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A thread to discuss the potential for a Southern winter storm affecting portions of the Tennessee Valley.

 

SHORT WAVE ENERGY TRACKING FROM THE NORTHERN ROCKIES CARVES OUT A
POSITIVELY TITLED LONG WAVE TROUGH ACROSS THE SOUTHWEST STATES AND
SOUTHERN PLAINS ON DAYS 2 AND 3. WHILE THERE WAS GENERALLY DECENT
AGREEMENT ON THE PLACEMENT OF THE LONG WAVE TROUGH...THERE WAS
SPREAD ON THE TRACK OF THE SURFACE LOW AND THE AMOUNTS OF MOISTURE
DRAWN NORTH FROM THE GULF OF MEXICO. THESE FACTORS INFLUENCE THE
NORTHERN EXTENT OF THE PRECIPITATION SHIELD AND PRECIPITATION
TYPE. AT THIS POINT...THE 12Z NAM/00Z ECMWF/ECMWF ENSEMBLE MEAN
BLEND APPEARS TO HANDLE THE MOISTURE FOR DAYS 2 AND 3 (THE 12Z GFS
SEEMS TOO SUPPRESSED WITH THE MOISTURE)..

 

Area forecast discussion
National Weather Service Morristown Tennessee
307 PM EST Friday Feb 13 2015

Short term...(tonight through Saturday night)...high pressure over
the area shift east tonight and early Saturday. An Arctic front
will move through late in the day Saturday...followed by a surge of
bitter cold air. Precipitation will move in Saturday
afternoon...especially north...and will likely start as some light
rain showers in valley areas before transitioning to snow showers by
Saturday night. However...freezing levels will be low enough for
most of the precipitation to fall as snow in the higher elevations...and
some light snow accumulation is expected there. Winds will increase
ahead of the front and will continue as winds shift to the northwest
behind the front. Winds will be stronger in the higher elevations
of the east Tennessee mountains...and will issue a Wind Advisory for the
mountains from Monroe north. The very cold air will combine with
the winds to produce wind chill values that reach warning criteria
in these same mountain areas...so will issue a wind chill watch as
well for Saturday night. May end up needing a Wind Chill Advisory
for some other parts of the area as well...especially the plateau
and SW Virginia...but will let later shifts evaluate this. Will issue an
Special Weather Statement highlighting the snow and cold potential.

Long term...(sunday through friday)... very interesting forecast
in the extended period...with a possible significant winter weather
event occurring on late Sunday night and Monday. However...this is
still an uncertain forecast given the fluctuations between forecast
models regarding the timing of the system and the amount of quantitative precipitation forecast
that will occur. Currently...my feeling is that the European model (ecmwf) /and its
emsembles/ has been more consistent in showing timing/track and quantitative precipitation forecast
for this event. To a lesser extent...the NAM is on board with this
European model (ecmwf) wetter solution...though its timing is earlier by around 12
hours. Combined with the sunfreezing air...this would support
widespread snowfall across the region during this time period. The
GFS also shows a snow solution...but is much weaker with quantitative precipitation forecast
amounts. Given its very erratic run to run track record of
late...have opted to weigh the European model (ecmwf)/NAM solutions more heavily for
this forecast package. At any rate...all model guidance suggests
that some snowfall /possibility significant/ will occur across the
Tennessee Valley and southern appalachian region from 06z Monday to
06z Tuesday.

The primary factor for generating this snowfall will be the very
cold temperatures in place at the surface and aloft Sunday night...
with lows /even with some cloud cover/ bottoming out in the teens
across the nearly the entire Tennessee Valley. Lift will also
enhanced by a fairly Stout 700 mb jet rotating over the region. The
main questions will be much moisture will be available to work with
and the timing of the precipitation. As mentioned...the NAM is
earlier with its timing and shows the heaviest precipitation moving
through the area faster...during a 06-18z Monday time period. The
European model (ecmwf)/Canadian shows a slower solution during the day and more of a
12z Monday to 00-06z Tuesday time frame. In each case though...quantitative precipitation forecast
amounts are high which would generate a good measurable snowfall
areawide. As a result...have introduced snowfall amounts for this
time period...though as mentioned timing is still a big question.

