National Weather Service of USA has issued warning that severe thunderstorms are expected todays evening and overnight from the lower Mississippi Valley to the Tennessee Valley.Heavy rain and some flash flooding is expected for portions of the Ohio and Tennessee Valleys through tonight. Drop in temperatures Much colder temperatures will be dropping south across the Plains and Midwest through the middle of the week. A frontal system ejecting out across the central Plains will advance through the lower Mississippi and Tennessee Valleys this evening and the overnight period. Increasing southerly flow ahead of the front will draw very moist and increasingly unstable air north up from the Gulf Coast region, which will set the stage for widespread shower and thunderstorm activity. Tornado Warning In fact, severe thunderstorms capable of producing damaging winds and tornadoes are expected across much of the lower Mississippi and Tennessee Valley region through tonight, and as a result the Storm Prediction Center continues to highlight an Enhanced Risk area of severe weather which includes areas of southeast Arkansas, northern Mississippi, northwest Alabama and much of western and central Tennessee. Risk area of severe weather A Slight Risk area of severe weather then expands outward to include the remainder of the lower Mississippi and Tennessee Valley region through tonight. In addition to severe thunderstorms, there is the threat for heavy rains and flash flooding, with the Weather Prediction Center highlighting a Slight Risk for excessive rainfall across portions of the Tennessee and Ohio Valleys through tonight. The front that will be driving the threat of heavy rainfall and severe weather will continue to move east, with showers and storms moving from the Midwest into the eastern U.S. on Tuesday. Potential for some storms to become strong to severe Again, there is the potential for some storms to become strong to severe, with the Storm Prediction Center indicating a Slight Risk for severe weather for a portion of the Southeast northward into the Mid-Atlantic region going through Tuesday evening. The cold front will move offshore most of the East Coast Tuesday night, however the same boundary will be slowing down and at least temporarily stalling out across portions of the Gulf Coast states going into Wednesday which will maintain the threat for showers and thunderstorms here. Rain and snow Back to the west, some rain and snow will accompany a frontal system advancing east across the upper Midwest through tonight and early Tuesday. Behind the system, much colder air will spill southeast from southern Canada and down across the northern and central Plains through the middle of the week, resulting in some well-below normal daytime highs east of the northern Rockies and across especially the Dakotas for Tuesday and Wednesday. Increase in periods of snow accumulation As the front banks up along the eastern slopes of the northern Rockies, this will encourage periods of some accumulating snow, with some locally heavy accumulations possible from the ranges of northern Idaho and western Montana to western Wyoming. Dry conditions will continue across much California, the Southwest and the Great Basin. Low daytime relative humidity, along with windy conditions are expected to elevate fire weather conditions for portions of the northern Sacramento Valley in California through Tuesday.