Friday, December 21, 2012

Wind subsides, so let's talk about Christmas weather!

As promised, the Thursday weather system brought a great deal of wind to the area, including some sporadic damage, but the storms themselves were generally more bark than bite and passed through the metro quickly in the 4am hour Thursday.  The cold air behind the departing system set us up for a chilly day for the first day of winter, which started at 5:12am CST today.  Moderating temperatures are expected this weekend as wind switches to the south on Saturday.

Showers, possible thunder to end the weekend

The next precipitation-inducing impulse will move through Sunday night and early Monday.  Scattered light showers are likely as early as Sunday afternoon as moisture increases ahead of the system, but temps will still rise to near 60 on Sunday.  A few weak thundershowers are possible Sunday night into early Monday as the impulse moves through, then most of Christmas Eve should be dry and seasonal with highs in the upper half of the 50s.

Snow for Christmas?

What has been getting many people's attention is the next big system to affect the eastern U.S. and how it will affect holiday plans and travel.  We've reserved comment to this point as we dealt with yesterday's front & post-frontal wind and while the computer models tried to better resolve the overall pattern.  The mid-range models of choice at - the GFS and Euro (or ECMWF) - are finally coming into agreement on the big picture, but those pesky details (that make all the difference) are still being worked out.

What we do know is that a low pressure system moving out of the Southern Plains will pass by to our south on Christmas night before heading up the East Coast.  In the winter, that means we'll be on the cold side of the low, which is a good setup for something other than liquid precipitation.  Two main ingredients are required for winter precip: 1) sufficient cold air and 2) available moisture, especially in the lower levels. When they are both in place, snow or other wintry precip can occur. Lacking one, we get nothing (or rain).  The latest Euro model, as well as the last couple of model runs of the GFS, are in enough agreement that we can now safely say that there is POTENTIAL for some light snow late Tuesday night into Wednesday morning (the 26th) in the metro area.  I've placed a couple of graphics below to demonstrate.

Last night's European model shows some snowfall potential mainly northwest of Memphis, however light snow would be likely in the scenario in the immediate area with an inch or more just across the river.

Friday morning GFS model output. Graphic is read RIGHT to LEFT. Green bars are rain. Blue or gray bars are snow. The red line is surface temperature.  The amounts of precip are SMALL from this model run, with a total of  under 0.20" of snow, but like the Euro, it shows the potential for some flakes.  The scale for temp & precip amounts is on the right. Click image to see much larger view.
Understand that this is about potential and neither of these graphics are an official forecast.  The MWN forecast itself calls for a chance of rain or light snow and will have to be refined as time draws near.  We do not hype for hype's sake, but when there is a decent chance of an event occurring, we let you know.

Bottom line: I would not be surprised by an inch of snow on grassy surfaces Wednesday morning.  I also would not be a bit surprised to see nothing. I don't expect a lot of travel issues with this system locally, though there is some potential for problems from OK into northern AR, southern MO on Christmas night and southern IL and IN on the 26th.  We'll keep you updated!

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