high-resolution model pressure difference of 0.070 mb km· • A plan view of winds is the improved representation of complex terrain, which (Z=2) in (Fig. 10) shows 26 m S·l easterly flow west of the is invaluable especially over mountainous regions. higher terrain of the Park Range and to the north of SBS. The RAMS forecasts initialized at 0000 UTC 23 April This spatial location of higher winds, which is displaced 78 National Weather Digest

• (m) rrn~:!:!Il=:rrnrrnTl'TrTTTTTTrrnTTT~::!J)'rrnrrrrrrnrrnTl'TTTTT']

8000.

-300. -250. -200. -150. -100. -50. O. x (km) W ...ich Puk Mouniabu Rance

20 25 30 - 500. -400. -300. - 200. - 100. O. 100. " Clem) I I I Fig. 10. RAMS 8-h forecast plan view at Z=2 of grid-point winds -.5 - ,25 -.1 -.05 .05 .1 ,25 .5 (m s" ), isotachs at 5 m s" intervals and shaded (shade scale in m s" below figure) and valid at 0800 UTC 23 April 1999. Topography Fig. 12. RAMS 8-h forecast vertical cross section of w (cm s") is indicated by bold lines with a contour interval of 500 m. Symbols: valid at 0800 UTC 23 April 1999. Downward w denoted by light Craig (CAG), Steamboat Springs (SBS), Fort Collins (FCL), Eagle shading as indicated by scale below figure. East-west cross sec­ (EGE), Grand Junction (GJT), Aspen (ASE) and Leadville (LXV). tion is atY=55 (see fig. 10) just north of SBS.

directly below this wind maximum. The e pattern also depicts a less stable layer in the middle troposphere from 5000-10,000 m MSL overlaying a stable layer above crest-level. The RAMS cross-section of the w component of the wind (Fig. 12) shows a wave structure similar to those described in earlier literature discussed in section 2. Three distinct downward maximums could be seen west of the Colorado Front Range with a downward veloc­ ity maximum of 0.5 m S·l displaced above SBS. Special note should be made of the intense downward velocity seen west of the Wasatch Front (far left, Fig. 12). Just prior to the valid time of the RAMS forecast, a gust to 51 m S·l was recorded near Brigham City, Utah. This mark established a new record for lower elevations in the state of Utah (Dunn 1999). Wuatch Puk The model guidance provided by RAMS closely resem­ Mouniahu Rance E.. bled the findings of previous observational and theoreti­ cal research. Several important mesoscale factors -200. -100. O. 100. x Clem) appeared to have come together in the SBS vicinity, to 0 5 10 15 20 25 30 35 40 45 50 create these damaging winds. Crest-level flow was strong, and isentropes illustrated a mountain wave pat­ Fig. 11. RAMS 8-h forecast vertical cross section of winds tern and favorable stability profile. RAMS also displayed (5 m s" isotach interval with shade scale below figure) and poten­ strong downward descent of the mountain wave in the w tial temperature (5 K contour interval) valid at 0800 UTC 23 April field, and the cross-barrier SLP gradient was significant. 1999. East-west cross-section is at Y =55 (see Fig. 10) just north of Finally, a critical layer was evident in the middle tropos­ SBS. phere. The use of RAMS, in concert with other NCEP guidance, showed a strong potential for a downslope west of the higher terrain of the Park Range compares windstorm west of the Colorado Park Range. well with the damage area shown in (Fig. 2). Several cross-sections were generated in this forecast, nmning 5. Comparison to the 25 October 1997 Forest along an axis from FCL to just south of SLC (approxi­ Blowdown mated by the 40.5 N latitude). The wind and e fields in (Fig. 11) clearly show the mountain wave pattern with The Routt National Forest blowdown of 25 October descending easterly winds of 28 m S·l west of the Park 1997 is the only other mountain wave event occurring Range. The enhanced damage area is located almost west of the Continental Divide in the northern Colorado Volume 26 Numbers 3, 4 December 2002 79

