ACGKST 1961 MONTHLY WEATHER REVIEW 290

THE WEATHER AND CIRCULATION OF MAY 1961 Persistent Cool Weather in the ROBERT H. GELHARD Extended Forecast Branch, U.S. Weather Bureau, Washington, D.C.

1. HIGHLIGHTS Alaskan area wheremean height' anomalies increased The outstanding feature of tlle wcat,l~rof May 1961 210 It,.from April (fig. 2). Atthe same timeheight was thepersistence of the cold regime of t,lw preceding anomalies decreased by 570 ft'. near Baffin Island, repre- month [I] through out^ most of the United States cast' of scnting a rather abrupt return to normal in that ares. the Rocky llountains and also in the Pacific Northwest'. Inthe Sout~hwest Pacificfilling of t,lle deepAsiatic In t'lle East numerous stations fronl St.Louis, hfo., ronstal t,rougll observedin April and intensificat'ion of eastward through the OhioV:~llcy t,o Youngsto\vn, t>llc subtropicalHigh downst~rearn were predominantly Ohio reported t'lae coldest average tcmpcraturcs on record t~li~oa.tologica1NS shown by the snlall anomalous cbanges for May. In t'he Far West unseasonably cold tcn~pcra- (fig. 2). However,this nornlal cllange, concurrent' with tures retarded t'hc maturity of' crops through most, ol deepening east of Kamckatka and blocking to t'he nort'h, the Great' Vtllle?- of Chlifornia and below normaltern- produced :L zone of confluence south of Karnchatjka which peratures ext~endedinto cxtreme soutllern(hlifornia. wts the tlonlinantfeature of thecirculation. Figure 3 A major contribution to the lowmean ternprratures shows that'the maximum mean 700-mb.west'erlies in for M~IJC~LIIIP fro111 MI extremely cold po1:r.r air n1sLss this region were from 4' to 5' of latitude north of t"rleir that blanketed theeastern half ol t,lle countryduring normal position (A) and 9 rrle.ters per second faster than the lastweek of themonth. Beginning on tlle 26tE1, normal(B). Downstre:tln, the effects of blocking on record low tenlpcraturrs were recorded through out^ the this high-inch Pacific circulation are sl~oumb>- a dirninu- Middle West'and e:st'ward, with frost, :as far soutll as tion of wind speeds in the broad diflucrlt area ol western Kentucky and snow in Ohio :tnd Michigan.Daily North Rnlerica and b~-the southward displacen~cnt~(from temperature anomaliesranged from more than 20' F. normal) of the 700-nab. jetstream t,hrough the extreme below normal in Xew England and the Mid-Westto southwestcrn Cnited States. IOo F. below norrun1 t'llrouqh t,he Gulf St,ntes (luring t,Elis Elsewl~crethe anonlalies of the general circulation were period. within 160 f't. of thelong-period normal, the largest Extremely heavy precipitation fro111 t'he ('cntral Plains 1nontl1-to-111ont11changes (fig. 2) beingindicative of a eastward through t'he Ohio Valley rtwllt'etl in 1nt1jor Aootls return to normal from the more tmonlalous state of throughout the month in most' of the Arkansas, Missouri, April [I]. Ohio, andMiddle Rlississippi River w:~tc:rshctls where amounts ranged from more than 8 incllcs at ('oncordi:t, 3. TEMPERATURE Kans. to a record12.22 inches at Ev:tnsvillc, Intl. On The cool tenlperature regime of April continued t'hrough the otherhand, drought conditions that prcvailetl in Ifay with nearly all of the contiguous United States aver- through the Southwest from l'cxt~sto southern aging below normal except Texas and t'he Rocky Moun- California continued(luring Xla~7 exceptinextreme tain States (fig. 4). Tn rtluch of t'he Mid-West and Ohio western Texas. There wasno rnettsurablca precipitation Valley the persist'ent' low temperatures of April and May in theext'rc'me Sout'hwest or at' ('orpus ('histi, 'l'ex., 1961 combined to make this period t'lae coldest since 1907. a new- record for Ala- at' thatst' 'I t'1011. The cool regime in t,he Pacific States and t'lle Great Basin, \vhile not of recordintensit'y, was extremelypersistent, 2. THEGENERAL CIRCULATION part,icul:trly inthe Great Valley of southernCalifornia. The generalcirculation for lfay 1961 st, 700 mb. The number of days in t,he month during which the aver- (fig. 1) does not' show man^- unusual features in spite of age daily t'emperature exceeded t'he normal ranged from the ext'remelyanomalous w~eather observed over most only six at) Sacramentoto four at San Diego. Santa of the Unit'edSt'ates. Blocking, which h:td dominated Maria recorded only OKW day above normal and that by eastern Clanada duringApril [I] andthe northeastern only 1' F. Atlantic during March [Z], retrogradedrapidly to the The unusual month-to-month persistenceof temperature

