February 1969 157 UDC 551.513:551.506.1(215-17):551.515.72(71-11) “1968.11” THEWEATHER ANDCIRCULATION OF NOVEMBER 1968 Continued Blocking Over Eastern North America L. P. STARK Extended Forecast Division, Weather Bureau, ESSA, Suitland, Md. 1. MONTHLY MEAN CIRCULATION and by a channel of below normal heights (fig. 3) from Blocking weakened considerably over and near North thecentral United States to themid-Atlantic (where Americaduring November andstrengthened inthe heights were 120 m below normal) to the eastern Medi- eastern portion of the Northern Hemisphere. terranean.This anomaly pattern with strong easterly Figure 1 shows changes inthe mean 700-mb height flow to the north and the strong westerly flow in middle anomaly from October to November 1968. Heights were latitudesis typical of lvidespread blocking. Westerly generally lower in November from the Maritime Provinces flow was farther south than normal with the maximum to the Beaufort Sea. Heights decreased most near Davis flow more than 10 mjsec faster than normal. The negative Strait where a strong ridge in October weakened about channel reflects the average path of storms this month 110 mas the Icelandic Lowbecame established near with a secondary track into the Mediterranean south of southern Greenland (fig. 2). the blocking in Europe. This track is also clearly defined The ridge formerlyover western Europeand the in the sea-level pressureanomaly chart (not shown) in United Kingdom m.oved eastward and acquired a strong which pressures averaged as much as 15 mb below normal negative tilt fromGreenland to southernRussia while for November in the mid-Atlantic and6 mb below normal heightanomalies increased 160 mover northernScan- in the Mediterranean. dinavia. This was accompanied by marked high latitude Amplification spread eastward from the Scandinavian warming, height anomaly decreases of 90 m near Spain, segment of the block. Cold air, confined to the Arctic in October,spread southward in November over western Siberiaproduced thestrongest vortex of themonth over the Northern Hemisphere. This resulted in, a maxi- mum change of 700-mb height anomaly of about 200 m and a strong maximum mean wind speed of more than 20 mjsec west of Lake Baikal (fig. 4). Weak ridging ex- tended from Sakhalin Island to the Arctic Circle where anotherportion of the blockbecame established as a 700-mb High, more than 100 m above normal. Theremaining major vortex in the Northern Hemi- sphere was also related to blocking. In October the Aleu- tian Low, about 80 m below normal, was located in the northern Bering Sea. As blocking increased, the Low moved to the southern Bering Sea and deepened as much as 150 m near the Aleutians. A strong mean wind maxi- mum of 24 m/sec near 40”N accompanied this deep Low. Very strong storms deepened south of the Aleutians with average sea-level pressure as much as 12 mb below normal. This flow resulted in very little increase in the ridge along western North Americawhere a weak ridge was located in October.Heights decreased across most of the United States as maritime Pacific air predominated with little intrusion of continental itirmasses through the strong westerly flow. This is usually true during this type of blocking regime in the absence of astrong ridge in western Canada. In thecentral United States 700-mb FIGURE1.-Mean 700-mb height anomaly change(decameters) heightsv decreased bv nomore than 40 m from October from October to November 1968. Novemberto as the flow changedvery little.The flow Unauthenticated | Downloaded 09/27/21 05:12 PM UTC 1 58 MONTHLY WEATHERREVIEW Vol. 97, No. 2 FIGURE2.-Mean 700-mb contours (decameters) for November 1968. FIGURE3.-Departure from normal of mean 700-mb height (deca- FIGURE4."Mean 700-mbwind speed(meters per second) for meters) for November 1968. November 1968. Heavy solid lines show axes of maximum wind speed and light dashed lines show the normal. Unauthenticated | Downloaded 09/27/21 05:12 PM UTC February 1969 L. P. Stark 1 59 FIGURE5.-Departure from normal of average surface temperature FIGURE6.-Percentage of normal precipitation (inches) for Novem- (OF) forNovember 1968 (from Environmental Data Service, ber 1968 (from Environmental Data Service, 1968). 1968). became more cyclonic in the eastern United States while TABLE1.-Unusual snowfall in November 1968 heightsincreased somewhat from western Canadato City Remarks lower Hudson Bay. These slight changes produced slight changes in the mean weather over the Nation. Albany, N.Y.. .. ~ ~ ~. 13.5 12,13 10 in. above Novembcr normal Lynchburg, Va.. .. .~ ~ ~ ~ 9,10,11,12 November record 2. MONTHLY MEAN WEATHER Raleigh, N.C ... ... ..... 1. 2 12 November record dllent,own, Pa-. .. ~. ~.~ ~ 6. 4 11,12 Heaviest so early in the season Anomaloustemperatures this month (fig. 5) were not Greensboro, N.C.---.-. 