MARCH 1962 121 THE WEATHER AND CIRCULATION OF DECEMBER 1961 Strong Blocking at High Latitudes RAYMOND A. GREEN Extended Forecast Branch, U S . Weather Bureau, Washington, D.C. 122 MONTHLY WKATI-IER REVIEW MARCH 19G2 FIGURE 2.-Change in mean 700-mb. height departurrs from nor- FICI-RE 3-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 a t middle and high latitudes of both the Pacific and dashed arrows their normal 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 f d l s of as nlucll tts 580 fret) (fig. 7) between the h;tlf-nlonth means as portion of tllc eastern Pacific ridge advt~nced into western Korth America. Further progression of t,he trough in the United States was accompanied by falls of several hundred feet with noticeable impact on the country's weather. Tenlperatures over the western two-thirds of the United States were general1~- eoltler than norllla,l during t,he first half of Decelllber (fig. j R ), in response t'o north- erly flow between tllc strong nlean ridge in the eastern Pacific arid the trough from I,akc Superior southwestward (fig. SA). Also, most of the area was blankct~etl t )y snow from storms early in the month. Durillg this period $111 unusually severe outhrcak of cold air flooIran 700-mh. contours (solid) and height de- pnrturcs from llormal (dotted), both in tens of feet, for December 16-30, 1961. (B) Departure of average surface temperature from norrnal for December 16-30, 1981. Temperature anomalies wnrrrled as the mean ridge moved irrl:tnd in the Wcst but cooled n.ith thc advance of the mcan trough into the Southeast. MARCH 1962 MONTHLY TITEATHER REVIEW 125 FIGT-RE S.-Dep:trturc of average surface temperature from norrnal (" F.) for Decrrnhcr 1961. Temperatures mere predominantly eoldcr than normal over the country. East,errl Canadian block- ing accounted for the relative warrnth i n the Northeast (from [3]). FIGURE 7.-Change in 15-day mean 700-1uh. height December 1-15 to December 16-30, 1961 i n tens of feet. Hlockirlg irlcreascd over eastern Asia and t,lle western Pacific, eastern I"ort,h America, and the eastern Atlantic as the month progrcsscd. occurrence of snow cover at8 t'llis tilno of ?-car is less than 50 percent. Over the Co1or:tdo Plateau, which rcrltairletl ;Lbnortllully cold long after surrounding ~r e a s hntl w:rr111ctl (figs. 5B and 6B), another factor WIS prcscnt. 'I'hcrc the (,old air wtm trtrppetl 1)y surrounding nlountt~ins a n d protcctccl from warming west'erlics to :I, varying cxtcnt, (tlcpentling on locatmion) by an overlying tcmpera,turc irlvcrsion. An extreme esnrnplc occwrretl at Winslow, Ariz., w1ltr.e temperat'ures renl:tirled far lwlow nornlal for mcelis following A snow storm on the 10th ant1 I I t h . 'I'lterc- after fog filled the local basin and n strollg inversion developed aloft that averngetl 13" C'. fro111 h s e t o t o p during t h e l a s t 1 1 d a y s of the tnontll. The t~vc~r:lg~ temperature at' Winslow WM a record low for the month, 11.9" F. below nortttal. In the coast,td vdleys of C'&forni;L there WLS R sind:trly slow response to ch:mges of the caircnlation which are ordinarily conducive to warrning elsewhere in the country. In this case the deterrent w t ~s vt~lley fog, which is typics1l.v prevalent in December and was observed almost ($on- tinuously after the first few d;~ys except for several tlt~ys around the 20th of this month. In California, valley fog oft'cn occurs in n ~a r i t i t ~~c a i r trapped t,here by :in inversion which is fornled atld sus- tained by subsiding air from t'he Great Basin High. The earth's surface is t'hus shielded from insolat'ion a n t i wtlvec- tive warming, much in the way observed over t'lw Colorado Plateau. While sizable variations occurred during December, per.sist8ence