Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 211957 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
257 PM CDT Fri Jul 21 2017

.NEAR TERM...(Tonight)
Issued at 257 PM CDT Fri Jul 21 2017

Strong upper level ridging layered across the mid CONUS was on a
weakening trend, as a weak upper low retrogrades westward along the
NE Gulf coast. The presence of this low was helping with the
formation of scattered convection well south of the Tennessee Valley.
Terrain and strong heating has also produced isolated convection just
to our northeast over the Middle Tennessee Cumberland Plateau.
Otherwise, partly cloudy skies prevailed across much of the Tennessee
Valley, with air temperatures into the low/mid 90s, and heat indexes
into the 102-106 range.

The very warm to hot conditions late this afternoon will cool with a
loss of daytime heating. Temperatures towards sunset should cool to
around 90. We`ll let the on-going Heat Advisory expire at 7 PM this
evening.

Strong daytime heating and resulting instability could result in
isolated showers/thunderstorms early in the evening. Any activity
that forms should fade by the mid evening. Otherwise no weather of
consequence is expected. That being said, could not rule out patchy
fog forming late tonight, especially in/near areas that receive a
wetting rain this evening. Mild to warm night time lows will "cool"
into the lower 70s with light/variable winds.

.SHORT TERM...(Saturday through Sunday)
Issued at 257 PM CDT Fri Jul 21 2017

Another very warm to hot day will end the week. The upper high over
the region will break down more as moisture from the southern low
weakens the ridge. The strong daytime heating and this moisture and
resulting instability will result in isolated to scattered showers
and thunderstorms, especially tomorrow afternoon. Precipitable water
amounts rise into the 1.7 to 1.8 inch range, with locally heavy rains
and gusty winds accompanying the more substantial convection. High
temperatures in the low/mid 90s, along with dewpoint values into the
low/mid 70s will yield more heat indices into the upper 90s NE
Alabama and adjacent southern middle Tennessee, to 107-109 across
parts of NW Alabama. As such, another Heat Advisory will be needed
for Saturday afternoon.

More clouds than sun on Sunday, will bring somewhat of a relief
regarding high temperatures. Better rain chances are also expected as
various upper level systems move across the region. The NAM in
particular was depicting a possible gravity wave moving southward
from convection to our north. This could bring a good chance of rain
to the area to begin a new week. Precip water amounts are even
higher, generally over 2 inches from the NAM; somewhat lower from the
GFS. Thus locally heavy rains and gusty winds are again the main
threats. Some storms in these situations can become very active
electrically wise.

.LONG TERM...(Sunday night through Thursday)
Issued at 257 PM CDT Fri Jul 21 2017

Strong shortwave trough will push southward across the Mississippi
Valley on Monday. This will help slide a cold front, albeit a weak
front, toward the region. Expect scattered to numerous showers and
thunderstorms across the region on Monday into Monday evening. Given
the rather robust heat/moisture in place along with the enhanced
upward vertical motion/forcing with the upper level system and a few
strong storms are possible.Forecast soundings from the GFS indicate
fairly steep low level lapse rates (as you would expect in this
regime) and DCAPE potential approaching 1 kj/kg.

On Tuesday, the southern stream extension of the trough may attempt
to cut off keeping the weather somewhat unsettled across the lower
Tennessee Valley and Gulf Coast Region. The 12z GFS is slightly
further south with this feature but it`s fairly well defined in all
of the extended guidance. Thus, will maintain chance pops through the
Tuesday-Wednesday time frame. Again, a few strong storms will be
possible on Tuesday.

Flow becomes northwesterly later in the work week as the upper level
ridge is established once again across the Plains. This should keep
the worst of the heat to our west and at least maintain a low threat
of storms through the period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1214 PM CDT Fri Jul 21 2017

Large ridge of high pressure will remain anchored across the Mid
Mississippi Valley over the next 24 to 48 hours. This will provide
mostly subsident, dry, and VFR conditions for both terminal sites.
Have included diurnal cumulus but the predominant sky condition
should remain mostly clear through the period.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory until 7 PM CDT this evening for ALZ001>009-016.

TN...Heat Advisory until 7 PM CDT this evening for TNZ076-096-097.

&&

$$

NEAR TERM...RSB
SHORT TERM...RSB
LONG TERM...15
AVIATION...15


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at weather.gov/huntsville.



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