DATASTREME DAILY SUMMARY

Wednesday, 20 September 2000


00Z Weather Systems


GORDON'S RAINS REACH THE NORTHEAST -- A large area of precipitation found across the Middle Atlantic States on Tuesday night represented some of the moisture brought northward by the low pressure system that at one time had been former Hurricane Gordon. This system, now a midlatitude storm, was located over New Jersey with an occluded front stretching east-northeastward along New York's Long Island. This occluded front represents a front formed where a faster moving cold front has caught and merged with a slower moving warm front. A point of occlusion was situated to the east of Long Island, where the warm and cold fronts merged. The cold front for this storm system extended southwestward off the Atlantic Coast.

As the storm system moved northeastward along the Eastern Seaboard from the Carolinas on Monday night, it was accompanied by widespread rains from the Carolinas north to New York City. Rainfall accumulations ranged between 1 to 3 inches at many locations along the coast. Daily rainfall records were established Tuesday at Bridgeport, CT (1.84 inches) and Washington-Dulles Airport in northern Virginia (1.24 inches).

By Wednesday morning this storm system should have moved to the northeast, reaching the vicinity of Massachusetts' Cape Cod, and by evening it is expected to have reached the Canadian Maritime Provinces. As much as an inch of rain could fall across the lower Hudson Valley of New York State and southern New England during the 24 hour span from Tuesday through Wednesday evenings.

During the late afternoon, Tampa was struck by a severe thunderstorm that produced high winds that caused damage.

A CHANGE IN THE MIDWEST -- A cold front moving across the Midwest on Tuesday brought a marked fall in temperatures. By early evening, some locations that had experienced the passage of the front reported temperatures that were more than 20 degrees lower than 24 hours earlier. The cold front trailed southward from an elongated low pressure system in western Ontario across the western Great Lakes before curving southwestward across the mid-Mississippi Valley and into the central Plains. This front marked the leading edge of cooler and drier air carried southeastward from the northern Plains by westerly and northwesterly winds. To the east of the advancing front, southerly winds helped carry warm and humid air northward across the eastern third of the country.

A widespread band of rain along the front stretched from eastern Lake Superior southwestward to northern Missouri before curving westward across the central Plains.

The surface low and the northern portion of the front are expected to move eastward, with the cold front crossing Lower Michigan by Wednesday morning. Widespread rain should continue across the western and central Great Lakes. Farther south the front is anticipated to continue southward into the lower Mississippi Valley and the southern Plains. A slight risk of severe thunderstorms exists during Wednesday for a large portion of the Ohio and Lower Mississippi Valleys.

THE HEAT CONTINUES IN THE WEST -- A dome of hot air continued across the West on Tuesday. Record high temperatures were set in California at Paso Robles (107 degrees), Ukiah (106 degrees), Red Bluff (106 degrees), Livermore (103 degrees), Stockton (102 degrees), Sacramento (101 degrees), San Jose (99 degrees), Oakland (96 degrees) and San Francisco (93 degrees) and in Utah at St. George (103 degrees). Numerous record high low temperatures across the Pacific Northwest also contributed to the discomfort as little nighttime relief was afforded. The high temperatures coupled with low humidities and gusty winds have kept the wildfire danger at seriously high levels across California and Oregon. Some relief is anticipated with cooler air moving inland from off the Pacific Ocean.

YESTERDAY'S TEMPERATURE EXTREMES ACROSS THE CONTINENTAL US --Tuesday morning's lowest temperature was 33 degrees at Bridgeport, CA, while the highest temperature on Tuesday was 114 degrees at Death Valley, CA.

ALASKAN WEATHER -- While high pressure was found across much of mainland Alaska, cloudy conditions were found over much of the state except for the Yukon-Kuskokwim Delta, the region surrounding Norton Sound and in the vicinity of the Alaska Bowl. A weakening storm located over the eastern Gulf of Alaska south of the Copper River Basin contributed to the clouds and rain along the southeast Panhandle. A storm system was progressing slowly eastward across the central Bering Sea. A trailing cold front crossed the central Aleutians, resulting in cloudy skies across much of the eastern and central portions of this island chain. Light snow fell at Barrow and at Bettles, where freezing drizzle and fog were also reported.

The state's lowest overnight temperature on Tuesday morning was 18 degrees at Arctic Village. The highest temperature by mid afternoon of Tuesday was 59 degrees at Port Heiden.

HAWAIIAN WEATHER -- A ridge of high pressure extended southwestward to just north of Kauai from a high pressure center in the Eastern Pacific on Tuesday afternoon. Winds across Oahu, Kauai and the other northern islands were from the southeast at approximately 10 mph, while winds over the Big Island and Maui were from the east at 15 mph. The high pressure ridge is expected to drift southward toward the islands and weaken, along with a corresponding weakening of the winds over the islands. With weak trade winds, the local sea and land breeze circulation regimes should develop, with onshore flow during the afternoon hours as the slopes of the islands heat and an offshore flow at night as the land surfaces cool more quickly than the surrounding water. Except for inland showers produced by these local afternoon circulation regimes, sunny conditions should prevail. The inland showers should be relatively light.

As indicated by a moored buoy to the northwest of Kauai, storm-generated ocean swell was approaching the islands from the northwest. Since this swell is expected to produce surf to heights of 10 feet on Wednesday, high surf advisories were posted for the northwest facing shores of the islands.

EYE ON THE TROPICS -- After apparently dissipating east of the Windward Islands last weekend, Tropical Depression 12 reformed Tuesday from a tropical wave in the Caribbean Sea south of Cuba. As of early Wednesday morning this system was moving westward across the Caribbean Sea at 16 mph approximately 175 miles east-southeast of Cape San Antonio, Cuba. Sustained near-surface winds were 35 mph. This system could intensify to form a tropical storm and affect the weather across the Gulf of Mexico and the Gulf Coast states.

REPORT FROM THE FIELD -- Eleanor Vallier-Talbot, an NWS forecaster from Charleston, SC reported on the passage of the remnants of former Hurricane Gordon over the Low country of South Carolina. She said that while Charleston and points to the southwest received only about 2 inches of rain, radar-estimated rainfall along the central South Carolina coast to the east of Charleston was on the order of 10 to 12 inches. As a result, flooding occurred in Georgetown, causing the closing of US Highway 17, a main thoroughfare between Charleston and Mrytle Beach.

A POINT OF CLARIFICATION -- The yellow dashed lines on the various fronts maps and the included satellite composite image are called "trough lines". A trough line is drawn on a surface weather map along the axis of an elongated "trough" of lower pressure. Oftentimes this coincides with some change in the wind speed and/or direction that tends to produce clouds or precipitation. This region does not represent a marked boundary between differing air masses as would a front. Sometimes, a trough line is analyzed after a front dissipates and the temperature contrast disappears, but a band of clouds and a shift in the wind still persist.

PROBING THE ATMOSPHERE -- In order to monitor the weather conditions in the free atmosphere, weather instruments are sent aloft twice daily attached to weather balloons launched from more than 70 sites across the country. Some of these data are available on the DataStreme Homepage as product under the heading of Upper Air Data-Text. If you would like additional information concerning how to read these upper air data files, please consult Wednesday's optional Supplemental Information .


HISTORICAL WEATHER EVENTS - 20 September

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


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