Area Forecast Discussion
Issued by NWS Huntsville, AL

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821
FXUS64 KHUN 241128
AFDHUN

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

.UPDATE...
For 12Z TAFS and Near Term.

&&

.NEAR TERM...(Today)
Issued at 628 AM CDT Wed May 24 2017

As of 6AM, the previously forecast line of thunderstorms is starting
to exit NE AL/S Middle TN. Scattered showers are noted back to the
west with an additional area of widespread rainfall moving slowly
E-NE (more north than east) back in NE MS. A few lightning strikes
have been noted in this area as well.

An upper low remains 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. Although WPC has the cold front
analyzed over N Central AL, thinking it is still west of Muscle
Shoals this morning. 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.

The HRRR/RAP are still slow with the current evolution of the precip
with the RAP slightly closer, with an enhanced area of precip at 11z.
If the RAP is correct, the next batch of rain won`t account for much
and be out of the area by 17z. Then just scattered light showers this
afternoon. The HRRR is also showing this second batch moving through
although it is wanting this second batch to have some stronger cells
in it. Looking at forecast soundings, we don`t seem to gain more
instability/break the cap until after 18/19z and I think the HRRR is
moving this second batch through too fast (despite the lag in the
current line). Overall, not confident on what happens after 18/19z
today. Will likely see scattered showers but how strong they become
is the question. There will be decent bulk shear but it will depend
if the atmosphere can destabilize in spite of the cold advection
behind the front. Any strong storm could produce strong wind gusts.

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
then.

.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
Tuesday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 628 AM CDT Wed May 24 2017

Tricky TAF forecast again today. CIGS are currently at 500ft at HSV
but do not think they will stay there long so included that in a
TEMPO group. Went around 1kft for the prevailing group to start both
TAFs this morning. Winds increase late this morning with occasional
gusts to 20kts and that should help increase CIGS a bit. Showers will
be on and off all day and added another TEMPO group for -TSRA. Not
very confident on this timing or the overall coverage so amendments
may be necessary. Did not include rain after 23z but there is still a
chance for some scattered showers and will wait till the next TAF
update to reevaluate. By the end of the TAF period, CIGS will start
to scattered out with winds lessening.

&&

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

$$

NEAR TERM...LN
SHORT TERM...LN
LONG TERM...70/DD
AVIATION...LN


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