178 MONTHLY WEATHER REVIEW h1AY 1961

THEWEATHER AND CIRCULATION OF FEBRUARY 1961' An Example of Attentuation in the Long-Wave Pattern

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

1. CONTRASTINGREGIMES IN WINTER 1960-61 strongly". cyclonic~ in January to slight81y-. cyclonic - in February. The result of t'hese changes in and near North Att.enuation of the long-wave features in t8he u-est8ern America was a decided increase in t'he average speed of portion of theNorthern Hemisphere in February 1961 the westerlies at mid-latitudes. The hemispheric zonal was in marked contrast to the patternstmh:Ltm had prevailed index (5' T.-175' E.) was 2 n1.p.s. greater in February inthe preceding wintermont,hs. December[I] and In than it was in Jarlutn-y. This increaseoccurred despite January [2] pronouncedamplification of thc mont~hly a regional decrease in westerlies from t'he cent'ral Atlantic mean 700-mb. flow patt,ern dominated the circulat,ion and to Europe wherethe, meridional component of t'he flow effectively controlled theweather in North Americ2L. dominated. Widespread cold conditionsin December were repeated In western Europe changesin the average circulation in January in the East and Southin response to thc strong at 700 mb. were large.Diffluent, cyclonic flowin Janu- ridge in western North America that extended from the ary was replaced in February by a strong ridge (fig. 1) Great Basin tothe ArcticOcean. Substantid changes in which there were maximum heightt rises of 390 ieet in in the circulation inFebruary 1961 (fig. 1) produccd a Spain (fig. 2).The evolution of this ridgewas simul- strong damping of the long waves in the mid-troposphere. taneouswith damping of thepattern int'he eastern As a result the country was subjected to mild Pacific air Pacific and western North America; its growth deserves masses instead of cold continental air; the westerlies at further comment,,based on 5-daymean 700-mb. charts mid-latitudesover North Americawere strengthened; one week apart (not shown). and temperatures in the East became warmer than normal On the 5-daymean chart' for -6, a high near the end of the winter season. center was located just to the west of Xorocco with a 2. SOME ASPECTS OF THEMONTHLY MEAN flatridge extending northwardto Scotland. One week CIRCULATION later the High was centered over Gibraltar and the ridge was still rather flat to the north, but the troughsouthward The changes in circulation from January to February fromGreenland had become quite deep near 40' N. are best examined by referring to figure 2, the difference Meanwhile a positive height anomaly center of 390 feet in 700-mb. heightdeparture from normal betweent'he hadmoved from Gibraltar to northernSpain and in- two months.The greatest changes were inthe Pacific, creased to 590 feet. where most of t.he area underwent height, rises in Febru- During the week of -20 the anomaly center ary, and the deep, cyclonic center of January at middln rcached a maximum of 760 feet above normal in latitudes weakened and moved northward. Consequently, and wasassociat,ed with a dominant' blocking High. In the ridge in western Nort'h America could no longer be t,he last week of February the High was observed over the sustained in such strength as was noted in December and Baltic Sea, still strong, with an omega-configuration well January. Heights fell throughout the area from January defined by a strong trough (negatively tilted) in the east- to February, with maximum change of -420 feet in the ern Atlantic and adeep resonant trough fromnort'hwestern Yukon (fig. 2). to t'hc Red Sea. The trough in eastern North America advanced to the Anothcr blocking High wasobserved on the 30-day centralAtlantic as flattening of t'he pattern progressed meanchart (fig. I) overnortheastern Siberia, but this downstream.Height anomalies increased over most, was considerablJ7 weaker thanthe onein Europe. Its of eastern North America (maximum of +310 ft'.in NewYork) where contour curvature changed from development occurred as the trough along the Asian coast deepened at middle and low latitudes and as the ridge 1 The weather of March, April, and will be described in theJunc, July, and issues of the Review, respectively. which was in the Arctic Ocean in January retrograded.

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FIGUREl.--Pvlean 700-mb. contours (solid) and departures from normal (dotted), both in tens of feet, for February 1961. The trough in the Southwest intensified as the wavelength and westerlies increased at mid-latitudes.

