365

THE WEATHER AND CIRCULATION OF Record Heat in the Mid-West

JAMES F. O'CONNOR

Extended Forecast Branch, U.S.Weather Bureau, Woshlngion, D.C

1. INTRODUCTION westernsector of thehemisphere, as ridges replaced troughs and vice versa. Surnrner seemed to come about a month early to States This reversal and its anlplification over North America east of the Rockies in May 1962 as temperatures soared to was apparentlythe result of along antecedent trend values much more characteristic of June, producing the evident as far bark as early April in the Pacific. At that hottest May on record over a large part' of this region. time a vigorous shearingof the mid-Pacific trough occurred By contrast, the West was unseasonably cool, especially associated with strong height rises southeastof Kamchatka later in the month. (fig. 2A). This trend evolved, in late April, into a strong Therainfall pattern also exhibitedmarked contrasts, blocking High in the north-central Pacific as the middle notably between the Nort'hern Plains, where the wettest and low latitude wave trains became completely out of May on record occurred in Inany places, and the South, phase (fig. 2B). where a severe drought set new records for consecutive Duringearly May, continued progression at higher days without rain. latitudes, relative to t'he stationary lower-latitude waves, The heat in central and east'ern sections was in sharp resulted in reanlalgarnation of the Bering Sea ridge with contrastto t'he cool weathera yetw ago [I]. While th:Lt theeastern Pacific subtropical ridge (fig. 2C). Corre- May was notable for persistence of an earlier cold regime, sponding realignment ol the downstream troughsproduced May this year showed rernarknbly little persistence from a full-latitudetrough along the westcoast of North ,4pril. Only 34 percent of thecountry failed to change Americttin themanner described by Namias [5]. This more than one temperature class (out of 5), in comparison was follon-ed by amplification over the United States in with the 1942-1957 average of' 62 percentfor April to the last llwlf of May- (fig. 2D) as the shearing trendbetween May changes [2]. high and IOIV latitudes again resumed. This marked reversal in temperaturewws dso duplicated in theprecipitation regime. For example rrlost areas of 3. SYNOPTICACTIVITY heavy April precipitation this year [3] in the South, East, and Northeast,had relatively light amounts in May, The very slo~movement of the deepeningplanetary while the West and Nort'h Central States, which 11nd been trough in the Far West during May result'ed in rather relatively dry in April, received sharply increased rainfall, persistent weather over many areas. However, significant ranging tothree times t'he normal in some areas. This short-periodvariability occurred, principally near the was especially beneficial in the Northern Plains, but in the zone of strong temperature contrast extending from the Southern Plains moisture remained critically shortin some centralRockies to the upper Mississippi Valley. This areas, expecially in Texas. was also the region of principal surface cycloneactivity, 2. THEPLANETARY CIRCULATION withmasinrum frequency occurring in South Dakota wheretlle most abnorrnal rains occurred. Conditions Theaverage circulation at' 700 rnb. in May 1962 TVRS were also rehtively changeable near the east coast due to one of considerableamplitude at' middle latitudes, with periodic"back-door" cold airintrusions which were deeper than normal troughs in the western United States favored by strong nort'herly anomalous componentsof the and OR the east coast, and strong ridges in the eastern average flow at 700 mb. (fig. 1). Pacific andeastern United States (fig. 1). ( Early in the nlonth, the only High to emerge from the had a strikingly similar circulation and temperature pat,- Rockies duringJIay crossed t'he South andbecame station- tern, butwith major differences in precipitationto be nryin the Southeast. It waspreceded by showersover discussed Inter). much of the East' and by t'he last rains to occur in many At high latitudes the pattern was flatter than usual due parts of tlleSouth and Southeast until the end of the to lower than normal heights in the Yukon ridge and abovemonth. West of this High to the Pacific coast, a warming normal heights over the Bering Sea and eastern Canada. trend developed with temperatures rising to 20' F. above This representeda dramatic reversal from April in tlle normal in tom^ on the 5t'h.

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FIGUREl.-Mean 700-mb. contours (solid) and height departures from nornlal (dotted), both in tens of fret for Nay 1962. Strong ridge in eastern Pacific maintai1Ic.d amplified TWVO pattcmr ovw the I-nited States.