of surface t8enlperature t111o111d;v over the Cnited St:tt,es from the preceding mont~h was unusually high wit811 85 percent' in the O+ I class change category (5 cltlsses at 100 represent~ntmive stst'ions). The expected change is 67 percent,. The t'rend was t'oward cooling with 42 percent colder by one or more classes compared to 24 pcrccnt8 wnrmer. Another int8eresting aspect of temperature was the wide rutlge of d d y readings in t h e interior of Alaska. At Fair- banks the monthly extremes were +25" F. and -62' F., ant1 a t ;2lcGrath tllc high was + 41" F., t'he low-67" F. The w m n extremes carw early and t,he cold lat'e in tmhe nlolrt,ll. Irlt'crlsificat8ion of the northerly component of 111cvtn flow (see t'he change field of fig. 7) as t'he mont'h progresscd corlt,ributed to the cooling t,rend over Alaska. 5. PRECIPITATION Vcry heavy precipitation, some of record-breaking pro- portions (see t8ablc 1 and fig. !I), occurred in t'he central Gulf Sttlt8es and the Tennessee Valley in December. The 1w:~vicst rain fell between the 4th and the 18th during t'lw approach of the mean trough and was largely pro- duced by over-running of u wtlrm southwesterly current from the Gulf of Mexico above cool air near t'he surface. \.Vcckly tot,& in this period ranged locally t'o 15 inches ttnd causcd flooding in Pvlississippi t t n d AltLbtrrrla. Record flood st,ages were reported :it Bovina and Jackson, Miss. on the 21st. River st':tgcs were 28 feet over flood level ant1 still rising on t'he 26th ;Itl Jackson, Ala. By tlle 29th however, cool dry air had swept into the flood area and tempcrat,ures t8hereafter remained mostly below normal wit,ll light precipitation. Flood watters were receding by tmlle end of the tnontll. At Yunla, Ariz., more rain fell in 24 hours around mid-month than had accumulated during the rest of 1961. FIGURE 9.-Total precipitation in inches for December 1961. Heavy rains caused locally severe flooding in the Soatheast (from [3]). This was the first, month with above normal precipitation a t t h a t s t a t i o n since June 1960. A Kona-t’ype storm at, Kauai, Hawaii, brought heavy rainfall from the 4th to the 6t,h. Lihue, Kauai reported 4.20 inches during the storm, over two-thirds the tot’al for the month. Heavy snowfall over t’hc Central Plains was more ex- tensive than usual. Omaha, Sebr., Kansas Cit- and S t . Joseph, Mo., and Des Moines, Iowa, all with long periods of record, reported their largest totals of December snow- fall. Snow remained corltirluous1.y on the ground at these and many surrounding stations after the 8th. In north- western Kew York snow accurnulat,ions were unusudly T .,\R m l.-A?nounts os heav!l precipitation at 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 t h e end of the lnonth when Buffalo reported rnorc than 2 I‘eet on t,lle 29th ttnd 30t,h. This snow did not’ extend to thc traditional snow belt of that region I~owever. Deficient r~lo~lthly amounts of precipitation (fig. 9) were reportcd just east’ of thc (lont~inental Divide from Montana to Texas, whcrc Incan west’-nort’hwesterlv flow is usuallp dry because of the loss ol rnoist>urc in crossing the Rockies. Other dry areas were the Los hngcles Basin ol California and parts of Florida, where Miami reported its driest December since records began in 1911. REFERENCES 1. C.Y. \&“cather Bureau, “Normal Weather Charts for the Northern Hemisphere”, Technical Paper S o . 21, Washington, D.C. 1952. 2. C. M. Woffinden, “The Weathcar and Circulation of Sovcmber 1961,’’ MonthZ!/ Weather Review, vol. BO, No. 2, Feb. 1962, 3 . U.S. Weather Bureau, W e e k l y Weather and Crop Bulletin, Na- pp. 71-78. tional S //n ~t n n r l /, vol. SLIS, KO. 2, Jan. 8, 1962. U s GOVERNMENT P R l N T l N G O F F I C E 1 9 6 2