JULY1977 WEATHER AND CIRCULATION OF APRIL 1977 933 WEATHER AND CIRCULATION OF APRIL 1977 Generally Mild and Wet but Continued Drought in the West A. ' JAMES WAGNERLong Range Prediction Group, National Meteorological 'Center, NWS, NOAA, Washington, DC 202331. Mean circulation Deepening of a strong mean low over the ArcticOcean led to markedly faster than normal polar westerlies over most of the Northern Hemisphere. Theremnants of the high-latitude blocking which hadcharacterized the circulation since late December 1976were manifested as weak ridges and positive anomaliesnorth of the Sea of Okhotsk and over Greenland(Figs. 1 and 2). The principal Pacific trough persisted in its Marchposition near Japan (Taubensee, 1977). Weakening ofthe strong ridge which had been over the easternPacific, along with the normal seasonal decline of thewesterlies, favored retrogression and a southwardextension of the weak mean trough which had beenjust west of the British Columbia coast. As a consequence, the 700 mb ridge over the Canadian RockyMountains rebuilt to stronger than normal values, butwas not as extremely amplified in a meridional configuration as it had been during much of the past falland winter. Although the mean 700 mb wind maximum associatedAPRIL 1977Fro. 1. Mean 700 mb height contours (dekameters) for April 1977.934 M O N T H L Y W E A T H E R R E V I E W VOLUME 105APRIl Fro. 2. Departure from normal of mean 700 mb height (m) for April 1977.Fro. 4. Departure from normal of mean 1000-700 mb thickness (m) for April 1977.with enhanced thermal contrast was slightly south ofnormal and somewhat stronger than normal over mostof the Pacific (Figs. 3 and 4), the southern branch ofthe normally split flow over southwestern NorthAmerica was so weak as to be indiscernible. Thenorthern branch was located in central Canada, farnorth of its normal position near the United Statesborder. The southern part of the trough which hadbeen over the Great Plains during March (Taubensee,1977) weakened and retrograded to the southern RockyMountains in the generally weak flow at low latitudes, Fro. 3. Mean 700 mb geostrophic wind speed (ms-~) for April1977. Solid arrows indicate observed axes of maximum wind speed;dashed lines show the normal.while the remains of the northern part sheared andjoined with the Hudson Bay trough and advanced tothe Atlantic Coast of North America in the strengthenedmean flow at high latitudes (Figs. 1 and 3). The low which had been south of Greenland crossedthe Atlantic and extended southward into Europe as adeep trough, joining with a pre-existing low-latitudetrough which retrograded slightly from the Black Seato the eastern Mediterranean. A strong 700 mb ridgein central Asia remained approximately stationarywhile amplifying; its associated height and thicknessanomalies strengthened somewhat and moved noktheastward (Figs. 1, 2 and 4).2. Temperature Temperatures averaged above normal over most ofthe Nation for the second consecutive month (Tau FIG. 5. Departure from normal of average surface temperature(-F) for April 1977 (from National Oceanic and AtmosphericAdministration and Statistical Reporting Service, 1977).JULY1977 WEATHER AND CIRCULATION OF APRIL 1977 935~.~/ze. ~ PERCENTAGE OF NORMAL PRECIPITATION~. , ~, [~] U;.dr:;Y [~ ~7~\' ~ ..... ..... Fro. 6. Percentage of normal precipitation for April 1977 (fromNational Oceanic and Atmospheric A'dministration and StatisticalReporting Service, 1977).bensee, 1977) in excellent agreement with the monthlymean thickness anomaly pattern (Figs. 4 and 5). Theonly area which was substantially cooler than normalwas southern Texas and the Rio Grande Valley, wheretemperatures averaged as much as 4-F below normal. The greatest warm anomalies were located over theIntermountain Region and northern Mississippi Valley,where temperatures averaged 6- to 8-F above normal.A number of stations in these areas reported one of thewarmest Aprils on record (Table 1). Cold air circulating around the strong Arctic lowkept temperatures along the north coast of Alaskabelow normal, but advection of maritime air ahead ofPacific storms continued the relatively mild weatheralong the south coast and Alaskan panhandle. Temperatures remained near to above normal in Hawaii,which continued under the influence of a stronger thannormal subtropical ridge (Figs. 1 and 2).TABLV. 