THE WEATHER and CIRCULATION of MAY 1960 Including a Discussion of the Unusual Retrogression of 5-Day Mean Polar Vortices

MAY 1960 MONTHLY WEATHER REVIEW 193

THE WEATHER AND CIRCULATION OF MAY 1960 Including a Discussion of the Unusual Retrogression of 5-Day Mean Polar Vortices

JAMES F. ANDREWS Extended Forecast Section, U.S. Weather Bureau, Washington, D.C.

1. MEANCIRCULATION temperaturesprevailed (fig. 4A). Thegreatest negative departures were observed in the Pacific Northwest where The monthly mean circulationat 700 mb. for May 1960 temperaturesaveraged 4' F. below normal.Greatest (fig. 1) was characterized by strong blocking over much positive departures were found in theNortheast where of the higher latitudes of the Northern Hemisphere. In Caribou, Maine, reported 9' F. above normal for May. terms of 700-mb. heightanomaly, the principal centers KO monthly mean temperature records were established, of blocking were located over eastern Canada (+320 ft.) one reason beingthat May hadtwo relatively well-defined and northern Siberia (+370 ft.) with another, somewhat temperature regimes (see section 3). weaker,center over southwestern Alaska (+ 180 ft.). The May temperature pattern was well related to the The effect of this blocking on the mid-latitude planetary meancirculation pattern. In general the cool weather circulation was to produce a pattern of truncated trough- was associated with below normal 700-mb. heightsand ridgesystems with rather short wave spacings. The westerlies well south of normal across the United States exception tothis was inthe Pacificwhere the weaker (figs. 1 and 2). The thickness of thelayer between Alaskan block was associated with a simple zonal pattern 1000 mb. and 700 mb. was also below normal over most and full-latitude mid-Pacific trough (fig. 1). of the Nation (fig. 5). The warmth in the Northeast was A further effect of the blockingwas to displace the related tothe strong ridge over eastern Canada and mid-troposphericwesterlies at 700 mb.south of their associated southeasterlyanomalous flow atthe surhce normalposition over most of the western part of t'he (fig. 3) and aloft (fig. 1). Thicknessvalues were also NorthernHemisphere (fig. 2A). Thegreatest displacc- above normal in this area. ment of the zone of nlaximurn west winds was over the Figure 4B shows the total precipitation and figure 4C easternUnited States and western Atlantic, t'he area the percentage of normalprecipitation for May 1260. south of theCanadian block. Windspeeds at 700 mb. Near record amounts fell in the Pacific Northwest where were as much as 7 m.p.s. below normal near Nova Scotia some areas received more than twice their normal tobals (fig. 2B).Another area of subnormal wind speeds was for themonth. Heaviest amounts fell along the coast inthe Gulf of Alaska,,a manifestation of the Alaskan and at t>hehigher elevations. Mt. Shasta, Calif., with a block. Thefastest windspeeds, upto 6 n1.p.s. above record dating back to 1888, recorded its greatest snowfall normal, were found in the eastern Pacific. for May, while StampedePass, Wash., set a new May The effects of blocking at '700 mb. were also apparerlt record for total precipitation with 9.12 inches. onthe monthly mean sea level chart for May (fig. 3). Precipit'ation was also heavy in portions of the Mid- Pressures averaged well above normal over much of the West and Middle Atlantic Stat'es(figs. 4B, 4C). La Crosse, higherlatitudes, with centers of greatestdeparture in Wis., experienced itswettest May since observations northern Siberia(+ 14 nlb.), the westernPacific (+S rllb.), began in 1873. Near record amounts fell in Green Bay, andeastern Canada (3-7 mb.). Sea level pressures and Wis., Marquette, blich., and Tulsa, Okla. Areas receiv- 700-mb. heights were both well below normalacross ing less than half theirnormal totals for May were most of the United States and Atlantic (figs. 1 and 3), a Wyoming,northern and eastern Texas, northwestern furtherindication of thestrong blocking in eastern Georgia, and central Florida (fig. 4C). Canada. Precipitation, as is usually the case during the warm 2. AVERAGEUNITED STATES WEATHER season, was not too well related to the mean circulation. A strongrelationship existed in the Pacific Northwest, May was a rather cool month over much of the con- however, where cool, wet weather accompanied stronger tiguousUnited States. Temperat,ures were generally than normal southwesterly flow from the trough in the below normal, except in t'he central and southern Rocky east'ern Pacific (figs. 1, 2B). Furthermore,the heaviest MountainStates, the h-orthern Plains, along the Cali- precipitation fell close to or justnorth of the zone of forniacoast, and in the Nort,hea,st,where a,bove normal maximum wind at' 700 mb. (figs. 2, 4B).The band of

