Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 121132

Area Forecast Discussion
National Weather Service Huntsville AL
532 AM CST Tue Dec 12 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 245 AM CST Tue Dec 12 2017

An "Alberta Clipper" system has arrived as a shortwave trough moving
within the prevailing northwesterly flow aloft/ It is moving over
the TN/OH Valleys. The front ahead of this shortwave trough is
bringing yet another reinforcing polar/arctic airmass. Looking at the
temperature profile, behind the front there is enough low-level
moisture to support light rain/drizzle/snow over portions of southern
middle Tennessee this morning. Due to the breezy conditions
(deposition, evaporation), quick moving system, and generally dry
conditions, no accumulations are expected. If any precipitation is
still occurring, it should end around the 11AM-noon time frame as the
boundary layer dewpoint depressions continue rising.

Speaking of breezy conditions, the strong pressure gradient will
mean winds will increase rapidly behind the front. Already seeing
upstream observations over the Midwest of 10-15 MPH with gusts up to
25 MPH. Made some adjustments to the wind forecast to increase the
forecast wind speeds as guidance is underestimating current
conditions. Clouds are already increasing which along with the breezy
and cold northerly surface winds will make for a cold winter like
day. Maximum temperatures for the day may have already occurred in
some locations (especially over NW AL) and have generally trended
temperatures lower--into the mid 40s today. Clouds should begin to
recede later this afternoon with areas of southern middle Tennessee
the slowest to recede due to the trajectory of the shortwave trough.

.SHORT TERM...(Tonight through Thursday night)
Issued at 245 AM CST Tue Dec 12 2017

After a cold winter-like day, winds should diminish and clear sky
conditions prevail atop the airmass as the eastern flank of a
surface anticyclone that quickly moves south across the Southern
Plains. Overnight lows will quickly drop below freezing with
overnight lows anticipated to be in the low to mid 20s. Some areas of
southern middle TN may experience lows in the teens!

Then, by Wednesday, another "Alberta Clipper" system, consisting of
a shortwave trough moving almost along the same trajectory as the
previous one (perhaps about 100 miles further north), will be
progressing across the Central Plains/Midwest. It is expected to
arrive on Thursday morning with another reinforcing cold front.
Before that happens expect sunny sky conditions and temperatures to
rise modestly into the upper 40s to low 50s as southerly winds
briefly return on Wednesday. Still seeing a SW to NE gradient in
temperatures as the cold airmass is briefly shifted north.

After the cold front arrives, cold air advection will arrive again
with daytime high temperatures forecast to be in the low 50s. The
cold air advection will not be as strong and it appears the airmass
is somewhat more modified than the previous one (arriving today)
Overnight lows will dip again below freezing on Thursday night.
Meanwhile, the mean trough pattern continues with a couple of
shortwave troughs merging into an elongated trough axis stretching
from the Upper Midwest to TX. This will become more important for the
extended portion of the forecast.

.LONG TERM...(Friday through Monday)
Issued at 245 AM CST Tue Dec 12 2017

On Friday, a positive tilt trough will drop southeast through the
OH, TN and lower MS valleys.  Mid level isentropic uplift in advance
of the trough will likely produce areas of mid and high clouds,
mainly just to our north.  The GFS is a bit more aggressive with a
more compact and stronger upper wave which produces better -divQ
fields over the TN valley.  Will go with the drier ECWMF solution
for now, and wait for better model consistency before going with any

After this wave passes, the pattern adjusts rapidly from a NW flow
train, allowing a strong trough to build across the southern U.S.
There are model differences in the handling of the Baja cutoff low.
The ECMWF ejects it as a weakening shortwave in advance of the
building trough, while the GFS holds onto it.  This results in a
more amplified ECMWF solution, and a more positively tilted wave on
the GFS.  As for sensible weather, surface high pressure will move
east along the Gulf Coast on Saturday, ending up along the GA/FL
Atlantic coast by Saturday night. Deep layer south-southwest flow
will prevail over the southern states owing to good warm air
advection. Gulf moisture advection will soon follow. GEFS plumes for
Sunday indicate dew points range in the upper 40s to middle 50s
possible Sunday into Monday. It is likely the unstable warm sector
will remain to our south with either ECMWF or GFS solution. So have
gone with all showers and no thunderstorms as of yet. Despite their
differences, it appears we have a good chance of beneficial rainfall
Sunday into Monday.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 531 AM CST Tue Dec 12 2017

VFR conditions should prevail for most of the next 24 hours as a cold
front passes over the region. There is the potential for MVFR cigs,
approaching from TN, to affect both terminals between 12/14Z-12/17Z
but not enough certainty to include at MSL. Did include a tempo
group for that potential at KHSV. Wind directions have already veered
to the NW with winds expected to increase to 10 kts gusting to 15
kts initially. Then, increasing to 10-15 kts gusting to 20-22 kts
between 15Z-00Z. Winds should diminish after 01Z.





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