Here we go, we all knew it would happen eventually. Now before I go any further, today is only Friday and we’re talking about a storm system that is over 7 days away. Will it pan out exactly how the models suggest it will today? Only time will tell and I’ll follow the new trends and let you know how things evolve. A lot can change between now and then, it’s a very chaotic winter pattern. Model guidance is extremely poor at times as they try to figure things out.

So, several of the models have turned an about face and bring in another upper level storm system for Friday the 13th. That system lingers into the weekend producing a lot of precipitation. The only problem is, it’s freezing rain. Why ice instead of snow? Let’s take a look below.

Upper level storm system approaching Friday.

Upper level storm system approaching Friday.

The first thing as noted above is a nice strong storm system coming in. As long as it stays on a more southern course, it’s bad news for Oklahoma. The next graphic demonstrates the very strong mid level lift occurring out ahead of this system for Friday morning. This will generate the deep cloud cover and light precipitation.

Mid level lift to create clouds and rain.

Mid level lift to create clouds and rain.

There’s only one problem and that is, arctic air has already started moving into the region before that system arrives. Surface temperatures (at ground level) are in the 20s with dewpoint values in the teens. Plenty cold for frozen precip. (These numbers will meet in the middle of the two once precipitation starts falling).

Surface temps.

Surface temps.

 

Dewpoint values, amount of dry air in place.

Dewpoint values, amount of dry air in place.

So why ice instead of snow? We need to look up a few thousand feet for the answer. Our first image shows the location of the shallow arctic airmass at 925 mb. Note the demarcation of the freezing line. East of this line will be where the frozen stuff transitions to just regular cold rain (E OK). You see how close that is to the I-35 and I-44 corridors, the typical location of these types of setups.

Freezing line at 925mb.

Freezing line at 925mb.

This image is at 850 mb showing warm air advection overriding the cold shallow arctic airmass below it. This pumps in the moisture as well from the Gulf of Mexico and allows much higher precipitation totals. The freezing line is way north and until that line moves SE to cover OK it’s going to be either rain or ice.

Warm moist air at 850mb overriding cold air.

Warm moist air at 850mb overriding cold air.

Here’s a vertical explanation of what I’m talking about above. The snow melts from the cloud layer into rain as it falls through this warm region of air at 850mb. Then as it enters the shallow arctic air, it refreezes on contact with the surface. If you notice in the 3rd column, sleet will form if the arctic air is thick enough to freeze the raindrops before they hit the ground. When it comes to ice storms, you’d rather have the sleet vs the freezing rain. It does a lot less damage overall. However, forecasting the difference between the two can be a really big headache. I wrote a blog about the process a while back you can read here: Why are winter forecasts so difficult.

Graphical explanation of how ice forms.

Graphical explanation of how ice forms.

So what does all of this mean? It means an Oklahoma ice storm. The European model is extremely aggressive with precipitation amounts around 1 to 1.5″ of ice across C OK. The Canadian model shown here isn’t that aggressive but none the less gives warning that things won’t go well with plenty of ice to cause problems. Eventually the system will move across and transition over to a tiny bit of snow, but not much as indicated by the maps.

Computer model ice accumulation prediction for Friday-Sunday.

Computer model ice accumulation prediction for Friday-Sunday.

 

Just light snow amounts forecasted.

Just light snow amounts forecasted.

So there you have it. This is going to be an interesting system to watch unfold. Two things can easily go awry. 1. The arctic airmass doesn’t make it in here. 2. The system comes out farther north or out of phase in timing with the arctic air. It’s very complicated so stay tuned!

This is also a good time to remind you to download my free weather app, ATsWeatherToGo. Link: iTunes and Google Play. Be sure to check out the Road Angel service built inside the app to help if you get stranded on the roads in this weather. Thanks so much and stay safe out there! -AT

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