I • ~ • M'I ¥ 2.20 10000 .. . . ) .. I - .. " • "I ~ 8000 l10 ~ ) . I " :[ ~ ., ~ ~ N I... " ... " I I II I .; ., i ' 0000 lOO I # ... ,( + t • • I I + • If " 4000 I -~ - ~ -. - ; - - - I 1')0 I. ~ t 2300 I J ~ ~ ~ ~ ~I ~ ·20 ·10 o , I Xlkm) , S t I ~ ~ + 160 I ~ ~ I . . ~ . Fig. 14. West to east vertical cross section on Grid 4 (1 .67 km grid , t ~ • I .1 J spacing) at Y = 185 (Fig. 13) after 34 h of simulation time, valid at , I 1000 UTe 25 October 1997, with potential temperature (2K inter­ , ~ ~ . vals). U-component of the wind is shaded (near 3500 m MSL: hor­ 170 I ~ • ~ , f' I I izontal stippling less than -35 m S·' and vertical stippling less than - T -r 7'-,r -40 m s·') (near 8000 m MSL: thick stippling greater than -5 m s·, and thin stippling greater than 0 m s·'). Height (MSL) is in meters. -.2.0 -10 SPL 0 10 .2.0 (Meyers et aL 2002) t,~ z~ . o III ~-I-3>- M~x X (kIll) ;H -:U bl ,,-1--3>- 1997 blowdown. Mountain top temperatures of -4 OF 14-:4:1 111"'-'-:- 7-14 IU ,,-I ..... were recorded in the 1997 event, whereas readings of 0- 7 itt. '1\-1 .. around 16 OF were recorded during the 1999 event. Also, surface temperatures at the mid-point of each wind event Fig. 13. Plan view of wind speed (m s·') on Grid 4 (1.67 km grid were taken from METAR observations at nearby spacing) after 34 h of simulation time, valid at 1000 UTe Hayden, Colorado (HDN, Fig. 2). Surface temperatures of 25 October 1997. Wind speeds are contoured every 5 m s·" with minimum contour 15 m s·' . Wind arrows are plotted every other grid the 1997 event were 14 OF colder than those observed in point with speed legend at bottom of figure. Symbols: Steamboat the April 1999 event (23 OF in 1997 vs. 37 OF in 1999). The Springs (S8S), Storm Peak Laboratory (SPL), Mount Zirkel (MTZ) much colder lower tropospheric air present in 1997 may and Mount Ethel (MTE). (Meyers et aL 2002) have been a significant factor in the magnitude of that event. The cold air may have also generated a stability structure to the lee of the Park Range, which inhibited Rockies that has been examined in detail (Meyers et al, the extent of the downslope winds to lower elevations, 2002). This event produced wind gusts in excess of 50 such as the town of Steamboat Springs, and thus, con­ m S·I, which downed 13,000 acres of old growth forest in centrated damage to the higher elevations. the Park Range. The toppled trees fell in a swath several Investigations of the 1997 event through numerical miles wide and 20 miles long. The findings of various modeling were conducted using the RAMS model. studies pertaining to this devastating blowdown will be Findings of these simulations indicated that strong discussed here and compared to those of the 22-23 April easterly cross-barrier flow with a favorable stability 1999 event, profile was the primary meteorological factor enhanc­ Analyses at 0000 UTC 25 October 1997 (Meyers et al. ing the mountain wave. Results of the finest- grid 2002) indicated a deep, closed low in northeast New RAMS simulations (1.67 km) depicted 28 m S·l east Mexico which produced lower and middle tropospheric winds to the lee of the Park Range at the surface (Fig. easterly flow of 15-20 m S·l across northern Colorado, This 13). A cross-section of the u-component ofthe wind and closed low was much farther east than that observed in e taken perpendicular to the Park Range showed a 42 the April 1999 event. Froude number calculations for m S·l maximum descending the west side of the barri­ both events were quite similar, with values falling in the er along with tight isentrope packing just above crest­ non-linear flow regime (-0.5). Surface observations dur­ level (Fig. 14). A high amplitude mountain wave struc­ ing the 1997 event indicated a cross-barrier SLP differ­ ture was evident in the isentrope pattern. Additionally, ence of only 0.026 mb km·1 between CYS and CAG. This a wave induced critical level was observed in the mid­ value was far less than the 0.070 mb km·1 difference dle troposphere (-8000 m MSL), characterized by a observed in the April 1999 event. Another observed para­ flow reversal (westerly wind). Despite the varying hor­ meter that varied greatly between the two mountain izontal grid spatial differences of the two RAMS fore­ wave events was lower tropospheric temperature. Meyers casts (12 km in 1999 and 1.67 km in 1997), the overall et al. (2002) showed how deep, very cold lower tropos­ structure of the wind and e fields were somewhat sim­ pheric air enhanced mountain wave formation during the ilar. The difference was the magnitude of the winds 80 National Weather Digest and amplitude of the mountain wave, some of which greater than that generated during the 1997 event. A may be attributed to the differences in the horizontal comparison of the Colorado Park Range windstorms and vertical grid resolutions. indicates that cross-barrier sea-level pressure gradient may not be a good predictor of event severity in that 6. Conclusions region for certain types of wind events. However, more research needs to be conducted concerning downslope This research was prompted by the occurrence of a windstorms west of the Continental Divide and the severe mountain wave event west of the Colorado Park relationship to cross-barrier SLP difference. Range on 22-23 April 1999, the second such event with­ Operational forecasters west of a mountain barrier in a span of two years in that region. Prior to the 25 should be attentive to cut-off lows south or southwest of October 1997 Routt National Forest blowdown little the forecast area that could produce strong synoptic knowledge or literature existed regarding mountain easterly winds. Wind profilers and the velocity azimuth wave events west of the Continental Divide in display (VAD) wind profile of the WSR-88D can supple­ Colorado. A detailed examination ofthe mountain wave ment upper-air reports to determine the presence of of 22-23 April 1999 and subsequent comparisons to the embedded wind maxima. This could prove especially 1997 event provide some insight to the synoptic and helpful in data sparse regions of the west and between mesoscale characteristics common to this phenomenon. radiosonde launch times. NCEP guidance observations served as a good diag­ nostic tool to infer the potential for strong winds on 23 Acknowledgments April 1999. The guidance depicted strong easterly cross-barrier flow and the presence of a mean-state The authors wish to thank John Snook and Douglas critical level evident in the middle troposphere. Wesley for their insights and comments on this However, the NCEP models did not forecast a downs­ research. The authors would also like to thank Dr. lope wind event most likely due to the coarse horizon­ Gregory Poulos and Charlie Liles for their thorough tal resolution and poor representation in the model. A review of the manuscript. A special thank you to Drs. local-area operational model (RAMS) was used in con­ Randy Borys and Melanie Wetzel of the Desert cert with the Eta model to forecast the magnitude and Research Institute's Storm Peak Laboratory (SPL) for location of the mesoscale phenomena. The 12-km graphs and data from SPL and for a thorough review of RAMS forecast added good predictive value in forecast­ the research. Also, thanks to NWS Forecast Office ing the severe winds. RAMS forecast fields of wind and Grand Junction Hydrologist Brian Avery and forecast­ e depict the mountain wave pattern and descending er David Nadler for their assistance with the manu­ winds (28 m S·l) west of the barrier. Forecast profiles of script. Additional thanks are given to Liz Page and middle tropospheric instability overlaying regions of Jonathan Heyl for providing archived data used to crest-level stability, and forecasts of downward vertical examine the event. velocity also gave operational forecasters strong indica­ Support for this research was provided, in part, by tions of mountain wave formation. the National Weather Service's Cooperative Program The results qualitatively resembled the overall wind for Operational Meteorology, Education and Training flow structure observed in the RAMS simulation (1.67 (COMET) under UCAR Grants S96-71867 and SOO- kID grid spacing) of the 25 October 1997 event. Strong 19126. The COMET funding is provided by a coopera­ synoptic and crest-level easterly flow, and the presence tive agreement between the National Oceanic and of a critical level were common to both episodes. In Atmospheric Administration (NOAA) and the addition, RAMS e profiles of the 1997 event clearly University Corporation for Atmospheric Research showed a well-developed mountain wave with a stable (UCAR). The views expressed herein are those of the layer, and with lower stabilities aloft. Both events authors and do not necessarily reflect the views of occurred simultaneously with a devastating snowstorm NOAA, its sub-agencies, or UCAR. along the Front Range of Colorado. Several factors appear to have caused the higher wind speeds forecast­ Authors ed in the 1997 event (42 m S-l to the lee of the barrier in the 1997 event, whereas a 28 m S-l maximum was fore­ Christopher N. Jones is the Warning Coordination cast during the 1999 event). First, Meyers et al. (2002) Meteorologist at WFO Riverton, WY. He previously showed how the presence of very cold, lower-tropos­ served at four NWS offices: WSFO Cheyenne, WY, WSO pheric air modified the stability profile, thereby Casper, WY, NWSO Paducah, KY, and WFO Grand enhancing mountain wave development by nonlinear Junction, CO. He received his B.A. in meteorology from effects. Mountain top temperatures in the 1997 event the University of Northern Colorado in May 1995. were 20 OF colder than those measured in the 1999 Chris' interests include mountain meteorology, public event. Secondly, 700-mb winds were stronger in the outreach, and operational forecasting. 1997 event than in the 1999 event. Contrary to the sta­ Jeffrey D. Colton is a Senior Forecaster at WFO bility profiles and forecast winds, which favor the 1997 Grand Junction, CO. Previously, he served as a jour­ event for higher wind speeds, the cross-barrier sea­ neyman forecaster at WFO Amarillo, TX, and as a level pressure gradient favored the 1999 event. The Meteorologist Intern at WFO Cheyenne, WY. He 1999 episode generated 0.070 mb kID-I pressure gradi­ received his B.S. degree in meteorology from ent between CYS and CAG, a value nearly three times Metropolitan State College in Denver, CO in May 1992. Volume 26 Numbers 3, 4 December 2002 81

