Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240458

Area Forecast Discussion
National Weather Service Huntsville AL
1158 PM CDT Wed Aug 23 2017

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 912 PM CDT Wed Aug 23 2017

In the wake of a cold front passage earlier today, cooler and drier
air continued filtering across the greater Tennessee Valley region.
In comparison to yesterday at this time, cooler and more notably -
drier conditions reigned across the region. The 9 PM temperature and
dewpoint at the Huntsville Int`l Airport was 76/64 and 67% relative
humidity, while yesterday at the same time it was 80/76 and an 87%
RH values. The area of high pressure extending southward from central
Ontario province to the Mid Mississippi Valley was the main
controlling factor with our sensible weather. Another feature, a
passing upper level disturbance to our west was spreading mid/high
altitude clouds over the region.

Model guidance at least through the rest of the evening and overnight
suggests the upper feature will translate east of the region,
resulting in mostly clear skies by daybreak Thursday. Radiative
cooling from the clear skies and cool air advection will result in
notably cooler temperatures overnight, with lows ranging from the
upper 50s in our cooler spots in the mountains of NE Alabama and
southern middle Tennessee, to the lower 60s south of the Tennessee
River. The warmest spots in the mid 60s or a bit higher will be
possible near and downwind of large bodies of water - with water
temperatures now in the 70s.

The forecast overall was in good shape. Only made minor changes,
mainly for short term changes, and to keep mid/high clouds in place a
few hours longer. Given that our western areas will clear sooner,
residual moisture from yesterday`s rain there could be enough for
patchy fog formation at daybreak. Not confident enough to add that in
the grids now, but will give it a mention here.

.SHORT TERM...(Thursday through Friday night)
Issued at 242 PM CDT Wed Aug 23 2017

Quiet and tranquil conditions will prevail across the mid TN Valley
thru the end of the work week, as a strong area of high pressure
builds ewd across the Midwest/OH Valley regions. Afternoon temps
look to remain slightly below seasonal trends Thu/Fri, with highs
mainly in the mid 80s for most areas while overnight lows Thu
night/Fri night remain in the lower/mid 60s. Moisture below H7 may
begin to creep back into the region Fri night, as low level winds
begin to veer toward the SE/E.

.LONG TERM...(Saturday through Tuesday)
Issued at 242 PM CDT Wed Aug 23 2017

Weak northwest flow will persist aloft on Saturday, with surface
winds generally to the east-northeast as high pressure builds along
the east coast. Moisture will be somewhat limited across the local
area, however there might be just enough to see some very isolated
showers and thunderstorms during the afternoon hours.

An upper level low will begin to dig southward and into the Upper
Midwest on Sunday, tracking southeastward in a more broad upper
trough extending across much of the eastern CONUS. Weak shortwaves
in the northwest flow aloft will move through the TN Valley on
Sunday, as greater moisture begins to lift northward across the
region. This will bring a greater coverage of showers and
thunderstorms during the day, though still scattered in coverage.
The TN Valley will generally remain under the influence of the broad
upper trough through the first half of next week, keeping diurnally
driven thunderstorms in the forecast for Monday and Tuesday.
Meanwhile, Harvey will meander around the SE TX coast through the
weekend and then lift northward and towards the TN Valley early next
week. It should be noted that models have come into much better
agreement in terms of the timing of Harvey`s movement (though some
timing and placement discrepancies remain), with the first impacts
to the local area expected as early as next Tuesday night. While
there may be periods of greater thunderstorm coverage, given the
uncertainties on where and how quickly Harvey tracks, will keep
scattered thunderstorms in the forecast through the extended period.
It`s still a bit early to look at the overall impacts to the local
area, but heavy rainfall does appear likely across portions of the
TN Valley during the latter half of next week.

Temperatures through the period will remain just below normal for
late August. The combination of northwest flow/low upper heights and
cloud cover will keep afternoon highs in the lower to mid 80s.
Meanwhile, overnight lows will generally cool into the 60s.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1157 PM CDT Wed Aug 23 2017

VFR weather should prevail across the TAF sites for the next 24
hours, as high pressure builds southward from central Canada. Light
northerly winds tonight, will become NE 5-10kt shortly after daybreak
on Thursday. Winds should become light from the ENE after dusk Thu
evening. Isolated to at times scattered daytime fair weather cumulus
are possible during the warmest part of the day, otherwise mostly
clear skies are expected.





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