The Weather and Circulation of November 1960'

FEBRUARY1061 53

THE WEATHER AND CIRCULATION OF NOVEMBER 1960'

Another November with a Mid-Month Reversal JAMES F. O'CONNOR

Extended Forecast Section, US.Weather Bureau, Washington, D.C.

1. HIGHLIGHTS tlaily tenlperat'ure departure from normal of -34' F. on t,lle 28th and Huron, S. Dak., -31' on the 30th. Temperatures averaged unseasonably mild throughout most of the United States during November 1960, con- tinuing the warn1regime which dominated the country 2. AVERAGEMONTHLY CIRCULATION during most of the fall season. This was in sharp contrast to the cold November and full season of 1959 [I]. Jn the The t~verage circulationat 700 mb. for November 1960 lat'ter part of the nlonth tcmperatures in many sections (fig. 1) was of tlle high-index t'ype; i.e., negrtt'ive height east of the Rockies rose t,o record high levels for so lat'e in depart'urcs from norrrlwl dominated t,he higher latitudes, theyear. At Rapid City, S. Dak., the average t,emper- while positive departures were general at lower latitudes. aturc on the 25th was 32' F. above normal, and the maxi- The onlysignificant exception was the area of blocking mum was 75' F. mtl posit'iveheight depart'ures oyer Greenland and t'he Precipit'at'iontot'als were in excess of norrnalin the Norwegian Sea. west,ern States, where many places experienced one of the The most anondous featureof this average circulation wettestNovembers on record. In Nevadareservoirs ~LSthe strongncgat'ive height anomaly near Ireland. were beginningto refill aft'er t'wo years of drought. In This center, together with the blocking to its north, had contrast to the West, some large areas eastof the Rockies also been aient'ure of t'heprevious month's pattern [2] hadone of theirdriest Novembers, particularly t'he when it \vas displaced slightly southeastward. Central Plains and extreme southern Rockies, as well :LS At sea level, t'he mont'hly average pat'tern (not shown) much of the eastern and southeastcrn partsof the country. contained n deep cyclonic center as much as 14 mb. below A marked change in circulat'iorl ne:umid-mont'h was normal west of Ireland, close t'o the -450-ft. center of reflected in int,ensification of st'orrniness over the country 11eigllt departureat 700 rnb.shown in figure 1. This in the second half of themonth, particularly from t'he center ol action was a manifestation of persistent daily Great Lakes westward. A rapidly deepening storm swept storminessduring the month. For example, systems as across the Great Lakeson the 15th tmd 16th. accompanied deep ils 960 mb. were observed on 10 days, while storms by high winds, thunder, hail, and t~ornudoes. In t'he Far as deep as 975 mb. were observed on 25 days during the West tlle weather was quitme st'ormy inthe last part, of the month somewllcrc in t'he Nort'h Atlantic. month with almost daily precipit'at'ion in Pacific the Nort,ll- InSovcnlber there WILS considerableshearing of the west. Heaviestamounts were reported lrorn wcst'crrl troughs which had been generally of the full-lat'itude type Oregon,where flooding caused considerable d:mlngc and in October. 771is was reflect,ed (fig. 1) in troughs at high requiredevacuation along tributaries of theWillalncttc latitudes superimposed over low-latitude ridges,especially river. in the At,lantic and Pacific. As a result the zonal index, Near the month's end,:L st,orrn movedout of Nevada on u ~ne:~surcof the strengt'h of t'he mid-lat'itudc westerlies tmhe27th and swept across t'he rlort8Jlern Plains,intcnsi- ;Lvcr:lgcd ovcr the Western Henlisphere at 700 mb., in- fying to blizzardproportions on tlle27th and 28tjhin c.rcwxd from 9.7 1n.p.s. (0.2 1n.p.s. above normal) in Octo- juxtaposition with t~ st,rong crescent-shaped continentd- ber to 12.4 1n.p.s. (1.9 1n.p.s. above normal) in November. polar Highcentered over western Canuda. High winds 'I'he greatest strengthening of the west'erlies occurred in (e.g., a recordfor November of 68 n1.p.h. atDuluth, t,he castern At'lantic, but increases occurred elsewhere in Minn.) and heavy snows in north central areas brought t,he West,ern Hemisphere as height, falls (relative to nor- the coltlesb weather of the season to many places cast of mal) at high lat'itudcs weresuperimposed on rises at theRockies. For exumple,Havre, Mont,., reported B lower latitudes, as shown in figure 2. I Artirles drscrihing tho wcetller of Decenlhcr 1980 and January and February 1961 will The height anomnly difference pattern bet'ween October appear in the March, April, and May, 1961 issues, respectively, of tile Monfkly Il'eutker Review. and Novemberover North Arncrica (fig. 2) is also of

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FIGUREl.-"I.an 700-mb. cont'ours (solid) and height dcparturrs from normal (dotted) (bot'h in tens of feet) for Novembcr 1960. High index was the outstanding feature of the circulation with nogst)ivr. drpwtrlrcls at high latitudes and positive drpitrturos at lower latitudes.

