OCTOBER 1982 J. KORSHOVER AND J. K. ANGELL 1515A Review of Air-Stagnation Cases in the Eastern United States During 1981 Annual Summary J. KORSHOVER AND J. K. ANGELLAir Resources Labs, ERL, NOAA, Rockville, MD 20852 (Manuscript received 11 June 1982)ABSTRACT The number and location of stagnation days within the eastern United States, as estimated mainly froma surface geostrophic-wind criterion, is presented by month and for the year 1981 as a whole. In 1981, thenumber of stagnation days exceeded the 40-year average everywhere except Florida, with a maximumfrequency of occurrence somewhat to the west and north of the usual location. There were unique occurrencesof stagnation in a belt extending from Minnesota to Maine, including extreme southeastern Canada.1. Introduction This is the first in a planned series of annual reviewsof air-stagnation cases in the United States east of100-W. Presented are maps showing, by month andfor the year 1981 as a whole, the number of stagnationdays in this region, as well as the average number ofstagnation days during the period 1936-75. Comparison of the two sets of data allows one to estimatehow typical a stagnation year 1981 was.2. Procedures The method used herein to delineate areas of stagnation is mostly objective, but also partly subjective,and other techniques might yield slightly different.results. Basically, the method involves delineatingthose areas where, for at least four days, the surfacegeostrophic wind (as determined from the DailyWeather Maps of the National Weather Service) isless than 8 m s-I. In general, this would correspondto a surface (anemometer level) wind speed less than~,4 m s-1 (e.g., Brunt, 1941, p. 260; Godske et al.,1957, p. 454). The choice of four days and ~4 m s-~is somewhat arbitrary, but has its origin in studiesand conclusions from the Donora, Pennsylvaniasmog disaster of 1948 (Fletcher, 1949; Willett, 1949;Hewson, 1951). Areas of stagnation, so obtained,were eliminated if fronts crossed through the region,there was precipitation (even a trace), or the windspeed at 50 kPa exceeded ~ 13 m s- ~. The wind speedcriterion tended to limit the stagnation cases to thosewith height ridges at 50 kPa. More details concerningthe method used for evaluation of stagnation casesmay be obtained from Korshover (1976). In Figs. 1-9, the solid lines show the number ofstagnation days (if any) per month, and for the year1981 as a whole, in the region east of 100-W andbetween 20 and 50-N. For example, if, in a givenregion, there were two stagnation cases during a givenmonth, one of four days duration and one of 8 daysduration, the total number of stagnation days duringthe month would be 12, and a 12-isopleth would enclose that region. The dashed lines in Figs. 1-9 showthe average number of stagnation days for the month,or the year as a whole, based on evaluations for theperiod 1936-75. Comparison of the solid and dashedlines allows one to determine whether 1981 was'atypical stagnation year, with regard to location ofareas of stagnation and, to a certain extent, whetherit was typical with respect to number of days of stagnation. The limitation in the latter regard results fromthe fact that, by our definition, one stagnation caseinvolves at least 4 stagnation days, which is then compared with a 40-year average value made up of manyzero values (no stagnation cases in a given month ofa given year). Hence, if a stagnation case occurs in1981, the number of stagnation days for 1981 is likelyto exceed the 40-year average in that region for thatmonth.3. Discussion There were no stagnation cases, as defined here,in January, February and March of 1981. There wasone case in April, in the usual location of southernGeorgia and northern Florida '(Fig. 1). Stagnationoccurred between 26 and 30 April as, following a coldfront passage, a cool high-pressure area was transformed into a warm high-pressure area resembling atypical Bermuda High. The flow at 50 kPa was weakfrom the west and northwest.1516 MONTHLY WEATHER REVIEW VOLUME 110 FIG. 1. Number of stagnation days in April 1981 (solid line) incomparison withthe April average of stagnation days during theperiod 1936-75 (dashed line). There was also one stagnation case in May, againin the usual area in the Southeast (Fig. 2). Stagnationoccurred between 22 and 26 May, as a cool polar airmass settled over the southern and eastern part of thecountry, following the northeastward movement ofan East Coast low. Fog was common in the southeastern states during this period. The flow at 50 kPawas weak from the northwest, west and southwest,respectively.FIG. 3. As in Fig. 1, but for June. The one stagnation case in June was centered inAppalachia, slightly north of the usual location forthis time of year (Fig. 3). Stagnation occurred between27 and 30 June, as a high-pressure area moved slowlyeastward behind a low moving down the St. LawrenceRiver. The flow at 50 kPa was generally from thenorthwest. The one stagr~ation case in July covered Arkansas,Louisiana and Mississippi, south and west of theusual location (Fig. 4). Stagnation occurred between~ay 1961, ,[. ,,~ ,. ~,. FIG. 2. As in Fig. 1, but for May.J, uly 1981 FIo. 4. As in Fig. 1, but for July.OCTOBER 1982 J. KORSHOVER AND J. K. ANGELL 1517)ctober 1981FIG. 5. As in Fig. 1, but for August.FIG. 7. As in Fig. 1, but for