THE WEATHER and CIRCULATION of FEBRUARY 1960 a Cold Stormy Month Over the United States Associated with Widespread Blocking L

72 MONTHLY WEATHER REVIEW FEBRUARY1960

THE WEATHER AND CIRCULATION OF FEBRUARY 1960 A Cold Stormy Month Over the United States Associated With Widespread Blocking L. P. STARK

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

1. THEWINTER SEASON heights averaged above normal from the Great Lakes to t'he North Pole andfrom the Gulf of Alaska tothe Intense blocking over North America in February was NorwegianSea. Theimpact of theseanomalies was of the climax of the winter season of 1959-60. In the three primary significance tomuch of the westernportion of months of December, Janunry, and February the patterns the Korthern Hemisphere and especially to the weather of circulation (fig. IA), precipitation (fig. IB), :and telrl- in t'he United St,ates. perature (fig. IC)had largedepartures fro111 normal. The area of posit'ive height DN was surrounded by s The 700-mb. mean pattern for winter 1959-60 consisted band of negativedepartures which WAS almostuninter- of aridge inwestern North America anda trough in ruptedaround t'he hemisphere. That particular con- eastern United States. Most impressive was the posit'ive figuration was accompanied by tan expanded circumpolar height departurefrom normal (hereafter referred to :is vortex of t'he classic t'ype [4]. DN) which reachedfrom 40' N. to beyondthe North A further manifcstation of the sout'hward displacement Pole, with a center of f480 feet near BtLffin Island. To of westerlies can be derived from the profile of zonal wind t'he south height's were lower than normal from 40' N. speed at 700 mb. (fig. 4). Thebelt of westerlywinds tosouthern Mexico, a patterntypical of blocking [I]. ret~ched :L maximum of ithost 15m.p.s. atlatitude Precipitation in t'he wint'erof 1959-60 (fig. 1B) was more 30-35' X. and remained stronger than normal down to than times thenormal in a bandfrom the Southwest 20' X. Normally themaximum has a speed 2 m.p.s. tothe Great Lakes. ReltLtive dryness W;LS confined to slower than and a location 5' north of that observed this parts of the OhioValley and t'he Pacific Northwestin February.Note also thatthe westerlieswere about an otherwise rather wet map. 8 m.p.s. slower than normalin the latitude band50-55' N., Thedepartures from normal of averagesurface tem- competwtting for t'hr excess west'erlies in lower latitudes perature (fig. IC) were well correlated with the 700-mb. 111. height DN (fig. IA)and result,edfrom successively The timevariation of the 5-day mean zonalindex, colder monthsthrough the season. The onset of t'he giving the speed of the 700-mb. temperate westerlies calendarwinter was a markedcontrast, to the cold fall (35-55' X.) from 5' W. west'ward to 175' E., is repro- seasonwhich hadpreceded. December was very mild duced in figure 5 for tlle entire winter season. Its general (see fig. 4 of [a]); January was considerably colder (see trend wt~sdownward, interrupted at mid-February by s fig. 7A of [3]); and in February (fig. 2) there was :L further temporaryrecovery. Weaker thannormal tempcrate increase in tlle intensity and extent of the cold air. westerlies arefrequently a sign of colder than normal Insummary the winter of 1959-60 was cold in the temperatures at mid-latitudesover continents, as was Southand warm in tlle North. The seasonbegan pre- observed in February (fig. 2). In addit'ion, below normal dominantly warm and ended with a very cold February. thicknesscsfrom 1000 to 700 mh. (fig. 6) were almost The circulat,ionover Korth America featuredhigh everywhere distributedin :L lilatl[ler similar t'o height latitude blocking, which in the month of February p1:Lyecl departuresfrom rlormd (fig. 3). IL mostinteresting role. By the end of February the 5-day mean index fell to 5.1 m.p.s., the lowest value of tlle month andof the season. 2. THEMONTHLY MEAN CIRCULATION Five-day mean t>emperatures were also the lowest of the season at the end of the mont,h when they averaged from The predominant featureof the circulatiorl in February, 25 t,o 32' F. below normal from eastern Kansas to Idaho. as shownin figure 3, was the largecenter of positive In general, the long-term t'rend of the index was down- height DN over northern North America. The maximum ward and very much like that of the surface temperatures height departure was 660 feetnear Davis St'rait, and in the United States for the winter season.