Jeff's interests include mountain meteorology, severe Cotton, W R, J. F. Weaver and B. A Beitler, 1995: An storms forecasting, public outreach and operational unusual summertime downslope wind event in Fort forecasting. Collins, Colorado on 3 July 1993. Wea. Forecasting, 10, Ray L. McAnelly is a Senior Research Associate in 786-797. the Department of Atmospheric Science, Colorado State University. He received his B.S. and M.S. degrees Doyle, J. 0., D. R Durran, C. Chen, B. A Colle, M. in Meteorology from Texas A&M University in 1976 Georgelin, V. Grubisic, W R Hsu, C. Y. Huang, 0. Landau, and 1980, respectively. His research at CSU has Y. L. Lin, G. S. Poulos, W Y. Sun, D. B. Weber, M. G. focused on convective storm systems and Wurlele and M. Xue, 2000: An intercomparison of model mesoscale/cloud-scale modeling. He helped develop and predicted wave breaking for the 11 January 1972 Boulder continues to maintain the realtime RAMS forecast windstorm. Mon. Wea. Rev., 128,901-914. operations for the Colorado region at CSu. Through the COMET Outreach program, he has collaborated with Dunn, L., 1999: Downslope windstorm case study NWS Central Region forecasters on many case studies. Wasatch Front -April 23, 1999. Western Region Technical Michael P. Meyers, Ph.D., has been the Science and Attachment NO. 99-09. Operations Officer at the National Weather Service Forecast Office in Grand Junction, Colorado since 1995. Durran,o. R, and J. B. Klemp, 1987: Another look at downs­ In addition, Michael held a Department of Energy lope windstorms. Part II: Nonlinear amplification beneath Global Change Distinguished Postdoctoral Fellowship wave-overturning layers. J. Atmos. Sci., 44, 3402-3412. through the Oak Ridge Institute of Science and Education from 1994 through 1995. Michael's educa­ ____, 1990: Mountain waves and downslope wind­ tional background includes a B.S. (1985) in storms. Meteorological Monograph, Atmospheric Meteorology from Millersville University of Processes over Complex Terrain, 23, No. 45, Blumen, W. Pennsylvania and M.S. (1989) and Ph.D. (1995) degrees Ed., Amer. Meteor. Soc., 59-81. in Atmospheric Sciences from Colorado State University. ____, 2002: Downslope winds. Encyclopedia of Atmospheric Sciences, J. Holton, J. Pyle, and J. Curry, References Eds., Elsevier, 644-649.

