JANUABY 1950 MONTHLYWEATHERREVIEW 13 THE WEATHER AND CIRCULATION OF

WILLIAM H. KLEIN

Extended Forecast Section, U. S. Weather Bureau Washinqton, D. C.

One of the most outstandingfeatures of the general inthe entireNorthern Hemisphere.2 At sea level the circulation of the Northern Hemisphere during January maximum positive anomaly in thisregion was 25 millibars, 1950 at the 700-mb. level (and throughoutthe troposphere) a value which has been exceeded only once (January 1909) was the strong ridge which extended ina northwest during the past 50 years. Although a discussion of the direction from a center of high pressure about halfway evolution and cause of this tremendous ridge is beyond the between and Lower to the Arctic Ocean scope of this article, its effects upon the general circulation (see fig. 1). The maximum positive anomaly of 700-mb. were of the utmostimportance for the weather of the height associated with this ridge, 900 feet in the Alaskan United States. In general agreement with the principle peninsula andsoutheastern Bering Sea, is byfar the of conserva.tion of vorticity, it was accompanied by deep greatest anomaly ever observed in this region onany troughs and large areas of negative height anomaly at monthly mean map during our 18-year period of record 700 mb. to both its east and west. Figure 1 shows that (1932-50), and has been exceeded only once at any place 2 A positive 700-mb. height anomaly of 910 feet was observed on the west coast of Greenland during February1947 (see articleby Namiae in MONTHLYWEATAER REVIEW, 1 Bee charts I-XI following p. 21, for analyzed climatological data for month. vol. 75, No. 8, , pp. 145-152).

FIOU~1.-Monthly mean 700-mb. cbrtrt for January 1950. Contours at 200-foot intervals are shown by solid lines, 700-mb. height departure from normal at 100-foot intervals by dashed lines with the zero isopleth heavier. Anomaly centers and contours are labeled in 10’s of feet. (Chart covers 30-day period Dec. 31, 1949-Jan. 29, 1950 instead of calendar month.)

Unauthenticated | Downloaded 10/08/21 10:03 AM UTC 14 MONTHLYREVIEW WEATHER JANUAEY1950 tbese c.ircula.tion features were only a part of a larger justnorth of the border of theUnited States, moved system of four roughly sinusoidal waves in the middle generally southeastward along the Alaskan coast through lat,itude zonal westerlies encompassing the entire hemi- Oregon, Idaho, Wyoming, Nebraska, and Kansas. North sphere. of this track the foehn effects were greatly weakened, and This system of planetary waves, with_"astrong trough intensely cold Arctic air and heavy snows predominated and below-normal heights in the West,and an intense in the Dakot,as, Montana,Washington, and Idaho. South ridge and above-normal heights in the East, represents a of the cycloneztrack anticyclonic conditions, light pre- reversal of the normal January circulation pattern in the cipitation, and,:milder Pacific air prevailed *:overmost of United States, which is characterized by a ridge in the the Southwest. Both at sea level and aloft-the West and R trough in the East. Comparison of charts I High was abnormallydominant over eastern United and V (see inserts following p. 21) with figure 1 shows that, States. It was instrumentalin steering the principal as noted in previous studies of the dependence of weather stormsnortheastward from Kansas through Iowa and upon the 700-mb. circulation, the area of subnormal Wisconsin to Labradorand in preventing cold polar surface temperatures had cyclonic curvature and negative anticyclones from from penetratingthe country height anomdies aloft, andthe region of anticyclonic appreciably except in the northern Plains. In fact,the curvatureand above-normal 700-mb. height was char- Bermuda High was sufficiently strong to reduce precipita- acterized by ebnormal surface warmth. Similarly, most tion to light along most of the Atlantic and Gulf Coasts. of the heavy precipitation was located in the region of The location of the principal 700-mb. troughs and ridges strong southwesterly flow aloft near the point of inflection in the entire Northern Hemisphere during January 1950 between the trough and the next ridge downstream. The was similar to that in .3 Thus, during both departure from normal of t'he direction of meaa air flow is monthstemperatures were abnormally cold in western assumed to be parallel to the dashed isopleths of equal and northwestern United States and abnormally 700-mb. height anomaly in figure 1 and is also related to warmineastern United States, theBritishIsles, andMexico; surface temperatureand precipitation. For example, while precipitation was generally heavy in the Hawaiian surface temperatures higher than thenormal were observed Islands and generally deficient in western Europe. Most in areas of below-normal700-mb. height in portions of of the centers of 700-mb. height anomaly in the western Colorado and Idaho because of strong southwesterly flow hemisphere, however,were farthernorth and more relative to the normal. Likewise, stronger-than-normal intense this Januarythan last. Consequently, westerly westerly wind components produced heavy precipitation wind components (relative to normal) produced wetter along most of the West Coast but created a rain-shadow weather on the West Coastand much warmer, drier east of the Rocky Mountains. conditions inthe Southwest during January 1950 than The tracks of centers of anticyclones and cyclones wereexperienced in January 1949,when easterly wind given in charts I1 and I11 are also helpful in explaining components prevailed. On theother hand, the greater the observed surface weather. These tracks for the most intensityand northward displacement of the Bermuda part closely paralleled the 700-mb. mean contours ob- High in January 1950 compared to January 1949 made served during themonth and were displaced south of this Jaouary the warmest on record in large portions of their normal position in western United States and north the eastern and southern United States. of normal in eastern portions. Thus, cyclones from the 8 For detailed discussion of January 1949, see article by Klein in Monthly Weveatlrcr Gulf of Alaska, which usually cross western North America Review, vol. 77, No. 4, . pp. 99-113.

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