The Weather and Circulation of July 1950' William H
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JULY 1950 WEATHERMONTHLY REVIEW 125 THE WEATHER AND CIRCULATION OF JULY 1950' WILLIAM H. KLEIN Extended Forecast Section, U. S. Weather Bureau Washington, D. C. July 1950 was a remarkably cool, cloudy, andrainy was the coolest July of the last 25 years in the United month in the eastern three-quarters of the United States. States east of the Continental Divide and probably just as In most areas from the Rocky Mountain States to the cool as any July of the past 40 years. Atlantic Coast temperatures werebelow normal (Chart The principal feature of the general circulation with I), skies were clear less than half the time (Chart IV),and which this weather can be associated is a mean trough in precipitation wasexcessive (ChartV and inset). These the constant pressure surfaces located through the Missis- conditions were most marked in the southern and central sippi Valley at all levels of the troposphere from 700 mb. to Plains States, in portions of which the negative departure 300 mb. (Charts IX to XI). The field of 700-mb. height of mean temperature from normal exceeded 6' F., the per- anonlaly drawn in figure 1 shows that this trough was centage of clear sky between sunrise and sunset was less deeper than normal and displaced well to the west of its than 20, and total rainfall was more than 8 inches greater normal position, which is along the Atlantic Coast. As a than the normalamount. Considering the extentand intensity of the below normal temperatures, this month 1 See Oharts I-XI, following p. 133, for analyzed climatological data for the month. FIGURE1.-Mean 700-mb. chart for the 30day period July 1-30, 1950. Contours at 2i"foot intervals are shown by solid lines, 700-mb. height departures from normal at 100-foot intar- vals by dashed lines with thezero isopleth heavier. Anomaly centers and contoursare labeled in tensof feet. Minimum latitudetrough locationsare shown by heavy solid lines. Unauthenticated | Downloaded 10/04/21 01:36 AM UTC 126 MONTHLY WEATHER REVIEW JULY1950 result cyclonic curvature prevailed in the circulation over and continued east-northeastward across the country into the central part of the United States, where the warm the Atlantic. Each was accompanied by widespread High and subsidence aloft typical of the dust-bowl decade cloudiness and showers and followed by a surge of high of the 1930's were conspicuous by their absence. Frequent pressure and cool maritimeair from the north Pacific. showersand thunderstorms were favored by horizontal These are not indicated in Charts 11 and 111, however, convergence, upward vertical motion, and instability in because their continuity was obscured in the mountains the vicinity of the trough, while daytime maximum tem- of western North America due to the absence of a closed peratures were kept low by thelarge amountof associated circulation. cloudiness. Thus comparison of figure 1 with Charts I, IV, After crossing the Continental Divide, these surges of and V (inset) reveals in general a good agreement between cool Pacific air were reinforced by outbreaks of cool polar the areas of cyclonic curvatureand negative height continental air from the Hudson Bay region. These out- anomaly at 700 mb. and the regions of much cloudiness, breaks accompanied the movement of several anticyclones, sub-normal temperature, and above-normal precipitation from centralCanada through the Dakotas, Minnesota, at the surface. Cloudiness and showers were also favored and the Lakes along tracks given in Chart 11. Southward by large-scale convergence in a sharp inverted trough in movement of cool Canadian air into the United States the 1014-mb. mean isobar at sea level (Chart VI) in the was facilitated by the fact thatpressures relative tonormal central and southern Plains. In addition, large quantities were generally higher in Canada thap in the centralUnited of moisture were carried into this region from the Gulf of States. This was a type of blocking action which showed Mexico sincesea level winds were more southeasterly than up at 700 mb. (fig. 1) as a westward protrusion of a strong normal (Chart I1 inset). positive height anomaly center over Greenland into north- The weather of the United States was also related to eastern Canada, while heights in southern Canada and features of the general circulation farther afield. It can the central United States were generally below normal. be seen in figure 1 that the 700-mb. westerlies in the east- At sea level, monthly mean pressures were above normal ern Pacific, between a Low center in the Gulf of Alaska in practically all of Canada, with the greatest departure, and a High center almost directly to the south, were con- +3 mb., in centralHudson Bay (chart notshown), whereas siderably stronger than