Black, T. L., 1994: The new NMC mesoscale Eta model: Klemp, J. B., and 0. Lilly 1975: The dynamics of wave Description and forecast examples. Wea. Forecasting, 9, induced downslope winds. J. Atmos. Sci., 32, 320-330. 265-278. Lee T. J., R A Pielke, R C. Kessler and J. Weaver, 1989: Borys, R D., and M. A Wetzel, 1997: Storm Peak Influence of cold pools downstream of mountain barriers Laboratory: A research, teaching, and service facility for on downslope winds and flushing. Mon. Wea. Rev., 117, the atmospheric sciences. Bull. Amer. Meteor. Soc., 78, 2041-2058. 2115-2123. Lilly, D. K., and E. J. Zipser, 1972: The Front Range wind­ Brown, J. M., A A Rockwood, J. F. Weaver, B. 0. Jamison storm of 11 January 1972 - A meteorological narrative. and R Holmes, 1992: An expert system for the prediction Weatherwise, 25, 56-63. of downslope windstorms. Fourth Workshop on Oper. Meteor., Sep 15-18, Whistler, B.C. Canada. Meyers, M. P., J. S. Snook, 0. A Wesley and G.S. Poulos, 2003: A Rocky Mountain storm - Part II: The forest blow­ Carruthers,o. J., and J. C. R Hunt, 1990: Fluid mechan­ down over the West Slope of northern Colorado moun­ ics of airflow over hills: Turbulence, fluxes and waves in tains - observations, analysis and modeling. Wea. the boundary layer. Meteorological Monograph, Forecasting, 18,662-674. Atmospheric Processes over Complex Terrain, 23, No. 45, Blumen, WEd., Amer. Meteor. Soc., 83-107. Peltier, W R, and T. L. Clark, 1979: The evolution and sta­ bility of finite-amplitude mountain waves. Part II: Clark, T. L., and R 0. Farley, 1984: Severe downslope Surface wave drag and severe downslope windstorms. J. windstorm calculations in two and three spatial dimen­ Atmos. Sci., 36,1498-1529. sions using anelastic interactive grid nesting. J. Atmos. Sci., 41, 329-350. Pielke, R A, W R Cotton, R L. Walko, C. J. Tremback, W A Lyons, L. 0. Grasso, M. E. Nicholls, M. D. Moran, D. A ____, and W R Peltier, 1977: On the evolution and Wesley, T. J. Lee and J. H. Copeland, 1992: A comprehen­ stability of finite-amplitude mountain waves. J. Atmos. sive meteorological modeling system RAMS. Meteor. Sci., 34,1715-1730. Atmos. Phys., 49, 69-91.

