Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240236

Area Forecast Discussion
National Weather Service Huntsville AL
936 PM CDT Tue May 23 2017

.NEAR TERM...(Tonight)
Issued at 920 PM CDT Tue May 23 2017

One batch of showers and storms has finally moved off to the east
into GA. However, another round will be moving across the MS River in
the next hour or so. These showers and storms are forming along a
cold front that extends S/SE out of a low pressure system over the
Great Lakes into the ArkLaMiss. Latest radar and surface analysis is
showing a secondary low pressure system forming over NE TX along the
front. This feature is forecast to lift NE into northern MS and then
into central TN tonight through Wednesday morning.

Last few runs of the HRRR seem to have a good handle on the movement
although believe they are a bit on the slower side in the progression
of showers across the area. Initial forecast had PoPs increasing
well after 06z tonight but have shifted those to around midnight with
60-70 PoPs going for much of the overnight period. Forecast soundings
and model guidance all indicate some limited elevated instability
(200-500 J/kg) and 30-40kts of shear. Which is enough to continue
thunderstorm wording for tonight. Main hazards would be heavy
rainfall and wind gusts of 30-40 mph. Lows in the lower 60s still
seem reasonable based on temps upstream and latest hires guidance.

.SHORT TERM...(Wednesday Night through Thursday Night)
Issued at 249 PM CDT Tue May 23 2017

The upper low pulls away Wednesday night which should end precip
completely from southwest to northeast. Thursday should be a fine
weather day with a surface ridge shifting east with corresponding
confluent northwesterly flow at 500 mb. Highs should bounce back into
the lower to middle 70s, followed by lows in the lower to middle 50s
by Friday morning. We will hold off on patchy fog, but it may be
something for consideration if the winds are able to drop off.

.LONG TERM...(Friday through Monday Night)
Issued at 249 PM CDT Tue May 23 2017

Upper level ridging and subsidence behind the departed front become
established over the Tennessee Valley into the northeastern Gulf of
Mexico on Thursday night into Friday morning. This continues into
Friday afternoon, as the storm system over Oklahoma edges east only
slightly, mainly due to the upper level ridging to its east over the
Tennessee Valley. Although models hint at some energy moving through
almost zonal flow over northern Tennessee and Kentucky, the
atmosphere should still be too dry for even any isolated shower/storm
activity. Could see high cloudiness move into the area in response
to this upper level forcing, but most of it will be in Southern
Middle Tennessee.

Models continue to show very moist conditions returning northward
quickly Friday night into Saturday, as low level winds become more
southerly. However, synoptic models diverge concerning how far south
a warm front extending from the storms system over Oklahoma sets up.
GFS is much further north, while ECMWF is much further south by late
Saturday afternoon into Saturday evening. Thus, if GFS is right,
timing of precipitation potential may be later than the current
forecast indicates and hold off until late Saturday night. At this
point used a blend of models. This pushes scattered showers/storms
into Southern Middle Tennessee and northern Alabama Friday night
into Saturday. Warm temperatures aloft should keep wet-bulb heights
fairly high. Models show decent bulk shear values between 40 and 50
knots Friday night into Saturday, but the lack of forcing should
limit coverage to mainly near and north of the Tennessee River.
However, a few storms could be strong during this period producing
gusty winds around 50 mph and frequent lightning. Breezy conditions
with winds of 10-15 mph with gusts up to 25 mph should return on
Saturday. Higher elevations may see wind gusts a bit higher,
especially Saturday night.

Expect more widespread coverage of showers and thunderstorms to occur
Saturday night into Sunday (as forcing associated with the surface
low moving northeast into the western Great Lakes approaches and then
moves into Alabama from Mississippi). Most synoptic models are not
too different concerning the timing of this frontal boundary and have
the actual front approaching northwestern Alabama Sunday afternoon.
PWATS increase to between 1.6 and 1.8 inches and remain in this
range most of this period. This may lead to flash flooding/river
flooding issues during this period as well, given the slow movement
eastward of the front. Plenty of surface based and elevated CAPE is
forecast by models Saturday night into Sunday, especially on Sunday
(2000-3500 J/KG). 0-6 km bulk shear values will be between 40 and 55
knots as well. Low level helicity is forecast to increase Saturday
night to around 200 to 300 m2/s2 (especially west of I-65). This
helicity quickly decreases on Sunday. Couldn`t rule out a low
potential for a brief, weak tornado spinup on Saturday night before
helicity decreases. However, we are more confident that we will see
strong thunderstorms producing wind gusts around 50 mph, heavy
rainfall, and frequent lightning Saturday night through Sunday. Wet-
bulb heights remain fairly high, but lower to around 10,000 feet.

GFS more quickly moves the front south of the Tennessee River, but
ECMWF keeps the back edge of precipitation further north on Monday.
For now, went with a blend of models. Kept 30 to 40 percent chance of
showers and thunderstorms in the forecast to cover any convection
along and just behind the front. Kept mostly cloudy conditions in
place most of Monday as the upper low and associated moisture will be
slow to exit the area. By Monday night, a few showers may linger,
but this may be overdone due to blending of model differences.
Either way, cooler and drier weather should return with lows in the
lower 60s and highs around 80s degrees expected.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 620 PM CDT Tue May 23 2017

The storms have moved off to the east and we are left with VFR
conditions through about 06Z tonight. A cold front and associated
showers and thunderstorms will move across the area tonight. MVFR
cigs and vis accompany the storms tonight. Showers may continue past
12Z Wednesday morning but most of the activity will be off to the
east. MVFR cigs continue through the morning with winds increasing
out of the NW. Gusts of 15kts are possible in the afternoon.




NEAR TERM...Stumpf

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