Weather Underground Forecast for Monday, April 30, 2012. Unsettled weather will continue from parts of the Southern and Central Plains into the Upper Great Lakes on Monday as low pressure over the Mid-Mississippi Valley lifts northeastward toward the Great Lakes. Moisture from the Gulf of Mexico will interact with an associated frontal disturbance lifting through the Southern Plains and an associated warm front lifting through the Ohio Valley, producing showers, periods of heavy rain, and thunderstorms throughout the day. Thunderstorms in portions of the Southern and Central Plains, as well as areas of the Mid-Mississippi Valley and Ohio Valley may turn severe through the evening. While large hail and damaging wind events are the main concerns with severe storms in these regions, a few tornadoes may also form in the Central and Southern Plains. In the Upper Midwest, low pressure along the Northern Plains-Canada border will kick up more rain showers across the Northern Plains as it lifts into Canada on Monday. In the Southeast, moist, easterly flow associated with a trough of low pressure over the western Caribbean Sea and the Florida Straits will continue across southern Florida. This will translate into light to moderate showers and chances of thunderstorms in the southern tip of Florida through evening. In the West, a frontal system in the Pacific will approach the Pacific Northwest coast, bringing rain to the Pacific Northwest and chances of rain and high elevation snow to the Northern Intermountain West. Temperatures in the Lower 48 states Sunday have ranged from a morning low of 8 degrees at Mt. Washington, N.H. to a high of 93 degrees at Swainsboro, Ga.