interest in connection with t'he changes of weather from showed that 41 percent warmed from 1 to 2 classes, while October to November. The general trend over the Unit,etl 44 percent rernairled in the same cl:m (out of five). This States shown in this patt'ern, i.e., falls in t'he West and representsunusually highpersistence from October to rises in theEast, WAS t'oward more southerly (or less Sovenlber. A sirnilar ahnorrnalit~y occurredlast Novem- northerly) components of the 2Lvcrag.e flow. This favored ber [I], when 85 percent of the country did not change by warmertemperatures, relative t'o normd, than t.he prc- 111ore than one class. For precipitation, the nortmllcrnand vious month, and increased precipit~nt~ion.A conlputut'ion western sect'ions reported hcnvier prccipitntion t'lmn t'hein of t'he temperature anomaly class changes for 100 stations previous month, while most of the rernaindcr of thc coun- distributedthroughout the contiguous Unit'ed States try became drier, except in the Kentucky-Tennessee area.

Unauthenticated | Downloaded 09/23/21 08:20 PM UTC FIGURE2.-Difference (in tens of feet) betwcen the mean monthly 700-mb. height departuresfrom normal of October and November 1960. Falls in the Pacific Northwest and rises in thc southeastern UnitedStates produced anomalous southerly or southwesterly components of the average flow overmost of the country and generalwarming, relative to normal,from Oet,ober.

3. AVERAGENOVEMBER WEATHER FIGURE3.-(A) Temperaturedeparture from normal (" F.) for November 1960. (B) Percentage of normalprecipitation for November 1960. (From [4].) Surface t'errlperatmures during November averaged up to 4' F. above normal over most of the contiguous United States and Ha,waii, but below norrrd over most of Cali- fornia and Alaska (fig. 3). Thegenerally mild wcathcr 4. THEMID-MONTH REVERSAL can be explained by the prevttlerlcc of anomalous sout>ll- westcrly componcnts of :~verrrge flow, as shown by tllc Although this November exhibited more t,han normal height departure pat,t#ernin figurc 1. persisterlcc frorn the previous nlont'll, cspecixlly in Precipitnt'ion (fig. 3) averaged heavier tllan normal in temperature,the circulation wit,hirl themonth was the western United States where many stations report#ed characterized by considerablevariability. The intra- one of the snowiestNovembers on record.The wetness rnont~hly vari:tbility is best portrayed by the patterns of in the West was related to the deeper than norrrld trough averagecirculation and temperaturefor the twohalf- off the Pacific Coast of Xort'h America and to frequent,, months,shown in figures 4 and 5, and by theheight arid at times int'ense, daily storminess which propagated differences between tllc first t~rrcllast halves of the month inlandperiodically from this t'rough. Very heavy pre- at 700 rnb., shown in figure 6. cipitationin southern California and Nevada occurred In t'he first h:Llf-rnontjll (fig. 48), a stronger than nornlal early inthe month inassociation with a strong cutoff ridge along t,he west coast of Cnrlada and a deeper than Low aloftover this area. Althoughprecipitation in thc normaltrough over t'hc Great Lakes facilitated the Mississippi Valley was mostlyless than normal, rwver- dcployrlerrt of coldcr than norrnal polar air from Canada theless it st'nnds out in figure 3 in contrast t'o the dryness southward over the United States. As a result tempera- int'he Plains and in the East'. Thisprocipithtion was t'ures avcraged IJClOW norrrrd (fig. 4B) over most of the nssociated wit,h amean trough in the area (fig. I) and country, exceptfor part's of the Southwest. The coldest was produced by daily troughsof large amplitude crossing rctgion in t,his pcriod, relat'iveto normtl,was the Kentucky- this region at about weekly intervals, on about Kovemhr Tennesseearea, wherc temperatures are known to be 1, 9, 15, 23, and 28. The troughs of thelst, 15th, and irlversel- correlated with heightdepartures in western 28th were associated with deep surface syst'erns Canada.

Unauthenticated | Downloaded 09/23/21 08:20 PM UTC FIGURE4.-(A) ;\lean 700-rnb. contours (solid) andheight dcpar- FIGURE 5.-(A) Ileal1 700-111b. contours(solid) and height departures from norrnal (dotted), both in tcrls of feet, for Pioverr1l)er tures from rlormal (dotted), both in tens of feet, for h'ovember 1-15, 1960. (B) Surface tempcraturc departurefrom norrrlal 16-30, 1960. (B) Temperature departure from norrrlal (" F.) for (" F.) for approximately same period as (A). Ridgealong west, approximately same period as (A). Deeptrough off west coast coast and trough in eastern hTorth Arrlerica produced coolcr than was associat,ed with increasedstorrniIlcss arid warmer than normal normal conditiorls irl rnost of the United States. corlditiorls in most of the United States.

