Weekend rainfall: May not end up with that much

Labor Day weekend is fast approaching and what's left of Isaac may make an appearance for some across the Mississippi River Valley and Great Lakes.  Now Tropical Storm Isaac has ever so slowly been moving northwest over the past 24 to 36 hours and still remains in central Louisiana.  Significant flooding and severe weather have plagued the gulf states and will continue to do so through the afternoon.

Isaac is forecast to slowly lift north into Arkansas and Missouri by the beginning of the weekend and then shift east into central Illinois and Indiana by Sunday and Monday.  While we're hopeful to get some rainfall from this, even if it's a holiday weekend, it appears as if the heaviest of the rain band may end up developing south of northern Illinois.  As the remnants of Isaac begin to lift north there will also be an area of high pressure drifting south out of Canada pushing a weak cool front through Wisconsin.  This front may pass through northeast Illinois late Friday but behind it will be a cooler and drier air mass.  Thanks to an easterly wind this weekend the drier air may inhibit the northward extent of Isaac and keep it a little further south.  There isn't much in the upper levels of the atmosphere to help steer Isaac so this is also contributing to the slow movement.  This will also cause it to slightly weaken as the system travels further north.  Also, with the track of the low moving through central Illinois the greatest lift and therefore greatest rainfall potential usually will lie just to the north/northeast of the low. 

This is the three day QPF (rainfall forecast) from the Hydrometeorological Prediction Center running through Sunday morning.  The red and orange colors indicate rainfall amounts in excess of five inches.  Notice how the bulk of this precipitation is mainly south of I-80 with a fairly sharp decrease in totals across northern Illinois.  This is due to the cool front and dry air that will be in place across the upper Midwest.  I've kept the rain threat in the forecast for the weekend but there is also a possibility that we might not see much at all; especially if the low shifts a little further south.