Saturday, December 20, 2014

Wiggle Room

Read the post below and you know about "wiggle room".  The forecast is never 100% until after it happens in the world of weather.  I'm going to have to tweak my forecast a little to include a little snow, but I think mainly north late Tuesday into early Wednesday.  Right now, I still think there will not be a white Christmas, but I reserve the right to have "wiggle room". LOL

I mentioned how horrible the models have been in the medium to long range and I'm going to give you a classic example below with this situation coming up Tuesday into Wednesday morning.  It should be mentioned, the models usually struggle this time of year anyway.

How do I think this will all play out?  Temperatures will go well above average Monday ahead of the cold front.  Rain will increase during the day and continue into Tuesday.  The high Tuesday will be set early with falling temperatures into the 30s and 40s late.  As the system starts to exit, moisture may wrap in from the north and the rain COULD change to some wet snow, but mainly north.  This would be brief and finished by Wednesday morning.  The American Global Forecasting System (GFS) is the most aggressive with this and the European is not.  The GFS brings flakes as far south as central Arkansas, but the Euro keeps most of it into Missouri.  I'll take a compromise between the two and say any brief transition will be mainly northern Arkansas, but given the variability in the data, let's watch this carefully.

Regardless, Christmas will be dry and cool/mild with highs in the 50s to near 60.  A new front arrives Friday into Saturday with another shot of cool air.

This is the GFS run from last Tuesday valid for Christmas Eve morning at 500 mb (@20,000 feet up).  This shows you the troughs and the ridges and all the pieces of energy (disturbances) working through the river of air in the atmosphere (jet stream).  You can clearly see the model showing a trough oriented from northwest to southeast from the Great Lakes to the eastern seaboard.  That places Arkansas in a dry northwesterly wind flow aloft.
Here's the GFS run from this Saturday morning valid at the same time.  WOW!  That's different and has major implications in the forecast.  The trough is carved right over the central United States and would bring us more unsettled weather.  
This is the GFS run from Saturday morning at the surface courtesy of  See the surface low over the KY/ Indiana border at 994 mb?  That's fairly strong.  The flow of air on the back side of that could change the rain to snow according to this model as it ends Tuesday night/Wednesday morning.  REMEMBER, THE EURO  IS NOTHING LIKE THIS!!!!  It keeps any snow north of the state.   At this time, the best forecast is for mainly rain with any transition northern Arkansas.  Stay tuned!
Check out the new experimental GFS.  This may have the best idea.  It's faster with the storm and only has the change over for northern Arkansas late Tuesday.

1 comment:

Anonymous said...

White Christmas according to Wikipedia is...

"In the United States, the official definition of a white Christmas is that there has to be a snow depth of at least 1 in or 2.5 cm at 7:00 a.m. local time on Christmas morning."

The key here is that snow is on the ground. Snow falling with nothing accumulating is NOT a white Christmas (unless you are desperate).

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