1. Record and near-record warm monthly meantemperatures observed during April 1977. Temper ature Anomaly- Station (-F) (-F) RemarksWilmington, N.C. 65.9 +2.6 3rd warmest AprilDetroit, Mich. 52.4 +4.7 6th warmest AprilMuskegon, Mich. 50.3 -I-5.0 Warmest April since 1955Evansville, Ind. 61.5 +4.8 Tied for 3rd warmest AprilFort Wayne, Ind.55.4 +6.1 2nd warmest AprilDes Moines, Iowa 58.5 +9.0 Warmest AprilWaterloo, Iowa 51.7 +4,3 Warmest April since 1955Sioux City, Iowa 55.6 - +6.2 5th warmest AprilRochester, Minn. 53.0 +8.5 2nd warmest AprilMinneapolis, Minn. 53.0 +7.9 3rd warmest AprilDuluth, Minn. 44.4 +5.8 Tied for 6th warmest AprilGrand Island, Neb. 56.2 +6.3 Warmest April since 1955Fargo, N.D. 49.5 +7.2 5th warmest AprilWilliston, N.D. 49.1 +6.7 Warmest April since 1952Missoula, Mont. 46.9 +3.0 Warmest April since 1952Walla Walla, Wash. 57.3 +4.5 5th warmest AprilPhoenix, Ariz. 73.5 +5.8 -Tied for 2nd warmest AprilTABLE 2. Record and near-record monthly and seasonalprecipitation totals established in April 1977. Amount AnomalyStation (inches) (inches) RemarksSan Francisco, Calif. T --1.59 Tied with 1949 for driest AprilMount Shasta, Calif. 1.18 -- 1.20 8th consecutive dry month 10.85 --23.69 Seasonal total since 1 July 1976Elko, Nev. 0.18 --0.64 7th consecutive dry month Only 27% normal since 1 July 1976Pendleton, Oreg. 0.18 --0.83 Drought continued Driest moisture season on recordWalla Walla, Wash. 0.20 --1.23 5th driest April , 5.15 --7.32 Seasonal total since 1 Sept. 1976Pocatello, Idaho 0.06 --1.00 Driest AprilHelena. Mont. 0.10 --0.83 2nd driest AprilMissoula, Mont. 0.08 --0.93 Driest April since 1952Grand Junction, Colo. 0.54 -0.25 1 lth consecutive dry monthWilliston, N.D. 0.53 --0.71 Driest April since 1952Brownsville, Tex. 6.62 +5.34 Wettest AprilWinslow, Ariz.4.8 -- Snowiest AprilValdez, Alaska 71.4 -- Snowiest AprilSault Ste. Marie, Mich. 11.3 -- Brought seasonal total 175.6 --- snowfall to new record highBuffalo. N.Y. 2.2 -- Added to already 198.9 -- record seasonal total snowfall3. Precipitation 'The trough in the southern Great Plains (Fig. 1)favored heavier than normal precipitation over mostof the important agricultural areas of the Nation'smiddle third (Fig. 6). Rains of 2-5 inches were beneficialto the winter wheat areas of the central Great Plainswhich had been dry previously, and also continued agood supply of moisture to sections where ample rainshad occurred in March (Taubensee, 1977). Heavyrains fell along the southwest Gulf Coast, whereBrownsville had the wettest April on record (Table 2).Excessive rains in the southern Appalachians wereassociated with damaging floods, but the Southeastremained dry near the subtropical ridge. In the absence of the normal 700 mb wind maximumand its associated storms near the United StatesCanadian border (Fig. 3), precipitation was less thanhalf normal over the northwestern third of the country(Fig. 6), where the severe drought continued withlittle relief (Table 2). Wells and streams were reportedto be drying up in some areas. Precipitation was generally above normal overHawaii, and along the south coast of Alaska. Anchoragehad over three times the normal total, and Valdez reported its snowiest April on record (Table 2). Winslow,Ariz., also reported its snowiest April, although accumulations were insignificant there and meltedquickly.936 MONTHLY WEATHER REVIEW VOLUME105~=~-f~ 'D~PARTURE OF AVERAGE TEMPERATUREFROM NORMAL (OF) 1 1 -,,U. ..................................... :2:,::. TOTAL PRECIPITATION, INCHES ..... :--~ I '-.,~-. =~ ~- ~ ......., "' ';-~' ~11 .... ~1 ................................................................ Fro. 7. (A) Mean 700 mb contours (dam) for 5-9 April 1977;(B) depsrture from norma! of average surface air temperature(-F); snd (C) tots] precipitation (inches) for week of z~-10 Apri!1977 (from Nationa! Oceanic and Atmospheric Administrationsnd Statistical Reporting Service, 1977).4. Weekly variabilitya. 