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FIGUREI.-Mean 700-rnb. contours (solid) and height departures frc-m normal (dot,ted) (both in ten;; of feet) for May 1961. Above normal heights in and Alaska were associated with frequent arlticyclogenesis and cool weather east of the Rocky Mountains.

is well illustrated by figure 5, t'hc change in temperature closely the negative height' ttnorr1tt1~-at 700 rnb. the rela- anomaly class (out of five classes) from Aprilto 51a>-. tive~nl~grlitutle of the temperature anomaly- was rnuch In R dist'ribution of 100 cities, 37 remained in exact1;v the greater than that of the hcight anornaly. Exarnination of same class as the previous rnonth, covering the areashomn the 1 !j-day cornponent~sof t'he rl~ontllly700-rnb. height by crosshat'ching. No city changed by as rnany AS :3 or rncan, figure 6, shows :L two-fold effect' operat'ing to muin- 4 classes, and 79 changed by no more than 1 class. The tain cool tcmpcrtttures in t,he Far West'. During the first latter figurecornpares with 59 expected bychance, 32 hdf of t'he rnonth low tcn1per:ttures were tmsoriated with observed from Marchto April 1961 [I], and 62 for the :morndous northerly flow from A blocking High centered average April-May change from 1942 to 1957 [4]. overAlaska (fig. BA), and with the mainwesterlies dis- Although the pat'tern of ternperature tlr1orrld-y follows placed well south of 40' N. During the latter half of the

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FIGURE2.--Change in 700-rnb. height anomalies (tens of fret) from April 1961 to May 1961. Relaxation of blocking in c:lstclrn Carl- ada is represented by the largest change area.

month tenlperaturcs modified corlsidcrwbly in thc North- west (fig. 8 C, E) as t,he blocking over Alaska rrlastd (fig. 7), and the circulation returned to the dorninunt spring pattern of t'roughsnear the east and west coasts and a strong full-lat'itude ridge through mid-continent (fig. GB). The extreme negative tilt' (northwest to southeast) oft11c west coast trough is c~orr~n~onlyassociiit,ed with tllrt south- ward displacerr~entof mid-latitude westerlies t~ndIlclp&l maintaincool temperatures in southern ('tllifornia. In contrast to the persist'ent temperature regime of the Far West, temperatures east of the Continental Uivitle, although predorninantly below normal, sl~owctl IIIO~C variability (fig. 8). Thisvariability can bc rc~lntc~tlto FIG(-RE3. -(A) Mea11 700-rnb. isotachsand (U) departurefrom the differences in t,he 700-~nb. hinlontllly c.ircn1il t'lolls normal wind speed(both in meters persecond) for May 1961. (fig. 7). Solid arrows in (A) indicateprimary axes of thewesterly jet During the first, half of the month most of the United stream, and dashed arrows indicate their normal May positions. States was dorninat'ed by a broad cyclonic. circulation :lug- The westerlies were displacd well south of the normal position mented by strong rlortllerly flow in western (hatla asso- over westernUnited States, and an importantsecondary maxi- IIIIII~Iwas located over northeastern Canada. ciated with a blockingHigh over Alaska. Following :in init'ial outbreak of cold airwhich affected no st of thc country except the Gulf States, succeedingpolar air masses followed morenortherly trwk with a wt~rn~ir~g areti of western North An~erica (fig. 8G). A succession of trend east of thetrough in central United States. The (*oldpolar anticyclones moved into the eastern half of the track of t'he "glttncing Highs" associatedwit'h these s11:dlow country, as shown by the sout~hernrnost track infigure 9A, polar out'breuksis shown in figure 9A by the MTOWS rcsultingin frost and lightsnow in the Mid-West' and through the Great Lakes and eastern ('anadtt. Esanlina- rxortl low minimum temperatures throughout the East. t'ion of the 5-day mean charts shows a transition period near mid-monthduring which thc ~nasil~llll~l-(~ur~~~~tu~e trough in central (!an:lda joined with the sldlow trough 4. PRECIPITATION in the XorthernPlains (fig. 6A) to form a full-latitudc trough near the east coast,and the blocking High over Of purtmicular interest, isthe unusually heavy precipita- Alaska moved east to form a full-ltltitudc ridge just east t,iorl whichoccurred in the CentralPlains and Middle of the Continental Divide (fig. 6R). Associated with the Mississippi watershed and resulted in floods throughout development of thislarge-amplitude trough and ridge t'hc area most of t,he month (fig.. 10). Heaviest rains fell , c_ system was rapidwarming throughout the mountainduring t'he first half of t.he rnont8h(fig. 8 B, D) associat'ed

