Posted at: 05/14/2013 11:38 PM
Updated at: 06/10/2013 8:57 PM

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Record Breaking Heat Recap

May 2013 will be one never forgotten.  From Record May snow, cold and heat all in a span of 12 days.  We've seen record low high temperatures set in Rochester this month with 33° on the 2nd/3rd and record highs as well with 97 today.

Highs ranged from 95° to 102° in most places across SE Minnesota and N Iowa.

Here's a complete list of local temperature readings from the National Weather Service in La Crosse, WI service area.

Records, smashed.  The picture at right shows a sampling of the records set today (white) and old records (gold).  For some, the warmest temperature ever recorded in the month of May.  For others, the earliest we have ever seen temperatures so warm.  In Rochester, 97° observed today is the earliest instance in the year we've ever seen that reading, besting the old date, May 22nd, 1922.  For Albert Lea, 102° was the earliest seeing triple digit heat with the previous record being on May 31st, 1936.

So why did it get so warm?  The two biggest reasons are Very Little Moisture and Wind.

It was an ideal setup really with the local area on the warm side of warm front.  Southwesterly winds gusting to 35 mph allowed for the atmosphere to thoroughly mix.  Temperatures above the surface of the earth ware unseasonably warm and the winds mixing up to levels 7,000 to 8,000 feet, we could tap into that warmth and bring it to the surface.  Also with the wind out of the southwest, the airmass came over the Rocky Mountains and as it descended down the lee side, temperatures also could warm through compressional warming.  Basically the air descending is squeezed because of increasing pressure and leads to higher temperatures.  That airmass then moved our way.

In addition, the lack of moisture.  Dewpoints were in the 40s for most of the day, which is very low with such extreme heat.  This left Relative Humidity, at right, in the teens and 20% at best.  Why so low?  Well, we weren't tapping into the Gulf of Mexico (our main moisture source in the Upper Midwest) and also, lack of latent heat (which is water) from transpiration of green vegetation like crops and trees, since they are still in their infancy in the growing stage.

And this helped the air warm how?  A drier airmass transfers heat more quickly than a moisture laden airmass does.  Think of boiling water on the stove.  It takes a few minutes to get the water boiling but the air around the burner is warm in seconds.  The same principle applies.  Not convinced?  Take a look at morning lows compared to afternoon highs at left.  A 43 to 53° swing, for that very reason!

Commonly in the summer you hear the term Heat Index used, which takes into account the moisture or mugginess of the air to give a feels like temperature.  Usually this number is higher than the actual temperature making conditions even more uncomfortable.  However today, there was so little moisture and a significant breeze, heat indices were actually about 5° cooler than the actual air temperature.  So it actually felt better than it really was! 

I'm sure there's more but that's all for now.  It's hard to believe what we've experienced.

-Storm Tracker 6 Chief Meteorologist
Chris Kuball