Area Forecast Discussion
Issued by NWS Huntsville, AL

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147
FXUS64 KHUN 211107 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
607 AM CDT Fri Jul 21 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 346 AM CDT Fri Jul 21 2017

It is a muggy early morning, with dewpoints in the lower to mid 70s.
Lows are expected to only drop into the 70 to 75 degree range in
most locations around daybreak. Would not be surprised to see
Huntsville only drop to 76 or 77 degrees. Most models rotate some
slightly deeper moisture from northeastern Tennessee into northern
Alabama this morning into the afternoon hours. At this point, most
models are showing overall fairly low relative humidity fields around
700 mb with this feature. Therefore, am a little skeptical that more
than a scattered deck of cumulus will form today (but could not rule
out some brief mostly cloudy conditions north of the Tennessee
River).

Most models forecast the upper-level ridge to set up directly over
Nashville and and extend southward into northern Alabama. Thus,
stronger subsidence should be in place today and help to further
limit cloud cover. Models continue to show warmer 925 mb temperatures
(26 to 29 degrees) spreading further east today directly underneath
the upper ridge. This should push temperatures higher than yesterday
across the region, especially with more sunshine expected east of
I-65. Highs up to 99 degrees is expected both in the Muscle Shoals
and Huntsville locations (couldn`t rule out a 100 degree high at
either location). Further east, highs in the 93 to 97 degree range
look reasonable in northeastern Alabama and Southern Middle
Tennessee.

Expect dewpoints to lower a bit, but remain in the 71 to 74 degree
range much of the day across the area. This will push heat index
values into the 105 to 109 degree range across most areas, except
Dekalb county (mainly due to lower temperatures reaching the 93 to
95 degree range) and expected lower dewpoints around 71 degrees. Will
add Jackson and Franklin counties to the current heat advisory given
the expected expansion of these values further east.

At the same time today, the weak upper low over the Florida peninsula
is forecast by models to push west into the northern Gulf of Mexico.
Models extend a weak inverted trough axis northward from this feature
into southern/central Alabama. This will likely produce some
isolated to scattered convection in these areas. Some of this forcing
may edge into our southeastern counties from the southwest, but only
included isolated chance of a shower or thunderstorm east of a line
from Hanceville to Guntersville to Scottsboro for this forcing.

.SHORT TERM...(Tonight through Sunday)
Issued at 346 AM CDT Fri Jul 21 2017

Models depict additional energy aloft rotating around the upper
level ridge tonight along this inverted trough axis. Models are a bit
split concerning how far west this activity makes it due to
positioning differences in the upper ridge. However, at least
isolated pops look warranted tonight given this forcing and still a
very moist boundary layer. Gusty winds and frequent lightning will be
the biggest threats with the strongest storms.

By Saturday, models continue to strengthen the convergence along this
inverted trough axis, as a weak longwave trough axis extends
southwest into it. With daytime heating and deepening moisture,
scattered showers and thunderstorms look possible. We should see a
little relief from the oppressive heat, thanks to more cloud cover
and a better chance of shower/storms. Strong storms producing gusty
winds to around 50 mph and frequent lightning look possible.

Looks like we`ll see only isolated showers/storms Saturday night, as
we lose the effects of daytime heating and the longwave trough axis
shifts east of the area.

However, as a frontal boundary approaches and then moves into the
area Sunday through Monday, chances of showers and thunderstorms
increase and temperatures moderate even more. Should see highs
only climb into the mid 80s to around 90s degrees by Sunday and
Monday. Little shear is forecast with this frontal boundary. However,
strong storms producing gusty winds to around 50 mph and frequent
lightning look possible. With the dry air aloft and moist boundary
layer, would not be shocked if we saw an isolated severe thunderstorm
that could produce damaging winds. However, mainly expect strong
thunderstorms with the potential of produce wind gusts around 50 mph.

.LONG TERM...(Saturday night through Thursday)
Issued at 346 AM CDT Fri Jul 21 2017

The upper-level ridge that has brought the dome of very hot and
humid air into the region will shift back to the west by Sunday
night into Monday. An upper-trough shifting from the Great lakes
into the Central Appalachians will try to push a weak front into
Tennessee on Sunday and eventually the Tennessee Valley of Northern
Alabama by Monday. There is still uncertainty as to how far south
this boundary will make it, but wherever it hangs up, it will serve
as a focus for showers and thunderstorms on Monday and Tuesday. In
addition to this boundary, a very weak upper-level disturbance along
the Northern Gulf Coast will help to advance some richer moisture
into the region. As a result, scattered to numerous showers and
thunderstorms are expected each day (favoring the afternoon and
evening hours). With MLCAPE values in the 1000-2000 J/kg range, some
localized strong storms will be possible. This rich Gulf moisture
will push PWATs around 2" by Monday, making heavy rainfall and
localized flooding a concern as well through Tuesday.

Thereafter, models diverge a bit in their solutions on how the
pattern will evolve for the later part of next week. The initial
upper-trough will shift well east of the region by Tuesday Night,
allowing high pressure to potentially build back in for Wednesday.
The ECMWF, however, does try to maintain some troughing over the
Eastern CONUS as it wants to push a broad, weak upper-level
disturbance into the Tennessee Valley late Wednesday and eventually
through the Southeastern United States. This would place the region
underneath a slightly cooler and wetter northwest flow pattern. The
GFS is more bullish on reestablishing the ridge, which would promote
a slightly warmer -- and more typical diurnally driven convection
pattern through the end of the period. Due to this uncertainty, have
steered the forecast toward the SuperBlend/CONSAll solution for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 606 AM CDT Fri Jul 21 2017

MVFR VSBYS should continue at KHSV until approximately 8 AM.
Otherwise expect VFR conditions and light winds at both TAF sites.
Some additional fog could form on Saturday morning, but left out
due to low confidence given the additional dry/hot weather expected
today.

&&

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

TN...Heat Advisory from noon today to 7 PM CDT this evening for TNZ076-
     096-097.

&&

$$

NEAR TERM...KTW
SHORT TERM...KTW
LONG TERM...AMP.24
AVIATION...KTW


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