Unauthenticated | Downloaded 09/26/21 08:10 AM UTC FIGITRE4.-Departure of average surface temperature (" F.) from normal for May 1961. (From [3].) Ternpc,ratllre averngcd above normal (shaded) in the Rocky Molmtain itreit but below rlormal in the remainder of the nation.

FIGURE 6.-Fifteen-day mean 700-nlb. height contours and depar- tures from normal (both in tens of feet) for (A) May 1-15, 1961. and (E) -30, 1061.

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FIGURE8.-Departurc of average surface tempcraure from rlornlal (" F.) and tot,al precipitation (inches) for the weekly periods ending (A,R) May 7, (C,ll) , fE,F) , and (G,H) , 1961. (From [GI.)

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FIG~.HElO.-€’ercr~nt:tge of norrrlnl pcrcipitation for May 1961. (From [SI.) KO mcaslmible precipitat,ionin parts of the South- west contrasted with above normal amolults in rna~lyother areas.

f1w~ut~nt1~-:ssoc*iatmetl with marked confluence of t,he mean 700-rnh. contours,such as prevailedover Indiana this month (fig. 1). Precipitat,ion U~Snormul to above normal irl the Pacific Xort,llwest under the influence of the weak co;lst;ll t,rougll tlespit,c the wetlkrless (relative to normal) of thc westerlies at mid-latitudes. During the wcck ending , :L major storm moved ‘out of t,hc lower Mississippi Valleyand produced moderate t,o Irr;1vy prccipit’ntion over most of t’llc eastern half of the country. Intense deepening of theparent storm asit rnovetl into ctlstmcrnCanada rcsult,ed in a fresh polar out- brtwk of cold air reaching t’o the Gulf States, and arather intense Witv(’ developed in response to t’he sharptempera- ture gratlient8 near nort811ernFlorida. However, a sudden nortllw-urd shift, of the jet stream in response to the deep c\-c.lonic vort8exnear Hudson Bay and anticyclogenesis to the south over the Great, Lakes resulted in cutting off the FIGURES.-Frequency of (A) arlt,icyclone and (B) cyclone p:tssagcts (within 5” squares at 45’ N.) during Rlay 1961. l’rirnnry tracks WIIVCncar Floridtl from t8he main westerlies. The major are indicated by solid arrows, secondary tracks by dashed arrows. portion of the mont,ll’s precipitation in the coast’ttl area of Frequent weakLows in the Sorthern Plains were of secondary South C:wolina to Virginia WAS produced by t,he slow importancecompared to major storm devcloprrlrut in Ccrltral northwart1rnigrwtion of this cut-off coastal Low from Plains. 1\Tny IO to M11y 1 ?I. Heaviestprecipit8nt~ion in the Northeastoccurred late t,Ilc month in conjunct,ion with wave development as a with the shallow trough t811rought,he PlainsShtes ant1 the cold front, movedof€ the Middle Atlantic coast in advance depressed axis of prevailing west’erlies. The effect of the of thck ext#rcmelycold polar airmass mentioned earlier. depressed westerlies in restrict,ing the northward intrusion Subsequent baroclinic deepening in the mean east, coast of Gulf moisture is shown indirectly in the frequency and t,roughresulted in precipitat,ion amounts in excess of 4 prevailing tracks of daily cyclones for the mont,h(fig. 9B). inches in parts of New England (fig. 8H). Although a maximum numberof closed Lows wasobserved As is often the case, precipitation, or the lack of it, is inthe northwestern Plains, few developed into well difficult t,o specify. In the Sout,hwest it seems likely that organized cyclonic syst’ems,while the major developments t,hc strong low-latitude west,erlies operated to produce a occurred justnorth of theprevailing west,erlics in the rainshadow effect andto prevent, moisture from being CentralPlains. An int,eresting effect of thenort,h\vard int,rotluced by large-scale sout811east8erlycomponents of t’he lim.it of Gulf moisturewas t,he range of precipitation general circulat~ion. More difficult to explainis the con- t,hrough Indiana from a record 12.22 inches at Evansville tinued drought in eastern Texas. Here lack of moisture in the south to only 1.19 inches ut South Bend in the north,at high levels tlnd antmicyclonic vort,icityjust sout,h of the t8hedryest there since 1939. Suchsharp gradients are jetstream limited precipit,at,iorl t’o scat,tered showers

