THE WEATHER and CIRCULATION of JULY 1959 the Second Consecutive July with an Unusual Circulation Pattern
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JULY 1959 MONTHLY WEATHER REVIEW 275 THE WEATHER AND CIRCULATION OF JULY 1959 The Second Consecutive July with an Unusual Circulation Pattern CARLOS R. DUNN Extended Forecast Section. U.S. Weather Bureau, Washington, D.C. 1. INTRODUCTION Theother aforementioned 30-day mean troughsand ridgeswere unusually placed to a similar degree. Note For the second successive July [e] many of the t,rouglls that the Wday mean trough line in the central Pacific and ridges on the 700-mb. 30-day mean map (fig. 1) were fellin a region of minimumtrough frequencyand in not in the climatologically preferred locations indicated western Alaska passed through an meaof zero trough fre- in studiesmade by Kleinand Winston [6]. This was quency [B]. This ,July 5-day mean troughs were located especially true of the western sectors of theNorthern at BOo K. in the 30" longitude band between 175" W. and Hemisphere inJuly 1958, andagain this July an ab- 145O W. 100 percent, of the time, compared to only 31 normal trough appeared over the central 7Jnited States. percent during July 1947-55. The 30-day mean ridge and Inthe eastern and westerncoastal areas, where weak trough in the United States each was located in a,n only ridges were observed, thecirculation was more anticy- slight,ly more favoredarea. The common situationin clonic than usual. This regime was associated with gen- ,July is for a continental anticyclone to persist over cen- erally light precipitation in the West, but heavy rainfall tral United States, but this Julya 5-day mean troughwas in several large areas east of t,he Rocky Mountain States located at, 40" N. between 105" W.and 85" W. 78 percent (figs. 2, 3). Rainfall from tropical cyclones Cindy and of tlle time (to be compared wit.11 20 percent during the Debra augmentedthe total precipitation in Texas and 1947-55 period). along theeast coast. Temperaturesaveraged below It, should be noted that, the 30-day mean troughs and normal in most of the Central and Southeastern States ridges (fig. 1) were located very close to the regions of and were above normal in most) other regions, with record lligh incidence of 5-day mean troughs and ridges, with heat reported in theFar Southwest (fig. 4). minor exceptions in Asia (fig. 5), implying a good deal of stability in the general circulation during the month. 2. 30-DAYAND 5-DAY MEAN CIRCULATIONS However, the mean circulation (fig. 1) differed in sev- The July circulation at, the 700-mb. level over the west- ern1 aspects from that of June 1959 [4]. During June an ern Northern Hemisphere (fig. 1) was one of small ampli- exaggerated summertime pattern, witha ridge over central tude, and many of the troughs and ridges were out of United States and a trough along each coast, was estab- phase with those on theJuly normal chart I!)]. The lished. That this trend did not continue into July is at- normal July troughs along both the east :~ndwest coast)s tested bytlle anomalous height, falls in central United of No1.th America were apparent this yearonly in eastern States, flanked by rises of more than 200 ft. off the New Canada and possibly along the California coast, where a England and Washington coasts (fig. 7). Although it is trough remnant was discernible. More interest,ing is the difficult to determine how or where these changes origi- unusual circulationin other areas. Theridges in the nated, they were accompanied by a tmin of height change western Bering Sea, western Unit,ed States, and north- centersextending upstream in the westerlies to about central At,lantic, and the troughs in nort,h-centralPacific, 120" E. In July1958 the trough in central United States central United States, and northern Atlantic were all in was likewise associated with abnormally located troughs climatologically infrequentareas [6]. Thedata of [6] and ridgesas far upstreamas Siberia [2]. show that 30-day mean ridges were extremely rare in the Figure 8 is the 700-mb. 15-daymean height change western Bering Sea in the Julys from 1933 to 1955, and from the first half to t,he last half of July 1959. During in the northern latitudes over eastern Siberiano ridges at all were observed during this period. The 30-day mean July there was considerable filling of the Arctic Low in ridge this July resulted from frequent incidence of 5-day theBeaufort Sea. In middlelatitudes over the Pacific mean ridges in this area(fig. 5). If the ridge occurrences the perturbations intensified as falls occurred in the Si- at 60" N. for two 10" longitude bands are combined, we berian andPacific troughs concurrent withrises in the ad- find that 5-day mean ridges were observed inJuly 1959 80 jacent ridges. Some retrogression of the western North percent of the time between 155' E. and 175" E., com- American ridge is indicated by the large rise center, at pared witah only 23 percent during the Julysof 1947-55. 