Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240948

Area Forecast Discussion
National Weather Service Huntsville AL
448 AM CDT Wed May 24 2017

.NEAR TERM...(Today)
Issued at 345 AM CDT Wed May 24 2017

As of 330am, rain is already overspreading NW AL and moving into
Huntsville. A few lightning strikes have been noted. The surface
front that this is moving along is in NE MS and will continue to
press east this morning. An upper low is centered over N. Missouri
with the main energy out to the west. However, a strong shortwave
ahead of it coupled with an increasing LLJ from the south will
enhance frontogenesis around N. AL/Central TN/S. Central KY,
possibly with an additional surface low forming off in E. AR/W. MS.
Current water vapor imagery shows these two areas nicely this
morning as well as radar showing two areas of showers with a break
in between. The exact placement of these surface lows will likely
impact the path of the heaviest rainfall today.

Hires guidance is in decent agreement that as the widespread
rainfall moves east, precip will become more discrete and
potentially take on a bit of a line shape, as is already noted on
the leading edge. There is some uncertainty on the timing of both of
these waves as it looks like the HRRR/NMM/ARW are already 1-3 hours
too slow. It looks like this first leading edge will be
overspreading NE AL by 12z. As the day starts to heat up, the lift
moves east and we see a brief break in widespread rainfall,
instability will increase and bulk shear of 40-50kts could produce a
strong storm or two, possibly a severe one. Damaging wind gusts
would be the primary threat. Originally was thinking eastern areas
would have the highest chances but this was because of the slower
progression. With the quicker one, we could see a strong storm two at
any location across N. Al/S. Middle TN. Even if we do become
slightly capped with the cold advection, we should get enough heating
to support scattered thunderstorms and and showers through the
afternoon/early evening.

These showers will linger through the evening with the continued
lift from the upper low moving Southeast. The previously mentioned
cold advection and lingering clouds/rain will keep highs today only
in the upper 60s/lower 70s. Winds will also be breezy today with 15-
20 mph gusts possible.

.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 06Z TAFS through 06Z Wednesday night)
Issued at 1220 AM CDT Wed May 24 2017

Isolated showers and storms continue to develop over the TN Valley
this evening. Latest observations show low clouds between 700-1,000
ft developing over NE AL and models suggest some possible westward
expansion of those low clouds towards the HSV terminal. Have kept
them out right now due to the approaching front, which should keep
the lower clouds off to the east. But CIGs will lower as the front
approaches with showers and thunderstorms impacting both terminals
between 08-14Z tonight. A BKN deck of clouds will continue for much
of the Wednesday with spotty light showers. Breezy northwesterly
winds are expected for much of the day with gusts of 15kts possible.

Cigs begin to improve after 00Z Wednesday evening.





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