DATASTREME DAILY SUMMARY

Thursday, 30 November 2000


00Z Weather Systems


A MIXED BAG OF EASTERN PRECIPITATION -- The storm system that deposited as much as 6 inches of snow across portions of the Midwest on Tuesday night and early Wednesday continued moving toward the East Coast on Wednesday. A combination of rain and snow spread eastward with this system. As of late Wednesday night the low pressure center was located over Lake Erie, with a warm front that extended to the southeast across the Appalachians to the Del-Mar-Va Peninsula, while a cold front trailed southwestward along the Appalachians, across the lower Mississippi Valley to south Texas.

The precipitation shield associated with this storm system extended around the north side of the low, from just north of the warm front along the Middle Atlantic Coast inland across Pennsylvania and western New York State into northwest Ohio and then southward to the Ohio Valley. Most of this precipitation along the coastal areas was in the form of rain, as the air near the ocean was sufficiently warm to allow the precipitation to fall as rain. From the northern highlands in Pennsylvania westward through New York State and across the southern Great Lakes, the temperature was sufficiently cold to support snow. Rain fell in warm air along the Ohio Valley.

This storm system is expected to move eastward to the New Jersey coast by Thursday morning and redevelop after it moves out over the Atlantic Ocean later in the day. As this system moves eastward cold air will be drawn toward the coast, causing the rain that was falling in many Northeastern communities to turn to snow. As much as 3 inches of snow could fall across the higher terrain in northeast Pennsylvania. With intensification of the storm system over the open waters of the Atlantic, gale warnings for anticipated winds between 39 and 54 mph have been posted for the Middle Atlantic coast including all bays and sounds from the North Carolina-Virginia border to Providencetown, MA

SNOW ACROSS THE ROCKIES -- The powerful storm system that moved onshore along the Pacific Coast on Wednesday was traveling rapidly toward the east. The cold front crossed the Cascades and Sierras, and as of Wednesday night, extended from northern Idaho south to Nevada before curving westward to the coast of central California. At that time snow was falling across the mountains of northern Idaho and western Montana, as well as across eastern portions of Washington State. Several inches of snow have fallen across the mountains of Idaho and Montana as of late Wednesday evening. Farther south, rain fell across southern Idaho. Earlier, high winds reached 71 mph at Reno, NV and overturned a cattle truck in California near Mono Lake. Ten inches of new snow fell at Donner Summit along the Nevada-California border.

While this cold front is expected to weaken as it moves across the northern Rockies, at least one low pressure system along the trough line marking the remnants of the cold front is expected to reintensify on the eastern slopes of the Rockies. A snow and blowing snow advisory was in effect for the north central mountains of Colorado through noon Thursday as 3 to 6 inches of snow are expected along with 20 to 40 mph sustained winds, gusting to 60 mph. A wind advisory was also in effect for southern Wyoming, northwest Colorado and northeast Utah for sustained winds between 25 and 40 mph along with gusts to as high as 60 mph, especially in high terrain and in the mountain passes.

MORE STORMS AHEAD -- More storm systems were moving eastward across the northern Pacific Ocean on Wednesday evening, taking aim on the Pacific Northwest. Strong southwest to west winds were found along the Washington and Oregon coasts well ahead of the next storm system that should reach the mainland by Friday. As a result of the winds, gale warnings were posted for the Washington and Oregon coasts from Cape Flattery, WA to Florence, OR. Heavy surf advisories have been posted for the coasts of Oregon and California from Florence, OR to Point Piedras Blancas, CA through Thursday as large swells have been generated by a series of storm systems that have moved across the North Pacific. High wind warnings were in effect for the Washington Cascades and the central Sierras of Nevada and California. In addition to the high wind speeds, the onshore westerly winds were generating orographic precipitation. Between 0.5 and 1.0 inches of precipitation, primarily as coastal rain, are expected along the Olympic Peninsula by early Friday morning. A variety of winter storm warnings and winter weather advisories run from the Canadian border in eastern Washington State and northern Idaho south to central California. As much as 6 inches of new snow are likely especially in the higher elevations of the Cascades.

The high temperature on Wednesday afternoon at Las Vegas, NV was a record 74 degrees.