Colle, B. A, and C. F. Mass, 1998: Windstorms along the Poulos, G. S., J. E. Bossert, R A Pielke and T. B. McKee, western side ofthe Washington Cascade Mountains. Part 2000: The interaction of katabatic flow and mountain II: Characteristics of past events and three-dimensional waves, Part I: Observations and idealized simulations. J. idealized simulations. Mon. Wea. Rev., 126,53-71. Atmos. Sci., 57,1919-1936. 82 National Weather Digest

____, D. A. Wesley, J. S. Snook and M. P. Meyers, Svejkovsky, J., 1985: Santa Ana Airflow Observed From 2002: A Rocky Mountain storm - Part I: The blizzard - Wildfire Smoke Patterns in Satellite Imagery. Mon. Wea. lrinematic evolution and the potential for high-resolution Rev., 113, 902-902. numerical forecasting of snowfall. Wea. Forecasting, 17, 955-970 .. Walko, R. L., W R. Cotton, M. P. Meyers and J. L. Harrington, 1995: New RAMS cloud microphysics para­ Smith, R. B., 1987: Aerial Observations of the meterization. Part I: The single-moment scheme. Atmos. Yugoslavian Bora. J. Atmos. Sci., 44, 269-297. Research, 38, 29-62. AUTHOR INDEX TO VOLUME 26

