Weather and Circulation of February 1971
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May 1971 439 UDC Ml. 606. 1: 661.613 (73) " 1971.02 " WEATHER AND CIRCULATION OF FEBRUARY 1971 A Stormy Month With a Marked Mid-Month Reversal in the Temperature Regime A. JAMES WAGNER National Meteorological Center, National Weather Senice, NOAA, Suitland, Md. 1. MEAN CIRCULATION were 230 m above normal. This was an increase of 350 m with respect to normal compared to the previous month The mean 700-mb circulation during February was (fig. 3) , when the expanding, retrograding block displaced characterized by an amplified planetary wave train from a deep Low that had been near Novaya Zemlya (Posey the central Pacific to eastern North America, with troughs 1971). and ridges near their normal positions but stronger than Heights fell south of the block over Asia and also down- normal (figs. 1 and 2). The amplification may have stream by as much as 180 m over the Bering Sea, where it been initiated by continued retrogression of the blocking had been the previous month (fig. 3). Another area of ridge that had moved into the Bering Sea in January falls centered over the Davis Strait was associated with (fig. 1 of Posey 1971). This block was centered over the northward movement of the westerlies over the Atlantic, Tamyr Peninsula in February, where 700-mb heights which was dominated by above-normal heights (fig. 2). FIGURE1.-Mean 700-mb contours (dekameters) for February 1971. Unauthenticated | Downloaded 10/01/21 06:13 PM UTC 440 MONTHLY WEATHER REV1 EW VOl. 99,No. 5 FIGUREP.-Departure from normal of mean 700-mb height (deka- FIGURE4.-Mean 700-mb geoatrophic wind speed (meters per meters) for February 1971. second) for February 1971. Solid arrows show principal axes ob maximum wind speed, and dashed lines show the normal February positions. The p&tern was quite complex over North America, with no well-defined continuous jet across the continent. Most of the cyclones afEecting the United States appeared to be related to the northern branch of the jet from tho eastern Pacific and the southern branch of the Atlantic jet beginning over Texas. Only weak storms were associated with the branch beginning in western Canada. Mote- worthy because of its unusual configuration was a small loop of wind maxima surrounding the strong block QVQ~ the Siberian coast (fig. 4). A wind-speed maximum ob almost 10 m/s from the east was defined south ob the block. 9. MONTHLY TEMPERATURE The monthly mean temperature anomaly pattern over the United States changed little from January to February, remaining generally warm in the West and cold in the East FIGURE%-Mean 700-mb height anomaly change (dekameters) (fig. 5). Areas of greatest change were the marked cooling from January to February 1971. in the Central Plains, where a severe blizzard laid down B heavy snow cover, and the Northeast where temperatures moderated considerebly after a bitter cold January (Posey 1971). Over the country as a whole, the temperature at Heights rose strongly over the eastern Atlantic and the 86 out of 100 stations remained within one class (much British Isles, where anticyclonic conditions again became above, above, near, below, or much below) ob the January established (figs. 1 and 3). value. The 700-mb jet stream was stongest over both oceans, Due to a marked mid-month change in circulation being somewhat south of normal over the Pacific and north (discussed later in the text), there were few extreme or ob normal over the Atlantic (fig. 4). Speeds were above record monthly mean temperature anomalies. Norne, normal by as much 8s 11 mls over the Pacific and 8 m/s Alaska, recorded its coldest February as bitter Arctic over the Atlantic. The jet axis split into two branches as it cold kept its grip on the northernmost State, Not until approached the continents of North America and Europe. the end of the month did milder air intrude with sufTicient Unauthenticated | Downloaded 10/01/21 06:13 PM UTC May 1971 A. James Wagner 441 ATURE DEPART from 30-YEAR MEAN FIGURE5.-Departure from normal of average surface temperature FIQURE6.-Percentage of normal precipitation for February (OF) for February 1971 (from Environmental Data Service and 1971 (from Environmental Data Service and Statistical Re- Statistical Reporting Service 1971). porting Service 1971). TABLEl.-MmthZy and seasonal temperature records established dur- TABLE2.-Record and near-record monthly precipitation and snowjall ing February 1971 during February 1971 Station Temperature Date Remarks Statlon Amount Anomaly Remarks F) (inches) (inches) Millord, 18.8 ~. 