Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 260525
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1225 AM CDT Sun Mar 26 2017

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 811 PM CDT Sat Mar 25 2017

Large area of showers encompasses much of central and north AL into
eastern sections of middle TN. Instability is quite low based on
MUCAPE analysis. Instability is a bit higher over MS where steeper
lapse rates reside in closer proximity to the shortwave pivoting
across the MS river. Short range mesoscale models suggest this
activity will slowly move northeast through north AL and southern TN
after 06Z as the warm conveyor belt precipitation shifts east. Our
going hourly PoP forecast is excellent in this regard. The only
change I made was to go with coverage wording, dropping thunderstorms
to isolated. Earlier, we made some hourly temperature and dew point
adjustments as well.

.SHORT TERM...(Sunday through Sunday night)
Issued at 311 PM CDT Sat Mar 25 2017

Some storms may linger over northeast Alabama as the last of the
squall line and one of the shortwave troughs moves northeast on the
southeast quadrant of the upper low. Though lift is rather weak,
there is likely to be weak instability with warming of the profile
aloft keeping deep convection mostly suppressed. Uncertainty exists
on whether there will be additional cells in the afternoon and
evening as the main upper low opens and lifts northeast towards the
Great Lakes. There is not an actual front that passes over the
region behind this trough-more of a dry line type boundary. So, the
TN Valley remains mostly in the warm sector for the weekend. It is
possible that there could be some brief breaks in the cloud cover and
also orographic lift that could support storms in the afternoon and
evening over northeast Alabama. However, there are so many
limitations to this scenario, anything more than scattered strong
storms (at worst) are forecast. Otherwise, since we are in the warm
sector, daytime highs should be warm in the 70s before another more
weakly amplified shortwave trough, that will be affecting the Plains
on Sunday, moves east. This trough should bring the next opportunity
for storms (some strong to severe) on Monday. More details on that in
the long term section.

.LONG TERM...(Monday through Friday)
Issued at 311 PM CDT Sat Mar 25 2017

The upper low continues to move east-northeast across southern
Missouri during the day on Monday towards the Tennessee Valley.
Models have shifted this feature slightly further north the last few
runs. However, some upper level energy/forcing just east of the
approaching shortwave axis itself remains to our west much of the
day. This could produce some scattered showers and storms given the
unstable nature of the air mass that will be over the area. Models
continue to show surface based CAPE values of between 1000 and 2000
J/KG and about the same amount of elevated (MUCAPE) instability. Bulk
shear values of 40-50 knots are expected. This is not overall
impressive, but likely enough for severe thunderstorm development.
The most impressive parameters continue to be steep low and mid-
level lapse rates and wet-bulb zero heights between 7000 and 9000
feet. Any storms that develop on Monday could produce damaging winds
or large hail. As we get into the late afternoon hours into the very
early evening hours, low-level helicity climbs enough to be concerned
about tornado potential. The better threat though should be large
hail (golfball or larger) given instability and lapse rates.

As the shortwave pushes further east through Tennessee/Kentucky
Monday night, enough elevated instability will likely be in place to
continue to produce more marginally severe hail through about
midnight. Lingering weaker scattered storms will continue on Tuesday,
as the disturbance continues to push further east into Georgia with
some trailing lift extending southwest into the area.

High pressure builds into northern Alabama behind the front on
Tuesday. Although some drier air will advect into the area by Tuesday
morning, warm temperatures in the mid 70s will continue due to the
strong ridging aloft. Lows in the 50s look reasonable with little
cloud cover.

Models continue to develop a weak shear region somewhere between
central and northern Alabama on Wednesday. A few showers or
thunderstorms are included in the forecast to cover any
precipitation. However, with a very strong upper ridge over the area,
not real confident this will develop, but left in for continuity. As
the ridge continues to strengthen Wednesday into Thursday, expect
highs to be able to climb into the upper 70s to lower 80s. Believe
this will hold off precipitation longer than even the Superblend is
showing, but to keep consistent, left in 30 to 50 pops as early as
Thursday morning. This could be a strong to severe thunderstorm
event, but model differences Thursday night into Friday looks like a
better bet for more widespread activity and more likely when severe
thunderstorm potential could evolve.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1225 AM CDT Sun Mar 26 2017

Bands of -ra/shra have moved e of the two main terminals going into
the early morning hrs, with cloud bases above VFR levels. Lower
clouds more in the 1-2K ft range are xpcted to spread ewd into the
area near daybreak ahead of an approaching cold front. Some
additional -ra/shra may also move back into the area from the w
along/ahead of the oncoming front into the morning period. Cigs will
then begin to lift/sct heading into the early afternoon hrs as the
front moves e of the local area.

&&

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

$$

NEAR TERM...17
SHORT TERM...SL.77
LONG TERM...KTW
AVIATION...09


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