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Snow May... No Way



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No you aren't dreaming, a rain/snow mix will be possible on the first day of May.  This isn’t the first time this has happened and certainly won’t be the last.  But before you pack all of your belongings in a U-Haul and permanently head south, chances for any accumulation are exceedingly slim… especially now that the ground has warmed substantially since even a week ago.  But that does tell you how cool the temperatures will be heading into the middle of the week with highs likely stuck in the 40’s for some of us through Friday.  I’ll explain why that is and what we are expecting for rain (and snow) in the blog below.

First off, April and May are typically amongst the wettest months of the year so rain is no stranger to South Dakota this time of year.  So the only thing that will be only semi unusual will be the fairly chilly temperatures to start the first week of May.  Lets start with the rain first.  Here is a forecast for rainfall through Thursday evening from the Weather Prediction Center (WPC), formally known as the HPC.



They have the southeast getting the bulk of the rain, with even heavier amounts from Omaha to Albert Lea, Minnesota.  This is much needed water so I doubt many will be complaining.


However, the potential for snow IS something to complain about.  As cold air wraps into this storm from the north, temperatures will really begin to fall.  Remember though, in order for snow to reach the ground, every layer of the atmosphere other than the few hundred feet nearest to the ground MUST be below freezing.  To check that variable, we usually look at the 850mb temperatures (about 5000 feet  above the surface).  This is typically where a “warm” layer will sit in the atmosphere and prevent snow from reaching the ground.  Below is a forecast for the 850mb temperatures Wednesday morning.



Take note that the entire area is below freezing… the 32° line is in red.  Every point north of that line is below freezing at 5000 feet.  So the key to get snow is how cold can the surface temperature get.  Well lets look at forecast surface temperatures for Wednesday morning.



According to this model, surface temperatures appear to be in the 30’s which can be cold enough for snow, but not always.  This is why we say that there is a possibility for a rain/snow mix because with temperatures this close, it really can go either way.


Other forecasting models though show a little bit different picture.  The image below is the snowfall accumulation forecast for one computer model.



Just seeing color on this map is not a great sight going into May because it shows the potential for accumulation.  Right now, it looks like the best chances for at least a little snow appear to be in northern Nebraska and parts of Iowa.


Here is what the National Weather Service has for their current precipitation forecast through Wednesday night.



So why the big pattern shift??  Well, it has to do with the combination of our next two storm systems.  The first of these storms rolls through North Dakota Monday night which could bring a few isolated storms to our north.  This opens the door, so to speak, for cold air to sink southward.   With just this first system, temperatures would cool temporarily and then jump right back above average.  But it’s the combination of this first storm and a much stronger wave moving through right after it that creates the very chilly and unsettled weather pattern.  Not to mention, this second storm will likely become cutoff from the steering currents in the atmosphere.  This can create a stagnant pattern and allow the low to meander where ever it wants to go.  This steering current is a lot like a car on the interstate.  Under normal conditions, there are no off ramps so the storm continues in one general direction that can be predicted to some degree of certainty.   Now picture driving down that same road, but now this road is expanded to fill the U.S.  So the car can change direction or speed at any time it wants to.  This is what a cutoff low is like, making it nearly impossible to predict its overall pattern and where it’s going to end up.  An illustration of this can be seen below along with 850mb temperatures.









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