Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221121 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
621 AM CDT Sat Jul 22 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 357 AM CDT Sat Jul 22 2017

A quiet morning across the Tennessee Valley with a strong upper-level
ridge of high pressure still generally the dominant weather feature
of influence. The result has been a mostly clear and calm early
morning, with a continued moist boundary layer, with dewpoints in
the low to mid 70s as of 09z. In fact, KHSV is still at a brutal
78/77! Due to these conditions, am expecting some light patchy fog to
develop, locally, along the valleys of Northeast Alabama and
along/near Lake Guntersville through daybreak.

This fog will quickly dissipate after sunrise and we shift quickly
to another hot and humid day across the Valley -- and a return of
scattered diurnal convection. A weak area of low pressure along the
northern Gulf Coast will try to sneak northward into the lower-
Mississippi Valley today (as the center of the aforementioned ridge
slips back toward the Ozarks and Southern Plains). As a result, we`ll
likely see some additional cloud cover and some scattered showers
and storms developing by midday through the afternoon. The HRRR is
hinting that this convection may initiate along a weak convergence
zone situated just south of the Tennessee River, extending NE into
the Cumberland Plateau. Either way, scattered convection is expected
across the entire region today. A few strong to perhaps marginally
severe storms cannot be ruled out, with gusty winds to 40-50 mph
possible with the strongest activity.

This convection should also result in more cloud cover and some
slightly cooler highs in the low to mid 90s. However, dewpoints in
the low to mid 70s (combined with the air temperatures) will make the
Heat Index miserable and downright dangerous for much of us across
the Valley this afternoon and evening -- as values peak in the 105 to
109 degree range again across all of Northwest and North Central
Alabama (as well as most of Southern Middle Tennessee). Left Dekalb,
Jackson, and Franklin (TN) out of the Advisory as cloud cover and
some slightly lower dewpoints should keep Heat Indices just below
criteria there.

.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. Eventhough 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.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory from noon today to 7 PM CDT this evening for
     ALZ001>008-016.

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

&&

$$

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


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