MARCH 1962 121

THEWEATHER AND CIRCULATION OF DECEMBER 1961 Strong Blocking at High Latitudes RAYMONDA. GREEN Extended Forecast Branch, US. Weather Bureau, Washington, D.C.

Unauthenticated | Downloaded 09/24/21 11:11 PM UTC 122 MONTHLY WKATI-IER REVIEW MARCH 19G2

Unauthenticated | Downloaded 09/24/21 11:11 PM UTC FIGURE2.-Change in mean 700-mb. height departurrs from nor- FICI-RE3-Mean 700-mb. isotachs for December 1961. Heavy mal (tens of feet)from November to December 1961. Chrtrlgc. solid arrows indicate primary axes of mean rnaxirrlunl winds, and patterns at middleand high latitudes of boththe Pacific and dashed arrows theirnormal December positions. Southward the Atlantic indicate the increase of blocking. depression of the westclrlies by blocking is apparent over eastern Asia, the western Pacific,rc'orth America,and the Atlantic.

Alaska accounted for height fdls of as nlucll tts 580 fret) (fig. 7) between the h;tlf-nlonth means as portion of tllc eastern Pacific ridge advt~nced intowestern Korth America. Furtherprogression of t,hetrough in the was accompanied by falls of several hundredfeet with noticeable impact on the country's weather. Tenlperaturesover the western two-thirds of the United States were general1~-eoltler than norllla,l during t,he first half of Decelllber (fig. jR), in response t'o north- erly flow betweentllc strong nlean ridge in the eastern Pacific arid the trough from I,akc Superior southwestward (fig. SA). Also, most of the area was blankct~etlt)y snow from storms earlyin the month. Durillgthis period $111 unusually severe outhrcak of cold air floo

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FIGURE5.-(A) Mean 7GO-mb. contours (solid) and height de- FIGI.REG.-(a) >Iran 700-mh. contours (solid) and height de- partures from normal (dotted) both in tens of fert, for Dcwmber pnrturcs from llormal (dotted), both in tens of feet, for -15, 1961. (B) Departure of average surface temperature from 16-30, 1961. (B) Departure of averagesurface temperature normalforDecember 1-15, 1961. Temperatureanomalies from norrnal for -30, 1981. Temperature anomalies were negative behind and along the mean trough and positive wnrrrled as the mean ridge moved irrl:tnd in the Wcst but cooled ahead of it. n.ith thc advance of the mcan trough into the Southeast. changes of 1nea11 700-1nh. height (fig. 7) were greatest over theSoutheast, where the largcst half-rnonth tcnl- perature changes occurred, exceeding 10" F. alongthe coast of Georgia and most of Florida. By the cntl of the rrlonthfreezing terrlperatures had penetrated well into Florida. 4. MONTHLYAVERAGE TEMPERATURES In general, average ternpcretures for Decwrlher (fig. 8) werecolder than nor.rtl:tl over theUnited States. 7'hc coldness conformed well in a gross sense with the me:Ln circulation of figure 1, with negtLtive hcightanornalies

Unauthenticated | Downloaded 09/24/21 11:11 PM UTC MONTHLY TITEATHER REVIEW 125

FIGT-RES.-Dep:trturc of average surface temperature from norrnal (" F.) forDecrrnhcr 1961. Temperatures mere predominantly eoldcr than normal over the country. East,errl Canadian block- ing accounted for the relative warrnth in the Northeast (from [3]).

