A NOTICE -- COMET, provider of DataStreme images, is experiencing data processing difficulties for radar and satellite information. For current imagery, we are linking you (via the Homepage) to an alternate site for the latest "Radar", and satellite ("Infrared - Latest", "Visible - Latest", and "Water Vapor - Latest") files. We are working with COMET to correct these problems. Please note that these images are not configured on the same base map as our other products.
AN OVERSIGHT -- Last Friday's Summary neglected to mention the occurrence of the Autumnal Equinox on Friday afternoon (at 1727Z, or 1:27 PM EDT, etc.). At that time the noontime sun appeared directly above the equator. Within the next several days the length of daylight will become noticeably shorter than that of nighttime.
WEATHER OVER THE WEEKEND -- On Friday morning, Tropical Storm Helene made landfall around 8 a.m. (EDT) in the Florida Panhandle. By Friday afternoon, as the system continued to move inland and weaken, it was downgraded to a tropical depression Friday afternoon. Helene was responsible for some heavy rains that fell across the Panhandle and southern Georgia, with Apalachicola receiving 10.58 inches by Friday afternoon and Tallahassee 7.91 inches. Some flooding was reported as a result of the excessive rain. Several tornadoes spawned from the system crossed the Panhandle. As the system moved into the Lowcountry of South Carolina by early Saturday, heavy rains produced flooding near Charleston. At least two tornadoes were reported to the northwest of Charleston, with one tornado causing an injury. A fatality and multiple injuries were sustained in the other storm. Both tornadoes caused property damage.
A storm system moved out onto the Plains early Friday. This system continued across the western Great Lakes on Saturday morning, the eastern Lakes by Saturday night and into the St. Lawrence Valley by Sunday morning. To the north of the system, a large band of precipitation fell across the Midwest with Rockford, IL setting a rainfall record on Friday. The cold front that trailed the low pressure center to its south and west contained some active thunderstorms as it swept to the southeast across the Plains and into the Ohio and Lower Mississippi Valleys. On Friday afternoon and evening severe thunderstorms containing hail with diameters as large as 1.75 inches and producing damaging winds to 70 mph moved across Oklahoma, Kansas, Missouri and Iowa. Additional severe thunderstorms with large hail and wind damage were reported on Saturday across Texas, Oklahoma, Missouri, Arkansas and Alabama. Two tornadoes were reported on Saturday afternoon south of Columbus, OH.
As the cold front went through north Texas, several tenths of an inch of rain fell at the official reporting station at the Dallas/Ft. Worth Airport, breaking the 84 day dry spell when no measurable precipitation (0.01 inch or greater) fell. This dry spell broke the previous record number of consecutive rainless days by slightly more than 3 weeks.
The first major snowfall of the season blanketed portions of the Rockies, over the weekend. A cold high pressure system moved southward from Canada, while low pressure was found across the southern Rockies. As a result, an east wind produced upslope snow conditions over the Rockies, initially over Montana early Friday and then progressing southward along the mountain chain into Wyoming and northern Colorado by early Sunday. Ascending air cools as it moves upslope, with a development of clouds and precipitation. The cold air insured that the precipitation fell as snow. As much as 15 inches of snow fell in the Medicine Bow Mountains of southeastern Wyoming. Cheyenne broke daily snowfall records on Saturday with 4.2 inches and on Sunday with 6.4 inches. The latter record was the greatest snowfall ever recorded in Wyoming's capital city in the month of September. Snow and icy road conditions forced the closing of 200 miles of I-80 from Cheyenne to Rock Springs across southern Wyoming. A 12 vehicle accident was reported east of Laramie on Saturday.
Unseasonably cold air spread across the Plains on Sunday. St. Cloud, MN had a morning record low temperature of 24 degrees, while Goodland, KS reached a daily record low of 30 degrees just before midnight. Afternoon high temperatures on Sunday were as much as 30 degrees below the long term average highs for the third week of September in western Kansas and Nebraska. Some stations either tied or set record low maximum temperatures, to include Goodland, KS with a high of only 41 degrees, Sioux City, IA (47 degrees), Garden City, KS (47 degrees) and Oklahoma City, OK (56 degrees). However, farther to the south, Houston, TX tied a record high temperature with 95 degrees and New Orleans (Audubon Park), LA had a record tying 94 degrees.
WEATHER FOR THE START OF THE NEW WEEK -- The following highlights of the national weather have been extracted from the surface weather map for late Sunday night.
A cold front extended southwestward from near the Canadian Maritimes across the Middle Atlantic States to the Rio Grande Valley of southern Texas. Several weak low pressure centers were found along this frontal boundary that separated a warm and moist tropical air mass in the Southeast from a cooler and drier air mass across the central Rockies. Temperature contrasts across the front were impressive, as evening temperatures in northern and central Texas were in the 50s and 60s, while within 200 miles to the south, temperatures remained in the 80s across South Texas. Several areas of precipitation were found along the cold front, to include the Ohio Valley, the Appalachians, the lower Mississippi Valley and much of southern and eastern Texas. Some active thunderstorms were detected on Sunday evening.
By Monday morning one of the low pressure systems along the front is expected to intensify over the Lower Mississippi Valley. As this low develops and moves eastward to the Appalachians by evening, the portion of the cold front to the east of the low would become a warm front that should move northward across the Southeast. To the west of this intensifying low the cold front should begin sweeping southeastward. In the warm and humid region to the south and east of the surface low, thunderstorm activity should develop. This region bounded by the warm and cold fronts is often called the "warm sector" of the low pressure system. A slight risk of severe thunderstorms is expected in the warm sector on Monday from central Alabama eastward into the Carolinas. To the north, as much of 1 inch of rain is anticipated across the Virginias for the 24 hours commencing on Monday morning.
