ginia and Maryland had fallen con- tage of runoff was unusually high. siderably and a fall had begun north This, coupled with the fact that the of Maryland. Conditions were ap- northern rivers were at or above flood proaching normal, but on March 17 a stage, and those in Maryland and Vir- storm over south-central North Caro- ginia, while not in flood were higher lina was causing general rains over than normal, gave the most disastrous the Middle Atlantic Seaboard and the floods of record in the James, Poto- upper Ohio Basin. These rains were mac, Susquehanna, Connecticut, and phenomenally heavy in the country Merrimack Rivers, in some of the to the west of the Coastal Plains and tributaries of the Ohio River in Penn- along the divide between the eastern sylvania, and in the Ohio from Pitts- rivers and the upper Ohio Basin. burgh, Pa., to below Wheeling, W. Va. Amounts of nearly 5 inches occurred The Ohio River flood gave a record in 12 hours or less over a large area crest-stage of 46.0 feet at Pittsburgh in the mountainous regions of Vir- on March 18. The previous high wa- ginia, Maryland and western Penn- ter record was 38.7 feet on March 15, sylvania, and more than 6 inches fell 1907. at some stations in a period of 48 All parts of the Atlantic Slope have hours or less. The storm moved slow- had heavier rains than fell on March ly in a northeasterly direction and 17 to 20, 1936, but they were not the rains were heavy over all of the caused by the same storm. Also, the North Atlantic Seaboard. To the maximum falls have not, in any part northeastward the amounts were not of the Atlantic Slope north of North so heavy,4 but from a flood-producing Carolina, occurred when conditions viewpoint the results were about the were as favorable for flood causation same, because the rivers in the south- as in March, 1936. ern part of the area under considera- tion were much lower at the begin- 4The rainfalls were locally unprecedented in ning of the rain than they were in New England, however, for at Mt. Washing- ton, Pinkham Notch, and Randolph, N. H., Pennsylvania, New York and New the mountains induced 13.86, 22.43, and 16.19 England. inches respectively of rainfall for the period March 9 to 22. See article by C. F. Brooks The rains fell on well saturated and and A. H. Thiessen on The Meteorology of Great Floods in E. U. S., Geogr. Rev., April, semi-frozen soil, or, in elevated re- 1937, pp. 284-88 for further discussion with gions, on a heavy blanket of snow of maps of this storm, also Dr. Brooks' com- ments appended to Dr. Byer's article in this a high water content, and the percen- BULLETIN infra.—Ed. Meteorological Conditions During the March 1936 and Other Great Flood Storms of the Atlantic Seaboard HORACE R. BYERS* U. S. Weather Bureau, Washington, D. C. (MS received Oct. 26, 1936) HE METEOROLOGICAL conditions in leading up to the earlier floods were ^ some of the notable floods of the reconstructed in accordance with the eastern United States have been so-called air mass analysis system. The examined, including those of the fa- 1935 and 1936 flood weather was ex- mous "Johnstown flood" of 1889, the amined from the analyzed maps and

November floods of 1927 in New Eng- *Paper presented before the New England land, the July floods of 1935 in New Water Works Association convening in New York, Sept. 23, 1936 ; the preceding article York State and those of March 1936. in this BULLETIN by Mr. M. W. Hayes deals with the rainfall and flood flows during the Weather maps for the several days same storms discussed here.—Ed.

Unauthenticated | Downloaded 10/07/21 03:06 PM UTC upper-air data prepared at the time vergence but this effect cannot be in the Air Mass Section of the Cen- estimated definitely. With the passage tral Office of the Weather Bureau. of the cold front the rain stopped. This meteorological condition—the movement of unstable tropical At- lantic air from the south-east over the eastern mountain ranges—seems to be a typical producer of floods dur- ing the warm season. Almost an identical type of situation occurred in the July floods of 1935. In this case, for which the map is shown in Fig. 2,

FIG. 1. WEATHER MAP OF 8:00 AM, EST, MAY 31, 1889.

