DATASTREME DAILY SUMMARY

Thursday, 6 September 2001


00Z Weather Systems


WELCOME TO DATASTREME - The Daily Summary file will describe the current weather pattern across the U.S. The Tuesday and Thursday files will also contain the "Concept of the Day" with a question to be answered on the weekly Chapter Response Form. Additional Supplemental Information Files will provide optional background material.

The following highlights of the national weather have been extracted from the surface weather map for Wednesday night:

RAINS SPREAD NORTHWARD ACROSS THE PLAINS -- Widespread heavy rainshowers and embedded thunderstorms spread northward from Texas into Oklahoma and Kansas. This precipitation was associated with an upper-air circulation regime that extended upward to an altitude of at least 20,000 feet above sea level. As of late Wednesday night most of the rain had moved northward out of Texas. However, on Wednesday afternoon thunderstorms produced several weak tornadoes in the Lone Star State. One tornado produced some minor damage to apartment complexes in the Dallas suburb of Carrollton, while another tornado was reported about 15 miles to the north. Thunderstorm winds also produced some minor damage near Longview in east Texas. The heavy rains were also responsible for the flooded roadways in south Texas near Corpus Christi.

The area of precipitation is expected to expand northward and eastward across the Plains on Thursday, reaching the Upper Mississippi Valley by evening. Between 0.5 to 0.7 inches could fall in the 24 hours ending on Thursday evening in an area that extends from the Texas Gulf Coast to Iowa. This northward flow of moisture on southerly winds (from the south, in keeping with traditional meteorological notation) is ahead of a cold front that is forecast to move across the Rockies and into the western Plains. A slight risk of severe thunderstorms exists for the central Plains of Kansas and Nebraska by mid-afternoon on Thursday. A flash flood watch remained in effect for southern and south central Texas, to include Corpus Christi, San Antonio and Austin.

SEVERE WEATHER IN FLORIDA -- The other area in the South that continued to receive significant rain and severe weather on Wednesday was the central Florida Peninsula. Much of the Southeast has been dominated by a warm and humid tropical air mass. Strong thunderstorm winds across the Sunshine State produced some wind damage to trees near Orlando and near Tampa. One-inch diameter hail also fell at Orlando, where heavy rain also was responsible for the collapse of a roof section on an industrial complex.

Thunderstorm activity is expected to continue across the Florida Peninsula on Thursday, where as much as 0.75 inches could fall in the 24 hours ending at sunset.

HOT AGAIN IN THE NORTHERN PLAINS -- The increasing southerly winds ahead of an approaching front, coupled with relatively sunny skies were responsible for the continuation of unseasonably warm weather across the northern Plains. Afternoon high temperatures in the central Dakotas reached as much as 30 degrees above the average highs for the first week of September. The high temperature at Minot, ND was a record 104 degrees. In contrast, the clouds and the rain farther to the south held down afternoon highs across Texas, where readings were nearly 10 degrees below the long-term average highs for the date.

PLEASANT WEATHER ACROSS THE NORTHEAST -- A cool and relatively dry air mass that had originated in Canada dominated the weather from the Great Lakes eastward to New England and the Middle Atlantic States. Relatively cloud-free skies were the rule, with afternoon high temperatures that were about average. This air mass was associated with a large high pressure cell that was centered over the eastern Great Lakes on Wednesday evening.

The high pressure cell is expected to drift slowly to the southeast on Thursday, moving off the New Jersey coast by evening. The southern periphery of the cool air mass, which is represented on the Wednesday evening weather maps by the cold front across the Carolinas, should move southward to coastal Georgia by Thursday evening.

UNSETTLED WEATHER ACROSS THE WEST -- A nearly stationary cold front stretched across the northern Rockies into the Great Basin. This front was found along a complex and elongated trough of low pressure located across the Intermountain West. A wide spread area of rain was found to the north of the front, extending across Montana and Idaho. Some rain was also detected across portions of Utah and Colorado, to the south of the front. The front represented a boundary between air masses--with warm air from the Southwest located to the south of the front and cooler air of Pacific origin found to the north and west. Behind the front afternoon high temperatures across the Pacific Northwest were as much as 10 degrees lower than the average seasonal highs.

Strong winds were found in the vicinity of the front. These winds are associated with the relatively strong pressure gradients or horizontal differences in air pressure as reflected by the tight packing of the isobars, or lines of equal sea-level corrected pressure found on the evening surface weather analyses. Wind gusts had reached 60 mph across portions of northeast Utah and southeast Idaho on Wednesday evening. A high wind warning was in effect for northwest Utah, to include the desert and mountains around Great Salt Lake. Blowing dust in northern Nevada near Winnemucca had reduced the visibility to near zero, necessitating the issuance of blowing dust advisories for a portion of the Silver State. Wind advisories were issued for Snake Valley in Idaho, where westerly winds could gust to 50 mph, causing reduced visibility in blowing dust. Wind advisories also existed for the northern Sacramento Valley for Thursday as northerly winds (from the north) could gust to 35 mph.

