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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 231612 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1012 AM CST Fri Feb 23 2018

.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 12Z TAFS through 12Z Saturday morning)
Issued at 420 AM CST Fri Feb 23 2018

VFR flight weather conditions are expected through this evening.
Southerly flow may gust around 16kt from midday into the afternoon
before diminishing by evening. Ceilings of ~025agl will spread
northward by 08Z with scattered showers possible. An isolated
thunderstorm is possible, but have left out of the forecast due to
its low probability.





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