Monday, August 29, 2016

More Heavy Rain Possible

Over the weekend multiple showers and storms brought heavy rain to the Stateline, mainly in Whiteside and Lee counties where we had Flash Flood Watches and Warnings. We aren't done with the heavy rain however, slow moving storms today will once again bring a threat for high rain totals and localized flash flooding.

While the atmosphere lacks distinct forcing, storms are still probable across the area because of a few factors. High pressure this morning is centered near the central Great Lakes, but will move east by mid morning. As this happens, return flow on the backside of the system will bring increasing humidity across the area.

Already modest dew points in the 60's will rise into the low 70's with southerly winds in place. With sunshine and these increasing dew points, showers and storms will then develop. Because there is not a lot of forcing or lift in the atmosphere it is harder to time out when and where the storms will develop. They will however be likely today across northern Illinois and northwest Indiana. The threat for severe storms is low but a few stronger storms can't be ruled out that will produce gusty winds. The threat we are tracking today is heavy rain producing storms and even localized flash flooding. In the low levels of the atmosphere there is a lot of moisture to work with, something referred to as PWAT values, which will be rising to 1.5" to 2". We like to use the analogy that if we think of the atmosphere as a sponge, and we were to squeeze out the available moisture or PWAT we could see up to 2" of rain fall.

Also in the low levels of the atmosphere the 'flow' or wind speed is rather slow, meaning the storms we see today will be slow movers. With these two factors, high rain totals and localized flash flooding is possible.

Storms will be diurnally driven, meaning with the heating of the sun storms will be able to be sustained, but once the sunsets the storms essentially lose the energy and will faded away. However, a couple isolated storms will linger into the overnight.

Another day with storms for Tuesday with an approaching cold front, this will bring scattered showers and storms in the area. A shortwave trough or small low pressure system in Canada will move eastward, but it's trailing cold front will sink south through the area tomorrow firing up scattered showers and storms. Once again the severe threat is fairly low, but a few stronger storms will bring gusty winds and heavy downpours.

Good news is, the weather turns quite Wednesday into the rest of the week.

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