Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 111134
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
534 AM CST Mon Dec 11 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today and Tonight)
Issued at 226 AM CST Mon Dec 11 2017

Another active weather pattern is forecast for this week as a mean
trough remains in place across the eastern half of North America
while mean ridging is mainly over the western half of North America.
As a result, several Alberta Clipper systems will track SW over the
coming days.

Before the first one arrives, the cold air that has been in place
over the past few days will be modified underneath a sunny sky and
weak southerly flow prevails as a surface high pressure system
centered over the NW Gulf of Mexico shifts south. This southerly
flow will advect somewhat warmer air northward, but only enough to
moderate the temperatures into the upper 50s to near 60 degrees
today.

Then, the first Alberta Clipper system/Shortwave trough moving
southeast from the Midwest will arrive late tonight bringing a polar
airmass reinforcement of cold air. Overnight low temperatures will be
a challenge and depend on how quickly the cold air arrives.

.SHORT TERM...(Tuesday through Wednesday night)
Issued at 226 AM CST Mon Dec 11 2017

There is enough moisture and weak lift on Tuesday morning across
southern middle TN (mainly Franklin County) to support a light
rain/snow mix between 12-17Z. Not much in accumulations is expected
given the dry air above the -10C level, but definitely something to
monitor. The most noticeable feature will be the breezy northwesterly
surface winds of 10-15 MPH gusting to 20 MPH as strong cold air
advection ensues. Depending on the speed of the cold air, could see
high temperatures actually occurring in the early morning hours on
Tuesday. Temperatures should rise modestly from overnight lows in
the low to mid 30s to the low 40s due to the strong cold air
advection and mostly cloudy sky conditions.

The mid-level shortwave trough should merge with another approaching
embedded shortwave trough moving southeast on the southern flank of
the Canadian upper low over Quebec/Ontario on Tuesday. Clouds should
clear on Tuesday night yielding a very cold night in the low to mid
20s (possibly teens over southern middle TN). Little solar
modification of the airmass will take place on Wednesday, but weak
southwest surface flow will begin to push the airmass northeast. This
will mean a SW (higher) to NE (lower) gradient in daytime highs in
the mid to upper 40s. And then, yet another fast moving shortwave
trough (second Alberta clipper system) will move over the Midwest on
Wednesday afternoon arriving over the TN Valley late on Wednesday
night. Forecast soundings show a rather dry airmass below 700 mb. So,
will keep dry forecast in place ahead of the front expected on
Thursday.

.LONG TERM...(Thursday through Sunday)
Issued at 226 AM CST Mon Dec 11 2017

Region will remain in a west-northwest flow pattern aloft Thursday
into Friday.  Clipper-type low will be dropping through the eastern
OH valley into the central Appalachians on Thursday bringing a cold
front through during the day.  At this time, the models indicate
little moisture and weak UVV to support measurable precipitation at
this time.  A shortwave dropping south through the Plains quickly
follows behind on Friday will dig a stronger positive tilt wave
through the southeast U.S. This will likely generate rainfall along
the front from the northern Gulf of Mexico northeast through
northern FL and southern GA. An upper ridge then quickly amplifies
over the Southeast on Saturday with good southwest low level flow and
warm advection. Highs should reach the m-u50s quite easily Saturday
afternoon. Another shortwave trough arrives on Sunday, this time
being of Pacific origin. Strong mid level warm air advection may be
enough to generate some mid level cloud Saturday night, but will
leave dry until Sunday when the surface cool front arrives. There is
a wide disagreement with this system between a more bullish/wetter
GFS and drier ECMWF. This looks to be mainly due to the ability of
the GFS to advect deeper moisture northward, while the ECMWF is a bit
delayed in this regard. Will stick close to suggested blended PoP
guidance for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 534 AM CST Mon Dec 11 2017

VFR conditions are expected to prevail for the next 24 hours. A cold
front is expected late in the period and have retained a veering of
the winds to the West from the SW by 04Z-05Z. Clouds will also begin
to increase with an additional wind direction change to northwest by
07-09Z.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...SL.77
SHORT TERM...SL.77
LONG TERM...17
AVIATION...SL.77


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