Severe thunderstorms and torrential rain generally develop within the warm sector, just ahead of an advancing cold front. Energized by a potent zone of low pressure and a dip in the jet stream, the cold air knifes beneath the warm, humid air, producing lift, heavy precipitation and potential tornadoes.
But storms can also develop as warm fronts move into cooler air along the back edge of atmospheric troughs. Over the past few days, a stationary front has curved through the Heartland, separating intense heat in the Southern Plains from relatively cool air across the Upper Midwest and central Mississippi Valley. Tied to a low over eastern Kansas, this front has begun to nudge to the northeast; in concert, counterclockwise winds around the low have been sweeping Gulf moisture across the front, producing waves of thunderstorms through Iowa, Missouri and southern Illinois.
While these warm front storms are not as intense as those along cold front squall lines, they may yield significant amounts of precipitation; in addition, since backside fronts often progress at a snail's pace, the storms redevelop over the same area and flooding ensues. Such has been the case this week as southeastern Iowa and south-central Illinois have been deluged with rain.
