DATASTREME DAILY SUMMARY

Friday, 13 October 2000


00Z Weather Systems


A WARMING TREND ACROSS THE EAST -- Much of the country east of the Mississippi experienced a pleasant October day courtesy of a large high pressure system centered over West Virginia as of Thursday evening. Following a relatively chilly night where numerous record low temperatures were set across the Southeast, temperatures rebounded quickly under sunny skies. Afternoon high temperatures across the eastern half of the nation were either at or above the average high temperatures for this time of year. The largest anomalies (differences between the observed and average highs) were found in the western Great Lakes, where temperatures were 15 degrees above the climatological average highs. This high pressure was the same high pressure that earlier in the week spread abnormally cold air across the region. The warming was a result of air mass modification and warm-air advection. The cold air mass had been modified as it moved across previously warmed ground and as a result of the relatively cloud-free skies that permitted solar heating. A southerly wind regime across the Mississippi Valley associated with the clockwise circulation around the western flank of the high pressure cell was responsible for the warm-air advection, or transport of warm Gulf air northward.

In addition to the warm-air advection across the Upper Midwest, the southerly winds also provided moisture advection as evident by the increase in clouds across the region by Thursday afternoon. As of late Thursday night, some light precipitation was also detected by radar across the Upper Mississippi Valley.

The high pressure cell should continue to dominate the weather across the East and Southeast as the center of the high is forecast to drift southward to the western mountains of North Carolina by Friday evening. A slow weakening of the central pressure is also anticipated, diminishing from an impressive 1044 mb reading last Sunday to a forecasted 1025 mb. Another chilly night is expected because of the enhanced radiational cooling.

A WESTERN COOL SPELL -- A large pool of cold air has become entrenched across the Western States, accompanied by clouds and precipitation. Afternoon temperatures across the West were below average, with the greatest anomalies being found across the northern Rockies, where Thursday's highs were between 10 and 15 degrees below the long-term average highs. Widespread clouds and precipitation were found in a counterclockwise swirl around several low pressure centers located in western Montana and northern Idaho. Great Falls, MT received 0.65 inches of rain, which was a record rainfall total for the date. Several inches of snow had fallen at Yellowstone Park in northwest Wyoming as of Thursday evening.

The eastern periphery of the cold air was a cold front that was moving slowly eastward. This cold front trailed across the Plains and the southern Rockies from a weak low pressure center over South Dakota. Some precipitation was found near the low pressure center and farther south along the front over New Mexico.

On Friday the cold front is expected to continue moving eastward across the Plains, with the northern portion of the front and associated low pressure system reaching the Upper Mississippi Valley by evening. Widespread light precipitation could fall across the region. Some thunderstorms are possible across the central Plains, but none should become severe. The precipitation across the northern Rockies and Intermountain West is expected to gradually dissipate. However, snow advisories were in effect for far western Wyoming and the Wasatch Mountains of Utah and southeast Idaho where 6 to 12 inches of snow could fall at elevations above 7000 feet. The total amount of liquid equivalent precipitation falling across the Plains and the Rockies between Friday and Saturday mornings is expected to be relatively light, ranging between 0.25 and 0.50 inches.

TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US -- Thursday's lowest temperature was 18 degrees at Grand Canyon, AZ. Thursday's highest temperature was 88 degrees at Mineral Wells, TX.

ALASKAN WEATHER -- Cloud-free skies were found across much of Alaska as the result of a ridge of high pressure located over northwest Canada. The only exception was the cloudy skies over the southeast and the extreme west. The clouds in the southeast were produced by a storm system located in the vicinity of Prince William Sound with an associated front that stretched along the Gulf coast into the Panhandle, along with a stationary front that stretched from the Alaska Peninsula into the southern Yukon Territory. These systems were also responsible for precipitation across the southeast. Rain was found along the southeast Panhandle, to include Sitka and Yakutat. Snow fell across the southeast mainland, such as at Northway and McCarthy.

Clouds across the west were the result of a warm front that extended southeastward across the Bering Sea from a strong storm system over eastern Siberia. In addition, a tight pressure gradient that developed between the Canadian high pressure system and the Siberia storm system produced strong winds especially across the Northwest. Point Hope reported sustained winds of almost 40 mph, with gusts to over 50 mph. Whittier also had peak gusts to 40 mph.

The lowest overnight temperature in Alaska on Thursday morning was one degree below zero at Deadhorse, while the mid-afternoon highest statewide temperature was 61 degrees at Petersburg and Wrangell.

HAWAIIAN WEATHER -- A tight pressure gradient developed across the islands on Thursday as the result of a subtropical ridge located to the north of Hawaii and a weak tropical disturbance passing to the south. This steep pressure gradient produced moderate to brisk trade winds that reached 25 mph. The passing disturbance also helped enhance the trade showers over the windward slopes of the Big Island. Because of the winds, small craft advisories remained in effect for state waters.

A WINTER FORECAST -- On Thursday meteorologists and climatologists at the National Oceanic and Atmospheric Administration (NOAA) released their initial winter outlook or forecast for the nation. Their press release indicates that following several abnormally warm winters, the upcoming winter (November, December and January) should be more "normal" since the anomalous atmospheric and oceanic circulation patterns identified as La Niña and El Niño appear to have waned.

THE HUNTER'S MOON -- A full moon occurred early this morning (more precisely, at 0853 Z on 13 October or 4:53 AM EDT, 3:53 AM CDT, etc.). This full moon is often called the "Hunter's Moon" since it follows the more spectacular "Harvest Moon" (this year on 13 September). Because of geometrical considerations, this full moon is not as impressive as its predecessor in terms of the string of long moonlit nights when the moon rises near sunset. The Hunter Moon may have been named because the moonlight may have assisted hunters in their autumnal hunt.


HISTORICAL WEATHER EVENTS - 13 October

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

14 October

15 October


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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.