KILLER TORNADOES HIT THE SOUTHERN PLAINS -- Severe thunderstorms that spawned fatality-producing tornadoes developed across the southern Plains from north Texas into Oklahoma and Kansas late Monday afternoon and early evening. These tornadic supercell thunderstorms initially developed in the Red River Valley of southwest Oklahoma along a "dry line", or a trough line that marks a region of strong contrast in humidity, and moved northeastward. As of Monday evening, the dry line extended from the Big Bend area of west Texas northward to near Grand Island in southern Nebraska. Dewpoints to the east of the dry line across central Texas and Oklahoma ranged from the 60s and to the mid 70s as a result of the advection of maritime tropical air northward on brisk southerly winds. West of the dry line across the Texas and Oklahoma Panhandles, dewpoints ranged from the 20s to the 40s. This contrast in humidity has the same effect as a cold front, serving as a trigger mechanism for the development of thunderstorms.
One of the hardest hit regions appeared to be the southwestern suburbs of Oklahoma City, OK. Many homes were destroyed, wide spread power outages were reported and I-35 and several Interstate Highways to include I-40 were blocked due to damaged vehicles. Other regions that also suffered major damage were in the Tulsa, OK metropolitan area and the south side of Wichita, KS. As of early Tuesday morning, at least 36 fatalities and more than 100 injuries were attributed to the tornadoes in Oklahoma and Kansas. At least one tornado was said to have been a half mile wide. Multiple vortex tornadoes were observed. Hail with diameters of roughly 4.5 inches was reported.
A line of thunderstorms continued well into the nighttime hours from the Dallas-Fort Worth area in north Texas through central Oklahoma and southern Kansas. While most of the storms were moving to the northeast, the whole area was moving slowly eastward. Farther north, thunderstorms were also producing severe weather across the Dakotas.
A high risk of severe thunderstorms remains across Oklahoma and Kansas through Tuesday morning, while a slight risk continues from the south Texas to the Dakotas. Severe weather activity is expected to shift eastward and continue through Wednesday across the Plains and the Mississippi Valley, with a moderate risk of severe thunderstorms expected to be located over Arkansas and Missouri.
UNSETTLED WEATHER REMAINS ACROSS THE WEST -- To the west of the tornado outbreak across the Plains, a cold front extended southward from one low in eastern Saskatchewan to a low in the Nebraska Panhandle to southern New Mexico. This cold front represented the leading edge of unseasonably cool air that was located over the Intermountain West and the Pacific Coast. Scattered regions of precipitation were detected. Snow fell over the higher elevations of Washington, Oregon, Northern California, Nevada, Utah and Montana. Snow levels descended to around 2000 feet in western Washington, which is unusual for this time of season. As of Monday, the Arkansas River in La Junta, CO remained flooded.
By Tuesday morning, the front should have moved slightly eastward into the High Plains. Winter storm warnings were in effect for the central mountains of Colorado, while snow and blowing snow advisories were posted for the mountains of New Mexico.
WET WEATHER IN THE NORTHEAST -- Coastal regions of southern New England and New York experienced unseasonably cool, cloudy and wet weather on Monday. The storm system that has lingered off the East Coast since the end of last week continued to show its presence. As of late Monday, the center of the low was several hundred miles east of the New Jersey coastline. An easterly to northeasterly wind continued to being rain into the Boston and New York metropolitan areas. The extensive clouds, coupled with the falling rain and an onshore wind component, meant that afternoon temperatures remained below seasonal averages. In sunny skies to the north, the afternoon high temperature at Caribou, ME reached 81 degrees, a record high temperature.
The clouds and precipitation should linger along the Northeast coast as the surface low continues its slow travel to the northeast.
LIFE UNDER A HIGH -- A large ridge of high pressure remained across the eastern portion of the country on Monday night. High pressure extended from the Great Lakes southward to Florida. The dry air, the lack of clouds and the relatively weak winds caused widespread nighttime cooling. Record low temperatures were either tied or set on Monday morning at Augusta, GA (41 degrees) and in Florida at Hollywood (62 degrees) and Key West (65 degrees).
This high should move slowly eastward on Tuesday.
UPPER AIR -- The upper air constant pressure charts for 00Z Tuesday show some of the features as to the development of the supercell thunderstorms across the southern Plains. On both the 500 mb and 300 mb charts, a height trough is found across the Intermountain West, associated with the cold pool of air that extends over a large portion of the West. A ridge of high heights is located over essentially over essentially the entire eastern half of the country.
On the 300 mb surface, strong winds with speeds in excess of 50 knots are found to the south of the upper level trough, extending from the Southwest eastward to the southern Plains. Over north Texas and Oklahoma, a split in the isotach pattern provides an mechanism for the storm development. The widening of the spacing of the height contours and the split in the 50 knot isotach contribute to vigorous uplift because upper level divergence is enhanced.
Furthermore, the winds turned clockwise with height, being southerly or even southeasterly at the surface, then turning to a more southwesterly direction in the mid troposphere (500 mb level) and finally from the west southwest at 300 mb. This directional change in wind contributes to the spin associated with a tornado.
TEMPERATURE EXTREMES IN THE LOWER 48 -- The lowest temperature in the lower 48 states on Monday was 22 degrees at Chama, NM, while Monday's highest temperature was 97 degrees at Lajitas, TX.
ALASKAN WEATHER -- An elongated area of low pressure extended from over the northeastern Gulf of Alaska near Middleton Island southeastward along the Panhandle and into northwestern Canada. Clouds and precipitation accompanying this system were fond primarily across the Panhandle. Another elongated trough of low pressure extended from northwest Alaska across the central interior toward the upper Tanana Valley. A snow advisory was posted for the Tanana Valley for as much as 4 inches of additional snow.
A ridge of high pressure was located over the western Bering Sea near Shemya. A relatively strong pressure gradient between this high and the low in the northeast Gulf produced strong winds gusting to between 20 and 40 mph across south central and western Alaska, as well as the Aleutians. A wind advisory was in effect for Cold Bay.
The lowest overnight temperature in the state as of Monday was 3 degrees above zero at Barter Island, Ugnu-Kaparuk and Nuiqsut. The highest midafternoon temperature in Alaska was 52 degrees at Valdez.
HAWAIIAN WEATHER -- A ridge of high pressure centered several hundred miles to the northeast of Hawaii produced light to moderate winds from the east to southeast across the islands. A trough and frontal band remain to the northwest of the islands. This southeasterly surface flow has brought higher humidity, resulting in muggy conditions on Monday. Clouds and showers were found along the windward slopes and mountainous areas of all islands. With lighter large scale winds a local sea breeze is anticipated with afternoon showers over interior regions. This situation is expected to continue through Wednesday as the ridge is forecast to remain relatively stationary.
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.