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FIGURE1.-Mean 700-mb. contours (solid) and height drparturcs from normal (dotted), both in tens of feet, for May 1960. Blocking was a prorninrnt feature of the circulation.

precipitationstretching from Nevada to New Mexico, fromnormal isoline (figs. 1, 4B, 4C). Amountswere with lower percentages to the north and south (figs. 4B, much greater in the surrounding area. 4C), appearsto bear some relationship to the difluent Alaska was unseasonablywarm during May with circulation pattern at 700 mb. (fig. 1). Most of t'hepre- mostly subnormal amounts of precipit'ation (fig. 4). Both cipitation in the eastern half of the United States can be Fairbanksand Juneau experienced theirwarmest Mays associated with the deep mean trough over t'hat area. It of record,while Yakutatreported its driest May. This is interesting to note, however, that the smaller amounts warm,dry regime was associated witha strong ridge, (and percentages) were observed in the trough itself and above normal 700-mb. heights, and positive thickness in generally in the area defined by the -100-ft. departure the layer 1000 mb. to 700 mb. (figs. 1, 5).

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FIGURE2.-(A) Mean 700-mb. isotachsand (B) departurefrom monthly normal wind speed (both in meters per second) for May 1960. Solidarrows in (A) indicateaxes of primarywest wind maxima with the normal position dashed. Because of blocking, the westerlies were displaced to lower latitudes.

FIGURE4.-(A) Departure of averagetemperature fromnormal (OF.) for M3y 1960. (B) Totalprecipitation (inches) forMay 1960. (C) Percentage of normalprecipitation for May 1960. (From [I].)

FIGURE3.--Mean sealevel isobars (solid) and t,heir departures fromnormal (dotted), both in millibars, for May 1960. Excess of pressure at highlatitudes and deficit at low latitudes were associatedwith high-latitude blocking.

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9 835 12 27 12 30 12 41 12 42 13 *41 13 *38 13 *42 13 *50 13 42 13 39 13 40 - *Also a record low for any May.

