JULY19G2 299

THE WEATHER ANDCIRCULATION OF APRIL 1962 A Month with Persistent Blocking in the Pacific

CHARLES M. WOFFINDEN Extended Forecast Branch, U.S. Weather Bureau. Washington, D.C.

1. INTRODUCTION trough west of . As can be noted from the mean cairculation (fig. I) this trough was direct'lysouth of the The rnost ~IIO~I~IL~OUSfeature of t,he gcnertrl circu1:ttiorl mean ridge and was associat'ed with the southern branch at 700 mb. for April 1962 (fig. I) was :t pronounced and of the westerly flow. persistent'blocking complex inthe ('entral Pacific. The Thc split in t,he westerlies is also evident from figure 3 dominant pattern over t'he IJnited States featured warm which shows the mean a.xes of maximum west winds at ridge in the West and :L cool trough in the East, a regime 700 rnh. forApril 1962. 'I'he strongernorthern branch which wasprominent during the first three weeks, but proceeded into the southern Bering Seabefore again drop- underwent a fundamental reversal in the final week of the ping southwmd to join the southern branch in the eastern month. During the latter period, the West cooled as the Pacific. Nornlally in April the westerlies blow across the East warmed, and ellst ol' the Rockies temperatures rose Pacific in a. well organized single rnaxinlunl whose axis lies to values nlore typical of sunln~er than spring. From the just about between the two branches which characterized standpoint ol severe storm activit?-, mostol' April WBS com- the April circulation. Jn fact the normal location of the parativelymild for 2~ springmonth, though >l r:lsh of' wnter of rnaxinlun1 speed in the mid-Pacific corresponds tornadoes during the last few days sufficed to bring the almost exactly to the region of slowest winds this month. storm total to near normal incidence. The core of the blockingridge averaged very warm for 2. GENERAL CIRCULATION its latitude as evidenced by the extensive area of greater than norlrral thicknessshown on figure 4. The central As mentionedabove, blocking in the Pacific was per- value of 190 ft'. above nornlal (t'he largestvalue on the haps the most striking aspect of the mean 700-irib. chart map) was situated just to the west of t'he corresponding for themonth (fig. 1). Theassociated positive anomd~- 700-mb. arlornaly rnaxinlunl (fig. 1). overspread all of the Bering Sea and a large ttrcn of the Downstream from t'lle blocking region the two branches northwestern Pacific, and its Intlxitnum, located nctrr the of t,hc westerlies again joined in the confluent zone to the end of the Aleutian Chain, exceeded the normal b?- 640 ft. south of the Gulf of Alaska.Wind speeds increased to In absolute magnitude, this WAS nearl~~t.wice as large as a maximum of 16 nl.p.s., which is 8 n1.p.s. abovethe the next' largestcenter, the 350-ft. negative anomaly rlortllal for that' location, and the axis of the flow returned which accompaniedthe depression in the po1:lr basin. to a pathmore in accord wit,h normel (fig. 3). The The meanridge in the Pacificdoes not appc:rr esc.ep- position of the Bering Sea ridge favored a trough in the tionally strong in the rnonthl?- contour field, but it is Gulf of Alaskawhich, though much weaker than t'he significant that it was located in a region norn1:rlly occwpicd ridge(anotnaly only - 150 ft.), was equallypersistent. by a trough. This rernarkable featurc persisted for nlrrlost Althougl~ the northern portion of thistrough was fairly the entire nlonth and it is indeed interesting that such constant, itsextension equatorward was not, and its circulation regimes, so out of keepingwith the nornlal, affiliation first, with the subtropical trough west of Hawaii are often so long lived and dominant. and then with the one over Lower California accounted In the preceding non nth [I] blocking activity mas con- for the only major reversal in the Nation'sweat'her during centratedmainly in the vicinity ol' the DavisStrait. the IllO?lth. Its shift westwasd t,o t'he Pacific during April accounted Over Sorth America the ridge in the West and trough for the two largest 700-nlb. anomaly change renters be- in the East were near their normal location but st'ronger tween the two months, namely a fall of 780 ft. over the than normal. C'onsequcntly,t'he jet core (fig. 3) followed Davis Strait and a rise of 730 ft. over thc Korth Pacific near its usual path but with somewhat larger amplitude. (fig. 2). As is usualwit'h large hcight increases in high In addition, a weakly confluent flow prevailedover the lat'itudes,700-rnb. heights fell awayto the sout,h, and States bordering the eastern Gulf of Mexico. reductions up to 300 ft. from the preceding month (fig. 2) Partly RS n result of this confluence, the westerlies led to average heights 100 ft. below normal in the mean increasedagain over the western Atlantic and reached