Much of the western half of the country was dominated by a sprawling ridge of high pressure that was centered in southern Wyoming as of Sunday night. This high pressure system was associated with the cold air mass that had drifted southward across the northern Rockies. A continued slow southeastward movement of the center of the high is anticipated on Monday. The cold air mass, coupled with clear skies and weak winds typical under high pressure, should enhance nighttime cooling. As a result, a variety of frost advisories and freeze warnings were in effect on Sunday night for portions of Upstate New York and Pennsylvania and in a band extending from Michigan southwestward across portions of Wisconsin, Iowa, Minnesota, Nebraska, Wyoming, Colorado, Kansas and Oklahoma to the Texas Panhandle. Additional advisories were posted for northern Arizona.
A high wind warning was in effect along the Guadalupe Mountains of southern New Mexico and west Texas as the pressure gradient between the high pressure system over the central Rockies and the trough of low pressure over the Texas Panhandle remains strong. The strong pressure gradient is indicated by the relatively tight packing of the isobars over the region on the Sunday night surface analyses.
TEMPERATURE EXTREMES ACROSS THE LOWER 48 -- On Sunday, the lowest temperature reported in the continental U.S. was 11 degrees at Potomac, MT, while Sunday's highest temperature was 102 degrees at Cotulla, TX.
ALASKAN WEATHER -- Most of Alaska was cloud covered on Sunday afternoon. Weak high pressure was found across the southwest, resulting in some breaks in the overcast in the vicinity of Bristol Bay and across St. Paul Island, producing a visible sun, which is a relatively rare event on that island. A weakening storm system situated over the North Pacific to the south of the Alaska Peninsula spread clouds and precipitation over south central and southeastern Alaska. An occluded front curved northeastward from the nearly stationary low pressure center located approximately 500 miles south of the Alaska Peninsula, reaching a point to the southeast of Kodiak Island, where a warm front extended across the Copper River Basin in southeastern Alaska into the southern Yukon Territory and a cold front stretched southward across the Gulf of Alaska. Rain fell at Northway, the Anchorage Bow and over the Panhandle, to include Juneau. Cordova had 1.97 inches of rain for the 24 hours ending at midday on Sunday. To the northwest, a weak low located over the Chukchi Peninsula of Eastern Siberia had a warm front that extended eastward across the Bering Strait and the Seward Peninsula. Snow fell at Kotzebue. A strong storm system located south of the western Aleutians represented the remnants of Typhoon Shanshan.
The lowest overnight temperature in Alaska as of Sunday was 9 degrees at Anaktuvuk Pass, and the midafternoon highest statewide temperature was 65 degrees at Klawok.
HAWAIIAN WEATHER -- Relatively pleasant weather with afternoon showers and light easterly winds were found across the islands on Sunday, courtesy of an elongated ridge of high pressure located relatively close to the northwest of the 50th State. This weather regime is expected for the first half of the upcoming week. Ocean swell generated from the storm system in the North Pacific is expected to propagate southeastward and reach the islands by Wednesday, producing surf with heights possibly exceeding 15 feet.
EYE ON THE TROPICS -- At the end of last week, Hurricane Isaac, the ninth named tropical cyclone (to include tropical storms and hurricanes) intensified quickly near the Cape Verde Islands and moved westward. During Sunday this system became a major hurricane as the system reached a Category 3 status on the Saffir-Simpson Hurricane Intensity Scale. As of late early Monday, Hurricane Isaac was moving to the west-northwest at 9 mph and it was located approximately 1470 miles east of the Lesser Antilles. Sustained near-surface winds were estimated to be 115 mph. Whether Isaac will affect the US Mainland or its territories later in the week remains uncertain.
With the demise of Tropical Depression Norman along the west coast of Mexico on Friday afternoon, the eastern North Pacific basin was relatively quiet over the weekend.
REPORTS FROM THE FIELD -- A special thanks is extended to Eleanor Vallier-Talbot, an NWS forecaster and LIT member from Charleston, SC who submitted a detailed report on Tropical Storm Helene that was used in the above weekend discussion. She noted that in Berkeley County to the north of Charleston, the radar-estimated precipitation totals between 18 and 23 September ranged between 10 to 15 inches of rain, from the passage of the remnants of former Hurricane Gordon and Tropical Storm Helene.
Bill Lee, LIT Leader, from Winifred, MT, reported a rapid change from summer to winter in central Montana, accompanied by freezing rain, snow and subfreezing temporaries on Thursday. Two inches of frozen slush covered the roads near Lewistown. He says this taste of winter is at least one month early, and contrasts with the blistering heat of this past summer that was accompanied by drought, wildfires and smoke.
Pat Ryan, a recent DataStreme participant from Anchorage, AK reported that on Thursday winds gusted to 84 mph. Relatively warm conditions were associated with these downslope Chinook winds that warmed as they descended from the mountains surrounding the Anchorage Bowl and the air compressed.
AN INVITATION -- If you would like more background information concerning how various temperature records are identified, please consult Monday's optional Supplemental Information .
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
Return to DataStreme Homepage
URL: datastreme:/learn/m_sum.html
Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.