Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 241748

Area Forecast Discussion
National Weather Service Huntsville AL
1248 PM CDT Wed May 24 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 1057 AM CDT Wed May 24 2017

Morning GOES-16 imagery shows the parent upper low centered over
Missouri, but the more potent-looking shortwave has moved across
northwest Alabama into middle Tennessee. Consequently, the surface
front has just passed through DCU on its way to HSV, marked nicely by
a line of heavier showers. However, the extent of the rain behind
the front has dwindled, with little to no rain extending south of
the Tennessee-Alabama state line. There are a few showers well behind
the front over west Tennessee, closer to the main upper low.

The precipitation and convection forecasts are complex as the upper
low slowly wobbles into Tennessee tonight. The HRRR is handling the
current situation with the frontal position and shortwave better than
the other convection-allowing models. However, despite the upper
low, the HRRR keeps late afternoon precip coverage pretty scattered.
This seems a little sparse given the steepening lapse rates. So the
ongoing forecast of PoPs dropping to around 50%, then increasing
again to around 60% for the late afternoon looks reasonable in light
of little other reliable guidance to go on. Other grid parameters
including wind gusts have been updated to reflect ongoing conditions
and the heavier HRRR scenario noted above.

.SHORT TERM...(Tonight through Friday Night)
Issued at 345 AM CDT Wed May 24 2017

The base of the deep upper trough will reach the area after 00z
tonight with the center of the upper low finally moving off to the
northeast, centered near IN/IL by 12z. This is actually faster than
previous forecasts. This means that most of the precip/moisture
should pull off to the northeast and be out of the area by 12z
Thursday. Can`t rule out an isolated shower Thursday but it looks
like dry air will be quick to move in and shut off those precip
chances. Kept thunder out of the forecast overnight due to the
increasing low level cap from the cold advection. Thursday should
therefore be quite pleasant with temps in the lower to middle 70s
with clearing skies. Lows Thu night will be in the mid 50s.

Dry weather returns for Friday and Friday night as heights quickly
rise and temperatures warm. With southerly winds and sunny skies,
temps on Fri will get back into the lower to middle 80s with lows in
the mid to upper 60s. ECMWF is showing a mid level wave/MCS making
it`s way into the area by 9-12z Saturday but as it is the outlier
right now and given the limited moisture, will keep it dry through

.LONG TERM...(Saturday through Tuesday)
Issued at 447 AM CDT Wed May 24 2017

Consensus of medium range guidance from the global models suggests
that a subtropical ridge will be centered across south TX and
adjacent portions of the western Gulf at the beginning of the
extended period...with a current of strong westerly flow aloft to the
north of this feature encompassing much of the southern CONUS. At
the same time, a high-amplitude trough in the northern stream of the
jet is expected to be spreading slowly east-southeastward into the
northern Plains on Saturday. A weaker 500-mb disturbance will likely
be ejecting northeastward across the OH valley and into the central
Appalachians ahead of the northern stream trough, with scattered
showers and thunderstorms possible across our region early in the day
in the strengthening warm/moist advection regime south/east of this
feature. Of more concern is the redevelopment of thunderstorms to our
northwest Saturday afternoon, which will occur as a developing
surface wave lifts northeastward into the lower Great Lakes and the
trailing cold front begins to push southeastward across MO/IL. If
convection is as widespread to our northwest as anticipated, the
formation of a large cold pool and subsequent development of an MCS
will likely occur during the evening hours. Deep-layer steering flow
and a favorable orientation of the low-level jet suggest that this
MCS will likely propagate southeastward and into the TN Valley early
Sunday morning...and with elevated CAPE remaining in the 1000-1500
J/kg range as bulk shear increases to 45-50 knots, damaging winds and
large hail will be possible.

As for the period from Sunday into Sunday night, models indicate that
the northern stream trough will deepen and begin to close off in the
vicinity of southeastern Ontario and the northwestern Great Lakes.
Although this will maintain a strong westerly flow aloft across the
CWFA, there is little evidence of any disturbance which would
reinvigorate convection -- especially if the local airmass remains
somewhat stabilized by widespread precipitation earlier in the
morning. Thus, although low-end likely POPs were carried throughout
the day in the official forecast, there is a distinct possibility
that the afternoon hours will feature a much lower coverage of
showers/storms compared to the morning. A strong vorticity max
pivoting cyclonically around the closed low will likely provide the
force to push the cold front through the region on Sunday
night/Monday morning. A second round of convection will be possible
along the cold front Sunday night...with a slightly cooler but much
drier airmass expected to push into the region for Memorial Day and


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1248 PM CDT Wed May 24 2017

Generally VFR conditions are expected thru 18Z/25. Ceilings may
continue to waver 2500-3500 ft for the next couple hours. Showers
and thunderstorms will become more scattered as the upper level
system moves into the area. Coverage and impacts remain highly
uncertain, so have gone with a VCTS for now and further amendments
may be necessary. The scattered showers/storms will continue thru
about 07-08Z, when the rain should taper off. Clouds will stick
around but should stay in the VFR range.





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