At this time...have favored the European model (ecmwf) solution more and show accumulation
occuring toward the late morning and afternoon on Monday. Although
it will most definitely be possible to see higher amounts should the
forecast continue to trend this way...have gone a little
conservative in snowfall amounts for now give the model flucuation.
This will allow for some flexibility and fine-tuning as more model
runs come...and confidence increases one way or the other. As
mentioned...models have wavered with this system quite a bit...so
exact totals/timing are still uncertain at this time. However...confidence
is high for some valley and mountain snowfall accumulations on
Monday. As such...have added a broad 1-2 inches in the grids across
the entire forecast area...with very slightly higher amounts across
the southern valley/plateau and mountains. The potential for a
widespread heavier snow event is possible for Monday...but do want
to see a couple more model runs before having enough confidence to
place this in the forecast.

Thereafter...the forecast remains somewhat in question...though the
favored European model (ecmwf) shows a drier period for Tuesday into Wednesday as the
base of the trough rotates across the region. Other than some light
snow showers across the mountains Tuesday night...the secondary low
that will form on the leeside of the mountains will track further
east and thus keep any additional precipitation to the east of the
mountains. The GFS shows some moisture working in on Tuesday...but
given its aforementioned back and forth track record have been
cautious with trusting this for now. Models do hint at the potential
for another round of precipitation moving southeast into the
Tennessee Valley early Thursday...though the magnitude and track are
still uncertain this far out. Overall...this upcoming week will be
very cold and much below normal for this time of year. Some relief
may occur by Friday into the weekend as southerly winds and perhaps
some weak ridging start to return.
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I don't buy the 18z GFS any more than I bought the other two runs of it today. It's not consistent at all and imo has to be discounted due to that. 

 

Will just have to see what it does with the storm over the next 36-48 hours to see how it all pans out. I like the consistency of the other models vs that of the GFS and it's major track shifts north and south for now. The best thing about it all is that so far, every model brings pretty widespread snowfall to the Valley.

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I don't buy the 18z GFS any more than I bought the other two runs of it today. It's not consistent at all and imo has to be discounted due to that. 

 

Will just have to see what it does with the storm over the next 36-48 hours to see how it all pans out. I like the consistency of the other models vs that of the GFS and it's major track shifts north and south for now. The best thing about it all is that so far, every model brings pretty widespread snowfall to the Valley.

I would suspect that the overnight run of the GFS would correct to more closely align with the UKMET, the ECMWF and the NAM imo

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I step away for a few hours and return to the gfs TOTALLY REDEEMING itself!  Welcome back to the party Goofus, we missed you! 

 

:guitar:

 

At this point I feel pretty strongly about a widespread 3-6 inches across the entire Tennessee Valley.  Will I be sweating bullets through the next two model suites?  Sure, but man it feels good to be where we're at right now!

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I also favor a NAM Euro/ECMWF blend, and not just to wish for snow. The lead isentropic lift often comes out faster and farther north than forecast. If something creeps back in here Tuesday, that would be a bonus. That MRX Friday afternoon discussion is a gem. They are definitely in the forecast zone.

 

Still like the conservative MRX amounts too. Though Monday snow is only 3 days out now, this is the South and everything must be exactly right. So far it looks like the ingredients are coming together.

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0z NAM is in and it is interesting to say the least. It has less emphasis on the first wave(well the precip is further south over southern TN and northern AL/MS). Then the main trough back over the Rockies picks up the Baja low (big development) and the whole precip shield over the Deep South is expanding and heading back north. At Hour 84 the storm is still cranking. It has northern Alabama and Mississippi buried.

Clown Map out to hour 84:

http://imgur.com/xLTNdvv

Precip from the hour 84 panel:

http://imgur.com/pCrZBqH

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