Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221538

Area Forecast Discussion
National Weather Service Huntsville AL
1038 AM CDT Sat Jul 22 2017

.NEAR TERM...(Rest of Today)
Issued at 1038 AM CDT Sat Jul 22 2017

Around daybreak, upper level ridging that was situated from the
Ozarks to the Mid South was in the process of breaking down. A
complex of weak upper level lows were moving westward across the
Gulf coast. At the moment, the Tennessee Valley was in-between the
above noted upper ridge and low. With strong daytime heating, cloud
formation (mainly "fair weather cumulus" at the moment) were forming
across the region, as air temperatures rise through the mid/upper
80s. Areal dewpoint values in the mid/upper 70s ~ (like Huntsville
and Scottsboro at 79) were producing Heat Index values in the mid 90s
to 101.

High temperatures should continue rising into the low/mid 90s most
locations, with dewpoints in the low/mid 70s. This should create
heat indices from the mid/upper 90s mountains, to 108-109 across
parts of northwest and north central Alabama. As such will keep the
Heat Advisory going.

This moisture and resulting instability in a less capped environment
could lead to isolated to scattered showers and thunderstorms from
the late morning to early evening (especially this afternoon).
Forecast soundings inverted-V profile and WINDEX output suggests the
stronger storms could produce strong gusty outflow winds, torrential
rainfall, and frequent lightning.

Did not make major changes to the forecast for this afternoon, except
for minor temp/dewpoint tweaks and delayed showers for an hour or to
most spots.

.SHORT TERM...(Tonight through Monday)
Issued at 357 AM CDT Sat Jul 22 2017

A change in the pattern is coming as the aforementioned high
pressure ridge is forced further to the west on Sunday. This will
allow an upper-level trough passing through the Great Lakes and Ohio
Valley to drive a front into Tennessee (Sunday) and into Northern
Alabama (Monday). The weak coastal low mentioned earlier will have
helped to bring some very rich Gulf Moisture into the region with
PWATs progged to be at or slightly above two inches. Thus, with a
weaker CAP, scattered convection will be able to easily develop on
Sunday -- and be capable of heavy downpours and lightning. As the
front drops south Sunday night into Monday, more widespread storm
clusters will develop along and ahead of the boundary. Despite the
fairly weak shear, models do show enough instability to result in
some strong storms capable of damaging winds. However, the main
concern could be localized flooding given the boundary to focus this
activity, combined with the slow movement of these storms. Will have
to watch this in the coming days.

.LONG TERM...(Monday night through Friday)
Issued at 357 AM CDT Sat Jul 22 2017

Models finally push the back edge of the slow moving frontal
boundary into the area Monday night. For now, will only include 30
to 40 percent chances of precipitation, given the loss of daytime
heating and very weak shear forecast. Even though the surface front
will push further southeast slowly on Tuesday, the upper level trough
axis looks like it may linger behind the front. With some added
daytime heating, this should allow coverage to increase again Tuesday
afternoon, especially east of I-65. There remains little shear over
northern Alabama and the better forcing should be southeast of the
area and in Georgia. So, not forecasting above scattered coverage of
showers/storms for now. The atmosphere will remain very moist ahead
of this boundary through tuesday with PWAT values around or above
2.00 inches.

Precipitation chances drop significantly Tuesday night, as primarily
subsidence behind the front becomes established over northern
Alabama and drier air pushes into the area. Some upper level energy
may still move into the area via northwest flow, so cannot rule
out an isolated shower or thunderstorm though. It should be slightly
cooler with lows dropping into the upper 60s to around 70 degrees.
Models develop a weak upper low along the front to our southeast in
Georgia on Wednesday. This should increase precipitation chances
again and expect mainly widely scattered showers or thunderstorms
then, with the highest coverage east of I-65. GFS is even drier than
the ECMWF and only indicates isolated precipitation chances. Cloud
cover and scattered rain chances should help to keep high
temperatures in the upper 80s to lower 90s. It may be a bit warmer if
ECMWF is right.

The eastern edge of upper level ridging begins to try to build back
into the Tennessee Valley Wednesday night into Thursday. Although
this should increase subsidence over the area, northern Alabama and
southern Tennessee will still be on the edge of the ridge upstream
of northwestern flow. GFS shows some weak upper level energy moving
through this flow into the area Thursday through Friday. Some
stronger storms producing gusty winds look possible with very
moist boundary layer conditions returning, ample instability, and
some dry air in place aloft. ECMWF is much more bullish with this
energy and keeps it mainly north of the area. Therefore, went with a blend
for PoPs, keeping isolated to scattered pop in the forecast during
this period. Temperatures are the tricky part, as 925 mb temperatures
really rise on the eastern side of the ridge. However, if GFS is
right, cloud cover and precipitation will keep highs from climbing
much above the upper 80s to lower 90s (despite the warm temperatures
just above the surface). However, if the ECMWF is right with little
forcing over the area, we could be looking at another brief hot spell
(maybe mid/upper 90s again).

As a frontal boundary sags south and stalls over the area on
Friday/Friday night, a stronger longwave trough axis swings
southeast from the Great Lakes region. Slightly better shear ~ 30
knots is forecast by models as this impulse of energy pushes the
stalled front south through the area by Sunday morning. Along with
SBCAPE values between 2000 and 3000 J/KG, some strong storms look
possible with wind gusts around 50 mph, heavy rainfall rates, and
frequent lightning.  So a fairly wet weekend looks to be in store
compliments of more organized/longer lived convection along/ahead of
the front pushing through.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 621 AM CDT Sat Jul 22 2017

VFR conditions will be the predominant flight category at each
terminal through the period. Scattered TSRA will develop by midday,
and could potentially impact a terminal with localized IFR
conditions. Given uncertainties in the spatial coverage, have opted
to handle this with a VCTS for now as do not have 50% confidence yet
that a storm will impact either KTRI or KTYS. Should rainfall occur
later today, patchy fog will be possible late tonight. However, did
not have enough confidence to mention at this time.


AL...Heat Advisory until 7 PM CDT this evening for ALZ001>008-016.

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




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