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FIGURE1.-Mean 700-mb. contours (solid) and height departure? from normal (dotted) (both in tells of fret) for April 1962. A blocking pattern dominated the central Pacific and the, flow over liorth A4~nc.ricn.fruturd a ritfgc in the West and trough in t,lw East.

speeds up to 7 1n.p.s.above nornlal in the area ol I'ast- est' flow. However,the westerlies separated once again intotwo parts in t'he east'ern Atlantic and assulllecl a difluerlt nature over Europe.This tenderlcv toward blockingwas not discernibleduring the first portion ol the rllonth (fig. 5A) and did not become wcll intrenched until the last week (fig. 5B). Accordingly, the associatcd positive anomaly innorthern Russia averaged only 250 ft. on the r~lonthly mean chart (fig. I), substantially less

Unauthenticated | Downloaded 09/25/21 05:03 PM UTC Unauthenticated | Downloaded 09/25/21 05:03 PM UTC 5.-Mean TOO-mb. contours (solid) andheight departures FIGURE E'I(;URE 6.-Meatr 100-n1h. contours irl hundreds of feet with a con- from nornlal (dotted) (both in tens of fret) for (A) -14, tour intc,rval of 400 ft. for (A) April 10-14 and (B) -28, and (B) April 24-28, 1962. Theblocking anticyclone near thc 1962. In the Pacific a blocking pattern was still in evidence on Aleutians remained nearly constant in position and intensity in c.wh map as high as tho 100-rnb. level. each period but thecut-off depression to its south moved eastward late in the month resulting in progression of low-latitude features and a complete change of phase over the Unitcd States.

In order to study tlle vertical extent of t'he circulatioll relations were a fun(-tion of thc scale of the oscillation feat'ures discussed above, a 5-day mean 100-tnb. chart was concerned, with the larger-scaletropospheric features preparedand is included as figure GA. A number of having the best chance of beingpreserved at the upper authors have COl~lJllellt~d on the coupling betweenflow the level. 'Phis appliesparticularly to the polar Low which at 100 rnb. a.nd that at lower levels. Jnlian, Krawitz, and is well known to be a feature of tlle stratospheric circula- Panofsky [2], forexample, found a close correlation tion,especidly inminter. These characteristics are well between the lleight's at 500 rnb. and 100 mb. The cor- illustrated I)>- the flow duringthis period. It isstriking