In the second half-month strong deepening it1011g and half of themonth. In consequence of thischange, st off the west coast of North America from the Yukon lrlarked reversal in wcat,tler occurred over the continent, southward (fig. 6) resultedin a major cll~ngein the as reflect,ed in warming over most of North America (fig. circulation RS a deepfull-latitude trough becillrlc est%ith- CiIZ), in contrtlst, to cool conditions earlier in the month. lished off thewest coast of Korth America (fig. 5A) replacing the ridge which had dominated t'llis ar~aenr1it.r 5. THE INDEX CYCLE inthe rnont8h. This deepening occurred in consonuncc Figure 7 shows the variation of the 700-rnb. westerlies with deepening of t81wAsiat'ic co:bstal trough and ridging (.%Jay averages) which occurred this fa11 in three latitude in thenorth-ccnt,ral Pacific and was thus primarily II hands of theWestern Hemisphere, together with the barotropic manifestation of vorticitypropagut'ion. normalvariation. The subtropical westerliesincreased As a result of realignment of the large-scale waves in the Pacific, abnormal southerly componentsof the average 2 "_\r~rmaIs"of thesc indires are averaged from the maps in Wcather Bureau Technicd Paper No. 21 151. Thcy are not based on the 30-year period recognized for climatologicdl flnw dpvelnnrd nver much of Nort,h America in the last nonnals.

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FIGURE6.-Mean 700-mb. heightchanges (tens of feet)between first and last halves of November 1960 (figs. 4A and 5A). Height rises in the Central Pacific and falls off the west coast of Xorth America produced a marked change in the mean flow. rapidlyafter the minirnum in late Scptembcr, re:&ing t,heir maximumvalue of 7.0 rr1.p.s. inearly Kovember. The temperate westerlies reached their rnaxirnunl of 14.6 1n.p.s. (4.0 m.p.s. above normal) almost two weeks later. This was t'he second highest November index since 1945, and was exceeded onlyby 14.8 m.p.s. inthe period November 13-17, 1948. As thenorrnal index curves suggest, the nmxirnurn yearly values usually occur later in theyear, generally late December or early Janutuy. The development of an abnornlally strong maximurn in 24 I 8 I5 23 30 6 I3 20 27 thewesterlies at mid-latitudefrequently signals the 5EPT OCT r;ov beginning of a longdecline associatJed with the wintcx- FIGURE7.-Time variation of 5-daymean 700-mb.indices in index cycle. this case, figure 7 shows a sharp drop in In mctcrs per second for the western zone of the Northern Hemi- theternperatme westerlies subsequent to the November sphere (&180" W.). Subtropical index applies between latitudes rnaxirnum, withthe decline still in progress as of rnid- 20' X. and 35' K., temperate index between 35' X.and 55' N., December. The abnormally early date of the rnaxinlurrl and polar index between 55' N. and 70" K. Values are for 5-day and subsequent' decline this year suggests that the kind periodsellding on dates indicated. "Norrrlal" indices (dashed of weathernormdly associated with the winter season lines) averaged from maps of [5]. commenced early. As was observed with thc maxima in December 1959 and again in April 1960just prior to major reversals in the course, asso(!iiLted with minima of the subt'ropical westerly trend of the indices [3], the index maximum at mid- index. Thus again we have the suggestionthat a nlinirnum lut'itudcs t'llis month lagged the subtropical nlaxirrlunl by in t'he subtropical index, associated witha positive anomaly about two wceks. In additionit was sirnultaneously maximurn in t'he subtropics, of about 100 ft., rnay be a associated with increased positive values in the subtropics signal for a 11l:tjor reversalin the index cycle. Further- of latitudinally averaged height anomalies in the western more, as occurred in latr December 1959, April 1960, and zone of this hernisphcre (not shown). By way of compnri- thismonth also, the index reversal is likelyto be sc- son, thesepositive anomaly averages were as follows: companied by a major upheaval in long-wave and weather 130 ft. in December 1959 at 35' N., 100 ft. in April 1960 patterns,manifested initially atthe surface by violent st 30' N., and 90 ft. in November 1960 at 30' N. Maxima storm activity traversing the country from southwest t,o of averagepositive anomalies at theselatitudes arc, of northeast.

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REFERENCES nied byContiguous nUnited theTemperature Reversal in States,” MonthlyWeather Review, vol. 88, So. 4, Apr. 1960, 1. J. F. Andrews, “TheWeather and Circulation of Xovernber pp. 158-166. 1959”UnusuallyHigh Persistence from October,” Monthly U.S. WeatherBureau, WeeklyWeather and Crop Bulletin, Weather Review, vol. 87, No. 11, Nov. 1959, pp. 418-425. 4. National Swnzmary, vol. XLVII, Nos. 49 and 50, Dee. 5 and 12, 2. J. F. Andrews, “The Weather and Circulation of October 1960,’’ 1960. Monthly Weather Review, vol. 89, Xo. 1, Jan. 1961, pp. 24-30, 3. J. F. O’Connor, “The Weather and Circulation of april 1960- 5. U.S. Weather Bureau, “Normal Weather Charts for the Northern A SharpMid-Month Drop inthe Zonal WesterliesAccompa- Hemisphere,” TechnicalPaper, No. 21, Washington,D.C., 1952.

U 5 GOVERNMENTPRINTING OFFICE.1961

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