The Nation's Weather 09.21.09

By WEATHER UNDERGROUND (AP)

There was forecast to be several main weather features that would drive weather activity across the nation Monday.

In the East, a low pressure system would lift northeastward from the Ohio Valley into the Upper Great Lakes. The system was expected to continue pulling significant moisture across much of the Eastern U.S., creating showers and thunderstorms from the Upper Great Lakes through the Southeast. Meanwhile, scattered showers and thunderstorms, gusty winds, and locally heavy rainfall would persist across southern Florida. Fair and dry weather conditions were expected across much of the eastern Mid-Atlantic and the Northeast due to an exiting ridge of high pressure.

In the Central U.S., a deepening trough of low pressure and it's associated frontal boundary would kick up active weather across the Plains. Rich gulf moisture would mingle with the system to produce a swath of moderate to heavy precipitation and strong to severe thunderstorms throughout areas of the Central and Southern Plains. Cooler air from the North would also accompany the trough, yielding to lower daytime temperatures across the region.

In the West, an amplifying ridge of high pressure was forecast to instigate a heat wave in California and the Pacific Northwest. Forecasters said hot daytime temperatures and warm evening weather may pose a threat to young children, the elderly and other heat-sensitive groups. Low relative humidity values coupled with high temperatures and offshore flow would increase the fire weather danger in the Pacific Northwest and the northern and southwestern regions of California.

Temperatures in the Lower 48 states Sunday ranged from a low of 26 degrees at Whitefield, N.H., to a high of 110 degrees at Death Valley, Calif.