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Weather Underground Forecast for Thursday, August 21, 2014
An area of low pressure will begin to weaken over the eastern third of the country on Thursday, while a cold front will extend across the northern tier of the country.
A cold front is forecast to extend across the upper Intermountain West and the northern Plains on Thursday. As this system interacts with warm, muggy air over the region, strong thunderstorms will develop across several states. Heavy rain associated with these thunderstorms will bring a chance of flash flooding to northeast Montana, western North Dakota and northwest South Dakota. Rain and thunderstorms will also develop over the upper Mississippi Valley and the eastern Great Lakes. Just to the south, an abundance of monsoonal moisture will initiate isolated showers and thunderstorms across the Intermountain West and the Desert Southwest. A ridge of high pressure over the eastern Pacific will keep conditions mostly clear across the West Coast.
Meanwhile, a low pressure system will move eastward over the southern Great Lakes, the Ohio Valley and the central Appalachians. This system will usher heavy rain and thunderstorms across the upper Midwest, the Ohio Valley, the Tennessee Valley, the Mid-Atlantic and southern New England. There will be a threat of flash flooding over several states, including southern Michigan, northeast Indiana, Ohio, West Virginia and southwest Pennsylvania. An onshore flow from the Gulf of Mexico will trigger thunderstorm activity over the Gulf Coast and the Southeast on Thursday. .