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The olmrvcd tracks of daily cyclones areillustrated in Chart X of [5] and should be comparedwith the observed 3o-dt~ymean sea levcl t~ndDN chart (fig. 7). In t,lw Pacific the Aleutian Low andsubt'ropical High were close to their normal positions [6] since that ocean was :tffccted little by the blocking over North America. In the Atlantic, however, t'hc storm tracks were depressed sout~hwnrtl,:IS were t,he 700-mb. westerlies. There blocking WLS quitenoticeable sincesea-level pressures south of 45" x. were less than norrnalwith a lllilXiIllUJ~lde- parturc of - I1 nlb. 1leilr the Azores. The 1004-mb. Low just west of the United Kingdom was evidence of thegreat clisplrtccment of the Icelandic Low, normally locnted near Cape Farewell, Greenland. In the Urlitcd States the Southeast is normally under the influence of a mean High at sea-level. In February, however, that area had an average sea-level pressure as much as 6 mb. lower than normal and was subject to a mem flow from the north (fig. 7). Thcre was an appre- ciable increase in the frequency and intensity of storms inthe east'crn United States compared to a long term average [7]. The procession of cyclones east of the Rockies andthe frequent, occurrence of cold ridges at sea level (see nlem ridge in fig. 7) led to a cold, snowy February for much of the Nation. 3. MONTHLYMEAN WEATHER Monthlymcan temperature anomaliesin the United FIGURE1.-(A) Mesn 700-mh. contours (solid) md height depar- Statesduring February (fig. 2) were defined, t'o agreat tures from normal (dotted) (both in tens of feet) for 1l)ecernt)er extent,by the blockingover easternCanada (fig. 3). 1959-February 1960. The largepositive height anomaly in Coldert'tlan normal temperatures prevailedin at least C:madahelped define the strong blocking. (B) Perceutage of t811ree-fourt~lmof t'hecountry. Kegrtive departures from norrnalprecipitation for December 1959-F1:bru:my 1960. Exccbs- normal averaged 8 to loo F. from the Central Plains to the sive precipitation was widespread and related to nwnerous daily GulfCoastm. cont8rast,portions of thenortheastern stormssouth of the block. (From [8].) (C) Temperature In qu:drant were nbove normal wit'h n maximum of +la" depart,ure from normal (OF.) for 1)ecernber 195$)-February IMO. P. Kote the warmth in the Northand cold in the South. (From [8].) in Alainc.

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FIGURE5.-Time variation of zonal index in meters per secolld for thewestern portion of theNorthern Hemisphere it1 the latitude z3ne 35-55'N. The downward trend of the zonal intlcs during the 1959-GO winter season was accxnpanied by a sinlilitr trend of temperature. The lowest index valuc in February and thecoldest 5-day meall temperaturesoccurred :tt :ibout the same time.

FIOT'RE8,"Tot:tl precipitation(inches) for February 1980. Much of theprrcipitation chitst of theRockies resulted from rcscord ;~rlcltIear-rrcord snowstorms. (From [8].)

PrnLrRE G.--hfean thickness (1000 to 700 mb.) departurefrom rlorrrd tlurirlg February. In fact, t,tlis circurnst,arlce occurred mudl (tens of feet) for February 1960. There was a close correspondrrlcc, of the winter season. beta-een cooler than rrornml thickness and coolrr thau tlornxtl The wmn t,emperatures in the Northeast were an ex- sea-level terrlper:tt,urrs (fig. 2). t'remity of the greater picture which encompassed all the provinces :md territoriesrtorth of t,he Carmdiarl border. The cold portion of the United States W:IS found where Thtw wwe no monthly mean temperatures below normal 700-mb. height's were generally less than normal and where (except for. ext~remesouthern Sasknt,chewan) from Van- upper level contours were cyclonic. The warm Northeast couver Isltrnd to Ellsmrre Island and from Newfoundland was also subject to a cyclonic flow, and since t'htrt' flowwxs to PoilltBarrow. The westwardgrowt'h of anticyclo- northwesterly, it would appear to have t'ransported cold genesis in C'tmadtl was concurrentwith the widespread air inton'ew England. However, 700-mb. heights were warmth just described. above normal and theDN flow was from an easterly direc- As eqwctcd, below nor~tlalvalues of monthly mean tion, so that onshore advectionof relat'ively warm air from 1000 to 700-111h.thicktlcss (fig. 6) practically coincided the Atlantic was observed more frequently than normal wit11twlow norttltll surl'alc~~t'cn1per:atures inthe United