periodfor any Slay. Snowfall amounts of 3 to 5 inches were common in much of Wisconsin and Upper Michigan. At' Oklahoma City,Okla., a windspeed of72 m.p.h. exceeded the previous record €or any May. As the storm moved slowly nort,heastward, it becameblocked by a FIGIJRE5.-Mean thickness (1000-700 mb.) drpart,ure from normal 1;~rgchigh pressurearea over Hudson Bay. Both high (tens of feet) for May 19BQ. Below normal thickness otrer much and low pressure syst>ems remained nearly stationary for of thecontiguous United States was associatrd with grrlerallp t8heperiod May 7-13. Cold air moving southward to the cool surface temperature. rear of thestorm resulted innumerous temperature records hcing cstttblished for the lowest reading so late in An unusualevent occurred in the HawaiianIslmcls the spring. Some of t'hese are listed in table 1. when the first, thunderst80rmsever observed during Xhy While theEast was experiencingt,emperat#ures more c.h;~rnc~t~eristicof ,2Zarch, the West was enjoying consider- were reported at Honolulu.They accompanircl the ably wtmncr weather during thefirst hlf of Xfay (fig. passage of a500-mb. Low over theIslands and werc 7A). Much of thiswarmth occurrd during tlle period hhy responsible for anew May precipitation recordat Honolulu. 11-13 when many cities established daily rnrtxirnum tem- 3. TWO CONTRASTINGWEATHER AND peraturerecords. In addition,several stat,ions set new CIRCULATIONREGIMES lnnsi~nunl temperature records for occurrence so early in t'he )-ear, RS shown in hble 2. Thisextreme warmth, May's weat'herand circulation was composed of two coupled wit,tl record cold in theEast, resulted from a large- rat'her well-defined regimes, particularly with respect to amplitude ridge-trough syst8em. temperat'ure and circulat'ion. In figure 6 areshown t'he The circulat8ion reversalwhich occurred near mid-month half-month mean circulation patterns at 700 mh. for hlay was pritnarilyrelated t,o est~ablishment of a newtrough 1960, along with the heightchanges between thetwo in the mid-Pacific (fig. 6). This new development in the periods, while figures 7 and 8 show t'he corresponding rnid-t#roposphericcirculat'ion effectively shortened the temperatureanomaly and total precipit'at'ion. During wavelengthbetween the p1anet'ttr.y waves, thus forcing the first half of May a deep trough was observed over the east'ward the downst,reamt,rough-ridge systems. As a eastern United States,while ridge condit'ions predominat'ed result, the trough over east,ern YorthAmerica was replaced in t8he,West (fig, 6A). Strong blocking existed in eastern by a ridge, whilc the opposit'e change occurredin the West. Canadaand helped maintain the west'erlies well south This t'rtmsitiorl in the mean 700-mb. circulation at mid- of theirnormal positionacross eastern North America. month cm,perhaps, best be seen by reference to figure CC, The trough was associated with temperatures well below which shows the height changes between the two halves normal over most of the east,ern United States, as much of Ala)-. The rather uniform spacing of change centers in as 8O E". in Kentucky and Tennessee (fig. 7A). Only the extremeNort'heast, under tlle influence of a ridge and positive height anomalies, was warm. Mwh of the cool weat,herwas the result of a single stormsystem whichdeveloped in the Southwestabout May 4. This storm brought heavy rains, hail, and high winds t,o many sect,ions,tornadoes in Oklahoma and Arkansas,and heavy snowfall in Wisconsin, IJpper Michigan, and the eastern slopes of the Colorado Rockies. Dubuque, Iowa, reported 4.38 inches of rain in ti 24-l~our period on the 5t.h and 6th, t.he largest amount in ti similar.

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MAY 1960 MONTHLY WEATHER REVIEW 197

FIGURE6.-Fifteen-day mean 700-mb. heightcontours (solid) and departures from normal (dotted) (both in tens of feet) for (A) May 1-15, 1960, and (B) May 1e-30, 1960. (C) Height change (tcns of feet) from (A) to (B). Circulation reversal over North America was accompaniedby a changein temperature regimes inthe Unitcd States.

('001 weather in thc West was the result of frequvrlt' intrusions of polar Pacific air massesassociated withthe mean trough and below normal 700-mb. heights (figs. BB, 7B). Temperaturcsaveraged as much as 8' F. below nornral in Oregon. The strorlgest of these cold intrusions brought tlle lowest temperatures ever observed so late in the spring to marly areas of the Far West. Some of these arc listed in t'able 3. The precipitation pattern (fig. 8) was not as well related to the mid-nlonth circulat'ion reversal as was temperature. This is notunusual in view of the discontinuous and sporadicnaturn of precipitation, especially duringthe warmseason. In general,however, tlle area from the Gulf States to Xew England received somewhat' less precipitation during the latter half of May, while the Plains the western part of the hemisphere reflects quite well thc Statesand MississippiValley received more (fig. 8). complete circulation reversal there. Heavier amount's also fell along the north Pacific coast as These changes in circulat'ion were accompanied by an equally pronounced reversal of temperature regimes in the UnitedStat'es (fig. 7).Most of theEast experienceda rapid change t'o warmer, while the mest reverted to a cold regime. Because of the persistence of blocking and above Station normal700-mb. heights ineastern Canada, the North- ature (" F.) eastremained warm for the entire month. Caribou, "___ 19 34 Maine,established several daily maximum temperature 19 31 20 37 records, and a reading of 91' F. on the29th was the 22 40 22 28 highest temperature ever observed there so early in the 22 33 22 27 season.Elsewhere in theEast the unseasonablywarm 23 26 weatherwas generally not of arecord-breaking nature.

