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An Active Holiday Weekend Leading To A Nice Memorial Day

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In 2011 Memorial Day proved to be an active one with severe storms, this year the weekend looks more active than the holiday itself.  On Saturday the conditions were right for severe weather to occur, except there was one problem, a huge cap or lid on the atmosphere, keeping storms from really developing.  On Sunday we won’t have quite as much moisture in place, with cape values around 2000 opposed to 4000, j/kg, but we’ll be more likely to break the cap and get severe storms going.  Initially the storms will be scattered or isolated in nature before transitioning over to a flash flooding event by night time.
Speaking of flash flooding this May has been a wet one, with many locations already meeting their average rainfall for the month.  The images below show just that. Notice on the graph below for Sioux Falls how rainfall amounts have varied significantly over the years in May, that is probably due to the fact that thunderstorms tend to be isolated or scattered in nature. 

***All time 24 hour rain event in SF....this one was recorded on the southwest side of town
*** second line shows 24 hour rain at the airport for the month of May

We typically get flash flooding when storms essentially continue to initiate over the same area over and over again.  You can also get flash flooding just from a very slow moving storm that has a lot of moisture involved.  Lately we’ve been getting flash flooding with a stalled front, which means it pretty much just sits there and allows moisture to continue to surge up from the south, giving us several rounds of rain over a short period of time.

As a matter of fact, come Sunday night once again a cold front will be draped across the I-29 corridor with heavy rain expected both ahead of the front and especially behind it.  The front will be out of our area by Monday afternoon.  In the meantime, we could see anywhere between 1 to 2 inches in northwest Iowa, with heavier amounts locally, the image below is one of our models.

Out ahead of the front that’s where we could see the potential for large hail, gusty winds, and a few tornadoes.  Typically it’s during the beginning stages of a severe weather outbreak where you get your more isolated activity.  In order to get large hail you need large updrafts (or the belly of the storm).  The smaller the updraft the smaller the hail will be.  Along those same lines, in order to get a tall updraft you need a lot of food, to feed the storm…or in this case heat and moisture.  On Saturday we had CAPE values up to 4000 j/kg, which is plenty of food to keep our updrafts going, only problem was we had a strong cap in place.  A cap acts like a lid, preventing thunderstorms from developing.  A cap is found with a warm layer of air overtop a cooler layer.  Which is kind of like fog, where the moisture gets trapped at the surface.  If you get enough heating during the day sometimes you can break the cap and get storms to fire up.  However, that was not the case on Saturday as many of us were socked in with overcast skies, especially along and north of I-90. 

Also interesting to note, you need a lot of heat and humidity to get tall updrafts, and a ridge a loft can be a great instrument to do so…the image below shows just that. 

By Sunday an area of low pressure with an attached cold front will slide eastward, creating a forcing mechanism to get storms going.  While the warm front was key in locating storms on Saturday, the cold front or dry line (which is found just ahead of the cold front) will be the focal point on Sunday.  Even though we won’t have quite as much instability as Saturday to feed the storms, 1500-2000 j/kg will still be sufficient to keep storms going. 

Good news though heading into your holiday, Memorial Day will remain dry and far less humid.  Only thing is it will be quite windy with sustained winds anywhere between 20 and 40 miles per hour.  Once we get past Sunday night, our next chance of rain doesn’t come into the forecast until Wednesday.
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