Unauthenticated | Downloaded 09/25/21 05:03 PM UTC that' the polar vortex, though also well developetl at 700 mb. (anonlaly "530 ft.), assurlled a,n almost overwhelming dominance at 100 1nb. Llost' of theother large-scde features were also reflected at 100 nlb. though with a west- ward displacelnent of 5" t'o 10' of longitude.However, except for the intense polar depression, all closed cent>ers at' 700 mb.disappeared at thc higherlevel. It is of interest, nonetheless, that the split in the westerlies in the Pacific was still very real at 100 rnb. ApparentlJ- bloclring systems of' this ntagnit,utle extend at least to the 100-lnb. level. B. FIVE-DAYMEAN PERIOD APRIL 24-28 This general pat't'ern represented by figure 5.A proved remarkably stableduring the middle two weeks of the month. However, by thelast meek someextcnsivc F~GIJRE7.-lkparture of average t,emperature from normal (" F.) changes t'ookplace. Thoughthe blockingridge in the for April 1962. Except for a small region from the southern Great, Pacific changedrelatively little either in location or in- Lakes eastward, temperatures aver:tgc,d warm for the western and tensity, the cut-off depression to its south moved eastward cool for thr. castcrn half of the country. south of t#he blocking Higharid at't'ernpted to arnalganmt'e with the Gulf of Alaskatrough. This connection was never accomplished,but' rather a lrlarldly confluent map ior -18 (not shown), its value exceeded 700 patt'ern ensued which in effect forced the Gull' of Alaska trough eastward to join with the trough which had pre- I't. abov(: norrnd. At 100 rnb. thepoltlr depression, so prominent previously viously persistentlyoccupied Lower California. 'l'hus a full-latitude trough of negat'ivetilt arid below norrnal (fig. 6A), divided into two centers by April24-28 (fig. 6B) at subpolar latitudes, each an image of its counterpart at 700-1nb. heightappeared off the west coast of North 700 Inb. Thestronger center lay over the northern America for the first time during themonth and introduced cooler and wetter weather. On the other hand, the t,rough Siberian coast and the weaker over northeastern Canada. which had previouslydominated the East, gaveway to AgtLin most' major systems at 700 rnb. were, also discernible ridging and a record-breakinghot spell. According to at 100 mb. and rnostclosed systems were not observed Beebe [3] this pattern of figure 5B, with a trough in the st, the upper level. The difluent flow at 700 rnb. in both southern Plains States and a ridge in the Southeast is an the Pacific andthe Atlantic, however, was preserved at 100 ideal producer of tornadoes, and thisperiod was no ex- JTI~.,again suggesting that' such large warm blocking ception. Numeroustornadoes were reportedin the last, systemsin the troposphere also extend well into t,he few days of thenlonth, mostly in eastern Texas and stratosphere.The westward displacement. of troughs Oklahoals. and ridgesfrom 700 to 100 rnb.also prevailed in most, The circulation evolution over tlle Atlantic during this inst,tLnces during this period, though it is noteworthy that period is dso of interest..Correspontling to tl~epro- the blockingridge in t'he Pacific,t,he cut-off Hawaiian gressionof North American features, the trough initially trough,and the trough along the westcoast of North near theAppalachians onApril 10-14 (fig. 5A) dso Amcricw all represent nearly vertical systems. moved eastward into the Atlantic (fig. jS). It derpmecl 4. TEMPERATURE considerably, particularlyin the southern Davis Strait where a cut-off Low appeared, and, since it was bloclred The temperature pattern for April 1962 (fig. 7) was one in the north bythe warm ridge east of Greenlttnd, the of wtmn conditions in the West and mostly cool in the trough acquired a negativetilt. The closed High asso- East, the one exceptionbeing an area of abovenormal ciated wit,h thisnew bloclring surgeoverlay the British temperaturesextending from the lower Great Lakes Isles during this interval and, althoughthe central cont.our eastwardto the Atlantic coast. This warm pocket was value differed from that of its Pacific counterpart,only in large measure the result. of ltn unusually hot spell the by about 100 ft., its anomaly WAS much weaker, only 470 lastweek of themonth. The warrnth in the West was ft. compared to 880 ft. for the Pacific mticpclone. 11 welcome change from the cold regime of March, and R The evolut'ion of the British anticyclone can be traced largearea in the Southwest, experienced a warming of to the large High just, nort,hof the Caspian Sea during the fourtemperature classes,from much below inMarch 5-day periodApril 10-14 (fig. 58).This system rnovrd to n~uch above normalin April. The cool weather in tlle irregularly but with an overall ret>rograde tendency and Southeast- on t'he other hand, was a continuation of the by thelast week of thenlonth was situated over the March pattern. British Isles (fig. 5B). Atone st,age, whcn theanomaly The Apriltemperature regime becarneestablished center was over c,erltral Scandinavia on the 5-dny mean earlyin the month and persisted in its general aspects

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FIGURE8.--L>eparturc of average temperature from normal(" V.) for wsek rnding miduight (A) , 1962, and (C) , 1962, and total precipitation in inches for week ending midnight (B) April 15, 7962, aud (1))April 29, 1962. The tcmperatrlre departure from normal averaged rnostly warm in the West, and cool in the East but dterrd significantlg thcs last week as the warm air spread eastward to the Atlantic Seaboard and was replaced in the West by coo1r.r conditions. Itstinfall during the first portion wits grnerally adequate in the eastern thirdof the Nation and heavy in the Southeast, but \vas wry light or nonexistent, in the West. However frontal activity the final week of the nlonth brought increased shower activity to thr: \.l'c~st and particularly the Far Northwest. (From [4]).

forthree weeks. Largest anomalies, however, occurred the warm air rapitilv eastward to the Atlantic coast (fig. duringt'he week endingApril 15 (fig. SA).Coolest 8C).Largest positivet,ernperature anomalies for this conditions during this interval prevailed over hZirmesottL 7-day period occurred in the States bordering thesouthern where temperaturesaveraged 12" to 15' F. cooler than Great'Lakes. Binghamton, K.Y., andDetroit,, Mich., normal. The metm flow into this area (fig. 5A) followed eachaveraged 14' F. warnwrthan normal. This hot a northerly trajectory from over the snow fields of the spell produced new record high temperatures for Canadian prairies. at several locat'ions, the most not'able of which were: 94' Temperat,uresin the West during t,his period rose to F. st Binghamton, N.Y., 91" F. at Boston, Mass., and new highs for the year with readings over 90' F. in the Concord, N.H., ant1 83" F. at Buffalo, N.Y. Although interior of California and in the Southwest desert region. each of theprevious three weeks averaged cooler than At Yurna,for example, the maxima exceeded 100' F. normal over most' of the area around the southern Great from the 12th through the 15th and reached 106' I?. on Lakes, it is of interest thtkt the warrnt'h of this last week Friday the 13th. During the following week the pattern was sufficiently pronounced to offset the previous period remainedsinlilar except that the warn1 air pushed east- and result'edin a warmer than normal pattern for tjhe wardt'o include t'he Plains States from the Dakotas to nlonth tis a whole (fig. 7). Texas.Denver reported a high of 83" F. onthe 20tl1, Inthe Far West,however, the trough development the highest, ever recorded on that date. along the west coast (fig. 5B) previously described, intro- As previouslyment,ioned, the anticyclogenesis in the duced ti much cooler regimeand brought an end to a East during tjhe last,week of t,he month effectively spread rather prolonged period of warm weat'her.