In Fig. 1 is reproduced the 8 a. m. position of fronts, air masses, low pressure center and stream lines of May 31, 1889, when the storm which caused the "Johnstown flood'' was at its peak. The rain in this case fell out of the tropical Atlantic air mass FIG. 2. WEATHER MAP OF 8:00 AM, EST. (labeled TA) which was moving JULY 8, 1935. rapidly against the mountain from the we had definite proof of the insta- southeast. This flow is practically bility of the tropical Atlantic air, in normal to the axes of the ridges, and the form of meteorological airplane since this air mass during the warm ascents at Mitchell Field, L. I. and at season is an unstable one, pronounced Lakehurst, N. J., on the morning of lifting and convergence over the July 8th. These data showed that a mountains occurred to produce heavy lifting of only 2000 feet would set off rainfall. The cold front moving from vertical currents in this air which the west brought in colder, drier air, could account for very heavy rains. but its movement was so slow that the The floods in New England during streaming of the moist, unstable trop- the first few days of November, 1927, ical air from the southeast prevailed were of the same type, but it seems for a long enough time to permit the to be stretching the point to call accumulation of an unprecedented these "warm season" floods. However, amount of rainfall. It is probable the stability or instability of the that the presence of this cold front tropical Atlantic air is determined accounted for active pre-frontal con- more by the seasons in the ocean than

Unauthenticated | Downloaded 10/07/21 03:06 PM UTC [March, 1937 on the land, and we know that the The Johnstown flood and the New seasons in the oceans are delayed a England flood of 1927 as well as the month or two after those of the con- 1935 phenomenon represented exceed- tinents, so that September represents ingly heavy precipitation. They oc- almost "mid-summer" conditions for curred as the result of run-off of wa- the ocean waters and October can ter provided by rain alone without be considered as a "warm" month. any thawing or frozen ground being Furthermore, snow and ice are not involved. The question that logically present over the land and if high arises is whether or not we are justi- temperatures have prevailed there for fied in assuming that such a situa- some time, as in the case of late Octo- tion could occur during a time of ber and early November of 1927, we year when frozen ground would be have conditions approaching those of involved, such as. for example, during the warm season. In the case of the month of March. From considera- these floods then, as also in the 1889 tions of the physical processes in- and 1935 storms, tropical Atlantic air volved in the rainfall, we would an- forced upward by the mountains, to- swer in the negative. Again the ques- gether with prefrontal convergence, tion of stability or instability of the played a dominant role. In the 1927 air mass is to be considered. Since case, however, there was more activ- we have found that the heaviest rain- ity connected with the cold front than fall occurs in the tropical Atlantic air in the other two instances, and much mass, this is the only one for which of the rain, especially that around these properties need be studied. In Burlington, Vt., seemed to have been March we find that tropical Atlantic associated with the action along the air moving into our area from the slowly-moving cold front depicted in southeast is subjected to a great Fig. 3, which represents the map for amount of cooling from below as it November 4th. moves northward and shoreward over the ocean. In examining charts of ocean surface temperatures it is noted that there is very little cooling of the warm Gulf Stream air as it moves toward the land in the warm season. In addition to oceanic cooling effects in March, if the warm air is moving over ground which is just thawing and which, therefore, has a tempera- ture of 32°F, the cooling of the over- lying warm air by contact with the cold earth imparts further stability. In the March, 1936, storm there was a pronounced transport of tropical Atlantic air into the mountains, where excessive precipitation fell from this air mass directly from orographic ef- fects alone, on account of the stabil- ity produced by its being cooled from below by contact with the cold inshore WEATHER MAP OF 8:00 AM, EST, NOVEMBER 4, 1927. ocean currents and the chilled earth.

Unauthenticated | Downloaded 10/07/21 03:06 PM UTC FIG. 4. WEATHER MAP OF 8 :00 PM, EST, MARCH 17, 1936.