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States. Similarly, above normal thickness attendcd above was weak in the, north, but quite deep in the south. Two normalsurface temperatures in Nort'h America. The severe storms deepened in the Southern Plains early inthe +360-ft. center near Labrador was t,hem:Lxirnurn absolute month, en route to the eastern Great Lakes. value in theNorthern Hemisphere. It was dso the Tn theAt'lantic the most important feature was thc location of the maximumsurface temperaturet~nomaly division of the westerlies into one branch northeastward ($24' I?.) reported bythe Ca.nadian Meteoro1ogic:d from Labrador and the other eastward from the Middlc Service.. Atlant'ic States. The height departures from normal show Precipitation amounts exceeding 150 percent of normal that blocking was firmlyentrenched near Labrador, (fig. 8) were generally confined to the Central Plains and despite the cyclonic nature of the upper flow. to a strip from coastal New England to cent'ral Florida. 'I'h circulation of the second half of February, shown In the Eastfour stormswest of the Appalachians and three in figure 9A, representsa drastic change from the first east of the mountains were responsible for the record 27.6 15 days. Consideringonly majortroughs in figures 9A inches of snow at Roanoke, Va., 21.5 inches at Charleston, and 9R, the readjustment of t,lle circulationresult,ed in W. Va., and 20.8 inches at Youngstown, Ohio. the following changes: (1) strengthening of thetrough In the Great Plains much of the precipitation can bc alongt'he Asiatic coast'; (2) demise of the trough in the attributed to overrunning initiated by :t storm in north mid-Pacific; (3) retrogression of the t'rough in the southern Texas early inthe month. Another factor in the meanwas UnitedSt'ates; (4) development of anew trough inthe cyclonic anomalous flow from an easterlydirection at easternAtlantic; and (5) effective loss of atrough in 700 mb. (fig. 3). Furthermore, there were 10 or 11 days Eurazk with fronts from Montana to western Kansas as Pacific As aresult of thesechanges the hemisphericwave maritime air overran Canadian Polar air. February snow- number was reduced by one. Considering both contours fall records were reported at Goodland, Kans., which had and height departures from normal, there were two major 25.4 inches, Kansas City, Mo., 20.7 inches, and Lincoln, troughs at high lat'itudes and four at middle and lower Nebr., 19.2 inches. latitudes in t'he first 15days of themonth. The wave Alaska recorded warmer than normal temperatures and number wasreduced by one atboth high andmiddle normalor subnormal precipitation. Themonthly mean latitudes during the last 15 days of February. temperature at Barrow averaged 9.6' F. above normal for In and near North America the change in height DN February. A new maximum for the monthwas establislled was of great nlagrlitude and is accentuated graphically in on the 28th when the temperature rose to 32' F. On the figure 9C. The blockingin easternCanada intensified same datethe temperature at Yakutat reacheda new by 560 feet and appeared to spread westward. The ridge February high of 49' F. Greatest mean temperature de- which had been over the continent was located some 35" parturefrom normal was +12.0' F. at Kotzebue.The longitude to the west. It extendedfrom the sub-t'ropics unusual warming over Alaska was related to above normal to the Polar Basin, the heights increased in t'he 2 weeks 700-mb. heights and southeasterly DN flow. by 1,020 feet in the Gulf of Alaska. That was the largest, In the Hawaiian Islands temperatures were also rather changein theNorthern Hemisphere, and its proximity extreme.Lihue reported a new maximum of 86' F. for to Sorth Americawas of considerableimport'ance to Februaryand a new all-time minimum of 53' F. At weat'1lc.r in the United States. Honolulu 56' F. was a new low for the month. An all-time WEATHER minimum of 55' F. WAS established at Hilo.The new The weekly march of cold weatherin February is. records in t'he Hawaiian Jslands were attained on those described by t,lle series of t,emperaturedepartures from days when the normal trade wind regime was absent'. normal in figure 10, taken from 181. For the week ending 4. ATRANSITION AT MID-MONTH February7 (fig. IOA), t'emperaturcs were abovenormal over most of the Nation, with a maximum of 18' F. in CIRCULATION NorthDa,kota. Acute cooling inthe cent'ral portion of The evolution of the circultrtion during Febnutry cannot the United States in the second week (fig. 10B) followed be assessed solely from nn inspect,ion of the monthIy metm the development of an intense cyclone which swept across, charts.Half-monthly mem cl~srt's,shown in figure 9, t'lw Rockits from east'ern Colorado,where it int'ensified, depict clearly the gretlt change which occurred in F~~Ix-t'o the central Lakes and into Canada (Chart' X of [5]). ary, and emphasize it's proportions. Its passagein t'he Centraland Southern Plains was First consider figure 9A, the 700-mb. mean for tdlc 15- accompanied by snow, strong winds, and record sea-level day period ending February 14. The trough in the central pressurc for many stations. One of the lowest was 28.70 Pacific terminated in an intense Aleutian T,ow, which was inches (972 mb.) at Ponca City, Okla. On the day prior 600 feet below normal.The ridge over North America to thestorm, temperatures in t'he upperRio Grande was relativelyweak. Although it wassomewhat higher Valley reached as high as 102' F.; two daysaft'er t'l~c* than normal in the United States,was it lower than normal storm's passage, maximum temperatures were in the high in Canada.The trough downstream had fractured and 50's in that area.