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For the most part, precipitation was subnormal over a large portion of the United States during April 1962 and much of theSouthwest received no rain at all (fig. 9). The onlyregions where rainfall totals substantially exceeded the normal were the Gulf States and the cotistd strip of the PacificNorthwest. Total accutnulntions ranged from4 to 10 inches in eacharea and exceeded 150 percent of nornlal in the regions of heaviest precipi- t’ation. TheAppalachian and Atlantic Coastal States also received adequate moisture with totals ranging from 2 to 6 inches, while showers in the Rockies were sufficient to accountfor near normal anlounts. Reference to figure 1 suggests that, the rainfall in the Gulf St,ktt’esand the East occurred in conjunction with the mean trough FIGIJREY.-Total precipitation ill inchesfor April1962. Largest. in theeastern States, while theprecipitation in the amounts accumulated in the Southeast and thePacific Northwest. Driest regions occurrrd in the Southwest and thc northernPlains. Northwestwas associated with t~bnorw:~lI?-strong on- (From [4]. shore flow. Ot’hcrwise very dry weather prevailed over the Sation, with sorne areasapproaching drought, conditior~s by month’s end. At Las Vegas, Ncv., no rain fell in April, the first’ time in the period of record. Other stations in the Far Sout’hwest also reported no rain, and the dryness the first substantial rains of the month fell in the North- which wasalso characteristic of March wasprolonged west’ and, thoughamounts werelow, shower activity throughout April. The arid condit’ionsin the Southwest, also occurredover most of California,the northern occurred undert’hc mean ridge with anticyclonic flow GreatBasin, and particulnrly the central Rockies. Ac- and abovenormal heights. The northern Plains States curnu1;tt~ionsof 2 inches or more were general along the were also very dry as much of the area received less than coastalstrip of Washingtonand Oregonwith some 50 percent of rlorrrdrainfall. The driest Aprilin 50 reports of over 4 inches. years was recorded atsuch widely sepmttedpoints 21s ‘The t’roughin the southern Plains States (fig. 5B), Dayton,Ohio, Burlington,Iowa, Topeka, Kms., and was also an effectiverainfall producer during the last Billings, Mont., andin some areaswinter wheat was week of thenlont’h and the central Gulf Statesagain beginning to show the effects of soil moisture deficiencies. experienced heavydownpours, mostly as showers and This dry weather was related t,o the ridge over the West tlluntlerst~ormsalong frontal systems. One such storm and thedrying effects of downslope flow east of the at Baton Rouge, La., producednearly 7 inches of rain Rockies. It is of interest, in thisconnection, that the accompanied by 60 n1.p.h. windsand minor flooding. axis of maximum westerly flow (fig. 3) coincided closely Similarly,locally heavy amounts occurred in eastern with the region of driest weather. r71exus. Shrtrnrock, forexample, totaled 6.05 inches and The precipitation distribution for the first three weeks Dallas 4.92 inches during this week. of the month roughly followed the pattern of figure 8B. Dry conditionscharacterized the West under the mean REFERENCES ridge and wet condit>ionsthe East under the mean trough. 1. J. W. Posey, “The Weather and Circulation of ,” Monthly Heaviest precipitationoccurred in the Stat’es bordering Weather Review, vol. 90, No. 6, , pp. 252-258. the central Gulf as trailing fronts set off thunderstorms 2. P. It. Julian, L. Krawitz,and H. A. Panofsky,“The Relation upon encounteringmoisture-laden Gulf air.Hail ac- Between Height Patterns at 500 mb. and 100 mb.,” Monthly companied a number of thesestorms and hailstones of Weather Review, vol. 87, KO.7, , pp. 251-260. “golf to baseball” size were reported in northern Louisiana3. R. G. Beebe,“Tornado Composite Charts,” MonthlyWeather Review, vol. 84, No. 4, Apr. 1956, pp. 127-145. and central Mississippi. 4. U.S. WeatherBureau, WeeklyWeather and Crop Bulletin, As the circulat’ion pattern wlt’ered during the last week National Summary, vol. XLIX, Nos. 16, 18, and 19, Apr. 16, and the trough developed along the west coast (fig. 5B) 30, and May 7, 1902.

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