SEVERE WEATHER DEVELOPS IN MIDWEST -- Severe thunderstorms along an active cold front produced damaging winds and spawned several tornadoes across the Midwest on Wednesday evening. The cold front trailing from a low pressure system near Hudson Bay stretched from Lake Ontario southwest across the Ohio and lower Mississippi Valleys into the southern Plains.
An unconfirmed tornado on Wednesday night appears to be responsible for one fatality and more than 100 injuries, along with substantial damage in Xenia, OH, the site of major damage sustained during the Superoutbreak of tornadoes in April 1974. Earlier, several possible tornadoes were reported in south central Indiana. Wind damage from thunderstorms was reported from western Ohio across Indiana, Kentucky, Missouri, Tennessee and Mississippi.
The thunderstorms were fueled by the flow of warm and humid air northward into the Ohio Valley and eastern Great Lakes on southerly winds. Afternoon high temperatures across the Lower Mississippi Valley and eastern Great Lakes ranged between 10 to 15 degrees above the local average high temperatures for mid September. The eastward advancing cold front provided the trigger to cause rapid upward growth of the clouds that comprise the thunderstorms. As of late Wednesday night, thunderstorms and rainshowers continued along the cold front from southern Ontario southward to Tennessee. Some additional showers were found along a weak troughline or secondary cold front moving across the western Great Lakes. These showers were triggered as cold air moved across warm ground.
By Thursday morning the northern portions of the cold front are forecast to have reached the New England and Middle Atlantic Coasts, with the southern sections of the front reaching the Tennessee Valley. A slight risk of severe thunderstorm continues through Thursday morning for those areas ahead of the cold front, extending from western New York State to eastern Kentucky. The southern portion of the front is expected to become stationary across the Gulf Coast States by evening while the northern section will be well offshore.
COOL WEATHER ACROSS THE PLAINS -- The cold front moving across the eastern Lakes and Ohio Valley was the leading edge of a cool and dry air mass that moved into the Plains. This air mass accompanied the large high pressure system that was centered across Kansas on Wednesday night. Afternoon high temperatures across the northern Plains were approximately 10 to 15 degrees below the long term average highs for the date.
This high is expected to drift southeastward into the mid-Mississippi Valley by Thursday morning and then along the Ohio Valley by evening. The low humidity and light wind in the cool air mass will allow overnight temperatures to fall as a result of strong radiational cooling. As a result, frost advisories were posted for central North Dakota for Wednesday night and Thursday morning.
SNOW IN THE ROCKIES -- A weak low pressure center moved into the Pacific Northwest on Wednesday night. The storm system is expected to be located over northeastern Wyoming by Thursday morning and to the southeast of the Black Hills of South Dakota by evening. With cold air and moisture available, as much as a foot of snow could fall in the mountainous areas of Montana and Wyoming, with several inches possible in the valleys. Windy conditions are also anticipated, causing blowing snow. Thus, a variety of winter storm watches and warnings were issued for much of Montana east of the Continental Divide, plus a portion of Wyoming.
MORE RECORDS ALONG THE WEST COAST -- Abnormally warm conditions continued across the Pacific Coast on Wednesday. Record high temperatures were either tied or exceeded in California at Sacramento (102 degrees), Bakersfield (101 degrees), in Oregon at Brookings and in Arizona at Flagstaff (83 degrees)
TEMPERATURE EXTREMES IN THE CONTINENTAL U.S. -- Wednesday morning's lowest temperature was 23 degrees at West Yellowstone, MT, while the Wednesday afternoon highest temperature was 113 degrees at Death Valley, CA.
ALASKAN WEATHER -- Cloudy skies across western and southern Alaska on Wednesday afternoon were the result of a large storm system in the Bering Sea. This storm consisted of a low pressure center located to the west of the Pribilofs and an occluded front that curved eastward and then southward to a point of occlusion in the eastern Bering Sea. A warm front extended eastward from this point across the Alaska Peninsula and into the northern Gulf of Alaska, while a cold front continued southward across the eastern Aleutians into the North Pacific Ocean. Light rain was reported at Nome, Kodiak and St. Paul, while a rainshower was reported at Sand Point. Gusty winds were also associated with the storm system, with southerly wind gusts to near 40 mph reported at Healy near Denali and on St. Paul Island. A ridge of high pressure that extended eastward into western Canada dominated the weather across interior Alaska and the Panhandle. Relatively cloud-free skies were found across the interior.
