Area Forecast Discussion
Issued by NWS Huntsville, AL

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626
FXUS64 KHUN 191740
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1140 AM CST Thu Jan 19 2017

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 1022 AM CST Thu Jan 19 2017

A stationary front south of the forecast area should return northward
as a warm front this afternoon and evening. Despite that boundary
being located south of the region, very warm for mid-January
conditions prevailed around the greater Tennessee Valley region,
with mid/late morning temperatures in the mid 50s to low 60s, and
dewpoint depressions running running 2-6 degrees below air temps.
Dewpoint values were low/mid 60s south of the boundary, and in the
mid/upper 60s towards the Gulf coast.

Showers and a few embedded thunderstorms continued to move from the
south-southwest across the region (the thunder mainly west of our
forecast area so far). This activity was mainly near and west of US
Hwy 43, or west of a Lexington (eastern Lauderdale Cnty), Courtland
(n/central Lawrence Cnty), and Double Springs (Winston Cnty) line.
The stronger storms at this time were occurring in the deeper and
more unstable regime well south-southwest of this region.

For the rest of today, the area of showers and storms will continue
overspreading the remainder of the area. The showers over the
western areas, as well as other activity moving NNE across
west/central Alabama will affect more of the Tennessee Valley into
the afternoon. Given on-going trends regarding instability and
shear, overall storm strength into the afternoon in the "strong"
category are expected, with 40-55 mph wind gusts. Per output from
the 12Z NAM, somewhat stronger, borderline severe storms look
possible early this evening as higher shear occurs in a marginally
unstable atmosphere. Locally heavy rainfall remains a potential
threat too, especially over our western areas. Ponding of water in
low spots, and a flash flood event or two cannot be totally ruled
out.

.SHORT TERM...(Tonight through Friday night)
Issued at 255 AM CST Thu Jan 19 2017

As the system shifts N/NE into the OH Valley tonight expect rain to
end from west to east. Models agree that virtually all of the precip
should exit the area by sunrise Friday. Drier air quickly filters in
aloft behind the system during the day Friday and given enough mixing
in the low levels could see some sun peak out behind the clouds. We
never really have any "cold" air arrive behind the system as winds
remain out of the S/SW. So, with some clearing skies expect temps on
Friday to be a bit warmer with values around the 70 degree mark.

Quick on the heels of the system today is a shortwave and low that
ejects out of SE TX towards the TN Valley Friday night into Saturday.
Models are in decent agreement in the timing of this feature.
However, the GFS and Canadian generate an MCS that remains tied to
the Gulf Coast. The ECMWF on the other hand keeps precip tied to the
shortwave and brings heavy rain into the region by early Saturday
morning. Due to the disagreement in the models will go with a blend
of the through early Saturday morning.

.LONG TERM...(Saturday through Wednesday)
Issued at 255 AM CST Thu Jan 19 2017

One or more MCSs will likely be ongoing across southern portions of
MS and AL Saturday morning.  This will likely suppress the northward
movement of the more unstable warm sector.  I am favoring the GFS
and NAM in this regard with lighter QPF and perhaps more stratiform
precipitation as opposed to mostly convection.  A "break" in the
widespread precipitation is expected Saturday afternoon and perhaps
early evening before additional waves of showers and perhaps
thunderstorms move in along and north of the warm front.  But,
limiting factors continue to be surface based instability and weak
lapse rates.  Still plenty of uncertainty with regard to the
evolution of the strong upper low, but it appears that the threat of
severe weather in the TN valley is still rather marginal given the
lack of expected instability.  The upper low is still being progged
quite differently between the "northern" GFS and a more "southern"
ECMWF which has a strongly deepening surface low moving out of the
Gulf and into SC Sunday afternoon and Sunday night.  This may be due
to effects of the MCS and the axis of the Low Level Jet in this
area.  The northern GFS solution may pose a good setup for cold core
low-topped supercells in the vicinity of the occluded front and near
the upper low in western TN/KY just northwest of our area on Sunday.
The NAM fully takes the dry slot and occluded front through our area
late Saturday night and early Sunday morning which would diminish
our threat of thunderstorms Sunday. With all this being said, given
the strength of the differential PVA, a good dry slot, and ample
moisture, will keep high PoPs going through the weekend.

Suggested model blends were used closely in the days 5-7 period with
our next chance of precipitation on days 6-7 with a weak cold front
dropping southeast beneath a broad southwesterly upper jet. Although
not as mild as it has been, we will continue to experience
temperatures well above seasonal normals next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1140 AM CST Thu Jan 19 2017

As a developing storm system and cold front approach from the west,
extensive clouds and inclement flying weather is expected for the
period. As of issuance time, scattered to numerous showers and a few
embedded t-storms were affecting areas mostly west of the I-65
corridor and south of the Tennessee River towards northeast/central
Alabama. This activity should overspread the region this afternoon
and evening. Thus mainly MVFR CIG/VIS values are expected, with
reductions to IFR in and near the heavier showers and/or storms. The
main area of showers should be east of the region before daybreak
Friday. However lingering low clouds and light post system rain will
produce IFR (mainly low ceilings) through late Fri AM.


&&

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

$$

NEAR TERM...RSB
SHORT TERM...Stumpf
LONG TERM...17
AVIATION...RSB


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