FIGURE7.-Change in 15-day mean 700-1uh. height December 1-15 to December 16-30, 1961 in tens of feet. Hlockirlg irlcreascd over per.sist8ence of surfacet8enlperature t111o111d;v over the eastern Asia and t,lle western Pacific, eastern I"ort,h America, Cnited St:tt,es fromthe preceding mont~h was unusually and the eastern Atlantic as the month progrcsscd. high wit81185 percent' in the O+ I class change category (5 cltlsses at 100 represent~ntmivestst'ions). The expected change is 67 percent,. The t'rend was t'oward cooling with occurrence of snow cover at8 t'llis tilno of ?-car is less than 42 percent colder by one or more classes compared to 24 50 percent. pcrccnt8wnrmer. Over the Co1or:tdo Plateau, which rcrltairletl ;Lbnortllully Another int8erestingaspect of temperature was the wide cold longafter surrounding ~reashntl w:rr111ctl (figs. 5B rutlge of ddyreadings in the interior of Alaska. At Fair- banks the monthly extremes were +25" F. and -62' F., and 6B),another factor WIS prcscnt. 'I'hcrc the (,old air wtm trtrppetl 1)y surrounding nlountt~insand protcctccl ant1 at ;2lcGrath tllc high was + 41" F., t'he low-67" F. from warmingwest'erlics to :I, varying cxtcnt, (tlcpentling The wmn extremes carw early and t,he cold lat'e in tmhe on locatmion)by an overlying tcmpera,turc irlvcrsion. nlolrt,ll.Irlt'crlsificat8ion of thenortherly component of An extremeesnrnplc occwrretl atWinslow, Ariz., w1ltr.e 111cvtn flow (see t'he change field of fig. 7) as t'hemont'h temperat'ures renl:tirled far lwlownornlal for mcelis progresscd corlt,ributed to the cooling t,rend over Alaska. following A snow stormon the 10th ant1 IIth. 'I'lterc- 5. PRECIPITATION after fog filled thelocal basin and n strollginversion developed aloftthat averngetl 13" C'. fro111 hse totop Vcry heavy precipitation, some of record-breaking pro- during thelast 11 days of thetnontll. The t~vc~r:lg~portions (see t8ablc 1 and fig. !I), occurredin t'he central temperature at' Winslow WM a record low for the month, Gulf Sttlt8esand the Tennessee Valley in December. The 11.9" F. below nortttal. 1w:~vicstrain fell between the 4th and the 18thduring In the coast,tdvdleys of C'&forni;L there WLS R sind:trly t'lw approach of the mean troughand waslargely pro- slow response to ch:mges of the caircnlation which are duced by over-running of u wtlrm southwesterly current ordinarily conducive to warrning elsewhere in the country. from the Gulf of Mexico above cool air near t'he surface. In this case the deterrent wt~svt~lley fog, whichis typics1l.v \.Vcckly tot,& in this period ranged locally t'o 15 inches prevalent in December and was observedalmost ($on- ttnd causcd flooding in Pvlississippi ttnd AltLbtrrrla. Record tinuously after the first few d;~ysexcept for several tlt~ys flood st,ages were reported :it Bovina and Jackson, Miss. around the 20th of this month. on the21st. River st':tgcs were28 feet over flood level InCalifornia, valley fog oft'cnoccurs in n~aritit~~c airant1 still rising on t'he 26th ;Itl Jackson, Ala. By tlle 29th trapped t,here by :in inversion which is fornled atld sus- however, cool dry air had swept into the flood area and tained by subsiding air from t'he Great Basin High. The tempcrat,urest8hereafter remained mostly below normal earth's surface is t'hus shielded from insolat'ionanti wtlvec- wit,lllight precipitation. Flood watterswere receding by tive warming, much inthe way observed over t'lw Colorado tmlleend of the tnontll. Plateau. AtYunla, Ariz., more rain fell in 24 hours around While sizablevariations occurred during December, mid-month than had accumulated during therest of 1961.

Unauthenticated | Downloaded 09/24/21 11:11 PM UTC T.,\Rm l.-A?nounts os heav!l precipitationat selected stations in December 1961

Homarks

Dec. record (since 1895). Dec. record. Dec. rrcord (sinca 1879). Dec. rrcord. Dee. record.

Dec. rccord (sinco 1873).

light until the end of the lnonth when Buffaloreported rnorc than 2 I‘eet on t,lle 29th ttnd 30t,h. This snow did FIGURE9.-Total precipitationin inches for December 1961. not’ extendto thc traditional snow belt of that region Heavyrains caused locally severe flooding inthe Soatheast I~owever. (from [3]). Deficient r~lo~lthlyamounts of precipitation (fig. 9) were reportcd justeast’ of thc(lont~inental Divide from Montana to Texas, whcrc Incan west’-nort’hwesterlv flow This was the first,month with above normal precipitation is usuallp dry because of the loss ol rnoist>urc in crossing at that station since . A Kona-t’ype stormat, the Rockies. Other dry areas were the Los hngcles Basin Kauai,Hawaii, brought heavy rainfall from the 4th to ol California and parts of Florida, where Miami reported the6t,h. Lihue, Kauai reported 4.20 inchesduring the its driest December since records began in 1911. storm, over two-thirds the tot’al for the month. REFERENCES Heavy snowfall over t’hc Central Plains was more ex- tensive than usual. Omaha, Sebr., Kansas Cit- and St. 1. C.Y. \&“cather Bureau, “Normal Weather Charts for the Northern Joseph, Mo., and Des Moines, Iowa, all with long periods Hemisphere”, Technical Paper So. 21, Washington, D.C. 1952. of record, reported their largest totals of December snow- 2. C. M. Woffinden, “The Weathcar andCirculation of Sovcmber 1961,’’ MonthZ!/ Weather Review, vol. BO, No. 2, Feb. 1962, fall. Snow remained corltirluous1.y on the ground at these pp. 71-78. and many surrounding stations after the 8th. In north- 3. U.S. WeatherBureau, Weekly Weather and CropBulletin, Na- western Kew York snow accurnulat,ionswere unusudly tional S//n~tnnrl/,vol. SLIS,KO. 2, Jan. 8, 1962.

U s GOVERNMENT PRlNTlNG OFFICE 1962 Unauthenticated | Downloaded 09/24/21 11:11 PM UTC