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Area Forecast Discussion

684
FXUS64 KHUN 280822
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
322 AM CDT THU JUL 28 2016

.DISCUSSION...

Latest water vapor imagery shows a subtropical high off the
southeast Atlantic Coast and a stronger high over the Desert
Southwest. Meanwhile, a weak trough axis extended through the
Mississippi Valley region. At the surface, a weak area of low
pressure extended across the southern MS Valley, with a weak
boundary trailing eastward and southward from this low. An area of
showers continues to slowly push to the northeast across western TN
and northern MS in response to a vorticity max in the base of the
longwave trough. Much of the forecast area remains under the
influence of the subtropical high, with slightly drier air noted
across the TN Valley. This will change as we move into the morning
hours, as the axis of the trough shifts east, as does the upper
ridge. Though for the next few hours, a few light showers may
develop, with cloud cover will keep temps from dropping too much
through sunrise.

The ongoing showers to our west are expected to enter west Alabama
toward the mid to late morning hours, as the trough axis shifts
northeastward. Although cloud cover will limit the daytime heating,
strong southerly flow will continue to push warm and moist Gulf air
into the area. Forecast soundings show PW values rising from 1.8 to
2.2 inches by the afternoon. Meanwhile, forecast CAPE values will
rise to near 1000 j/kg. These factors, combined with the enhanced
lift from the upper trough, will lead to a greater coverage of
thunderstorms than we have previously seen. However, confidence is
still somewhat low and therefore will keep PoPs to the likely
category. Locally heavy rainfall will be possible, though the dry
soil moisture will limit the likelihood of flash flooding, unless
precip rates are high. Max temperatures will greatly depend on the
coverage of thunderstorms, though highs will be a few degrees below
those observed yesterday due to the cloud cover expected.

The upper trough will weaken by sunset, with the main piece of
energy shifting to the east. The coverage and intensity of lingering
showers and thunderstorms will become limited and will likely
diminish entirely by midnight. The forecast area will remain under a
broad and weak upper level trough through much of the weekend, as the
subtropical ridge moves well south of the area. Several disturbances
are expected to develop and move through the longwave trough, which
will provide for scattered thunderstorms, generally in the afternoon
through early evenings. Much of the forecast will
depend on mesoscale features, as the synoptic lift will generally
remain weak. Have maintained the 50 PoP in for Friday through Sunday,
with lower rain chances during the overnight periods.

The upper level high over the desert SW will strengthen over the
weekend, and push eastward during the day Monday. The broad upper
trough over much of the eastern CONUS will strengthen slightly in
response to the upper ridge. This will put the forecast area in weak
northwest flow aloft Sunday night and Monday, with guidance showing
a vort max developing over the upper midwest and moving southeast
into the area. However, given the uncertainty on day 5, will
maintain our 40 PoPs in for this period. The remainder of the forecast
will be dominated by the upper level ridge, with rain chances
depending on mesoscale interactions.

73

&&

.AVIATION...
(Issued 1157 PM CDT WED JUL 27 2016)
For 06Z TAFs:

Ceilings are expected to oscillate btwn VFR and MVFR over the next 24
hours as a disturbance over the AR/MS/TN region moves northeast
affecting N AL/S Middle TN later tonight into tomorrow. Low MVFR
cigs will be the first to move over the TN valley especially close to
KMSL. Have included SCT prevailing for the next 8 hours but will
likely see BKN025 at KMSL. Also could see this drop in cloud cover
at KHSV but not as sure on timing. Then, -shra should begin sometime
btwn 28/14Z-18Z and increase in coverage throughout the day with
drops to IFR possible. Have included prevailing MVFR starting after
28/17Z through the rest of the forecast period for the reduced VIS
likely with any shra. In addition, could also see -TSRA btwn
28/19Z-28/24Z which would act to reduce the flight category to IFR.

SL.77

&&

.PRELIMINARY POINT TEMPS/POPS...
Huntsville    86  71  89  71 /  60  50  50  40
Shoals        83  71  89  71 /  60  50  50  40
Vinemont      84  70  87  70 /  60  50  50  40
Fayetteville  83  70  86  70 /  60  50  50  40
Albertville   86  70  88  70 /  50  50  50  40
Fort Payne    86  70  87  70 /  50  50  50  40

&&

.HUN Watches/Warnings/Advisories...
AL...NONE.
TN...NONE.
&&

$$

For more information please visit our website
at weather.gov/huntsville.



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