October.13 and 17 July, in association with a typical summertime high-pressure system extending from theMidwest into the Gulf of Mexico. Initially, the 50kPa winds were light from the southeast, later fromthe southwest. There were three separate stagnation cases in August, generally to the north and west of the usuallocation (Fig. 5). The first episode occurred between30 July and 4 August, as a cool anticyclone movedslowly from the Great Lakes to the East Coast whilea stationary front persisted in the Southeast. Stagnation was found in the Midwest, with longest duration near Chicago. The weak 50 kPa flow was generally from the west. The second episode occurredbetween 17 and 23 August, as an unusually large (forthis time of year) polar anticyclone moved into theNorth Central States from Hudson Bay. The area ofstagnation extended from the upper Midwest to theEast Coast. This episode is unique in that during theperiod 1936-75 there were no cases at all of stagnation in the belt extending from Minnesota to Maineduring the month of August. A light northwesterly~EPTEMBER 1981I?vember ,lg81FIG. 6. As in Fig. 1, but for September. FIG. 8. As in Fig. 1, but for November.1518 MONTHLY WEATHER REVIEW VOLUME 110 FIG. 9. Number of stagnation days in 1981 (solid lines) in cOmparison with the annual average of stagnation days during the period 1936-75 (dashed lines).flov~ persisted at 50 kPa during this episode. The thirdepisode occurred between 25 and28 August as a polaranticyclone moved southward from eastern Canadainto the eastern United States (associated with a socalled "back door cold front"). The stagnation areawas limited mainly to Kentucky and West Virginiabecause of rain showers. Winds at 50 kPa were lightfrom the west and northwest during this time. There were also three separate stagnation cases inSeptember, generally to the southwest of the usuallocation (Fig. 6). The first episode occurred between10 and 14 September, in association with an extensiveanticyclone which covered the whole southeasternpart of the country following a cold front passage.Stagnation was found within the triangle formed byMaryland, Louisiana and Georgia. The 50 kPa windswere light northwesterly.. The second episode occurred between 19 and 23 September as a large anticyclone moved into the Deep South from centralCanada and then became nearly stationary. Stagnation was confined mainly to Mississippi and Georgia.Again light northwesterly winds Prevailed at 50 kPa.The third episode between 24 andS27 September wasassociated with an intense anticyclone which movedsouthward from Hudson Bay to the Greak Lakes andthence southeastward to the East Coast. The stagnation area was limited to the southern states of Alabama, Georgia and South Carolina because of relatively strong west winds at 50 kPa. Note the consistency of location of all three stagnation episodesin September. There were two stagnation cases in October, withthe most important one to the north of the usuallocation (Fig. 7). The first episode occurred between3 and 6 October as a polar anticylone moved fromthe Great Lakes into the Southeast. Because of relatively strong west winds at 50 kPa, the stagnationarea was limited mainly to Alabama. The more important episode occurred between 9 and 15 Octoberin association with an intense anticyclone which extended from Hudson Bay to Virginia. Stagnation occurred throughout this entire region. This episode wasunique because during the period 1936-75 there wereno cases of stagnation so far north into Canada. Thisstagnation episode was also of relatively long duration, related as it was to a nearly stationary cut-offHigh at 50 kPa over Hudson Bay. There was only one stagnation case in November,and none in December. The case in November, centered in the Mississippi and Ohio watersheds, wassomewhat west of the usual location (Fig. 8). Stagnation occurred between 12 and 15 November, inassociation with an anticyclone extending from eastern Canada to the Gulf of Mexico, and to the westof an East coast low. The winds at 50 kPa varied fromsouthwest to northwest.4. Summary In average for the year 1981, stagnation tended tooccur slightly west and north of the usual location(Fig. 9). Only in Florida were the number of stagnation days fewer in 1981, than the average for theperiod 1936-75. Thus, in.general, 1981 must be considered a year with an above-average number of stagnation days in the eastern United States, as well asa year with unique stagnation episodes in northernUnited States, and extreme southeastern Canada. REFERENCESBrunt, D., 1941: Physical and DynamicalMeteorology. Cambridge University Press, 428 pp.Fletcher, R. D., 1949: The Donora smog disaster--A problem in atmospheric pollution. Weatherwise, 2, 56-60.Godske, C. L., T, Bergeron, J. Bjerknes and R. C. Bundgaard, 1957: Dynamic Meteorology and Weather Forecasting. Amer. Meteor. Soc., 800 pp.Hewson, E. W., 1951: Atmospheric Pollution. Compendium of Meteorology, T. F. Malone, Ed., Amer. Meteor. Soc., 1139 1157.Korshover, J., 1976: Climatology of stagnating anticyclones east of the Rocky Mountains, 1936-1975. NOAA Tech. Memo. ERL ARL-55, 26 pp.Willett, H. D., 1949: Some meteorological aspects of air pollution. Trans. Conf. on Industrial Wastes, Ind. Hy~. Found., Amer., Trans. Bull. 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