Andretta, Thomas A., 2002: Climatology of the Snake River Meyers, Michael P., Brian A. Avery, Christopher N. Jones and Plain Convergence Zone. Natl. Wea. Dig. 26:3,4, 37-51. Jeffrey D. Colton, 2002: A Southwest Colorado Mountain Flash Aune, Robert M., Anthony J. Schreiner and Timothy J. Schmit, Flood in an Enhanced Southwest Monsoonal Environment. Nat!. 2002: Maritime Inversions and the GOES Sounder Cloud Wea. Dig. 26:3,4, 9-18. Product. Nat!. Wea. Dig. 26:1,2, 27-38. =-__----:-:-' Christopher N. Jones, Jeffrey D. Colton and Ray Avery, Brian A. , Christopher N. Jones, Jeffrey D. Colton and L. McAnelly, 2002: An Examination of a Severe Downslope Michael P. Meyers, 2002: A Southwest Colorado Mountain Windstorm West of the Colorado Park Range. Natl. Wea. Dig. Flash Flood in an Enhanced Southwest Monsoonal 26:3,4, 73-82. Environment. Nat!. Wea. Dig. 26:3,4, 9-18. Miskus, David, and Robert S. Robinson, 2002: Results of the Bernardet, Ligia R., 2002: Verification of High-Resolution NWA Agricultural Weather Survey. Natl. Wea. Dig. 26:1,2,3-6. Precipitation Forecasts for the 1996 Atlanta Olympic Games. Moneypenny, Michael, and Douglas Schneider, 2002: The Nat!. Wea. Dig. 26:3,4, 19-36. Mesoscale Seeder-Feeder Mechanism: Its Forecast Implications. Blechman, Jerome B., 2002: Surface Wind Directions Nat!. Wea . Dig. 26: 1, 2, 45-52. Associated with Snowfall in Upstate New York: Update for Noel, James, and Jeffrey C. Dobur, 2002: A Pilot Study 1994-2000. Natl. Wea. Dig. 26:3, 4, 63-72. Examining Model-Derived Precipitation Efficiency for use in Byrd, Stephen F., and Catherine M. Zapotocny, 2002: An Precipitation Forecasting in the Eastern United States. Natl. Examination of the Eastern Nebraska and Western Iowa Flash Wea. Dig. 26:3,4, 3-8. Flood Event of6-7 August 1999. Nat!. Wea. Dig. 26:1,2, 7-26. Przybylinski, Ron W., David A. Skerritt and Ray A. Wolf, 2002: Colton, Jeffrey D., Brian A. Avery, Christopher N. Jones and A Study of the 6 December 1995 Midwest Snow Event: Michael P. Meyers, 2002: A Southwest Colorado Mountain Synoptic and Mesoscale Aspects. Nat!. Wea. Dig. 26:3, 4, 52-62. Flash Flood III an Enhanced Southwest Monsoonal Robinson, Robert S., and David Miskus, 2002: Results of the Environment. Natl. Wea . Dig. 26:3,4, 9-18. NWA Agricultural Weather Survey. Nat!. Wea. Dig. 26:1,2,3-6. ____" Christopher N. Jones, Ray L. McAnelly and Schmit, Timothy J., Anthony J. Schreiner and Robert M. Aune, Michael P. Meyers, 2002: An Examination of a Severe 2002: Maritime Inversions and the GOES Sounder Cloud Downslope Windstonn West of the Colorado Park Range. Natl. Product. Natl. Wea. Dig. 26:1,2,27-38. Wea. Dig. 26:3, 4, 73-82. Schneider, Douglas, and Michael Moneypenny, 2002: The Dobur, Jeffrey C., and James Noel, 2002: A Pilot Study Mesoscale Seeder-Feeder Mechanism: Its Forecast Implications. Examining Model-Derived Precipitation Efficiency for use in Natl. Wea. Dig. 26:1,2,45-52. Precipitation Forecasting in the Eastern United States. Natl. Schreiner, Anthony J., Timothy J. Schmit and Robert M. Aune, Wea. Dig. 26:3,4, 3-8. 2002: Maritime Inversions and the GOES Sounder Cloud Ellrod, Gary P., 2002: Estimation of Low Cloud Base Heights at Product. Natl. Wea . Dig. 26:1,2,27-38. Night from Satellite Infrared and Surface Temperature Data. Skerritt, David A., Ron W. Przybylinski and Ray A. Wolf, 2002: Nat!. Wea. Dig. 26:1,2, 39-44. A Study of the 6 December 1995 Midwest Snow Event: Jones, Christopher N., Brian A. Avery, Jeffrey D. Colton and Synoptic and Mesoscale Aspects. Natl. Wea. Dig. 26:3, 4, 52-62. Michael P. Meyers, 2002: A Southwest Colorado Mountain Wolf, Ray A., Ron W. Przybylinski and David A. Skerritt, 2002: Flash Flood in an Enhanced Southwest Monsoonal A Study of the 6 December 1995 Midwest Snow Event: Environment. Natl. Wea . Dig. 26:3,4, 9-18. Synoptic and Mesoscale Aspects. Natl. Wea. Dig. 26:3, 4, 52-62. ____, Jeffrey D. Colton, Ray L. McAnelly and Michael Zapotocny, Catherine M., and Stephen F. Byrd, 2002: An P. Meyers, 2002: An Examination of a Severe Downslope Examination of the Eastern Nebraska and Western Iowa Flash Windstorm West of the Colorado Park Range. Nat!. Wea. Dig. Flood Event of6-7 August 1999. Natl. Wea. Dig. 26:1,2,7-26. 26:3, 4, 73-82. Features in Volume 26: McAnelly, Ray L., Christopher N. Jones, Jeffrey D. Colton and NWA Bylaws. 26:1, 2, 55-56. Michael P. Meyers, 2002: An Examination of a Severe NWA Weathercaster Seals of Approval. 26: 1, 2, 61-64. Downslope Windstorm West of the Colorado Park Range. Nat!. Author Index to Volume 26. 26:3, 4, 82. Wea. Dig. 26:3,4, 73-82. 2002 NWA Annual Awards. 26:3,4, 89-92.