RAIN CONTINUES OVER MISSISSIPPI VALLEY -- Rain fell across a large portion of the Midwest and the Middle Atlantic States on Tuesday, courtesy of a large, slow moving storm system. As of late Tuesday evening, the system responsible for the rain consisted of an occluding low pressure center located over the mid-Mississippi Valley near St. Louis, MO. An occluded front ran in a southeastward direction to a point of occlusion in western Kentucky. A warm front quickly turned into a stationary front extended to the southeast toward the Carolinas from the low in Illinois. A cold front curved southwestward to southern Arkansas, where it merged with a warm front associated with a low pressure center in southern Colorado.
As part of the clockwise circulation regime around the northern flank of this large Midwestern low pressure system, bands of rain moved to the northwest across Indiana, Illinois and southern Wisconsin. The slow moving nature of this storm system meant that many areas received excessive amounts of rain. Before the rain ended on Tuesday, some locations in northeast Kansas and southeastern Nebraska had received more than four inches of rain since Monday afternoon. As a result, flash flooding occurred on some rivers. The Missouri River south of Omaha, NE and the Mississippi River near Burlington, IA were above flood stage. The heavy rain across eastern Kansas over the last day brought the monthly precipitation total at Topeka to 8.69 inches, a new monthly record precipitation total for April.
Thunderstorm activity developed to the southeast of the low. On Tuesday evening severe thunderstorms with large hail moved eastward from southern Illinois into southwestern Indiana.
Only slow eastward movement of this storm system is anticipated by Wednesday morning. Heavy rain should continue across the mid-Mississippi and Ohio Valleys, near the low pressure center. A slight risk of severe thunderstorms continues until Wednesday morning across the Ohio Valley, and farther to the southeast over the Carolinas, in the warm sector.
SOME RELIEF --Rainshowers and thunderstorms continued across the rain-starved Southeast, to include portions of central and south Florida on Tuesday. Severe thunderstorms with large hail were reported near Ft. Lauderdale and Miami. Other thunderstorms were also reported near Palm Beach. Heavy rains accompanied the storms in the Miami area. While these rains were not sufficiently widespread to provide general relief to south Florida, they did indicate that the ridge of high pressure that had become entrenched over the Southeast for the last several months had eroded sufficiently to permit thunderstorm development. Concern remains that the lightning could still initiate wildfire.
Elsewhere across the Southeast, severe thunderstorms developed over Alabama and Georgia in the warm sector of the storm over the Mississippi Valley. High winds associated with the thunderstorms caused damage across Alabama and west central Georgia. Farther to the east, thunderstorms along the cold front that pushed southward across South Carolina produced large hail and caused wind damage as winds reached 60 mph.
COOL WEATHER IN THE NORTHEAST-- An elongated ridge of high pressure extended southward along the East Coast from a high center over Hudson Bay. Cool, dry air from this high pushed southward along the Atlantic Seaboard behind a cold front that reached the Carolinas before becoming stationary.
AND COOL IN THE NORTHWEST -- Cool air continued to push into the Pacific Northwest behind a cold front that moved eastward on Tuesday. However, by evening the front had essentially stalled over the Intermountain West. The northern end of the front was a low pressure center located over western Saskatchewan The front continued southwest toward northern California. Widely scattered precipitation was found across the Northwest.
On Tuesday morning, a funnel cloud was reported near Klamath Falls in south central Oregon, an indication of cold, and unstable conditions aloft. A record low temperature of 25 degrees was set at Pendleton, OR on Tuesday morning.
The stationary front is expected to remain essentially in place through Wednesday morning across the northern Rockies and the Great Basin. Snow is expected across the mountains of western Montana and Wyoming.
UPPER AIR -- One of the main features on the upper air charts for 00Z Wednesday was the cyclonic or counterclockwise circulation regime situated in the center of the country. This feature, found on both the 500 and 300 mb charts, indicates a deep storm system that extends upward from the surface low in Missouri through the entire troposphere. The system is also "vertically stacked", a characteristic of occluding systems where the circulation regimes at essentially all levels in the troposphere are directly above one another.
The wind flow across the country in the mid to upper troposphere showed highly meridional wind pattern, with large north-south excursions. The cyclonic feature in the center of the country was imbedded within a large ridge. A trough of lower height of the pressure surface was featured on the West Coast. A similar feature was also situated along the East Coast. These troughs on the coasts reflected the cold air that was found near the surface.
TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Tuesday, the lowest temperature reported in the continental U.S. was 21 degrees at Klamath Falls and Redmond, OR and Saranac Lake, NY. Tuesday's high was 103 degrees at Laredo, TX.
ALASKAN WEATHER -- A weak low pressure center located near Prince William Sound spread clouds and precipitation across the north Gulf Coast to southeast Alaska. Rain fell at Cordova, while snow was reported at Glennallen, Whittier and Valdez. A weak trough of low pressure was located over the central Aleutians, resulting in widespread clouds across the Aleutians and the south side of the Alaska Peninsula. Snow fell over Cold Bay.
A large ridge of high pressure was found across the northern portion of Alaska on Tuesday. Fog and low clouds were found along the Arctic Slope north of the Brooks Range and also along the western Seward Peninsula. Elsewhere, much of the state experienced relatively cloud-free skies.
The lowest overnight temperature in the state on Tuesday was 17 degrees below zero at Barter Island, while the midafternoon highest temperature was 53 degrees at Klawock.
HAWAIIAN WEATHER -- The islands continued to experience light to moderate trade winds on Tuesday as the result of a large ridge of high pressure located well to the northeast of Hawaii. Evening showers are expected over the windward sections of the Big Island and Maui. Elsewhere, dry weather should prevail until Friday.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.