140" W., west of the ridge. In North America the trough Unauthenticated | Downloaded 09/26/21 12:47 PM UTC 276 MONTHLY WEATHER REVIEW JULY1959 FIGUKE l.-Mean 700-nlb. contours (solid) :md height clc1)artnres from normal (dotted), both in tens of feet, for July 1039. Many of the troughs and ridges were located in climatologically unfavored areas. filled in the north butintensified over the Southern Plains thatarea. Speeds werealso well below normalin the Statesas July progressed. Over Europe an incipient western Pacific. ,IIhe largest, nlonthly height anomalies, R t330 ft. and block is discernible. The 700-mb. 30-day mean jet stream (fig. 9) was rat,ller -320 ft. couplet,were associated withthe Bering Sea, ill-defined and displaced north of its normal position in ridge and a deep Arctic Low over the Beaufort Sea (fig. many longitudes. Thisnorth\mrd shift. produced above 1). This corlfigura,tionIhoduced a, super-normd north- normal wind speeds over(hnda, but in the TJnited States west,erly flow over the Bering Strait, w*C-hichbrought cool wind speeds were nearto below normal.Negative de- thicknesses (1000-700 mb.) to much of Alaska (fig. 6), partures along the east coast and in the western Atlantic quitesimilar to March 1959 [3] when many places in (fig. OB) reflect the easterly regimewhich prevailed ill Alaslm reported record cold. Overthe United States Unauthenticated | Downloaded 09/26/21 12:47 PM UTC 1 Unauthenticated | Downloaded 09/26/21 12:47 PM UTC 278 REVIEW WEATHER MONTHLY JULY 1959 July (fig. 10). Verylittle rainfall was directly related to Lows, which were confined mainly to the higher lati- tudes (fig. 11H). However, one cyclone, hurricane Cindy, entered the South Carolina coast on July 8. Although it subsequently weakened, it produced copious rainfallalong t,he Atlantic Seaboard from July 8 to 11. New England mts north of the anomalous easterlies and was predomi- nantlydry, except for the extreme southeastern part where near record precipitation amounts were reported. lVe&er, pa~t~iculx~rlyvisibility, wa,s unusuallybad in Nantucket, Mass., where fog or fog and rain occurred on 27 days of ,July. The other major area of rainfall was centered over the SoutllernPlains and South-Cent,ra,lStates. It was as- sociated wit,h the abnornxdly cyclonic 700-mb. flow (figs. FIQURE6.-Departure frommonthly normal of the mean thick- 1, 5)and the anomalous southerly winds in the lower ness (700-1000 mb.) for July 3959 in tens of feet. Isoline inter- val is 50 ft. with intermediate lines dashed.Below normal troposphere (fig. 1). PITO extratropical cyclones were ob values are stippled. Pattern is similar to the surface tempera- served in this area, and most rainfall resulted from heavy ture chart (fig. 4) with air masses generally cool in the central localshower activityand hurricane Debra. This short- United States but warm in the West and Northeast. lived, tropical cyclone deepened explosively on the 24th, just off the Texas coast (see Chart X in [8] ) and filled rapidly as it moved northward into Oklahoma the next da,y. Debra produced heavy rainfall in Texas and Loui- height departures from normal, though relatively small, siana.Galveston, Tex., for example,reported over 5 were importantin determining the weather anomalies. inches. They were oriented effectively to produce an anomalous, West, of the 30-day mean trough (fig. 1) and the region meridional flow across the normal thickness lines (1000- of frequent 5-day mean troughs (fig. 5) rainfall was gen- 700 mb. [9], generallynortherly in the western and erally light (fig. 8). A broad band of little or no rainfall southerly in the eastern halfof the country. extendingfrom California northe:rstward to Montana was associated withpersistent anticyclonic circulation 3. PRECIPITATIONAND TEMPERATURE and northerly anomalousflow at 700 mb. over the western OVERTHE UNITED STATES TJnitetl States (figs. 1, 5). Kecord (or near record) dry- Except for the showers in Arizona, the bulkof the pre- ness \\-as reported at many stations in Colorado, Idaho, cipitat,ionfell east. of the 30-daymean trough in two Montana, the Ilakotas, and Wyoming (tableI-B) . ratherdistinct areas. Record amounts over the Middle The light,precipitation in the northern Appalachian Atlantic States were associated with greater than normal Mountains was consistent, with a weak mean ridge over easterly andsoutheasterly flow. This is well indicated that, region (fig. I),but. an explanation for the pocket of by the anomalies of 700-mb. height (fig. 1), wind speed light, rainfall in the soutJ~ern Appaluchiansis not readily (fig. 9), thickness (fig. 6), and sea level pressure (Chart extractedfrom the meancirculation. However, it does XI in [8] ). Moisture-laden tropical air masses predom- correspond with a center of anticyclonic relative vorticity inated, facilitating convective showers.