MARCH 1961 109

THEWEATHER AND CIRCULATION OF DECEMBER 1960' An Unusually Cold Month in the United States

ROBERT H. GELHARD

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

1. HIGHLIGHTS era1 circulation apparently began with the rat'her sudden dexlopmcnt of the Azores High late in h'ovenlber. Sub- Average temperatures throughout most of the contigu- sequentlythis amplification seems to havespread up- ous United Statesduring Deccnlbcr 1960 underwent n stream, first to west'ern XorthAmerica and then to marked reversal from the warn1reginle wllich 1r:td nort~lleastcrn Asia. dominated thefall season [ 1, 21. Subzero temperatures Relatively little change occurred in the eastern part' of were frequent in the nort1wr.n hwlf of the country east of thehemisphere except in northeastern Asia. Nearly all the Rocky LIourltains, :mtl freezing tcnlpertltures t hretlt- of Asia was dominated by a vast SiberianHigh at sea ened winter veget'able and citrus crops fromsouthern level,extending from theBlack Sea toJapan (fig. 3). California eastwardthrough tlle Rio Grandc Valle?- to Genrrdlystorrlly conditions existed inEurope and the central . lfcditcrrtrncan area, but the modifyingeffects of rnarit'ime Snow cover, which had been restricted to tlle eastern air dit1 not penetrate beyond European Russia. slopes of the Rockies and extrerlle northern Plains, spread throughout the northern hnlf ol' thecountry early in 3. THE INDEX CYCLE December and for the nlost partrenlaincd througllout themonth. Irl tllc southern Plains, precipitation irr excess Tlle chmge in the mesterlv circulat,ion from Noventber of normal resulted in sotne flooding around mid-n1onttr, to December was tl. classic cxtrnlple of an index cycle as while drought conditionsin t,he Great Basinwhich h;d tlescribcd by Narnias [4, 51. Figure 4 showsthe march been alleviated somcw-hnt in Sovenlber resumed during of int1ict.s of temperate (35O-55" X.) and subt'ropical December. (20"-35" X.) west'erlies at TOO III~.,as defined by over- lapping 5-tl:ty mean maps computed three times per week. 2. THEGENERAL CIRCULATION The index referred to here is for the western half of the The general circulation at 700 nrb. for December 1960 Korthern Hemisphere from 0" to 180" W. The temperate (fig. 1) represented a distinct change from the predomi- westerlies reached A peak near the end of the third week nantly zonal or high-index state of the preceding month in Kovember and then began a four-week decline before [l], particularly in the western portion of the Sorthern makingrapid recovery late in December. The dotted Hemisphere from western Europe to the central Yacific line,giving the daily components of the mean index,is Ocean. A comparison of figure 1 with the nlem 700-mb. included to show the very sharp drop in t,he index which height pattern for t'hepreceding month shows that the is dntnped sornewllat, hJ- the averaging process and coin- Museplanet'ary wave system of Soverllber became re,- cided with the first of several severe st'ortns in the South- solved into a series of well-defined troughsand ridges west.The decline in thet'emperate latitude index was located in climatologically preferred areas [3]. accomptmied bv southward displacement of the westerlies The field of anornalous 700-nlb. height changes (changes exceptin the Nort'h At'lantic, so thatthe subtropical after the normal change has been removed) from n'oveln- index rose as rapidly as the zord index fell, reaching a ber to December (fig. 2) helps delincatc the areas of great- peakDecember 21, a dt1.v afterthe minimum in the est amplification. Thegreatest change, +680 feet, took temperate index. place in the central Atlantic with the development of the Althoughthere seems to be a strong preferenc,e for Azores High at a higher than norrnal latit'ude. This was index cydes to occur in late wint,er,they are not uniquely associated with a northward shift of the mean westerlies fixed with respect to month. One necessary condition for and subsequent deepening of tlle downst'rearn trough in theonset of an index cycle seems t'o be an abundant the eastern Medit'erranean where anomalous height's fell supply of cold air in polar regions. Figure 5A shows that by 310 feet (fig. 2). The 5-da~-mean chart's (not show-n) the 1000-70O-nlb. thickness for Xovernberaveraged 130 show that amplification of the wave pattern of the gcn- feet below norrnal in northwestern Sorth America, thereby sat,isfying this condit'ion. However, in order to rnaint'ain 'Articles describing the weather of January. Fehruary, and will appear In the April, Ma.y, and issues. rcspcctively. of thr Monthly libnther Kezic/c.. the ntmospllericheat balance, the containment of polar

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12

FIGURE 2.-Difference betweenmonthly mean 700-mb. height 6 anomalies for November and December 1960 (Decembcr minus November) intens of feet.Anomalous rises inwestern Sorth America and falls in the east were associated with amplification of the long-wave pattern. 4

2 31 7 14 21 28 5 12 19 26 2 NOVEMBER DECEMBER NOVEMBER

FIGTJRE-l.--Tirne variation of 5-daymean 700-mb.indices in meters per second for the western zone of the Northern Hemi- sphere from 0" to 180" n'. Tempcrateindex applies between 35" and 55" X., andsubtropical index betuern 20' and 35' S. \':dues arefor 5-day periods ending on datesindicated. The dashed line rcqmwntsa daily index, and the dotted lines are thenormals based on thedata of WeatherBureau Technical Puper So. 21.