Ordinarily the effect of blocking shows up in a migration 3. WEATHERRELATED TO THEMEAN CIRCULATION southward of the westerlies and a decrease in the average TEMPERATURE zonal wind speed. Butthe blocking Highs t'tlis month were too far removed t'o have an appreciablc cfl'ect on Departures of average surface temperature from normal the zonal index in the westernport,ion of the Sortllcrn (fig. 3) were predominantly positiveexcept for a small Hemisphere or on the weather anti circulation in North area in t'hc Southwest where it was 1O-2' F. cooler than America. Dominant in the weat'her of the normal. The correspondence was close between the flow were the generally westerly flow from t>heeastern Pacific patternand the temperature anomaly. Compare the to the western Atlantic, and especially, thc position of the height departurefrom normal (dotted lines fig. 1) with Aleutian Low at an extreme nort'lleast location. thetemperature depart>ures in figure 3. Considering

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TABLEl.-Class changes of monthlyaverage surface temperature anomalies in 100 cities from .January to February 1961

Class chango* Frequency (percent)

+4 0 +3 21 +2 23 +1 30 0 "1 1; -2 0 -3 0 -4 0

*+indicatrs a change toward higher average temperature. Classes are much below norrnal, below normal, new normal, above normal, and much above normal.

interpretation of thisstatistical result is dependent on the well-known fact that theposition of the Aleut.ian Low has a controlling effect 011 temperatures in muchof North America. InFebruary it is normallylocated over Karnchntka. When it is displaced far to the east (asit was in February 1961), 700-mb. heights are usually lower thannormal in northwestern North America(as they were thismonth) (fig. 1).The resultingwesterly flow preventsoutbreaks of Arct'ic airfrom penetrating into FIGURE2.-Change of mean 700-mb. height departures from normal latitudes below 40° or 50° N. Furthermore, increased (tens of feet)from to February 1961. Attenua- westerlies at theselatitudes in the eastern Pacific tion of flow pattern near North America in February is accented (4-8 by falls in the ridge in western North America and rises in the m.p.s.stronger than normal this February) undergo troughs in the eastern Pacific and western Atlantic. strongkatabatic warming aft,er striking the middle Rockies. Thetotal effect is frequently unseasonable warmth east of the Divide at middle latitudes. The changein temperature regime fromJanuary to Februarywas determined by the circulation change (fig. 2).Consequently, persistence of temperature was large west of the Mississippi River, where height changes were small; to the east, temperatures changed radically where height changes were relatively large. A summary of the January to February changes in monthly average temperature (in terms of five classes) at 100 cities (table I) indicates the following: (a) 74 percent of the temperatures becamewarmer, (bl 7 percent became cooler, and (c) 21 percent became warmer by three classes (from much below normal to above normal).

PRECIPITATION

FIGURE3.-Departure of average temperature from normal (OF.) Precipitationin February (fig. 4)was much more forFebruary 1961. Heightanomaly (fig. 1) andtemperature extensive and generally hcavier (in 64 of 100 cit.ies) than anomalyare almost identical in pattern. (From [5].) it wasin January [5]. Fromthe Ohio Valley tothe westernPlateau and from the Canadian border to the Southern Plains, little or no precipitation fell in January, sign only, the similarityin the patterns is remarkable; when northerly flow at 700 mb. and sea level inhibited considering magnitude, the analogyis weaker. thetransport of moistair masses into the country. In The correlationbetween local 700-mb. heightsand February the average flow aloft (fig. 1) and atsea level [6] surface temperatureanomaly is generallypositive, but encouraged the advection of moisturenorthward from averagecorrelations in the wintermonths explainonly the Gulf of Mexico and eastward from t'he Pacific. about 35 percent of thetemperature variance [3]. By In the Pacific Northwest there was a deeponshore flow statisticallyintegrating the behavior of the atmosphere fromsea level to 700 mb.This current, the core of at variousdistances from the local area,the explained maximumwind speed or jetstream" at 700 mb., sup- variance can be considerably increased [4]. The synoptic ported a very high frequency of frontal systems, 8 to 12