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FIGCRE7,"Departure of average temperature from normal (" F.) FIGURE8.-Observed precipitation(approximate) in inches for for (A) May 1-15, 1960, and (B) May 1G-30, 1960.Areas of (A) >fay 1-15, 1960, and (Bj May 16-30, 1960. Areas of amounts negative departure greater than 2 are stippled. Sharp tempera greater than 2 inches are hatched. ture reversal is evident.

the mean trough in the eastern Pacific moved toward the drawn thrice-weeklyin theExtended Forecast Section. coast (figs. 6, 8). Poshions shown are one week n.part, and are plotted on the Weather and circulation reversals of this t'ype are not middle day of the 5-day period. unusual and have beendiscussed in previous articlesof this One striking ieature of these tracks is the continuous series. In fact, this is the second consecutive month t'hat retrogression of thevortices around t'he pole. Only the thishas occurred. Thus, duringApril-May 1960 ahalf- path of the upper-levelanticyclone appeared to be dis- month periodicity was observed, primarily in the t,empera- cont'inuousover theCanadian Arctic near mid-month ture and circulation patterns in the United States. How- (fig. 9A). At that time the major 700-mb. High departed ever, while April [2] was predominantly a warm rnont,h, from its polarorbit and moved southward into eastern May was cool. This wasbecause the zonalwesterlies Canada. At the same time, however, the primary surface across the United States were much fart,hersout,h in May. highpressure areacontinued in its circumpolar path, although a break-off High (fig. 9C) did move southward in 4. RETROGRESSION OF 5-DAYMEAN POLAR association with the upper-levelcenter (fig. 9A). Anin- CYCLONICAND ANTICYCLONIC VORTICES spection of daily Northern Hemisphere sealevel charts for May also disclosed the predominantly continuous nature During May 1960 the polar circulation of the Northern of the retrogression of the surface cells. Hemisphere was dominated primarily by two 5-clay mean A complete physical explanation of the unusually per- cells, one an anticycloneand the other a cyclone. In sistent retrogression of t,hese mean polar vortices cannot figure 9 are shown the tracks of these high andlow cells at be attempted here. However, a brief study was made of 700 mb. and sealevel, as prepared from 5-daymean charts how well the polar trough was predicted by the daily and

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FIGURE9.-Tracks of 5-day mean polar vortices during May 1960 at 700 nlb. for (A) anticyclone, (B) cyclone, and at sea level for (c) anticyclone, (1)) cyclone. Top number is middle dayof period; lower numberon (C) and (I)),intensity of center (millibars). Continuow retrogression of high-latitude vortices around the polc was the most unusual circulation feature of May.

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-2 - u) P -z X z0 -I6 z 0

SEA Lz'VEL POLAREAST€RLlES 0

-2 23 30 7 14 21 28 2 APRIL MAY 1960 JUNE

FIGURE10.-Time variation of 700-mb.polar westerlies (top) and sea level polar easterlies (bottom) (both in meters per second) over thewestern sector of theNorthern Hemisphere between latitudes 55" and 70" N. Solidlines connect 5-day mean index values (plotted at middle of 5-day period and computed thrice weekly), anddashed lines show variation of corresponding normal indices. The polar circulation featured two index cycles during May 1960.

90 80 75

55 - - 50 Y sc 45 -I J 40

30 FIGURE 12.-Observed 5-daymean temperature classes overvthe United States for periods corresponding to those in figure 13.

25 23LL4& 30 "q14 28 4 APRIL MAY 1960 JUNE time variation during May 1960 of the 5-day mean values FIGURE11.-Time-latitude section of 5-day mean zonalwind of 700-mb. polar west,erlies and sea level polar easterlies components at 700 mb. averaged over the western part of the averagedover the western part of theNorthern Hemi- NorthernHemisphere. Isotachs are in meters per second with sphere between 55O X. and 70' N. Clearly there were two easterlywinds stippled. Primary windaxes (botheasterly and westerly) are shown as solid arrows. The alternating periods of complete index cycles which were closely related to posi- net easterly and westerly flow at the higher latitudes reflect the tions of the sub-polar cells. As an example, notethat periodicity in the polar circulation. when the cyclonic vortex occupied the Canadian Arctic

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FIGURE13 -5-day mean 700-mb. contours(tens of feet)for (A) May 5-9, (B) May 17-21, and (C) May 26-30, 1960. Recur- rence of the circulation was an interesting feature of the month.