0~1~-in the extremenorthern Plains weretemperatures above nortnal for December. TheGrmt Basin area ard ('alilorniarenlained cool under the irlfluence of large st'agnrtntant'icyclones. In spite of abundant sunshine, record daily rnirlitnum tem- peratures in the cool dry air mass kept mean temperatures below normal.An int'eresting effect of thesepersistent, FIGURE3,"hverage sea level pressure (solid lincs)and its de- Basin Highs was reported from Boise, Idaho, where the parture fromnormal (dotted lines) in millibars for Deccnlhvr avc~agewind speed was only 4.6 m.p.h., the lightest in 1960. Highpressure was accompanied by cold nrather irlthe. 21 yet1r.s. contiguous United States. The most unseasonably cold part of t'hecountry was the East ant1 South as far west' as Arizona (fig. 7). Figure SA shows the trajectorv of migratory anticyclones carry- dist,ributed uniformlyacross the nation, 78 percent tle- ing cold air masses from Canada just west of the Great creased in average tenlperatureby at least one tctrlpcrature Lakes ant1 through the Ohio Valley. Throughout' t'he area class (out oI five classes) from Sovemhcr to December from t'he?rlississippi Valley eastwardsome record daily (fig. 6A). Of the remaining 22 percent, only 3 percent ~rrinirnurn temperatureswere est'ablished along with record increased. Even in thearea of nochange or increase, low t'empertbt'ures for the month, such RS -9" E". at three

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FIGURE5.--?IIean departures from normal of 1000-700-mh. thick- ness for (9) Xovember 1960 and (B) December 1960, both in tens of feet. Sote the completercversal in phase ns the illrk~s cycle progressed.

st,at,ions inKentuck?- and "7.5' F. atEvansville, Intl. For most st'ations, average December t~ernperatures \wro the lowest in from 10 to 25 years md close to record intensity.In Florida, freezing temperatures tllreatrrletl cit'rus and vegetable crops AS far sout'h as Tampa on five occasions. Richnlond, Va. reportedDecember 1960 as t'heonly month on record with every daily nlirli111~111 temperature below 32" F. Inthe lowerMississippi Valley and Southern Plains nnticyclonicconditions prevailed at sea level (fig. 3), chiefly emanatingfrom Great Basin Highs. HOWCVCI', t'here was also some contribution from polar Rir rllt~sses,21s FIGURE7.--Tempc~rat~;re departure from normal (OF.) for Decem- evidenced by subfreezing temperatures in thc Rio Grtl~ltlc hr 1960(from [GI). Widespread cold TV~Sassociated with Valley and resultant crop da~n:~ge. circnlntior~ shown in figurcls 1 and 3.

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Below normal ternperatures in Arizona andNew Alexic-o were maint'ained by persistent cloudiness due t'o cyclonic conditions aloft (fig. 1) which were quite pronouncctl tlur- ing the first three weeks of the period. In addition, heavy snows early in the period probably cont'rihut'cd to lower than normaltemperat>ures. Late in the month a warm- ing trendbegan in the Southwest', coincidcnt with the rising ~odindex. While new records for daily minimum temperature werc common t'hroughout most, of thecountry, warm air in advance of a severe st'orm in the centrd Grcat Plainspro- duced record high maximum temperatures it] the Xorth- east near the end of the first werk in December. The amplificat'iorl of t'hetrough-ridge syst,em in tho eastern Pacific and western North America, w1lic.h pro- vided the mechanism for bringing cold air into tllc con- tiguous United States,was also responsible for thc warmest' December on record for most' of Alaska. h strong anom- alous southerly component ol t'hc average flow, shown by the height anomaly pattern in figure 1 (coit~citlent with the sea level isobars, fig. 3), produced an influx of warm maritime air through out^ most' of the month. The extent of the exchange of air masses is w~llillustratctl by the change in t'he 1000-700-mb. tlliclmcw charts for Sovc~rn1)c~r and December (fig. 5). 5. PRECIPITATION