2d snowiest Feb. on record Raleigh, N.C. 6 2 Lowest Feb. min. on record Utah _______ _____ Tulsa, Okla. +2.41 4th wettest Feb. on record Tallahassee, Fla. 14 10 Do. 4.18 Topeka, Kans. +2.44 2d wettest Feb. on record Red Blut?, Calif. 81 11 Highest 50 e& in season 3.49 __.__.__ ~. 2d snowiest Feb. on record Long Beach, Calif. 91 12 Highest Feb. mar. on record a4 - __ - Grand Island, Nebr. Wettest Web. on record Winnemucca, Nev. 69 12 Tied highest Feb. mar. 3.39 i-2.88 +O. 62 26 wettest Feb. on record Cape Hatteras, N.C. 76 22 Highest Feb. mas. on record Rapid City, 5. Dak. 1.00 St. Paul Island, Alaska 13, -15 1e,n Lowest Feb. mins. on record 16.7 ________._____ 2d snowiest Feb. on record - wettest Feb. record Nome, Alaska -42 25,B Tied lowest Feb. mln. Bionx City, Iowa 2. e4 +l. 77 3d on Waterloo, Iowa a. +2.68 26 wettest Feb. on record -11.6 __._Coldest Feb. on record M Rochester, Minn. 2.21 +l. 41 Do. Boston, Mass. ____ 26 Ended 85 consecutive days with mlns. 32PF or below, longest St. Cloud, Minn. 21.0 ~ ____ ~._____ ~ - Snowiest Feb. on record 4th wettest Feb. record string on record Duluth. Minn. 237 +1.41 on Wettest Feb. on record Santa Marla, Calif. 22 n Lowest Feb. min. on record Sault Ste. Marie, Mich. 3.74 +2.24 32 6 ___________ ~. - 2d snowiest Feb. on record Houghton Lake, Mich. 3.38 +2.07 Wettest Feb. on record Alpens, Mi&. 3.17 +l. 66 Greatest Feb. preclp. since strength into New England to keep minimum tempera- 1687 Coiumbus, Ohio 16.4 .______ __ ~ ____ 3d snowlest Feb. on record tures above freezing at Boston, where a record 85-day Cinchmati, Ohio 6.76 +2 96 2d wettest. Feb. on record stretch of subfreezing minima finally came to an end (Abbe Observatory) 14.6 __ - - ~ - - - ~.- 3d snawiest Feb. on record Providence, R.I. 6.36 +2.% 3d wettest Feb. on record (table 1). Allentam, Pa. 6.44 +2.80 2d wettest Feb. on record WilliamSpOrt, Pa. ti60 +a 99 Wettest Feb. on record Wilmington, Del. 6.28 +3.34 26 wettest Feb. on record 3. MONTHLY PRECIPITATION ?dDunt Shasta. calif. 0.71 -6.70 Tied with Feb. 1898 for driest on record In contrast to the relatively dry conditions prevailing over the country during January (Posey 1971), large portions of the central and eastern United States had anomalous in California, where less than an inch of unusually heavy precipitation during February (fig. 6). precipitation fell at normally wet Mount Shasta and only Several major storms gave extensive precipitation as 0.01 in. of rain fell during the first half of the month at the mean 500-mb trough retrograded from its January Red Bluff. The unusually strong ridge just off the coa& position near the east coast to the Great Plains during (figs. 1 and 2) deflected the Pacific storms, which normally February (figs. 2 and 3). Due to generally low tempera- affect this area in late winter, far to the north into the tures in the Ohio Valley and Great Plains, a large part of Gulf of Alaska. the precipitation in these areas was in the form of snow. A large number of stations from the Great Plains 4. WEEKLY WEATHER through the Great Lakes and Ohio Valley eastward to the Middle Atlantic States reported record and near- FEBRUARY 1-7 record precipitation and snowfall totals during February The 700-mb circulation over North America during the 1971 (table 2). Only in the Far Southwest were conditions first week of February featured an amplified full-latitude appreciably drier than normal. The dryness was most trough extending from Hudson Bay southwestward Unauthenticated | Downloaded 10/01/21 06:13 PM UTC 4 MONTHLY WEATHER REVIEW VOP. 99, No. 5 mean temperatures averaged as much as B5OF below P~QIP- mal over the Northern and Central Plains. Several stations in the Northeast reported record low daily minimum tem- peratures on the first few days of the month. Beadings were as low as -%OF at Green Bay, Wis., on the 2d, -27OF at Concord, N.W., on the 3d, and -225OF at Port- land, Maine, on the 3d. Heaviest precipitation for the week, as much as 4 in. in Alabama (fig. 7C), fell in connection with a vigorous storm that swept from the Rockies to the Great Lakes around the middle of the week. Severe thunderstorms with flash floods and a few tornadoes occurred over the Southeastern States, and blizzard conditions developed from Hansas to Minne- sota. As the storm center deepened to 973 mb over Wis- consin, giving the lowest sea-level pressure since 1902 at Green Bay, Wis., the Great Lakes area experienced gale- force winds with gusts reaching5Q to 66) rni/hr. Two storms in quick succession moved rapidly north- eastward up the Atlantic Coast States from the Gulf OP Mexico on Sunday and Monday. They produced tornadoes at several locations in Florida and a snowstorm locally of near-blizzard intensity over the Ohio Valley and northern Appalachians.