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Flash Flood Watch
9/20/2016 10:05 AM

Wisconsin is bracing for another excessive, heavy rain event. 1-3″ of widespread rainfall with locally higher amounts expected in western Wisconsin by Friday (lesser amounts east). The National Weather Service in La Crosse has already issued a Flash Flood Watch. This forecast may turn out understated for some unlucky few. There is the possibility this becomes a major heavy flooding rain event with 6″+ inches of rainfall for localized areas on par with what we experienced in summer.

Flood Watch until 7PM Thursday

Flood Watch until 7PM Thursday

Thunderstorms and rain will start early Wednesday morning. There may be a lull Wednesday morning before more t-storms and rain ramp up Wednesday afternoon into Thursday. By Thursday rain chances increase south and east before diminishing on Friday. The rain may come in bursts or occur continuously.

MAJOR EVENT?

It also remains unclear where precisely the heaviest rain will fall and whether the flooding threat will translate east? Most models indicate that along and south of interstate I-90 will be most at risk. There is a natural tendency towards southward relocation of heavy rain bursts over time. My concern is that if the heavy rain axis does not relocate south Thursday rain amounts could be much higher than advertised turning this into a major event. The GFS shown below is the best case scenario showing the non-coincident heavy rain corridors. Even this model run is consistent with location and configuration of the drivers suggesting there may not be enough relocation. We will see.

12Z GFS showing two separate axis of heavy rain.

12Z GFS showing two separate axis of heavy rain by Friday.

BACKSTORY

There are locations especially near the Mississippi River in southwest and far northwest Wisconsin are already at record levels of yearly precipitation with three full months left to go! Over the last 90 days southwest Wisconsin is 6 to 16 inches above average and expected to worsen than trend this week. Similar precipitation bonuses in the far northwest from July. Right now I don’t see any way this works out positively for Wisconsin.

AHPS estimated rainfall showing massive surplus across southwest Wisconsin.

AHPS estimated rainfall showing massive surplus across southwest Wisconsin.

 

The post Flash Flood Watch appeared first on Wisconsin Weather.