In general,excessive precipitatiotlin tllt cbotltiguous United St'ateswas restric.tctl to the (mtral third of the count'ry antl t,o the North Atlantic coast (fig. 9). AII un- usual feature of December's prccipitation regime W:LS the large amount of snowfall occurring so early in the season. In the Southwest a,nd the Korthcast many new motrtllly snowfall records were cstablisl~cd,such as 10 incllcs at El Paso, Tex., antl Winslow,Ariz., 17 inches at Dayton, Ohio, and 21 inches at Worccstcr, )lass. Slotlcr;tt(> amounts were reported eIsew1m-e except for the Great Lakes and most' of the Great Basin here rc~presrntativf~ stations such as rmlsing, Mich.,and Salt 1~h-c('ity, I'tah, reported theirdriest Deccmhers on rocorcl. In c*ontrast, Wac0 and Dallas, Tes., rcportctl the wettest Dccernl)t~r on record. It is interesting to compare the precipitation pattcrr~s for November [I] and Decembc.r in terms of high a11tl low index circulations. Xovcmbcr's pattern was wetin thc west and relatively dry cast of the Rocky AIourltairls-a pattern oft'enassociated withfast w.csterl?--flow. How- ever,in Decemberamplification of the ridge it1 xwsterlt North America and dwpening of tlw upstrcslm trough pro- duced a strongly meridional steering current aloft, ant1 thc westerlies were weaker than normal tlmughout the W\rcst (fig. 10). The intensity of the western ridge was great enough tfo bloc,k effectivelythe pnssn.gc of migratory cyclones from the west (fig. 8B), so that thcl Pacific Sorth- FIGIRE 9"Percrnt:tge of normal precipitation for December 1960 west during this low-index regime received from 25 to 50 (from [Ci]). Amounts were generally in escess of normal between percent less prccipitat'ion than normal, ant1 thc Grcut thc Mississippi River and the ContinentalDivide but deficient Basin received less than half of normal. On thc otI1c.r elsewhere.

Unauthenticated | Downloaded 09/26/21 11:25 PM UTC hand,the weakness of the Westerlies diminished foehn drying arid favoredupslope precipitat’ion in the Plains region between the Mississippi River and the Continerltal Divide (fig. 10B). Anotherco~rm~on feature of low-indexcirculat,ions is the presence of areas of mid-tropospheric confluent flow at fairlylow lat’itudes such as t’hat found in the area (fig. 1). Whilethis was not tin unusually strong confluent zone, there was sufficient contrast between cold andwarm air musses tolead to threemajor storms in thesouthern Plains and upto 400 percent of normal precipitationwith some flooding ineastern Texas and southernArkansas. These same storms produced heavy snows as theymoved tllrough the Ohio Valley, roughly dong the track shownin figurc 8B. The)- were responsible forrecord December snowfalls from northern Virginia through New Eng1:md as they deepened in the area east of the me:m trough near the Middle Atlantic coast(fig. 1). Figure 6B illustrates well t’he general change from the precipitation regime of November to that of December; that is, from wct to dry in the West, a general increase in the Plwirls States, ant1 littlechange clsewhere. While it has been mentioned that Xovernber had n typical high- index precipitation p:~tterrl, December’s pattern was not necessarily typicd of low index since distribution of pre- cipitation is dependent on the precise geographical locs- tion of thequasi-stationary trough and ridge systems, wf1ic.h c.tm exhibitcorlsidcrable variation during periods of low index. REFERENCES

1. J. F. O’Connor, “The \\’eather andCirculation of ,” Monthly IVenther Review, vol. 89, No. 2, Feb. 1961, pp. 55-38. 2. JV. H. Klein. “The Circulation and \I’eathcbr of 1960,” Weather- wise, vol. 14, KO. 1, Feh. 1961. :{. IT. H. Klein and J. S. M’irtston. “GeorrranhicalFrequency of ~-,I Troughsand Ridges on Mran 700-1nh. Charts,” mfonthly FIGURE10.-(-4) Mean 700-mb. isotachs in meters per second for Weather Review. vol. 86, So. 9, Sept. 1958, pp. 344-358. December 1960. Solid arrow indicate primary axes and dashed 4. J. Narnias,“The Indrs Cycle and Its Role in the General arrowssecondary axes of maximum wind speed. (B) Departure Circulation,” Journal of Meteorology, vol. 7, No. 2, Apr. 1950, from normal of mean wind speeds for December 1960 (in meters PI). 1:30-139. per second).Weaker thannormal westerlies were associated 5. J. Samias, “Thirty-Day Forecasting: A Review of a Ten-Year with wet weather in the Great Plain: and dry weather west of Experiment,” Meteorological Monographs, vol. 2, No. 6, the Continental Divide. AmericanMeteorological Society, Boston, July 1953, 83 pp. (See pp. 35-40, and 63-66.) 6. U.S. \Yeather13ureau, Week!yWeather and Crop Bulletin, hrutionalSummary, vol. XLVIII, Kos. 1 and 2, Jan. 2 and 9, 1961.

U 5 GOVERNMENTPRiNTlNG OFFiCE 1961

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