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[September 21, 2010] (AP) Showers and thunderstorms were expected to move into the upper Midwest and Great Lakes on Tuesday.

A low pressure system was forecast to strengthen as it moved off the northern Rockies and into the Plains. The system would start taking a turn to the north and is expected to move through central Canada.

(Click on map for larger image.)

Flow around this system was expected to create a warm front that would lead the system to the east, while a cold front would follow close behind, tracking through the upper Midwest and upper Mississippi River Valley. The system would obtain energy by pulling moisture in from the Gulf of Mexico. Scattered showers and thunderstorms were expected to develop along the cold front, while widespread rain showers would develop over the Great Lakes and Northeast. Cool temperatures were expected to follow this cold front, with the northern Plains only reaching into the 60s. The Midwest and Northeast would remain in the 70s on Tuesday.

To the South, warm and humid conditions moving in from the Gulf of Mexico would allow for more storms to develop over eastern Texas and the Gulf states. Highs in the 90s were forecast to persist with heat indexes near 100 degrees.

Out West, high pressure building over the Rockies would bring more hazardous fire weather conditions. High pressure would allow for warm and dry surface conditions with periods of strong winds. Thus, fire advisories would remain in effect across the central Rockies and Great Basin.

Temperatures in the Lower 48 states Monday ranged from a low of 23 degrees at Mammoth Lakes, Calif., to a high of 111 degrees at Ocotillo Wells, Calif.

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[Associated Press article from Weather Underground]