5.9 9, 10, 11, 12 Heaviest so early in theseason well relatedto the anomalous 700-mb heights.Over Columbia, T 11 Earliest in 45 yr most of the Nation, heightswere below normal even where temperatures were abovenormal; in California where temperatures werebelow normal,heights mere slightly Nation. The band of precipitation that was twoto six abovenormal. Temperatures that werea few degrees timesnormal from the Southwest to Illinois was along below normal in New England and in central and southernthe mean track of five Lows that originated in the South- portions of the Nation seemed to fit the flow very well. west. Two other Lows, one from the Southwest and one In theseareas temperature changes from Octoberto from the Gulf of Mexico, were responsible for the heavy November were greatestand showed cooling of one or precipitation in the South, Southeast, and New England. two classes. Cooling in New Englandamounted t,o two Despite the severity of the latter two storms and.the high or three classes where temperatures had been above nor- frequency of theothers, few precipitationrecords were for 1). mal the previous 2 mo. Greatest warming occurred in the established and those were snowfall (table Ohio Valley andin the Pacific Northwest where mean 3. WEEKLY WEATHER temperature anomaliesincreased 3'-6'F this month. RELATED TO CIRCULATION CHANGES Except for a few daily records, no monthly records for warmth mere established.Tallahassee andWest Palm NOVEMBER 4-1 0 Beach,Fla., reported the lowestNovember mean t.em- Blockingwas strongestthe first week of November peratures of record, 54.8' and 67.8'F respectively. (fig.7A) as 700-mb heightsin the ridge across Canada Many stations in the easternhalf of the Nation reported were more than 140 m above normal near northern Hudson much cloudiness or low percentage of possible sunshine Bay.The ridge inwestern North America was slightly under the broadly cyclonic 700-mb mean flow. At Nan- above normal but its connection to the block helped keep tucket, Mass., possible sunshine was a record November heights below normal over most of the United States. low of 25 percent,and its temperature was less than a Temperaturesthis week (fig. 7B)were 3'-6"F above degree below normal. At Indianapolis, Ind., and Reading, normal over the West Coast States and the Great Basin. Pa., possible sunshine was arecord low for any month Elsewhere, temperatures werelower thannormal with and for November, respectively, and temperatures were maximum departures of 3"-6'F from the Northern Plains above normal. States to more than 9°F below normal on the Gulf Coast; Above normal precipitation fell from the Southwest to temperatures fell below zero in Wyoming and Iowa. The New Englandand over most of theSoutheast (fig. 6). freezing line reached the northern Gulf Coast States as a This was a responseto the broad mean trough andcyclonic cold ridgeextended from Hudson Bay to the Southern curvaturethat covered theeastern two-thirds of the Plains States most of t'he week. Unauthenticated | Downloaded 09/27/21 05:12 PM UTC 160 MONTHLYREVIEW WEATHER Vol. 97, No. 2 FIGURE7.-(A) Mean 700-mb contours(decameters) for Nov. 5-9, FIGURE8.-Same asfigure 7 except (A) forNov. 12-16, 1968; 1968; (B) departure fromnormal of averagesurface temperature (B) and (C) forNov. 11-17,1968. (OF) forNov. 4-10, 1968(from Environmental Data Service, 1968); (C) totalprecipitation (inches)for Nov. 4-10, 1968 (from Environmental Data Service, 1968). to New England. Near the endof the week a storm formed Precipitation was ratherheavy this week(fig. 7C) in the Gulf of Mexico as astrong short wave moved exceptin theSouthwest and Midwest. Early in the week throughthe mean ridgeand intothe Southwest. The a Pacific storm spread rain in the Far West; .as it moved surface Low deepened as it moved along the east coast eastward snow fell from the Rockies through the Lakes where it was accompanied by widespread rain, with some Unauthenticated | Downloaded 09/27/21 05:12 PM UTC February 1969 L. P. Stark 161 snow in the Northeast. Strongwinds caused some damage along the coast. NOVEMBER 11-1 7 The mean 700-mbflow this week (fig. 8A)ridged stronglyin the eastern Pacific, andtwo sharp daily troughsmoved into the West and changed the mean curvaturefrom anticyclonic to cyclonic. The longwave spacing of last weekfrom the Gulf of Mexico to the eastern Atlantic was shortened somewhat as the trough deepened off theeast coast and pronounced ridging occurred inthe mid-Atlantic. Blockingover Canada persisted and storminess continued along the east coast. Cool air moved into the Far West (fig. 8B) with two majorupper level dailytroughs, buttemperatures remainedwithin a degree or two of normal. Along the east coast temperatures loweredsomewhat this week and averaged 3"-6"F below normal. Elsewhere, marked warming predominated except for little change in the cool conditions of the week before in the Central Plains. Two days after the vigorous storm of last week another wave developedover the Gulf of Mexico.