From the 6th to t'he 10th two successive Highs wossed to 15" F. below 11orrna1in Xevada, while the hot airspread sout,hernCanada and the Great Lakes north of a front to the east coast although the warmest, 24" F. above nor- whichbecame stationary near 40" X. Precipitation was Jnal, occurredin Michigan on t'he 15th. Rain again oc- widespread in the north due to overrunning of' the cooler curred printarily in northern sections, due to disturbances air which ranged to 14" F. below normal in the X'orthern along a very strong front near the Continental Divide. Plains.The southern half of thecountq- was mostly The only major change from the 16th to the 20th was dry and persistently hot with te~nperatllres up to 20" E". cooling in the Upper Mississippi Valley from as much as above normal in Kansas and A-ebraska. 32" F. above normal on the 17th to slightlybelow normal From t'he 10th to rnid-montmh the deep planetary trough three days later. Also notable was t.he excessive rainfall aloft near the Pacific coast moved inland, producing cool on the 19th at St. Joseph, Mo., where over 5 inches fell in a showeryconditions in the Far West. Temperatures fell 2-hour period during a thundershower.

Unauthenticated | Downloaded 10/01/21 02:45 PM UTC FIGURE2.-Average half-month contours (solid) (tclns of feet) and 15-day height changes from preceding period (dashed) (at 200 ft. inter- val, zeroline hravy) for (A) earl!- April, (H) late April, (C) early May, and (I>) late May 1962. Evolution shows shearirlg and re- amalgnnlxtiou in Pacific followed by amplification over North America.

From the 21stto the 25th t,he dccpcst storm of the of theIllonthly average clepart'ures from normal. The month spread more heavy precipitation fro111 the central hottest area relative to normal was centered near Kansas Rockies tothe Upper Mississippi Valley,maintaining City, No., which reported an average maximum ternpera- relatively cool conditions north of its trailing front which ture of 85.2" F. Another hot, spot was Birmingham, Ala., was located along 40"N. on the 25th. However the coolest' where tlle rnaxinla averaged92.3 O F. or 0.1 O F. warmer than air still remained in theFar West, primarily over Sevada, Yuma, Ariz., which had daily maxima of 100" F. or over while the warmest t'ernperatures were in the South. from the 2d to the10th. Many citiesreported this Late in the month cooler air, ranging to 17" F. below >lay tobe the warnlest on record. Some of theseare normal, invaded the North Central Stat,es as the strong listed in t,able 1. planetaryridge aloft in theEast weakened, permitt'ing the first. shower activity since the beginning of the Illont'h to traverse the deep South and thus ending the mor~tll-long drought. The associated cloudiness also helpedto allc- viat'e the extreme heat which had prevailed in the South since early May. 4. TEMPERATURE Withtemperatures inthe "11luc11 above normal" category over most of the United States eastof the Oonti- nental Divide, t,his was the hottest May on record in a large part of that region. Many sectionsregistered average monthly temperatures close to tlle normal June values, and some were even warmer, notably Birmingham and Mobile, Ala., Thornasville, Ga., Wichita, Kans., and Greenville, S.C. Figure 3 shows the observed distribution

Unauthenticated | Downloaded 10/01/21 02:45 PM UTC FIGITRE3.--Mean temperat~~redepartures from normal (OF.) for

Unauthenticated | Downloaded 10/01/21 02:45 PM UTC FIGC:RE5,"Percentage of normalprecipitation for May 1962. (From [GI.) Most notable features are heavy rains in thc Plains and drought in thc South. 3 hail on 5 days were the most, recorded in any month since records began in 1890. Excessive short-duration rains were dso common, some notable ones being the 7.12 in. in 24 hr. on the 18th tmtl 19th at St. Joseph, Mo., and 4.6 in. at' Dubuque, IOWAon the 28t'h and29th. Farther to the east Reading, Pa. received 3.27 in. in 2 hr. on the 31st. In the South, w dry spell of rword duration occurred in many places lasting 3 to 4 weeks. The tlrought ended in Texas and Louisnna on the 28th with rains of 3 to 4 in. in some localities, and general showers fell throughout the Gulf St'at,esand Sout,lleast on the last3 days of the rnonth. After no measurablerain at Birmingham,Ala., sincc , t,lle 1.21 in. which fell from the 29th to 31st' was referred to us "The Million Dollar Rain" in that region. In relation to the average circulation (fig. 1) it appetm that the heavy precipitat'ion in the Nort'h Central States was associated with stronger than norrnul sout'herly flow, while t'he dryness in the East and Southeast was favored by t'he abnorn~allystrong northerly flow east of the planetary ridge.The abnormwl drynessin the Southern Plains can be explained, however, only by a very subtle characteristic of t,he circulationthere; viz., the sligl~tly above normalheights in northwestern Mexicowhich resulted in a more westerly flow thn usutd favoring drier air in Texasand New Mexico. This is to some extent reflected in thelowest average humidityon record reported at El Paso, Tex. Turning now to the major difl'erences in precipit A t'1011 between this hlay and May of 1959 which ht1d a strikingly 1