4-10 April The strong ridge over the eastern-Pacific at the endof March (Taubensee, 1977) moved eastward as strongwesterly flow over the Pacific broke down into a highlyamplified pattern from North America to Europe.Strong ridges were located over the Rocky Mountainsand the central Atlantic, while deep troughs werefound over eastern North America' and central Europe(Fig. 7A). Temperatures averaged above normal everywherewest of the Mississippi River with the exception ofLouisiana and southern Texas (Fig..7B). Under theinfluence Of the mean ridge, temperatures reachedrecord highs for so early in the season at numerouscities from Washington to Minnesota (Table 3) andaveraged as much as 12-F above normal in some places.The strong, tro. ugh steered cold air from Canada intothe East, where averages were 6~9-F below normal andfrost occurred as far south as the southern Applachians. A vigorous short wave early in the week triggereda storm which produced snow in the Great Lakes,tornadoes in the Southeast,, and torrential rains leading to flooding over much of the southern Applachianarea (Fig. 7C). Heavy rains had fallen in much of thesame area the previous week (Taubensee, 19.77) sorunoff was-substantial due to saturated soils.TABLE 3. Stations reporting record ea. rly season warmth during April 1977. TemperatureStation (-F) Date Remarks*Spokane, Wash. 81 7 87, 90 23, 24 Equaled highest on 23rdMedford, Oreg. 83 7 Equaled highest so early 'Pendleton. Oreg. 87 7 91 24Kalispell, Mont. 74 7 82, 84 23, 24Missoula, Mont. 82 8Helena, Mont. 79 8Glasgow, Mont. 82 8Ha.zre, Mont. 83 8Casper, Wyo. 78 9Grand Junction, Colo. 83, 84 9, 10Bismark, N.D. 87 9Rapid City. S.D. 88 9Huron, S.D. 91 9Sioux Falls, S.D. 88 9Duluth, Minn. 84 10St. Cloud, Minn. 87 10MinneapoliR, Minn. 88 10Rochester. Minn. 85 10Grand Rapids, Mich. 80 10 Equaled highest so early 86 18Detroit, Mich. 86, 87, 8910, 11, 12Allentown, Pa. 87 12Philadelphia, Pa. 92 12Bridgeport, Conn. 86 12Madison, Wise. 86, 87 17, 18Toledo, Ohio 88 18Burns, Oreg. ,80, 84 23, 24Stampede Pass, Wash. 76 24 Highest for monthYakima, Wash. 92 24Lewiston, Idaho 94, 97 23, 24Boise, Idaho 91 24Pocatello, Idaho 85 24Winnemucca, Nev. 89 24 Highest for month* All are highest so early in the season except as other~,ise noted.JuL'/1977 WEATHER AND CIRCULATION OF APRIL 1977 937b. 11-17 April The strong mean ridge and its associated warm airmoved eastward from the Rocky Mountains to theGreat Lakes area, while the trough which had beenover eastern North America progressed to the westernAtlantic (Fig. 8A). Fast westerly flow over the easternPacific between a deep trough in the Gulf of Alaska and ~.~: .. DEPARTURE OF AVERAGE TEMPERATURE FRoM NORMAL (-F) ............... '~ . / [ ~ ..... 1 ...... ~-~ 72~'5'~_?,,, ~ ~ ~~ ~./ ~.....~ ~ .~ ,~> ~ ,~ ~*~~:~ % .~ _ ~,~ , ,~ .~. ~ ~ -~r~ ... --~ : ~.. ~ ~~~'.~-~ N~ .'~q . ': . ~ 7:'~xz': ~~- . ~ ~ ~ ~_~ .~:~;~)~:.~ '~~' .W- ', / I '.~ ~, ~ ~'~ ,~ ~ ~ ~ :~. ~ ~~ ~ ~ :~~ ~' '~:~-~ ~.. ,-.::/~~--~~ ~' . ~ ~ .~ ~+ ' ~ '...... f ~ I ~;~1 ~~-~ ...... , 5 .... ~' "'?~ .... ,,~4':'~,.... ........ TOTAL PRECIPITATION, INCHES[2Z]N',?:;:;:2~2~:::::;.2Fm. 8. As inFig. 7 except for (A) 12-16Aprfl1977 and (B) and (C) week of ll-17April 1977.Fro. 9. Mean 700 mb height change (m) from first half to second half of April 1977.a strong subtropical ridge to the south pushed a weaktrough into the southern Rocky Mountains. Temperatures averaged above normal over practicallythe entire country, where 700 mb heights were abovenormal and a strong ridge extending from the Southeastto the Great Lakes kept out Arctic air. The weeklymean temperature was more than 18-F above normalover part of the northern Mississippi Valley and adjacent areas (Fig. 8B). Additional records for earlyseason warmth were set in the Northeast (Table 3).Indianapolis, Ind., had nine consecutive days withmaximum temperatures above 80-F from 10 to 18April, the longest such April heat wave on record.Chicago's Midway Airport reported a total of eightdays with maxima above 80-F, the greatest April totalon record. The rapidity of the transition from the cold weatherearlier in April to the warm spell was rather remarkable, and quite similar to events almost exactly oneyear previous (Wagner, 1976). As. the warm air movedeastward from the Great Plains, temperatures rosefrom 13- on the 9th to 81-F on the 10th at Lansing,Mich. Other two-day temperature changes from the10th to the llth were 27- to 90-F at Richmond, Va.,and 20- to 87-F at Dulles International Airport westof the Nation's Capital. At the latter location, thesewere the lowest and tied for highest