Snow Forecasting: Model Flip-Flop! - WKRN News 2

Snow Forecasting: Model Flip-Flop!


In this day and age, many weather savvy folks are able to go on the Internet and see the weather computer models that we use daily.

The buzz  started on social media yesterday when the GFS & ECMWF models both showed a chance for snow next Wednesday (the day after Christmas).

The scenario was for rain Monday, then again PM Christmas Eve, changing to snow on Wednesday as low pressure passed by.

The top graphic shows the evening run of the GFS output for noon next Wednesday.  The blue line is approximately (not always!) the rain/snow line (the 5400 meter thickness between 1000-500 millibars when thickness is proportional to the mean temp of that layer).  It shows that running south of Tennessee.  Cold enough for snow!

However, the second graphic is the overnight run of the GFS model.  It still shows the low, BUT does NOT have that thickness (proportional to temp) low enough for snow!  It shows it north of Tennessee.

That does not mean that snow will not occur next Wednesday.  The next model run may indeed show snow again.  Welcome to our world!

This is an example of the "flip-flopping" you hear us talk about when we are 4-7 days out.

However, both runs still agree on the low just to our northeast by then.  These are the generalities that you have to take away when "model watching" several days out.

That's why we say that we will keep you posted when we get closer to the time period.

My forecast for right now is rain changing to snow next Wednesday with temps falling through the 30s, but you can see how that might change!

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