In this flood nearly all of the precipi- tain lifting of unstable air are not tation was associated with fronts. present— that is, there is relatively The series of low-pressure centers little rain in the warm air mass it- which caused the March, 1936, floods self up to the crest of the mountains. forms an interesting study. Only the This is taken as an indication that the 8 p.m. map of March 17th is repro- tropical air was stable. Temperatures duced here (Fig. 4). It shows the along the coast support this conclu- peak development of the fourth and sion and a meteorological airplane flight showed definitely that the air last disturbance which moved along was stable and a greater amount of the stationary or slowly moving front lifting would be required to make it which lies across the area. The over- unstable than could be afforded to it running of the tropical Atlantic air by the mountains. over the polar air to the west in the While the heavy rains of New Eng- vicinity of the mountains is striking. land on March 18th and 19th occurred It will be noted that in this case with southeast winds and therefore there was a strong outbreak of trop- seemingly as simply an orographic ical Atlantic air flowing over the effect in the tropical Atlantic current, mountains, but rain in appreciable in reality these rains accompanied the quantities occurred only in connec- cold front, noted near Washington on tion with frontal action. The torren- the 8 p.m. map of the 17th, which was tial rains associated with pure moun- passing over New England the next

Unauthenticated | Downloaded 10/07/21 03:06 PM UTC day from the south. Later rains on the observations and automatic rec- the 19th to 20th occurred with the ords made on Mt. Washington (6300 passage of the second cold front from ft.) and its windward and leeward the south and the main front from the base stations, Pinkham Notch (2000 west. The rains accompanying these ft.) and Randolph (1300 ft.). The developments were locally as great as heavy rain began while the front was in some of the earlier warm, season over Washington, D.C., some 20 hours floods, but the area covered by these before there was evidence of its arri- excessive amounts was not nearly as val in New Hampshire. The passing large. The charts presented by Mr. of the front intensified the rainfall Hayes in the preceding article em- somewhat, but it appears that the phasize this difference. topographically forced ascent of the Accordingly, it seems reasonable to very strong SE wind was already so conclude that rainfall as great as oc- great that frontal action could not curred in the Johnstown flood is not accomplish much more. Whether or to be expected under conditions of not the orographic ascent touched frozen ground. off instability convection, as I first DISCUSSION (Communicated, March thought,2 is not indicated by the rec- 27) :—Although one may accept Dr. ords, in the absence of a recording Byers' main point that frontal action raingage. It is of little consequence was the dominant cause of the flood- in this case, however, for an ascent ing rains of March 1936, the greatly of but 2000 feet of the wind reaching increased falls on exposed highlands the mountain at 45 mi/hr would have east of the principal front along the provided the observed 0.25 inch of crest of the Appalachians, such as in rainfall per hour, assuming a preci- northeastern Pennsylvania and the pitating layer 3300 feet deep and the White Mountains, suggest a marked ascent spread over 6 miles. orographic contribution. Also to the If the 1.55-inch rainfall in 11 hours rain that fell on and west of the while the front was passing over Blue Appalachian divide, the ascent over Hill, March 18, can be taken as a the highlands must have made some rough measure of the non-orographic contribution. Furthermore, the stall- frontal rainfall for New England, ing of the major front over the Ap- there remains as the non-frontal rain- palachian divide seems to have been fall from 7:30 p.m. the 17th to 7:30 definitely favored by the mountain a.m. the 19th: 7.55 of the 9.10 inches wall.1 at Pinkham Notch, 2.15 of the 3.70 on The non-frontal character of the Mt. Washington, and 4.50 of the 6.05 greater part of the 10-inch rainfall of at Randolph.—Charles F. Brooks. March 17-19,1936 in the White Moun- JC/., BULLETIN, Dec., 1936, p. 359. tains seems to be clearly indicated by 2lbid., p. 360.

The Meteorology of Great Floods in the Eastern United States" an arti- cle by Charles F. Brooks and Alfred H. Thiessen has been published in the Geographical Review for April, pp. 269-290. This 22-page paper, including tables, 19 maps and diagrams, covers in some detail the great flood of Janu- ary, 1937, and in lesser detail the floods of March, 1936, July, 1935, Novem- ber, 1927, April, 1927, and others in 1922, 1916 1915, 1903, 1889, and 1882. Reprints of this article have been obtained and will be sold to mem- bers of the Society at cost, namely—15 cents including postage. Address the Secretary, American Meteorological Society, Blue Hill Observatory, Mil- ton, Mass.

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