5-day mean 700-mb. charts for the Northern Hemisphere forthree selectedperiods. The similaritybetween the circulation patterns for May 5-9 and May 26-30 is quite apparent. Note in particular the low centers over central Asia, southern Europe, the North Atlantic, andEllesmere Island. The patterns in North America were also similar, particularlywith respect to the trough-ridge positions. The magnitude of this similarity is expressed statistically intable 4, whichshows the correlation coefficients between the 700-mb. height anomaly patterns for selected periods along each 10' of latitude from 30' N. to 80' N. over the entire hemisphere, along with the overall pattern correlation. As might be expected, the patterns for May 5-9 andMay 26-30 were veryhighly intercarrelated, especially at the higher latitudes. On the other hand, the May17-21 circulation, approximately midway between the two periods cited above, was areaon the, 7thand 28th of May (fig. 9B),the polar quite different.While theMay 5-9 andMay 26-30 westerlieswere at a maximum and polar easterlies at a periods had strong ridges centered over Scandinavia, the minimum (fig. lo), but the reverse was true whenhigh May 17-21 circulationfeatured a deepsub-polar Low. pressureoccupied thesame area. The relation between Other major differences are apparent elsewhere over the position of thepolar cells andthe zonalcirculation is hemisphere. Table 4 shows that the latitudinal correla- furtherillustrated by figure 11, whichshows the time- latitudevariation of 5-daymean zonal windspeeds averaged over the Northern Hemisphere at 700 mb. The TABLE4.--Correlation of 5-day mean 700-mb. height anomalies over theNorthern Hemisphere between selected periodsduring May alternating periods of easterlyand westerly circulation 1960 at the higher latitudes reflect the index cycles previously mentioned. Latitude (" N.) Overall Periods correla- It is evident that a recurrence of approximately 3 weeks tion 80 70 60 50 40 30 existed in thepolar circulation. This wasalso true of -___"~~___ the mid-latitudecirculation patterns over much of the May59 and May%-30" 0.94 0.9G 0.69 0.36 0.27 0.30 0.67 May5-9andMay17-21" -.72 -.90 -.27 .34 -.41 .09 -.a hemisphere and of thetemperature patterns in the May17-21andMay26-30. -.8G -.81 -.24 .21 -.15 .30 -.04 UnitedStates (figs. 12 and 13). Figure 13 shows the

Unauthenticated | Downloaded 10/01/21 10:43 PM UTC 202 MONTHLY WEATHER REVIEW MAY 1960 tions between May 5-9 and May 17-21 were quite similar thetemperature patterns in the United States were to those between May 17-21 and May 26-30. At higher related,in part, to theconcomitant circulation changes latitudesboth sets of correlationswere very strongly over the polar regions. negative. In order to determine howwell this circulation recurrence REFERENCES was reflected in the temperature patterns in the United 1. U.S. WeatherBureau, WeeklyWeather and Crop Bulletin, States (fig. la), a count was made of the number of stations NationalSummary, vol. XLVII, No. 23, June 6, 1960. (out of 100) whose temperature class either remained the 2. J. F. O’Connor, “The Weather and Circulation of April, 1960- same or differed bynot more than oneclass. It was A Sharp Mid-MonthDrop in the ZonalwesterliesAccompanied found that 75 percent of the stations satisfied this condi- by a Temperature Reversal in the Contiguous United States,” tion for the similarcirculation patterns, May 5-9 and MonthlyWeather Review, vol.88, No. 4, April1960, pp. 158-166. May 26-30. However, a count between May 17-21 and 3. J. Namias and Collaborators, “Applicationof Numerical Methods each of the two periods May 5-9 and May 26-30 gave toExtended Forecasting Practices inthe U.S. Weather percentages of only 53 and 69,respectively. Alt’hough Bureau,” Monthly Weather Review, vol. 86, No. 12, Dec. 1958, otherfactors are involved, it would appear,then, t’hat pp. 467-476.

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