normal. Likewise the westerlies in the United States lesser positive or negative pressure in eastern North America and the Atlantic were stronger anomalies were found (Chart I1 inset). The thickness of than normal. As a result the zonal index at 700 mb., the air column between sea level, where pressures were measured between latitudes 35' N. and 55' N. from 0' above normal, and 700 mb., where heights were westward to 180°, was more than 2 m/sec higher than below normal, was therefore below normal in most of cen- normal, and the strength of the mean jet stream at 700 t,ral and southern Canada. This meant that the source mb. for July 1950 in the Western Hemisphere exceeded region for polar continental air entering the United States the normal by as much as 3 m/sec (fig. 2). This jet was was unusually cool, a favorablecondition for below-noms1 centered between 45' N. and 50' N. in thehemisphere as surface temperatures. a whole, but was weaker and farther south in the United In eastern andsoutheastern sections of the country States proper. Consequently during the month five dis- generally cool, wet weather occurred with anticyclonic tinctfrontal systems entered the Pacific Northwest, curvatureand above-normal heights at 700 mb. More- dropped southward and intensified east of the Divide, over, the Bermuda High was well developed and west of its normal position both at sea level and at 700 mb., a 900 condition generally associated with warm weather in the eastern UnitedStates. This discrepancy can be attri- 6$2: 5O W 60° buted to the presence of a trough just west of the Appa- 5 550 lachians in the fields of both the departure of monthly 50° mean sea level pressure from normal (Chart I1 inset) and 5 450 the meanisotherms at 700 mb.(Chart 1X). These -I 40° troughs probably furnished the convergence and insta- bility necessary to release large amounts of moisture, wa 350 w transported by stronger-than-normal southwesterly flow a 30' (3 at 700 mb. (fig. l), withfrequent showers, abundant 25O cloudiness, and cool surface temperatures asa consequence. 2 60 The displacement of the isotherm trough well to the east -8 "6 "4 -2 0 2 4 6 0 IO 12 14 of the contour trough at 700 mb. in this area is a very un- ZONAL WIND SPEED (mps) usual circumstance since contours and isotherms are RQmE2.-Monthly mean 700-mb. geostrophic zonal wind speed profile in meters per nearly in phase on most monthly mean maps at upper secondaveraged from 0" westward to 160° longitude. Solid curve is for July 193, dashed line is July normal. levels. As a consequence the contour trough sloped to Unauthenticated | Downloaded 10/04/21 01:36 AM UTC JULY 1950 REVIEW WEATHER MONTHLY 127 the east with increasing elevation, at least from 700 to Oregon, California, Nevada, andArizona, with the greatest 300 mb., (compare Charts IX, X, and XI), temperatures departure, 4.8', at Red Bluff, Calif. This abnormal sur- at 700 mb. were below normal where heights were above face warmth was intimately related to the existence of a normal in the Southeast, and the Bermuda High was cold well-developed thermal low at sea level (Chart VI), in and therefore quite weak in the upper troposphere. This the center of which pressures were as much as 3 mb. below anomalous nature of the Bermuda High is difficult to normal (Chart I1 inset),and to strong anticyclonic explain, but it probably contributed toward making this a vorticity at 700 mb. (fig. 1). In addition airflowwas generally cool, rainy July in theeastern United States. more northeasterly than normal at sea level and northerly The principal exceptions to the cool, wet regime east of relativeto normal at 700 mb., so that warming was the Divide were southeast Texas and portions of the favored by downslope winds and by abundant insolation Northeast. Chart I shows that temperatures were (ChartIV). In coastal sections of northern California, slightly above normal in coastal sections of New England Oregon, and Washington, on the other hand, temperatures and the Middle Atlantic States, and also in parts of New averaged slightly below normal because of the predom- York, New Hampshire, and Vermont, while precipitation inance of strong sea breezes (see wind roses, Chart I) and was generally deficient in these areas (Chart V inset) as stratus cloud conditions (Chart IV),induced by abnormal well asin portions of Pennsylvania and Maryland. At warmth in the interior valleys. Portland, Maine total precipitation for the month was It is perhaps surprising that total rainfall during the less than half of the normal amount and only .01 inch month exceeded the normal amountthroughout the more thanthe driest of record. This relatively warm, southern half of California and most of Nevada, Utah, and dry condition in the Northeast was associated with the Arizona since dry weather usually goes with the warm fact that 700-mb.