Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 221419

Area Forecast Discussion
National Weather Service Huntsville AL
919 AM CDT Tue May 22 2018

.NEAR TERM...(Rest of Today)
Issued at 919 AM CDT Tue May 22 2018

Morning satellite imagery is showing a weak upper level circulation
centered in central AL. As this moves north, it will be the main
driver of scattered to numerous showers and thunderstorms this
afternoon. LAPS soundings and cross sections show plenty of moisture
available with PW values already over 1.5". As of 9AM, we`re already
seeing light showers develop in areas east of Huntsville. Hires
guidance is less bullish in overall coverage of storms today but
them, along with the GFS/ECMWF/NAM all show a gradient of higher POPs
to the east, closer to where the upper low will pass, and lower in
the west. At this point, there seems to be no reason to edit the
earlier POP forecast with chances increasing throughout the day. Do
agree with the earlier assessment that hail does not look likely and
with a saturated sounding and low DCAPE, it will be hard to get
severe winds. Can`t rule out a few strong storms with gusty winds
though. Heavy rain will be the highest threat today due to the wind
profile all the way through the atmosphere being less than 10kts. Did
make some slight edits to the temps today, showing more of a
gradient of lower temps (upper 70s) in the east and lower to middle
80s in the west.

.SHORT TERM...(Tonight through Thursday)
Issued at 353 AM CDT Tue May 22 2018

The shear zone to our NW will gradually shift SE today, and move into
the area this evening. This will bring in some drier air aloft
overnight, but there will be little noticeable change at the
surface. This trend will continue into Wednesday. Dry air and some
subtle ridging to the west will not be enough to remove rain chances,
and NE AL will continue to carry the highest PoPs. The introduction
of mid-level dry air will increase the chance for strong downdrafts
for any storms that do manage to form. Increased sunshine will allow
temperatures to rebound into the upper 80s.

The region will remain in a transition zone for Thursday, with drier
air (and mid-level ridging) to the N/W, versus greater moisture to
the S/E. PoPs reflecting this gradient will persist, since
subsidence and dry air still do not appear sufficient to suppress
convection. Highs Thursday should be comparable to Wednesday. With
low-level moisture remaining consistent, low temperatures will stick
with persistence through the entire period as well.

.LONG TERM...(Thursday night through Monday)
Issued at 353 AM CDT Tue May 22 2018

The ridge that lingered across the Southeast throughout the week
will finally move off to the east as an upper-level trough swings
through the Northern Plains and another upper-level low lifts into
the Central Gulf of Mexico. Rich Gulf moisture will start streaming
into the region bringing an increase in cloud cover and higher POPs
into the forecast starting on Friday. This will help cool temps down
by a few degrees, with highs reaching the low to mid 80s Friday

Model runs are coming in weaker with the Northern Plains trough and
agree with a low developing in the Central Gulf. However, there is
model disagreement on the strength and track of the low in the Gulf.
The favored solution is the ECMWF that has the weaker Northern Plains
trough than the GFS, causing the Gulf low to take more of a western
track and stall out over the lower MS Valley. This also has the
potential to take on tropical characteristics. Regardless it will
stream tropical moisture in, as PWATs will increase to 1.8-1.9" (the
99th percentile for May) by this weekend and further increase POP
chances heading into Memorial Day. Weak wind profiles will keep shear
to a minimum, but the scattered to numerous showers and
thunderstorms will have the potential to produce heavy rainfall with
that moisture in place. Daytime highs over the weekend and for
Memorial Day will be in the low to mid 80s with overnight lows
staying steady in the mid to upper 60s. Models still have some time
to resolve their disagreements, so stay tuned to the forecast if you
have any outdoor plans this weekend.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 626 AM CDT Tue May 22 2018

MVFR ceilings at KHSV and KMSL will develop/persist until around
20Z, though they will lift above 2000 ft around mid-morning.
Confidence in showers and storms near KHSV has increased, so the TAFs
include VCSH after 16Z, and VCTS after 20Z. Confidence is lower at
KMSL, so only VCSH is included for the afternoon period. Confidence
regarding timing remains too low to include beyond VCSH/VCTS at this

Low clouds and showers should thin out after dark with a return to
mostly VFR conditions. However, fog may form at KHSV, particularly if
it rains, so a TEMPO for MVFR visibility has been included 08-12Z.
Confidence is lower at KMSL so it will not be included there.





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