The lowest temperature in Alaska on Wednesday morning was 12 degrees at Arctic Village. The highest statewide temperature as of Wednesday afternoon was 64 degrees at Klawock.
HAWAIIAN WEATHER -- A cold front associated with a storm system moving eastward across the North Pacific was approaching Kauai and Oahu on Wednesday afternoon. An elongated ridge of high pressure extended from a high pressure center in the eastern North Pacific southwestward just to the north of the islands. As a result, the winds across the 50th State had weakened and turned from an easterly to a more southeasterly direction, especially over the northern islands. Most of the islands experienced a few clouds, except for the higher elevations along the windward mountain slopes.
The moored buoy northwest of Kauai continued to monitor northwest ocean swell generated by the low pressure system. This swell was responsible for the 6 to 10 foot swell on the northwest facing shores. As a result, high surf advisories were continued for the northwest facing shores of the islands through at least Thursday.
EYE ON THE TROPICS -- Tropical Depression 12 continued to move slowly across the Gulf of Mexico on Wednesday. This system did not intensify and become a tropical storm as expected. As of late Wednesday night, Tropical Depression 12 had sustained winds of 30 mph and moving to the northwest at 16 mph approximately 420 miles south of Pensacola, FL.
Another cloud cluster in the Gulf was moving toward the northeast and was expected to contribute much needed rains to the Gulf Coast states.
In the eastern North Pacific, Tropical Depression Norman was moving northwest at 5 mph at approximately 45 miles east-southeast of Manzanillo, Mexico. Sustained near-surface winds were 35 mph. Heavy rains were found in Mexico.
REPORTS FROM THE FIELD --Marsha Rich, LIT Leader from New Hampshire, reported on a trip that she made to Mount Washington last weekend. Temperatures at the Observatory on top the 6288 foot mountain remained at 29 degrees. During much of the weekend, the skies were cloudy, with freezing rain and occasional ice pellets. On Saturday night, a peak gust of 81 mph was recorded. Icy road conditions delayed her descent on Sunday.
Renee Miller, LIT Member from central South Dakota, reported about the noticeable change in the weather on the High Plains. She noted that last weekend she experienced 100 degree temperatures, but the forecast for this upcoming weekend is for a chance of snow mixed with the rain. She did mention that the cooler weather that arrived on Tuesday with the passage of the cold front across the Plains was welcomed. The local fire department had to battle grass fires that were ignited by lightning and burned around 25,000 acres of grass besides some farm buildings. Rain would be welcome.
Simply watching clouds grow and change during any time interval brings home the idea that weather is not just limited to conditions measured at the earth's surface. To fully understand weather systems we must make measurements of various weather elements up through the atmosphere as well as over widely separated distances at the surface. Balloons carrying instruments and a radio transmitter are released twice each day (00Z and 12Z) at about 70 stations across the continental US. These instruments measure the atmospheric temperature, humidity, and pressure as the balloon ascends, until finally bursting at an altitude of about 20 kilometers or higher. The balloons are also tracked by ground based direction finding equipment to infer winds at various levels. These instruments radio their observations of the vertical patterns ("soundings") back to the launch site as they travel, hence the name radiosondes, or when tracked for winds, rawinsondes.
On the local scale, atmospheric conditions aloft may determine if the air were "unstable", resulting in vertical motions that could lead to thunderstorms or even tornadoes, or "stable", a condition that would suppress vertical cloud formation. On the larger scale, the moisture supply for major storm systems can be judged, or circulation patterns for continued fair weather or even forecasts of hurricane movements can be made. To fully know the weather now and for the near future, the total atmospheric picture must be seen. And long-range forecasts can only be attempted with knowledge of the complete three-dimensional structure of the global atmosphere.
For an explanation of how the sounding information retrieved from a radiosonde or rawinsonde is displayed on a specially prepared chart, call up the Thursday's optional Supplemental Information .
To be submitted on the lines for Thursday on the Study Guide, Part B, Applications', Week 2 Chapter Progress Response Form, under section B. Daily Summary.
From the files of the Aviation Weather Center, Kansas City, MO and Intellicast
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2000, The American Meteorological Society.