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FIGFRE9.-hIean 700-mh. contours (solid) andheight departures from normal (dotted) (both in tens of feet) for (A) Jan. 31-Feb. 1.2, 1980, and (H) Feb. 15-29, 1960; (C) 700-mb. heightchange from Jan. 31-Feb. 14, 1960 toFeb. 15-29, 1960. Themost signific:tnt cl~arlge occnrred in the Gulf of Alaska where heights increased as much ns 1,020 feet.

Theconsequences of thechange incirculation over North Anlrriea favored severe cold in most of the United NorthAmerica in the last two weeks of February werc Statcs. The strong northwest'erly flow from the Beaufort pronounced. Figure9B indicates the strength of thc Sea to thc Rocky Mountain States (fig. 9B) was respon- great ridge in the eastern Pacific wvllich propelled cold sible for a prolonged outbreak of Arctic air into the United air into the United States. By the end of the third weck Statts. Irl figure IOD, week endingFebruary 28, tem- (fig. lOC), weekly mean temperatures had dropped as muchpcratures nvcraged below normal in most of the United as 6' F. over a large part of the country. A storm which States. Meantemperatures more than 20' I?. below developed over southeastern Texas on the 17th deepened normal W~IYobserved from northern Texas to Montana. rapidly as it traversed the Gulf States, crossed t'he Middle Rather weak storms in tjhc southern and central portions Atlantic States, and moved into the Bay of Fundy. In ant1 one major st'orm in the East were responsible for as its wake was a broad band of heavy snowfallfrom the much as 16 inches of snow in Kansas and lesser amounts central Mississippi Valley to the Maritime Provinces. t,o the east. 'Phe continued rise in 700-mb. heights in northwestern In allinstances just discussed (fig. 10) the mostper-

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FIG~RE10,”Departure from normal of averagetemperature (OF.) for weeks ending (A) Feb. 7, 1960, (B) Feb. 14,1960, (C) Feb. .21 1960, and (D) Feb. 28. 1960. Aft,er the mild first week, ten1per:ttnres ovrr it large area becameprogressively colder. (From [SI).

sisterlt feature common t,o all weeks was t’lw warmth in theNortheast. In n’ew Englandtmemperaturt~s avcragctl from 6 to 12’ F. warmer than normaleach week. This area was influcnccdby mild Atlantic air urlder the blocking regime.. Thus, while other regions of thecountry were experiencing cold weather in February, it was an excrp- tionally mild month in Kew England.

REFERENCES

1. J. NaInias, “The Iudex Cycle and Its Role in the C‘enerd Circn- lation,” Journal of Meteoroloyy, vol. 7, No. 2, Apr. 1950, pp. 130-139.

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