DATASTREME DAILY SUMMARY

Wednesday, 29 November 2000


00Z Weather Systems


A WINTRY STORM IN THE UPPER MIDWEST -- A storm system that contained snow, freezing rain and strong winds traveled eastward from the western high Plains into the Upper Midwest on Tuesday. As of late Tuesday night, the low pressure system was located in west central Minnesota, to the southeast of Fargo, ND. This storm system was in the occluding stages of its life cycle, with the occluded front extending southeastward over the Upper Mississippi Valley before becoming a cold front over northeast Iowa. The cold front continued southward to the Ozarks of southern Missouri, then curved westward across the Southern Plains in west Texas, before turning northward to parallel the southern Rockies in eastern New Mexico.

A reasonably narrow band of precipitation wrapped around the storm system. The type of precipitation associated with this storm system depended upon the temperature. Snow fell in the cold air from the Dakotas southeastward to south central Wisconsin by late Tuesday night. Farther south, the snow changed first to ice pellets (sleet), then freezing rain and then to rain across the slightly warmer air in northern Illinois and southeastern Wisconsin. By Tuesday night as much as 7 inches of snow had fallen near Fargo, ND. Aberdeen, SD reported 4.8 inches of snow, with 0.48 inches of liquid equivalent precipitation, which tied the daily record.

In addition to the wintry precipitation, the storm also generated some strong, gusty winds especially to the west of the system where a tight pressure gradient was created between high pressure over the Rockies and the low. Winds had reached 40 mph in gusts across the western Dakotas earlier in the evening.

The storm is expected to continue to the east-southeast on Wednesday, reaching western Wisconsin by daybreak and northern Ohio by evening. Winter weather advisories remained in effect for portions of southeastern North Dakota, where 3 to 5 inches of total accumulation were expected and across western Wisconsin, where 2 to 3 inches could fall by morning. As much as 0.25 inches of rain could fall over northern Ohio by Wednesday evening on the warm side of the storm as it passes eastward, while an inch or two of snow is anticipated across Michigan on the cold side of the storm track.

A NEW STORM TAKES AIM ON THE NORTHWEST -- An intense storm system with central pressure less than 980 mb was moving across the North Pacific toward Vancouver Island on Tuesday night. This storm system, with a center approximately 600 miles west of the Washington State coast, had a warm front that extended southeastward to the Oregon coast, while a trailing cold front remained several hundred miles off the Oregon and Washington coasts. Some light precipitation marking the leading edge of the storm system was detected across the Olympic Peninsula and some other coastal locations to as far south as northern California late Tuesday night. Winds had increased in strength. A high wind warning was in effect for the west slopes of the Cascade foothills of Washington State, as sustained winds to 40 mph with gusts to 70 mph were reported east of Tacoma near Enumclaw on Tuesday evening.

This storm system is expected to reach the Northwest coast during Wednesday. By evening the cold front is forecast to have crossed the Cascades and moved into the interior. In anticipation of the approaching storm, gale warnings remained in effect for the Pacific coast from Cape Flattery, WA to Point Arena, CA. A snow advisory was posted for interior Washington east of the Cascades and the northern Panhandle of Idaho for 1 to 3 inches of snow through Wednesday. Winter storm warnings remained in effect for northern California through Wednesday as 18 inches of snow could fall on the mountains at elevations above 5000 feet. Some locations could also experience southwest winds gusting to 60 mph, creating whiteout conditions. Freezing rain could present another problem in the interior valleys of Washington and Oregon. A shallow pool of cool air was located across this region as a result of the high pressure system situated over eastern Washington and Idaho. As southerly and southwesterly winds bring warm, moist air over this pool of cool air, the rain that initially fell in the warm air would freeze as it reaches the ground, especially in Oregon and southern Washington along the Columbia River. Along coastal southern Oregon and northern California as much as 2 inches of rain could fall by Wednesday evening.

UPPER AIR -- The mid and upper tropospheric weather charts (500 and 300 mb, respectively) for 00Z Wednesday indicate that a height trough had developed across the northern Plains as a result of cold air being carried into the storm system over the northern Plains. To the west, a height ridge developed. Hence, the prevailing westerly winds at both levels curve northeastward as the flow onshore over West Coast, then turn to become northwesterly over the Rockies and the northern Plains. To the east of the upper trough, the winds become more westerly.

YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US --Tuesday morning's lowest temperature was 15 degrees below zero at West Yellowstone, MT, while the highest temperature on Tuesday was 85 degrees at McAllen, TX.

ALASKAN WEATHER -- Arctic air associated with a large ridge of high pressure centered over eastern Siberia remained over northwestern, western and interior Alaska on Tuesday afternoon. Midday temperatures, which were relatively seasonal, ranged from about 20 degrees below zero in northwest Alaska to single digits above zero in the interior. During the morning and early afternoon, the wind-chill equivalent temperature at Deadhorse hovered around 50 degrees below zero. The southeastern boundary of this cold air was along a nearly stationary front that extended from a low pressure center near the mouth of the Mackenzie River in northwest Canada to another low pressure center situated over the Alaska Peninsula near Katmai National Park. This latter low pressure system was responsible for precipitation across Kodiak Island, the Kenai Peninsula and south central Alaska. Some of these locations especially near the coast reported rain. High pressure was also located over the southern Yukon Territory and southeast Alaska.

The state's lowest overnight temperature on Tuesday morning was 26 degrees below zero at Anaktuvuk Pass and Selawik. The highest temperature by mid afternoon of Tuesday was 44 degrees at Seldovia.

HAWAIIAN WEATHER -- A ridge of high pressure continued to drift slowly southeastward across the subtropical North Pacific north of Hawaii. As a result of the weakening of the pressure gradient over the islands, the easterly trade winds began decreasing in speed from 20 mph to between 10 and 15 mph on Tuesday afternoon. The proximity of the ridge also meant that most of the islands would remain dry, except for some clouds and scattered showers that were on the windward slopes of the Big Island. The weak shear line to the north of the islands that marked the remnants of a cold front continued to dissipate.

TORNADO SEASON -- With one month remaining in the year, approximately 863 tornadoes have been reported across the United States during 2000, with 27 fatalities. For more information concerning this year's tornado statistics, consult Wednesday's optional Supplemental Information.


HISTORICAL WEATHER EVENTS - 29 November

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, 2000, The American Meteorological Society.