Unauthenticated | Downloaded 10/01/21 12:02 AM UTC MAY 1961 MONTHLY WEATHER REVIEW 181 crossing thecoast in February. As much :ts 20 in. of rain fell along the Washington coast as moist, Pacific air was forced to ascend the mountain slopes. Precipitation records forFebruary werebroken at Olympia,Wash. (7.02 in. above normal), Sexton Summit, Oreg. (6.75 in. above normal), and several other stations in Washington and Oregon. Flooding was reported in the Puget Sound area, the Olympic Peninsula, and the Willamette Basin. In the Southeast the distribution of precipitation was related almost ideally to the 700-rnb. pattern [7]. Heavy precipitation occurred inthe southwesterly flow ahead of the trough and through the weak ridge (fig. 1). There was additional support from the southeasterly anondous component of the 700-mb. flow and from the warm, moist air at sea level that hada long trajectory from the Tropics. FIGITRE 4,"Total precipitation, inches, for February 1961. Locally Locally severe flooding from Rlississippi to Georgia severe flooding in the Northwest and in the Southeast was closely resulted from 8 in. to more than17 in. of precipitation that related to strong southwesterlyflow in advance of mean troughs. fellin a narrow band from Louisiana to western South (From [51.) Carolina (fig. 4). Several cities in Alabama and Georgia reported the wet'testFebruary of record.From early reports precipitationwas heaviest in and vicinity, where the total was 17.67 in., 12.50 in. greater for January 31- is shown in figure 5C. From than normal. the Central Plains westward, temperatures ranged from In contrastto the wet weather just described were 4'-18' F. warmer thannormal. Most of thiswarmth continued drought conditions over most of the Southwest. was a function of the ridge aloft; some was related to the Many cities hadnear-record dryness for February and increasingwesterlies andthe deepLow in the Gulf of for the winterseason. At Cheyenne,Wyo., and Grand Alaska. In the easternportions of thecountry cold air Junction, Colo., February was the 16th and 11th consecu- was closely associatedwith the anomalous northerly tive month, respectively, in which precipitation was less components of the flow at 700 mb. and at sea level. The than normal. A deficit of precipitationin February. source and trajectory of the cold air observed in the East contributed to the driest winter of record at Chicago, Ill., was implicit in the strong ridge at sea level from Hudson Lansing, Mich., and Burbank, Calif. Bay to the Gulf Coast (fig. 5B).

4. INTRA-MONTHLY TRANSITION FEBRUARY15-MARCH I During the last half of February broadscaledamping JANUARY31-FEBRUARY 14 of themajor perturbations occurred inmid-latitudes The700-mb. circulation in the first half of February from the central Pacific to the eastern Atlantic (fig. 6A). (fig. 5A) was transitional between the large-amplitude TheAleutian Lowcontinued to weaken as heightsin- circulation of January and t,he higher index state during creased asmuch as 480 feetin the eastern Pacific. At the last half of February. thesame time heights decreasedover Canada,and cy- In theeastern Pacific, despiteappreciable filling, the clonic flow prevailed from Alberta to Labrador, in place eastern lobe of the Aleutian Low was still rather strong. of the ridgeformerly there. Inthe westernAtlantic The sea level counterpart (fig. 5B) was 11 mb. below height rises were widespread and reached a maximum of normal, as intense as any vortex on the chart, but con- 390 feet where t'here had been a trough the first half of siderably weaker than the same Low in January, when themonth. This nearly simultaneous growth of the pressures for the month averaged20 mb. below normal [2]. Bermuda High and the Highin the eastern Pacific suggests The ridge over North America had muchless amplitude a tcleconnection often noted in a period of rising index. than in January and was somewhat farther east. Height Generalheight falls in higher latitudes and rises in falls at 700 mb. eroded t,he western portion of the ridge at middle latitudes contributed to a pronounced strengthen- middle and high latitudes as the center of positive height, ing of the zonalindex in temperatelatitudes. At mid- departure from normal jumped from British Columbia in month the 5-day mean 700-mb. zonal index (5' W.-175' January to Hudson Bay in thefirst half of February. E.) was10.5 m.p.s., only 0.3 m.p.s. above normal. By In the Atlantic the strong troughat 700 mb. progressed the end of February, however, the index had jumped to from the east coast and deepened as cold air was injected 13.6 m.p.s., 3.8 m.p.s. above normal. from Canada. The Icelandic Low at sea level was near its Deepening of the trough in the lee of the Rockies was normal position, but was 11 mb. deeper than normal. assisted bythe increasedwesterlies impinging on this The distribution of temperature departures from normal naturalbarrier. Accompanying the increased cyclonic

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FIc;rxE 5.-(A) Xlean 700-mb. contours (solid) and departures from normal (dotted),both in tens of feet. (B) Mean sea level pressure (solid) anddepartures from normal (dotted), both in millibars. (C) Ikparture of averagetemperature from normal (OF.). All chartsfor January 31-February 14, 1961.

flowwere vigorous frontalsystems that crossed the A great' warming in the East and a cooling in the West Rockies and intensified just to its lee. In fact',there were the principal reactions t'o the reversed flow.Figure were 16 days of frontal activity in Montana during the 6c showst'llat average tcmperatures for - month, a maximum for the country. March 1 werc generally 4'-8' F. warmer than normal The sea level chart for February 15-hfarch 1 (fig. 6B) over most of the country and a few degrees cooler in the should be compared with that' of the first half of February Southwest.But the changesfrom the first half of (fig. 5B). Changesindicated are similar to thosefound February to the last half wcre quite large. In the East, onthe 700-mb. chartsfor the same periods. The com- forexample, t>emperature anomalies showed positive plementaryanticyclogeneses in theeastern Pacific and changes of 12'-16' F. ina belt from Florida to New the western Atlantic are clearly portrayed by the dotted York; west of t'he Plains States negative changes ranged lines showing the sealevel pressure departures from from 4'-10' F. normal. In the sameperiod sea level pressures fell 5-9 As the t'ransitionin the circulationproceeded, there mb. from the Gulf of Mexico to Baffin Bay. The direc- was a definite change in the tracks of cyclones in and near tion of the flow andits anomalous components are North America. This might' be anticipated from a study indicative of the changedregime. Note especially the of figures 5 and 6, but for additional emphasis and clarity, northerly direction in the West and the southerly direction figure 7 is presented.This figure was prepared by in the East. separating the tracks of migratory cyclones published in

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FIGURE&“(A) Mean 700-mb. contours (solid) and departures from normal(dotted), both in tens of feet. (B) Meansea level pressureand departure from normal, both in millibars. (C) Departure of averagetemperature from normal. All chartsfor February 15-March I, 1961.

FIQURE 7.-Paths of migratorycyclones (A) Feb. 1-14, 1961, and (€3) Feb. 15-28, 1961. Notegeneral shift of cyclones northward in both oceans and increased frequencyof tracks originating in the Southwest from the first half to the lasthalf of February.

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[6] into halves;those originating in the first half of REFERENCES February (A) and those originating in the second half (B). 1. R. €1. Gelhard,“The Weathcr and Circulation of December These tracks clearly illustratethe changes discussed 196O”AnUnusually Cold Month in the United States,” above. Thetransfer of cyclonic activityin the Pacific MonthlyWeather Review, vol.89, No. 3, Mar. 1961, pp. 109-114. to amore northerly locus was a typical result of anti- 2. R. A. Green, “The Weather and Circulation of January 1961,” cyclogenesis in the eastern Pacific. Inthe Atlantic t”ne Monthly Weather Review, vol. 89, No. 4, Apr. 1961, pp. 137-143. average storm trackwas displaced to the northin a similar 3. D. E. Martinand W. G. Leight,“Objective Temperature manner for a similar reason. Estimates from Mean Circulation Patterns,” Monthly Weather Storminesseast of the Rockiesreached its highest Review, vol. 77, No. 10, Oct. 1949, pp. 275-283. 4. W. H. Klein, B. M. Lewis, and I. Enger, “Objective Prediction frequency during the last half of February. Essentially, of Five-Day Mean Temperatures During Winter,” Journal of there were two majorstorm tracks4ne along the Meteorology, vol. 16, No. 6, Dee. 1959, pp. 672-682. Canadianborder, the other from the Southwest to the 5. U.S. WeathcrBureau, WeeklyWeather and Crop Bulletin, Northeast’.Both tracks can be reasonably‘lfitted” to National Summary, vol. XLVIII, Nos. 6 and 10, Feb. 6, and the flow at 700 mb.and to the mean sea level chart. &far.6, 1961. 6. U.S. WeatherBureau, ClimatologicalData-National Summary, Along the sea level track, associated with the new trough vol. 12, No. 2, Feb. 1961, Chart IX. in the Plains, was found the most intense storm of t’he 7. W. H. Klein, “Winter Precipitation as Related to the 700-mb. winter season in the East, as well as those syst’ems re- Circulation,” Bulletin of theAmerican Meteorological Society, sponsible for record-breaking precipitation in the South- vol. 29, No. 9, Part 1, Nov. 1948, pp. 430-453. east.

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