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height differences (fig. GB) overSort11 America, the REFERENCES distinct’iorl between the two cases is highliglltetl by the 1. 11. H. Gelhard, “Thr PVcwthrr and Circulation of - arrowsin figure 6B, which are quantitative in direction Persistrrlt Cool Weatherin the United States,” Monthly only.These show t’hat the Mity 1962circulation was Il’euther firviruu, \rol. 89, So. 8, Aug. 1961, pp. 299-305. more northerly in thesoutheastern United States and 2. J. Sarnias,“Pcrsistrnce of lfid-TroposphericCirculation be- hencedrier than in 1959, and morewesterly in the twwn Adjacent AIonths and Srasons,” The ltossb?/ Memoriul SouthernPlains, also a drier influence dueto the drier 170i,lrnc, The RockefellerPrcss in wsociation with Oxford sourceand the downslope mot’ion. In theSorthern I-uivrrsity l’rrss, Sew York, 1959, pp. 240-248. 3. C. FVoffinden, “The W‘c:tther and Circlllation of - was If. Plains the flow this May Inore southeasterly nnd hence A R.Iont,h withPersistent Blocking in the Pacific,” Jfonthly wet’t’erthan in 1959, probably because of the greater up- lt’eathcr Review, vol. 90, So. 7, .July 1062, pp. 29iJ-305. slopecomponents over the cooler air to thenorthwest. 4. C. AI. Woffindrn, “The ffeatller and Circulation of May 1959- wit’hsome addit’ional orographic lifting. The more Including an Analysis of Precipitation in Relation to Vertical southeasterly nature of t’his nlonth’s flow correspondingly Jlotion,” Monthly FI’eath,Pr Hroieu), vol. 87, KO. 5, May 1959, influenced thetracks of cyclones andvorticity rnaxinln ~>p.196-205. ernmating from the Rockies, also favoring heavy precipi- .5. J. Sarnias, “Thirty-Day Forc~asting-A Review of a Ten-Year Espcsriment,” Meteorological Monograph, vol. 2, KO. 6, Amer- tationfarther to the northwest (western Dakotas). By icanMeteorological Society, July 1953. (Srr pp. 61-62.) the same token, lesseningof the heavy precipitation to the 6. P.S. FTeathcr Burr~:tu, N‘rekly Weather and Crop Rulietin, westof the Continental Dividein the northern Rockies this -V~tionaZSummary, vol. XLIS, So. 23, .June 4, 1962, and 1701. Maycan be attributedto the downslope influence of SI,\’I, No. 23, . anomalous easterly flow components.

CORRESPONDENCE (Continncd from p. 350) frequencyend ofspectrum the is far from satisfwtory REFERENCES owing to errors in t’ransosorlde positioning. 1. CJ. Newton,“Axial Velocity Streaks in theJet Stream: As an explanation for the small magnitude of inertid iL-. Ageostrophic‘Inertial’ Oscillations,” Journal Mcteorology, oscillations, Mantis and Huch have hypothesized the of [4] vol. 16, So. 6, Dec. 1959, pp. 638-645. existence of small-scalepressure gradients which we 2. J. S. Sawyer,“Quasi-Periodic Wind Variations with Height in undetectable on the synoptic scale. The possible influence theLower Stratosphere,” Q~carlerly Journal of the Royal of the vertical advection of velocity is noted from the fwt MeteorologicalSociety, vol. 87, No. 371, Jan. 1961, pp. 2433. thatinertial oscillations are time dependent and hence, 3. J. K. Angcll, “Use of Constant Level Balloons in Meteorology,” if the wind speed varies with height, the wavelengths of dduanccs in Geophysics, vol. 8, Academic Press Inc., h’ew York, theinertial oscillations will varywith height and ttw 1961. undulations will not line up in the vertical. The frictional 4, 11. T. >fLlantisand 117. F. II~~~~,‘~hIcteorological studies using interplayresulting from vertical motions could then have ConstantAltitude Balloons,” TechnicalReport No. AP-14, alarge damping effect upon the horizontaloscillations at School of Physics,Universit?- of hIinnesot,a, 1959, 63 pp. a given level.

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