AccuWeather.com

By Jim Andrews, Senior Meteorologist

Sep 5, 2010; 9:33 AM ET

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Many people across the United States have outdoor events planned for Labor Day, the unofficial end to summer. The nation's midsection is at greatest risk for the weather to spoil those activities.

Vigorous low pressure trailing a cold front through the nation's midsection will spark a few showers and thunderstorms over the Upper Midwest to central Plains this Labor Day. Some thunderstorms threaten to turn severe during the afternoon in an area centered upon southern and eastern Iowa, including Cedar Rapids.

A few showers will spill well eastward from this low, perhaps putting a damper on outdoor activities from the upper Great Lakes to northern Maine.

A steadier rain, locally heavy, will wet much of the Dakotas. On top of this, temperatures across these two states will be generally held to the 60s.

Cooling will sweep into western Kansas and eastern Colorado behind the cold front, but 90-degree heat spread over the southern Plains will reach as far northeast as about Kansas.

The expanse of cool air will also reach across the northern and central Rockies to the Pacific Northwest, holding temperatures well below highs that are typical for early September.

California and Arizona will feel little or no cooling. Instead, bright sunshine will abound. Southern California will have a cooling onshore flow and also some low clouds.

Eastern high pressure will begin a warm-up from the Ohio Valley and southern Great Lakes to the Eastern Seaboard. It will start cool, and humidity will stay fairly low, but temperatures will recover enough to benefit beachgoers.

Low humidity and strong sunshine will also dominate most of the South with low humidity offsetting the above-normal daytime temperatures.

Farther south, steamy air will remain in place across the Florida Peninsula to the western Gulf Coast, providing fuel for scattered, drenching thunderstorms.

Low pressure having tropical characteristics near eastern Mexico, being watched for further development, could unleash numerous drenching storms over South Texas.

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