Unauthenticated | Downloaded 09/26/21 08:10 AM UTC BEGCS'I' 1961 MONTHLY WEATHER REVIEW 305 t8hroughmost' of the period. During the lat,ter Idf of the in others. This wouId indicatethat other parameters mont,h a strong easterly flow at low levels associut~cd wit,ll such as available moist,urc, warm air advection, thickness a series of largepolar ant~icy~lones moving dong the advection, the extent of over-running of a cold stable air southern track of figure 9A resultedin a considerable mass, etc., should be included in any specification scheme. amount, of up-slopeprecipit:rt'ion in western Texas. A n L~nfortunatelythe measurement and data processing of interesting note on the discont8inuousn:~tur~ of precipittl- these parameters is not us easy as for vertical mot,ion. t>ionis t'l~difference betwccn t,he mont'llly totals at' Hous- This experiment is similar t'o that done by Woffinden ton, Tex. of 0.63 inches at the tit>- office and 3.59 inc~hes [5], and was repeated onlybecausc of the differencein at' the Airport, St'ation. t'he circulat~ion and weather regime. However, the general In an effort to specify more clearly the month's prwipi- results and conclusions are the same. t'ation, a study was made of t'he BOO-mb. vertical rnotio~l REFERENCES charts as comput'ctl t'wicc daily from the baroclinic motlcl I. .J. F. O'Connor, "The W'cathrr and Circulation of April 1961- currentlyused by the Nurnrrical Weather Prctlic~tion Pc~rsistc~rltBlockillg in Eastern Canada. and Cool Weather in Branch (NWP). Values were read at' 5" intervals of the Unitc>tl States," Afonthl?]Weather Review, 1701. 89, KO. 7, latitude and longitude, and chart's werc constructed of .Jdy 1961, pp. 256-262. Iveekly meanvalues and mea11 positivevalues only lor 2. ,J. F. Andrews, "The Weather nnd Circulation of - .2r1ot,htsr Mild h2orlth in thc United States," Monthly Weather periods corresponding to t'hosein figure 8, ant1 meall Reuiew, vol. 89, hTo. 6, ., pp. 205-210. values ant1 mean positivevalues only for the period 3. C.S. Wc,ath(sr Bureau, H'eekly Weatherand Crop Bulletin,Na- corresponding to figure IO. The results show little corre- tional Sumnzary, vol. XLVIII, Yo. 23, June 5, 1961. spondence between the observed precipitation pat terns 4. J. F. Arldrcws, "The Wr:tt,her and Circulation of May 196G Including a Discussion of the Unusual Retrogression of 5-day and the patterns of mean values of verticdmotion 011 12lr:rrl Polar Vortices," Monthly Weather Review, vol. 88, Yo. 5, either a weekly or a monthly basis. Or1 the ot,hcr II:LII(~, M:Ly 1960, pp. 193-202. the patterns of positive valuesonly show a fair corre- 5. C. Rl. \TToffinden, "The W:cather and Circulation of - spondence with the observed precipitut'ion in some areas. Including an Analysis of Precipitation in Itelation to Vertical However, thereare many inconsistencies; e.g., the dry lfotion," MonthlyWeather Review, vol. 87, KO. 5, Sept. 1959, areas of Texas and tllc Great Lakes show definite maxima pp. 196-205. 6. U.S. \\.'eather Bureau, WeeklyWeather and Crop Bulletin,Na- in several cases, and the ltcavy rains in t'hc Ohio Vallt~y tional sum mar?^, vol. XLVIII, Nos. 19-22, May 8, 15, 22, 29, and New England lie in areas of minimum vertical mot'ion 1961.

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