Cold Front Brings Storms and Heavy Rain

The unseasonably cool air that is far from customary for July 4th in Southern Illinois has moved on, Sunday saw noticeable changes in the surface airmass. At the surface, southwest flow has taken back control and pushed dewpoints into the upper 60's Sunday afternoon with actual temperature readings in the low 80's. While southerly flow has taken control at the surface, aloft in the upper atmosphere, northwest flow is easily visible. Typically, upper level northwest flow in the summer months leads to an active pattern.

Winds are from the north/northwest across the Middle Mississippi Valley. Jetstreak visible upstream.

Over the next few days (Monday & Tuesday), a cold front will start to slowly sag south into parts of the Ohio River Valley. The front will move into the St. Louis area likely Monday evening. Ahead of the front, heat and humidity will build across Southern Illinois with surface based instability pushing 3000J/kg by Monday afternoon. While instability will be quite high, little to no lift and little in the way of wind in the atmosphere leaves uncertainty on the amount of storm activity. If storms develop Monday afternoon in Southern Illinois, it poses a threat for possibly large hail, and isolated damaging wind gusts. Highest likelihood of storm development looks to be near the Wabash River into Southern Indiana.

Sounding from Cape Girardeau late Monday shows high (CAPE) instability

Late Monday evening, a jetstreak (area of high winds) in the middle of the atmosphere will help fuel the development of an MCS (Mesoscale Convective System) across parts of southern Iowa and northern Missouri tracking it southeast along the stalled cold front in the Middle Mississippi Valley.

500mb jetstreak centered across N-Central IL steering MCS.

850mb (low level of the atmosphere) showing a 50knot wind barb west of St. Louis

While the MCS tracks near Southern Illinois, it will likely bring heavy rain and a lot of lightning in addition to a couple strong storms accompanying a Low Level Jet (LLJ). Precipitable water values in excess of 2" Tuesday morning is a good indicator that heavy rain will occur.

Preciptable water values early Tuesday morning

Behind the MCS, the front will likely start progressing south once again through Southern Illinois, pooling high amounts of instability and high precipitable water values ahead of it. Models have been trending towards a faster solution, possibly pushing the front to the Ohio River by Tuesday evening. Showers and storms are once again expected to fire along the front Tuesday evening, but frontal position will be the dependent factor. My personal opinion leads to storms firing farther south than some agencies are suggesting, though far Southern Illinois may see storm initiation. 

I personally am more concerned with the potential for heavy rains than I am with severe weather in Southern Illinois at this point, but we'll continue to monitor as new data becomes available. 

Starting Wednesday behind the front and continuing for the remainder of the week, dry conditions are expected across Southern Illinois.