Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231737 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1137 AM CST Fri Feb 23 2018

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 1012 AM CST Fri Feb 23 2018

Quiet weather still ongoing across the Tennessee Valley with high
pressure still established to the southeast of the region and a
stalled boundary off to the northwest. Continued SW flow off the Gulf
of Mexico will raise/maintain dewpoints into the low to mid 60s.
There will be some patchy low-level cloud cover (around 850 mb) and
3kft that will continue to filter into the region from the south
today. However, drier air noted on model soundings should be enough
to break up some/most of this by later this afternoon. As a result,
temperatures should easily warm into the upper 70s to around 80
degrees once again. Have a forecast high of 80 degrees at bot KMSL
and HSV, though this will remain below the record values of 83
degrees. Late in the afternoon, mesoscale models are hinting that a
weak boundary (warm front) will pivot into far northwest Alabama
around 00z. This would serve as a focus for a few isolated
thunderstorms late in the day into the evening hours. Have updated
the forecast to account for this possibility.

.SHORT TERM...(Tonight through Sunday)
Issued at 259 AM CST Fri Feb 23 2018

For tonight, a secondary warm front lifts northward through the area
as the broad area of south-southwest low level flow increases to
30-35kt from the central Gulf Coast into the TN valley. This
generates theta-e advection and moisture transport, and at least
scattered elevated showers. Based on soundings, these showers would
not be surface based, and instability aloft does not look very
impressive at this point with little to no positive area on the
forecast soundings. These showers will lift northeast of the TN River
by Saturday morning.

As the upper trough advances into the lower MO valley and upper
Midwest, the broad LLJ will intensify to 40-55kt across the lower MS
and TN valley during the day. The warm sector appears capped
initially on Saturday morning. The GFS develops a skinny and tall
CAPE of only 200-400 j/kg, while the NAM keeps the atmosphere rather
capped. Will keep a chance northwest to slight chance southeast
going, but have my doubts on the amount of thunderstorms we will
experience until Saturday evening. Then it gets more interesting to
our west near the MS River as a pre-frontal trough/line of
thunderstorms should rapidly translate east-northeast into the mid
South and lower TN valley region. The main limiting factor will be
somewhat lower dew points that the models indicate could mix down
and/or advect into the TN valley from the south-southeast during the
day Saturday into the evening. This narrows the better CAPE values
to our west. As such, pre-frontal supercells cannot be ruled out in
our western counties during the evening hours, likely moving
northeast from MS from the better instability. And deep layer
shear/low level helicity profiles both support supercell structures.
In a nutshell, I certainly agree with the SPC DY2 outlook with regard
to placement of the SLGT risk for our area mainly along and W of
I-65 and marginal risk further east. We will still forecast the line
of thunderstorms moving east-southeast across the entire area with
the weakening cold front. The question is timing. With the best
upper trough/QG lifting into the eastern corn belt and western Great
Lakes, the portion of the cold front in our area may tend to weaken
and slow down as depicted by the NAM/ECWMF, while the GFS is a bit
more progressive. Will go with a compromise, with showers exiting our
eastern counties Sunday morning.

.LONG TERM...(Sunday through Thursday)
Issued at 259 AM CST Fri Feb 23 2018

The frontal boundary will exit our forecast area during the day on
Sunday and get hung up in Central AL by Sunday night. With the
frontal boundary still near by and a weak disturbance moving through
ahead of a shortwave, precip chances linger in the forecast until
Monday morning. Models differ on this however, as the ECMWF is more
aggressive with the shortwave compared to the GFS, lifting the front
northward a little bit and anticipates more rainfall. Took a
compromise of the two for this forecast package.

Drier conditions are expected to last into Tuesday with sfc high
pressure across the region. We will feel a slight cool down behind
the front, but temps will technically still be above normal. For
Daytime highs will be in the mid/upper 60s on Monday and Tuesday.
While overnight lows will be in the lower 40s on Monday, warming up
into the lower 50s by Tuesday night. From here, models agree on the
large scale pattern but not the timing of the next system. This is
mainly due to the pace and evolution of the trough swinging out of
the Southwest on Wed/Thurs. Sfc cyclogensis will develop Wed Night in
the Southern Plains and lift into the Great Lakes by Thurs. Rain
returns to the forecast on Tuesday night with the warm front.
Plentiful moisture returns when we are in the warm sector, as PWATs
reach over 1 inch. Widespread showers with a few thunderstorms are
possible Wed and Wed night as the cold front approaches with a good
amount of wind energy throughout the wind field. Kept closer to
guidance and kept lingering showers in on Thurs.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1137 AM CST Fri Feb 23 2018

VFR conditions will prevail this afternoon through early this
evening, with passing decks of SCT/BKN Cu. A weak front will
gradually move into NW Alabama late this evening and overnight and
serve as a focus for scattered to numerous rain showers that will
impact KMSL and potentially KHSV. Models are hinting that this
activity may begin to impact KMSL by 04z and possibility KHSV by
06z. Reduced MVFR visibilities and lower MVFR/IFR ceilings are
possible through early Saturday morning (until this activity tapers
off). Some lower clouds will likely persist through the remainder of
the period, though they will become more SCT to BKN later in the





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