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Snow analysis

| 11:47 am November 27, 2011 | Comments (5)

Wow, what a complicated forecast!  Two things are  pretty sure…it will be damp and occasionally precipitate from now through Tuesday morning…and it is about to get very cold.  It was in the 70s yesterday afternoon…same time tomorrow it will be in the 40s.

The big question is whether snow will fall.  With an upper-level low like the one shown above (forecast at midnight Monday night), due to dynamical cooling the air gets very cold aloft.  One of the main indicators we look for in terms of the atmosphere being cold enough for snow to fall instead of rain is the 1000-850 mb thickness (in cold denser air the pressure changes quickly with height.  According to the latest 12Z models, the 1000-850 mb thickness will be below 1310 m (cold enough for snow) from Monday at 6 pm through Tuesday at 6 pm.  Another indicator is a sub-freezing temperature at 850 mb (about 4,000 feet)…this occurs from 3 pm Monday to 6 pm Tuesday.  So, this means that any precip that falls Monday night or Tuesday will mainly be in the form of snow over north and central Alabama.

It is nearly impossible to pinpoint where the precipitation will fall, as the upper low is a complicated blob of upper-level disturbances, and with the air getting so cold with height, the air will unstable enough for a few bursts of heavy snow (almost like summer thundershowers) at times. 

All of this is to say that many Alabamians will see snow fall sometime Monday night or Tuesday.  However, it has been a warm 2 weeks overall, and the ground is very warm.  Also, precipitation amounts will not be that heavy.  If it snows hard, it won’t last but 10 or 15 minutes.  So, any accumulations will be light and probably temporary, since surface temperatures will stay above freezing (in the mid to upper 30s) during the time frame for snow.  We do not expect travel problems at this time, but it will be a neat event to follow.  If it had been down in the 20s last week, it might be a different story.

I’ll have another update tonight around 930 pm or so when 00Z models come in, but models, other than showing the overall picture, aren’t much good at this point in showing exactly where the heavier snow will fall.


Category: Alabama's Weather, Meteorology 101, Winter Weather

Comments (5)

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  1. John Talbot says:

    Thanks Tim! Good stuff as always.

  2. Brent In Atlanta says:

    Tim, What about Atlanta, we don’t have the best forecasters here in market number 6…..????

  3. Pickled Ick says:

    Snowfall is a gift for us southerners.

  4. JustReleased says:

    Hi Dr. Snow White.

    I am back as it is snow season in ‘Bama. However, I should have posted something when those April tornados almost swept me from the planet. I lost some $$ on them. Also, got to watch one split into two storms, one a F4, the other an F? aloft rather than vacuum me up. But, snow is more exciting than tornados.

    These complex snow storms are more interesting to me because they have clearer boundaries as compared to tornado storms. Not so much swirling going on.

    From your site, I see that the talk about snow south of Huntsville was what I thought it was “wild rumor”.

  5. Richard Viola says:

    Thank you Tim for the details. That always helps.

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