Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221936
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
236 PM CDT Mon May 22 2017

.NEAR TERM...(Tonight)
Issued at 236 PM CDT Mon May 22 2017

Deep convection is redeveloping in the warm sector south of the cold
front across central and eastern AL. A drier and more stable airmass
is limiting cumulus development over north AL and south TN. There
could be a few showers or t-storms over our southeast counties
closer to the instability axis. Mid and high clouds will continue to
stream east-northeast into the TN valley this afternoon into this
evening associated with the next in a series of weak shortwave
impulses. In response, the 850 mb boundary will lift north into north
AL this evening with a LLJ. The NAM indicates 30-40kt but the new
GFS has lower velocity of 15-20kt. This may be a bit more realistic
given the limited amplitude of the upper wave. Deep convection will
lift northward into mainly our southeastern counties overnight, with
lighter stratiform precipitation in our northwest counties. Locally
heavy downpours could occur with the deeper convection closer to
I-59.

.SHORT TERM...(Tuesday through Wednesday Night)
Issued at 236 PM CDT Mon May 22 2017

Deep convection should exit very early Tuesday with scattered
lingering showers possible. Will carry a low PoP through the day, but
due to lack of synoptic forcing, there are no major features to key
in on that would trigger organized precipitation. A much better
chance of more numerous showers and thunderstorms will arrive late
Tuesday Night into Wednesday morning with the stronger impulse. This
feature is dropping southeast through MT/WY this afternoon and will
deepen and amplify the upper trough over the MS valley. Deep layer
bulk shear values will become stronger (50-60kt) late Tuesday Night
into Wednesday morning with a rather moist airmass. Cyclogensis over
the OH valley will be ongoing, so the surface pressure fields/fronts
over the mid South and TN valley will be undergoing adjustments. The
warm front will not be overly well- defined, but may be over north AL
by Wednesday morning as the cold front tracks east into western AL
and middle TN. The warm sector will shift east before stronger
daytime heating ensues Wednesday, but we will need to monitor
convective trends closely late Tuesday Night, especially in our
southeastern counties.

Light showers and rather cool/dreary weather will occur on Wednesday
into Wednesday night. This being in the wake of the frontal passage
and the arrival of the 5h low and cold pocket and 8h temps dropping
into the 5-7C range. Highs on Wednesday should only be around 70F
with Thursday morning lows only in the lower 50s.

.LONG TERM...(Thursday through Sunday)
Issued at 236 PM CDT Mon May 22 2017

Isolated to scattered showers will linger mainly in northeastern
Alabama and Southern Middle Tennessee on Thursday, mainly in the
morning. Clouds will likely hang on as well through most of the day
in these areas, before quickly clearing by Thursday evening. The much
more abundant sunshine south of a line from the Muscle Shoals to the
Cullman communities will make it feel more comfortable, but still
cool (mainly highs in the lower 70s - upper 60s further northeast).
Some patchy fog may be possible Friday morning, but winds may stay up
just enough to limit this significantly.

Upper level ridging over the Gulf will produce northwesterly flow
over the area. This should drop drier and cooler air into the area
during the day Thursday night into Friday. Lows in the lower to mid
50s look possible Friday morning. During the day on Friday,
subsidence under upper level ridging and abundant sunshine should
allow highs to climb back into the upper 70s to lower 80s. Return
flow originating from the Gulf of Mexico by Friday night into
Saturday should advect more moisture back into the area ahead of a
developing area of low pressure over Texas. Isolated to scattered
showers could push into Southern Middle Tennessee by Friday night as
upper level energy skirts the area. However, models do not show much
instability (surface or elevated due to some drier air still in
place).

However, upper level ridging will keep most of the energy with this
system north of Alabama (and maybe Southern Middle Tennessee) until
Saturday night or Sunday, depending on which model you believe. For
now, going with blend of both for rain chances. A more moist airmass
(courtesy of the return flow off the Gulf of Mexico) keeps low
temperatures a bit warmer, at least in the lower to mid 60s,
Saturday and Sunday morning.

By Saturday into early next week, rain/storm chances increase, as
better forcing ahead of the surface low pushes east into northern
Alabama. At this point, although shear and surface based instability
is impressive at times, a very strong low level jet is not shown and
no significant helicity is forecast by models. So strong storms look
possible, but not sure much more. PWATS climb to around 1.7 inches
at times as well. So some heavy rainfall is possible during this
time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1147 AM CDT Mon May 22 2017

VFR conditions are expected through early this evening before an area
of showers and possibly thunderstorms reaches near the KHSV vicinity
(~00Z) and KMSL (~05Z). The chance of showers and lower ceilings
(015-025agl...MVFR) will be greater at KHSV, but we have held off on
the lower cloud forecast until 05Z. At KMSL, the chance of showers
and lower clouds is lower, so have kept conditions VFR there. The
area of showers will shift southeast Tuesday morning with ceiling of
~025agl possible at KHSV through 18Z.


&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...17
SHORT TERM...17
LONG TERM...KTW
AVIATION...17


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