AN EARLY WINTER STORM -- One of the first major winter storm systems moved out into the Plains from the Rockies on Wednesday. While the calendar would indicate late autumn, this storm system would be considered a winter storm because of the snow accompanying the system. At least 36 inches of snow had fallen by early evening at Deadwood in the Black Hills of South Dakota. Other nearby locations had greater snowfall accumulations.
As of late Wednesday night, the storm system was undergoing a typical occlusion process in its life cycle. The surface low pressure center was located over south central North Dakota. An occluded front stretched to the east to a point of occlusion where a warm front curved southeastward across the Upper Mississippi and Ohio Valleys. A cold front stretched southward from the point of occlusion across the Missouri Valley and southern Plains to south Texas.
The storm system was intense, with a central pressure of approximately 989 mb. The low central pressure meant a strong pressure gradient with higher pressure located over Ontario to the east and high pressure over Idaho and southern British Columbia to the west. The tight packing of the isobars, especially in the northwest quadrant of the storm reflect this strong pressure gradient. Because of the tight gradient, strong southerly winds were found across the Mississippi Valley, while northwesterly winds prevailed across the high Plains and the Black Hills of South Dakota. The winds coupled with the snowfall forced the closing of many roads in western South Dakota and eastern Wyoming, including I-90.
Relatively warm air was carried northward along the eastern flank of the storm. Temperatures remained in the 60s as far north as the Twin Cities in southeast Minnesota at 10 PM CST. To the west of the storm, cold air was being fed into the western and southern side of the storm. Temperatures over the Dakotas and eastern Montana fell from the 40s to the 30s.
Rain and imbedded thunderstorms were situated along and ahead of the cold front, extending from east Texas northward to the Arrowhead of Minnesota. Rain also fell to the north of the storm across north central North Dakota, but farther west in the cold air, this rain turned to snow across the western Dakotas and eastern Montana.
The storm system is expected to continue its slow motion toward the northeast, reaching the Red River Valley near Grand Forks, ND by Thursday morning and western Ontario to the north of Lake of the Woods by evening. The cold front is expected to move eastward, crossing the western Great Lakes on Thursday. Snow is expected to fall across the Dakotas in the wake of the storm system. A winter weather advisory was posted for western and north central North Dakota, while snow advisories and winter storm warnings were in effect for northeastern Montana. A blizzard warning was issued for southeastern Montana. Northerly winds with gusts reaching 50 mph were expected, along with several inches of snow that could produce white-out conditions.
SEVERE WEATHER ACROSS THE PLAINS -- Thunderstorms developed in the warm sector of the storm system on Wednesday. This region, located to the southeast of the surface low pressure system, is a region with warm air and abundant low level moisture. Some of the thunderstorms turned severe. At least one tornado moved through Bismarck, ND and neighboring communities on Wednesday afternoon. Two minor injuries were sustained by the tornado, along with minor to moderate damage to buildings. Other tornadoes where also reported in eastern North Dakota and in southwest Minnesota. Damaging thunderstorm winds were reported in northeast Kansas near Topeka and Leavenworth during the morning and in Oklahoma. Later, severe thunderstorms with high winds moved through eastern Iowa, especially near Cedar Rapids and in western Arkansas near Ft. Smith.
A PESKY STORM -- The East Coast storm that brought snow to New England at the beginning of the week continued to generate windy conditions along the New England coast late Wednesday. While this system was located to the south of Nova Scotia, the pressure gradient produced strong northeasterly winds over coastal New England. Consequently, gale warnings for winds between 39 and 54 mph were posted to run through Thursday morning for coastal waters of Massachusetts in the vicinity of Cape Cod from Nantucket Island to Provincetown, and along the Maine coast from Eastport to Stonington.
THE UPPER AIR -- The suite of standard upper air charts for 00Z Thursday indicated that the strong storm system in the Dakotas extended upward through the entire troposphere. In addition:
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 11 degrees at Eureka, NV, while the Wednesday afternoon highest temperature was 88 degrees at Kingsville, TX.
ALASKAN WEATHER -- Most locations in Alaska were cloud covered on Wednesday afternoon, except for some locations along the coast near Yakutat and Cordova. A series of storm systems in the Bering Sea and the North Pacific helped produce wet weather across southern Alaska and the Bering Sea on Wednesday. One storm system was moving northward across the Bering Sea near Bristol Bay. A front stretched from the low to a second, and more intense, low pressure system to the south of Cold Bay. The front accompanying this system continued to a third system in the Gulf of Alaska south of Yakutat. High pressure was building across the eastern interior near the Alcan border. Along the North Slope, snow fell at Barrow, and Deadhorse. The lowest temperature in Alaska on Wednesday morning was 15 degrees below zero at Ugnu-Kuparuk. The highest statewide temperature as of Wednesday afternoon was 47 degrees at Sitka.
HAWAIIAN WEATHER -- The subtropical high pressure cell pushed farther south in the eastern North Pacific, turning the prevailing winds across the islands from easterlies to east-southeasterly. As a result, more tropical moisture was available for the heavy showers and an occasional thunderstorm that occurred over the windward slopes of the Big Island and Maui on Wednesday. One location had nearly 3 inches of rain. Because of the rain, flood watches were in effect for portions of the windward slopes of Maui and the Big Island into early Thursday morning. These areas had experienced some flooding last Sunday.
Not all weather occurs at the surface. The troposphere (weather "layer" of the atmosphere) and those imbedded weather systems depicted on surface weather maps as Highs and Lows may extend to heights of several kilometers above the Earth's surface. At upper levels, far above the effects of surface friction on wind flow, distinct Highs and Lows usually give way to wave-like air currents that stretch around the globe. Meteorologists have long known that the atmosphere is three- dimensional and the understanding of weather requires knowledge about the atmosphere throughout its breadth and depth.
Meteorologists routinely monitor the atmosphere using essentially horizontal upper-air charts drawn at several different levels. These charts are drawn twice daily based on data collected at 00Z and 12Z by radiosondes launched from a network of stations. Each chart is drawn at a particular pressure level. Constant pressure surfaces are used as radiosondes report the data in terms of pressure, pilots often fly aircraft using pressure altimeters, and meteorological calculations are somewhat easier when expressed in terms of pressure values. A surface of constant pressure undulates slightly in altitude from place to place primarily due to the temperatures of the underlying atmosphere.
An often-used upper-air chart is the 500-mb map. This level is about half way up in terms of atmospheric mass and occurs near an altitude of 5,500 meters (18,000 feet) in the mid-troposphere. Winds at this level correlate well with the movements of large-scale weather features that dominate the lower troposphere. Surface Lows generally travel in the same direction and at about half the speed of the 500-mb winds above them.
For a more complete look at features of various upper air charts available on the DataStreme homepage, see the optional Thursday Supplemental Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 8 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast