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[September 22, 2009] (AP) An active system was forecast to continue tracking over the central U.S. and into the eastern half of the country Tuesday.

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A strong low pressure system over the Plains was expected to pull warm and moist air in from the Gulf of Mexico and over the Mississippi River Valley. At the same time, flow along the back side of the system would pull cool air in from Canada and create a strong cold front that would stretch from the Great Lakes and into the southern Plains, moving eastward over the Mississippi throughout the day.

These moist conditions, triggered by the cold front, would allow for scattered showers and thunderstorms to pop up, with the strongest storms and heaviest rainfall remaining in the southern Plains and Gulf states. Rainfall amounts would remain less than a half of an inch in the North, with up to 2 inches likely in the South. As the front passed, strong winds were expected with gusts up to 30 mph. Wet weather would cover most of the eastern U.S. with light and scattered showers stretching clear to the East Coast. Rainfall amounts along the coast were expected to be less than a quarter of an inch. Highs would remain in the 70s over the Great Lakes region, while the South would reach into the 80s again Tuesday.

Meanwhile in the West, a ridge of high pressure building over the Rockies and into the Plains would allow for cold air to pour in from western Canada. This would allow for overnight lows to dip into the 30s; thus, many regions of the Rockies have been places under frost and freeze warnings. Along the West Coast, forecasters said to expect another taste of hot and dry conditions Tuesday. Heat advisories and fires would threaten the region as highs approach 100 in California and reach into the 90s in the Pacific Northwest.

Temperatures in the Lower 48 states Monday ranged from a low of 17 degrees at Stanley, Idaho, to a high of 108 degrees at Palm Springs, Calif.

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[Associated Press article from Weather Underground]