temperatures observed in the whole month. Between the llth and 12ththe temperature went from 13- to 79-F at Concord,N.H.; 25- to 84-F at Albany, N.Y.; and 36- to 90-Fat Kennedy International Airport in New York City.Before the warm spell was over, temperatures soared'to daily record values of over 90-F at some cities in themiddle Atlantic States, although failing to surpass theheat wave of April 1976 in most localities.938 M O N T H L Y W E A T H E R R E V I E W VoLo~E 105 ~. .... b., ~ Most of the western and eastern thirds of the country - ridges.hadc. 18-241ittleApril-r no precipitation, under the influence of~ During the second half of April, 700 mb heights increased markedly over the Arctic, Canada, Alaska,lpnl'~i9% ' ' ,~'*L DEPARTURE OF AVERAGE TEMPERATURE FROMNORMAL (-F) ~ .. ~ _~-~._~__~_.,~__ ~ .~"- ~.~ .. _':.!-~---_:.~_~~;.~.,,:~_. .,,~ ...... ~-~:---~ _.~_:~z~ ~ " )' ,~~:~~~~ 'J , ; . ..... ~ ~~::.~:~ :~: ,..:. :.~_:5 ~'~ ':~. :~:~- ,.~' 3~i~:~: ~ ~ -, f __ _~- ~::~" ~...... .......... ....PRECIPITATION, INCHESF~ %?:27.'2,~2Fro. 10. As in Fig. 7 except for (A) 19-23 April and (B) and (C) week of_ 18-24 April 1977. Strong southerly flow between the trough in thesouthern Rocky Mountains and the eastern ridgebrought generous quantities of moisture northwardfrom the western Gulf Coast to the northern MississippiValley. Rains were beneficial in most areas, except inTexas where severe weather, hail, and tornadoes withup to 5 inches of rain contributed to flooding (Fig. 8C). IOTA~. PRECIPITATION, INCHESFro. 11. As in Fig. 7 except for (A) 26-30 April and (B) and (C) week of 25 ~pril-1 May 19~.JtJL-1977 WEATHER AND CIRCULATION OF APRIL 1977 939and the northern tier of states as the flow became moreblocked (Fig. 9). Vigorous troughs became establishednear Kamchatka, over the eastern Pacific, and in theIcelandic area. The warm ridge progressed only slightly from itsprevious week's position to the east coast of the UnitedStates, while the mean trough moved from the RockyMountains to the Great Plains and weakened. A stronglow west of Vancouver Island contributed to renewedridging over western North America (Fig. 10A). Temperatures were again above normal over mostof the country, although departures were not so extreme as during the previous week. Greatest anomalouswarmth was over the Great Lakes area, where the weekaveraged as much as 12-F above normal (Fig. 10B).It was cool at the beginning of the week in the Northwest, but the strongly amplifying ridge brought recordhigh temperatures for so early in the season for thesecond time this month to parts of the Northwestinterior (Table 3). The trough in the southern Great Plains broughtheavy rains to the lower Mississippi Valley, where upto 10 inches in Louisiana caused floods (Fig. 10C).Phasing of a high-latitude .t. rough from Canada with thesouthern trough over the Ohio Valley at the end of theweek brought rains of over 2 inches to parts of the Northeast, where it had previously been rather dry. Most of the West was again rainless. d. 25 April-1 May Broadening eastward of the western ridge and deepening of the phased troughs into a full-latitude troughnear the East Coast gave a circulation pattern similarto that of the first week of the month (cf. Figs. 7A andllA). The temperature anomaly distributions of thetwo weeks were also quite similar, the main differencebeing less extreme cold in the Great Lakes area and New'England at the end of the month (Figs. 7B and liB).Precipitation was distributed over most of thecountry and no excessive amounts fell anywhere (Fig.llC). The best rains of the month fell in parts of California but did little to end the long-standing drought. REFERENCES National Oceanic and Atmospheric Administration, U. S. De partment of Commerce, and Statistical Reporting Service, U. S. Department of Agriculture, 1977: Weekly Weather and Crop Bulletin, 64, Nos. 15-19, (12, 19, and 26 April, and 3 and 10 May 1977). Taubensee, Robert R., 1977: Weather and circulation of March 1977-Record and near-record precipitation eases drought in parts of the Great Plains and Midwest. Mon. Wea. Rev., 105, 793-799. Wagner, A. James, 1976: Weather and circulation of April 1976 Unprecedented spring heat wave in the Northeast and record drought in the Southeast. Mon. Wea. Rcv., 104, 975-982.