Monday Brings Chance for Heavy Rain and Strong Storms

ANALYSIS: Over the last few days, persistent cool, damp weather has plagued Southern Illinois on the northern side of a stationary front. While surface winds have generally been from the northeast, upper-level winds have been predominately from the southwest creating overrunning rainfall and resulting in a considerable amount of clouds. A strengthening storm system will move through the Rocky Mountains into the Plains Sunday, pushing the stationary front and associated energy to the east. Ahead of the system in the Plains, dominate south flow will return as we await the arrival of the system Monday.

As the storm system arrives Monday morning drawing a warm front north, storms are likely to develop, but will be below severe limitations. As the warm front lifts north, surface dewpoints, temperatures, and winds will begin to go up as Southern Illinois becomes planted in the "warm sector". By early afternoon, we'll be turning our attention to showers and storms that have developed along and ahead of the cold front as they begin to push into western parts of Southeast Missouri. Models still differ in timing, but I'm currently leaning towards a mid-late afternoon arrival in Southern Illinois. As storms move in, models are indicating a very impressive wind field, but only limited amounts of instability. These situations are common in the lower Ohio Valley throughout the Fall season and have a history of producing the potential for violent weather.

HiRes NAM 925mb 00Z Tuesday (Monday evening) -- Strong low-level winds can be seen over Southern Illinois as warm air gets wrapped counter-clockwise around the parent low.

HiRes NAM Skew-T Monday afternoon showing the wind field (along the right side) strengthening from the south/southwest and a stiff southeast surface wind. CAPE 1000J/kg+ still a few hours ahead of the front.

HiRes NAM Radar Simulation 21Z Monday (4PM). A line of strong storms can be seen entering Southeast Missouri.  Disclaimer: Radar Simulations are far from exact and simply one model's solution.

SUMMARY: My current thinking at the time of this writing: A "squall line" is likely to push through during the mid to late afternoon hours in Southern Illinois. I feel the biggest threat is for embedded line segments called bow echoes creating damaging straight-line winds. With the amount of wind, and the directional shear involved a few tornadoes are possible, especially along any bookends of bowing segments. In addition to severe weather, very heavy rain is likely and may cause some localized flooding.