Heavy rain & severe storms possible Sunday into Monday

While it has been a pretty masterful holiday weekend, it looks like it could end with a bang… literally.  Storms are expected to develop late Sunday and continue through early Monday that will likely produce very heavy rain, some gusty winds, and even some hail.  While severe weather is a possibility, the threat for blinding rains is probably going to be the most noticeable and widespread component to these storms.  So how do we know that atmosphere is primed for not only severe weather but also heavy rain??  Well, there are 2 scales we look at that will give us the best picture of the overall state of the atmosphere.  The first one is CAPE.  If you are a frequent reader, then you have seen this before, but CAPE is an acronym for Convective Available Potential Energy.  It’s a big long fancy word for the amount of instability (the force) in the atmosphere.  The higher the number, the stronger your storms can become.  For July, since there is so much moisture around, we can actually get severe weather at a much lower number on the scale then in other months.  While there is no set threshold to the amount of CAPE you need since every situation is different, typically this time of year, if you are over 1000, then you could probably get severe storms.  Check out the forecasted values for late Sunday afternoon…

I have outlined in black the areas that have over 1000 CAPE.  BUT, there are pockets inside this area with over 4000! 4 times the amount needed to produce severe weather.  So does that mean even more severe weather? Simply… yes it does.  The more CAPE you have, the stronger the severe weather can be, so you can get even bigger hail, even stronger winds, and even heavier rain.  So how do we know that there is a heavy rain potential then?  Well, a couple factors play into this, but we can sum most of them up by looking at the precipitable water of the atmosphere.  Yes, another big word.  Precipitable water is a term used to describe the amount of water that is in a single column of the atmosphere.  Think about it like this… say you have a towel.  Now imagine that towel is long enough to stretch from the ground all the way to the top of the troposphere… roughly fifty thousand feet.  Now imagine that this towel is only 1 air parcel wide.  EXTREMELY SLIM right? But also very tall.  Now imagine that you can wring out that very tall but very slim towel.  The precipitable water value is the amount of water that you would ring out of that towel at any given time.  So let’s say the precipitable water value is 1 inch.  Then, if you were to ring out that towel, you would end up with an inch of water.  So now, let’s view our forecast map…

This shows forecast precipitable water amounts for late Sunday night.  Check out our bullseye right over central Minnesota.  Our precipitable water is over 2 inches and as much as 2.5”.  That means for every slim and tall column of the atmosphere, you could get 2.5” of water.  To put that into perspective, that’s an ENORMOUS amount of water for something so incredibly tiny.  The state of Minnesota has NEVER had a precipitable water hit 3” before and is typically just above an inch this time of year.  So Sunday night, the atmosphere would have 2 to 3 times the typical amount of July moisture in it.  That’s a lot!!  Because of this, heavy rain looks likely in thunderstorms.  Check out the rainfall potential for the area through the afternoon Monday.  Remember that this is only to get an idea of what is possible and NOT A FORECAST.  Your totals can and will vary greatly.

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