The front is expected to move eastward and southward as a cold front on Thursday. By evening the cold front is forecast to extend from northern Minnesota southwestward to eastern Colorado before curving westward across the Four Corners area. Over an inch of rain is anticipated across the mountains of southern Montana, eastern Idaho and northwest Wyoming by Thursday evening. This rain would be welcomed by firefighters battling the numerous western wildfires.

TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 35 degrees at Big Piney, WY and Bridgeport, CA, while the Wednesday afternoon highest temperature was 113 degrees at Death Valley, CA.

ALASKAN WEATHER -- Clouds covered most of Alaska on Wednesday afternoon, the result of a large complex low pressure system that lay across the state. This system had several widely-spaced low pressure centers. One center was located in the vicinity of Kotzebue Sound and the Seward Peninsula, a second center was situated near Norman Wells in Canada's Northwest Territories, while a third low was found in the southwest corner of the Yukon Territory near the Alaska Panhandle. A front extended southward from this latter low pressure center along the Panhandle. Another separate low pressure system was located over the North Pacific Ocean south of Dutch Harbor in the Aleutians. Significant amounts of rain fell across the Panhandle. Wrangell, Ketchikan and Petersburg received more than 0.5 inches of rain since early morning. Gusty winds were found across southern portions of Alaska. During the afternoon, Skagway had gusts to 51 mph, Downtown Juneau had gusts to 45 mph, while Middleton Island, King Salmon and Homer had gusts to about 35 mph.

The lowest temperature in Alaska on Wednesday morning was 30 degrees at Barrow. The highest statewide temperature as of Wednesday afternoon was 66 degrees at Ft. Wainwright.

HAWAIIAN WEATHER -- A band of clouds that was oriented in a northwest-southeast direction had moved from Kauai toward Oahu during late Wednesday afternoon. Showers associated with these clouds were expected to move eastward across the islands overnight. Once these showers pass, relatively dry weather should persist across the islands for the remainder of the week. High pressure to the northeast of the islands should maintain northeast trade winds across the Aloha State.

EYE ON THE TROPICS -- In the tropical Atlantic Tropical Storm Erin began to weaken during Wednesday afternoon and was ultimately downgraded to a tropical depression with sustained winds of 35 mph. Its position when the final advisory was issued by the National Hurricane Center was approximately 230 miles east-northeast of Barbuda.

In the Eastern North Pacific, Tropical Storm Gil intensified, becoming Hurricane Gil , while the tropical depression TD-9E became Tropical Storm Henriette. As of Wednesday evening, Hurricane Gil had satellite-estimated sustained surface winds of 75 mph as it moved westward at 3 mph some 1200 miles west-southwest of Cabo San Lucas. Tropical Storm Henriette, located 630 miles southwest of Cabo San Lucas had surface sustained winds of 45 mph and was moving to the west at 16 mph.


CONCEPT FOR THE DAY - AIR MASSES

Frequently, you may hear of a cold air mass moving southward from Canada or of Gulf (of Mexico) air dominating the Midwest. What is an air mass, and how can we detect the presence of an air mass on the surface weather map? Air masses are large volumes of air that are relatively uniform in their horizontal temperature and humidity characteristics. They are generally identifiable by the "Highs" or "H's" written on maps which represent the centers of large high pressure regions. These centers of high pressure are at the same time often locations of the centers of air masses.

Air masses form when air resides over a land or water surface with fairly uniform surface properties for several days or more. The air gradually assumes the temperature and moisture characteristics of that underlying surface. Sooner or later, these broad expanses of "conditioned" air begin to move. Masses of air from northern latitudes are usually cold and termed "polar" while those from southerly latitudes are warm or "tropical". If the surface of the source region were land, the air would probably be relatively dry or "continental", whereas ocean air would be more humid or "maritime".

Coterminous U.S. weather is mainly prone to continental polar air masses from central Canada, maritime polar air masses from the northern Pacific Ocean, or maritime tropical air masses from the Gulf of Mexico and their transitions. While rare, continental tropical air would develop over the desert Southwest and the Mexican Plateau.

For an additional explanation of air masses and their properties, call up the Thursday's optional Supplemental Information .

QUESTIONS:

To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Chapter 1 Progress Response Form, under section B. Daily Summary (found in Week 1 section of Part B and also on DataStreme Homepage).

  1. People living in Ohio could reasonably expect relatively [(warm and dry) (cool and dry)] conditions when an air mass passes through from Canada.
  2. The type of warm, humid air mass most likely to be found over Mississippi in July would be a [(maritime tropical) (maritime polar)] air mass.

HISTORICAL WEATHER EVENTS - 6 September

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


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URL: datastreme/learn/r_sum.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.