Sunday, December 23, 2012

Sunday update on Christmas night snow event

Today, computer models are giving us a little more to work with regarding the potential for snow on Christmas Day that we have been talking about since Friday. Unfortunately, the news is not all good for snow-lovers, but all hope is not lost!  If you have followed us for long, you know we don't hype the weather in hopes of drawing more followers.  We shoot straight.  Here's as straight as we can get:

The setup and result

A low pressure system will move out of the Texas Christmas morning and turn northeast as it strengthens.  By 9pm Christmas night, it will be positioned over northern Mississippi.  In the meantime, a very potent upper level disturbance will move across northern AR.

All along we've talked about the need for moisture (preferably the falling kind) and cold air at the same time.  The upper-level disturbance will act to rapidly cool the atmosphere as it moves toward the region.  The northwest side of the low is also where the best moisture is relative to that cold air.  Thus it appears areas north and west of Memphis (or roughly north of I-40 across AR into the northern half of west TN) will be in perfect position to see the highest snowfall totals.  With the low moving so close to Memphis, it will take longer for the "sufficiently cold" air to arrive, thus delaying the onset of the rain/snow changeover enough that much of the best moisture will be moving away.

As of 4pm this afternoon, the NWS has issued a Winter Storm Watch for the area shown below effective 3pm Tuesday until 6am Wednesday.  The metro is NOT included.  This is due to the fact that a "significant accumulating snow event" is not likely here.  It does not mean that snow will not fall however. Read on...

The graphic below shows the position of the low at 9pm Christmas night, where the precipitation is expected to be falling, and where the rain-snow line should be (approximately).  Based on consistency of the model and the fact that it initialized well, this model (the 12Z GFS) is our model of choice for now.

Low pressure moving northeast through northeast MS. Rain/snow line marked in solid blue. The  rain/snow line will shift east overnight but the precipitation areas will move northeast, leaving minor amounts of precipitation left after the rain/snow line passes through the Memphis metro.
Given that the GFS is our best option for how the atmosphere will "play out" Christmas Day, the graphic below shows the expected snowfall amounts from that model:

Projected snowfall totals from the GFS. The metro could see anywhere from less than  an inch in north MS to 2-3" in  northern Crittenden Co.  An inch or so in the city is possible

MWN's Forecast

Rain will begin by noon Christmas Day with temperatures in the mid 40s.  The heaviest precipitation will fall as rain before temperatures start to fall into the upper 30s by late afternoon or early evening. Precipitation will begin to changeover to snow by late evening (8-10pm) across the northwestern metro (north Crittenden Co into western Tipton Co) then gradually mix with and change to light snow in the rest of the metro, including Memphis and Shelby Co, around midnight.

Snowfall totals will range from a dusting on grassy areas over north MS to about an inch in Shelby County to 2-3" in areas that change over to snow the earliest (north Crittenden and Tipton Co).  By Wednesday morning, flurries may linger but accumulating snow will be over.  Temperatures will remain in the 30s Wednesday with a cold north wind and plenty of cloud cover.  Places that get more than an inch of snow could see it stick around into Thursday.

Travelers warning

If you plan to drive Tuesday evening or early Wednesday in northern AR (anywhere north of I-40), northwest TN (west of US 51), southern MO, western KY, or southern IL, please keep a close eye on forecasts and prepare a "plan B" now - either traveling earlier or delaying a day.  In addition to snow, very gusty wind will meaning blowing snow and reduced visibility in these areas, making travel difficult to treacherous, especially Tuesday evening.

How to stay in touch with MWN

As always, this post is disclaimed by the fact that we are still 48+ hours from the snow beginning and lots of things can, and maybe will, change.  It does not appear that this will be a significant snow for the metro, but a little bit is better than nothing!  Follow us on Facebook, Twitter, or Google+ (links below) for the latest on this developing winter weather scenario and we plan to update this blog again on Monday afternoon with the most recent information.

Finally, the app will give you everything you need while away from your computer.  StormView Radar shows where it's raining and where it's snowing, weather alerts from MWN StormWatch+ will advise you of any changes to watches, warnings, or advisories, and the MWN Forecast will tell you exactly what we expect to happen, from a local perspective (national apps tend to not do well in these dynamic events - you need a local source).

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1 comment:

Angela said...

Wonderfully crisp and always interesting!