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Flooding Rain – And Sleet/Snow/Freezing Issues

14 Jan 2013, 11:13 am

Freezing Rain And A “Sleety” Start In Texas

 

Sleet In Fort Worth
Sleet In Fort Worth

 

Pretty neat sleet pellets in that shot above!  I’m sure residents of Fort Worth, Texas, probably have mixed reactions to seeing scenes like that either via photography or in real life!

A lot of questions have been posed lately, due to the influx of sleet and freezing rain reports in regions that aren’t too commonly used to seeing those types of scenarios.  Why are we seeing sleet?  How does it form?  What is freezing rain, and how is that different than sleet?

Well, for the next couple of paragraphs we’ll attempt to answer those questions!

Let’s start with sleet.  To fully forecast sleet vs. rain vs. freezing rain, you need to have an idea of the atmospheric conditions – in terms of temperatures.

 

Flooding In Baton Rouge
Flooding In Baton Rouge

 

The photo above is from Baton Rouge, LA.  Flash flooding took place in that region, and that is where we’ll start our journey – explaining precipitation types.  In the mid-latitudes (basically everywhere in the United States, besides Florida) most of our precipitation starts out as snowfall.  Ice crystals form in the clouds, and you end up with snow as the starting point.  Pure, non-frozen, rainfall will come as a snowfall in the beginning, and then melt fairly quickly and fall to the surface as water droplets.

Things are different when you get sleet.  You’re talking about a higher-elevation warm sector in the atmosphere.  It is warm enough to melt the snowfall and turn it into liquid, and then that liquid drop falls into a deep enough cold sector to re-freeze the droplet… but this time it falls as a sleet pellet.

 

Sleet Formation Explainer
Sleet Formation Explainer

 

The process is similar for freezing rain.  Take a look at this vertical sounding of the atmosphere:

 

Atmospheric Sounding Near Jonesboro, AR
Atmospheric Sounding Near Jonesboro, AR

Notice the area that we highlighted.  That is the region that is just barely above freezing.  If you know how to interpret vertical soundings (measurements of temperature/dewpoint/wind speed & direction with height) you will easily know what is happening.  But to summarize, you have about 15% of the atmosphere at above-freezing temps, and then a HUGE drop-off to well below freezing surface temps after that.  This area happened to find itself under the gun for a freezing rain threat for the day today, and when you look at that sounding it is certainly no surprise!

 

Record Cold In The Southwest

 

Another big topic is the cold in the Southwest!  Many spots saw dry air intruding over the course of the weekend – and unfortunately for them it was dry, COLD, air.  Low temperatures, as a result, fell to record levels both on Sunday and also on the morning today.

Take a look at some areas in the Southwest that saw record lows this morning:

 

Record Lows: SW This Morning
Record Lows: SW This Morning

 

Tucson making it all the way down to 22 degrees!  Incredible cooling in some of these areas.  It continues even to areas to the North – and check out some of THESE temperatures from this morning:

 

Record Lows: NW This Morning
Record Lows: NW This Morning

 

How would you like to wake up to a -31° temperature?  I know I wouldn’t!

Stay warm out there, folks, and be sure to keep watching us on WeatherNation – we’ll make sure to mention when we’re all eventually going to warm up!  Residents of southern California can rejoice in knowing that 70s are only a few days away!  Residents of the northern Rockies and the Midwest may just have to wait a little (as in a couple months) before any rejoicing over warmth can take place…

WeatherNation Meteorologist Aaron Shaffer @ashafferWNTV

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