UPPER AIR -- As indicated by the 500 and 300 mb charts for 00Z Thursday, the trough of low heights at each level has moved eastward from the Great Plains to the Great Lakes States. This trough is associated with the surface low pressure system that was located over the eastern Great Lakes. A weak ridge is found across the Rockies, while farther west, another trough can be found along the west coast. This trough is associated with the surface storm system that moved across the Pacific Northwest earlier on Wednesday. A relatively strong jet stream, with winds exceeding 100 knots over northern California, was found on the 300 mb surface.

YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- Wednesday morning's lowest temperature was 15 degrees below zero at West Yellowstone, MT, while the Wednesday afternoon highest temperature was 85 degrees at McAllen, TX.

ALASKAN WEATHER -- The arctic high pressure located over eastern Siberia was moving toward Alaska, spreading more cold air across much of Alaska on Wednesday. The temperatures at many locations were several degrees lower on Wednesday morning than at the same time on Tuesday. During the early afternoon, the wind-chill equivalent temperature at Anaktuvuk was 60 degrees below zero, while those at Barter Island and Kivalina were only several degrees higher.

A large storm system was located in the southern Gulf of Alaska near the Queen Charlottes and the British Columbia coast. The occluded front associated with this system was responsible for maintaining the cloud cover and relatively mild conditions across the southern Panhandle. Downslope winds from the northeast reached 51 mph near downtown Juneau and in the Lynn Canal at Eldrod Rock Lighthouse. A powerful storm system with central pressure of approximately 965 mb was moving across the north Pacific east of the Kamchatka Peninsula. Strong southeasterly winds brought relatively warm air across the Aleutians and the Alaska Peninsula.

The lowest temperature in Alaska on Wednesday morning was 38 degrees below zero at Selawik. The highest statewide temperature as of Wednesday afternoon was 47 degrees at Ketchikan.

HAWAIIAN WEATHER -- Trade weather continued across Hawaii on Wednesday courtesy of a large high pressure system located just to the north of the islands. Relatively light easterly winds were the result of a weakened pressure gradient over the islands when the high moved closer to the islands. Most of the islands experienced relatively cloud-free skies, except for some clouds located on the windward slopes of the islands. This pleasant trade weather is expected to continue into the weekend.

CLOSING THE BOOKS ON NOVEMBER -- The National Climate Data Center in Asheville, NC has posted a listing of some of the notable extremes in temperature, precipitation and other weather elements across the nation for the month of November.


CONCEPT FOR THE DAY - FLASH FLOODS: A CONVECTIVE HAZARD

A flood occurs when water overflows the confines of a stream or other body of water and accumulates over low-lying areas. Floods are classified as flash floods, river floods and coastal floods.

A "flash flood" is a dangerous rise in water level of a stream in a few hours or less caused by heavy rain, ice jams, earthquake and earthslide, or dam failure. A flash flood usually occurs within 6 hours of the rain event, typically a thunderstorm. In some cases, the heavy precipitation can produce a wall of water, moving at incredible speeds and with sufficient force to roll boulders, tear out trees, destroy buildings and bridges and scour out new channels. In many cases, flash floods can cause automobiles to be swept away in just 2 feet of moving water. Since the 1970's, flash flooding has caused an average of 200 fatalities per year in the United States.

These flash flood events often differ in rapidity and violence from "river floods" that are seasonal, resulting from spring rains and melting snow. River floods may crest slowly and persist for as much as a week. The record spring 1997 floods on rivers in the Dakotas and Minnesota, notably the Red River of the North, were the result of rapid spring thaw of the heavy snow cover from a record number of blizzard-producing snowstorms. The great Midwest floods of 1993 were essentially a combination of river flooding produced by a persistent weather pattern, but also by the many local flash flood events on tributaries to the Mississippi and Missouri Rivers, all contributing a large, but slow, increase in the water volume in the entire river basin.

"Coastal floods" are usually associated with tropical weather systems and will be described subsequently.

The Thursday optional Supplemental Information file describes the terminology used in the official statements issued by the National Weather Service to inform the public of floods and flash floods. Also included are major meteorological factors leading to flash flooding.

QUESTIONS:

To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 11 Chapter Progress Response Form, under section B. Daily Summary.

  1. A flash flood crests within [(hours) (days)] of the rain event.
  2. The danger in flash floods is due to the [(depth of the water) (power of the moving water)].

HISTORICAL WEATHER EVENTS - 30 November

From the files of the Aviation Weather Center, Kansas City, MO and Intellicast


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URL Address: datastreme/learn/r_sum.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.