Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 250621
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
121 AM CDT Sat Mar 25 2017

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 915 PM CDT Fri Mar 24 2017

Surface analysis this evening continues to show high pressure ridging
over the region from the east. Further west, a potent surface low
with attendant cold front continues to gradually track eastward
through OK. Forecast track has this system lifting toward the Middle
Mississippi Valley by tomorrow, as the cold front begins to shift
through the TN Valley.

No significant changes were made to the forecast this evening.
Anticipating a QLCS to affect the western CWA beginning by the late
morning hours. Intensity will depend on moisture advection from the
south and west which would promote some intensification along the
line. Otherwise, forecast is on track.

.SHORT TERM...(Saturday through Sunday)
Issued at 1255 PM CDT Fri Mar 24 2017

Not much change in previous thinking with regard to the severe
weather forecast on Saturday. In general, following closer to the NAM
solution with a morning QLCS initially weakening upon entry into
middle TN and northwest AL as it encounters relatively lower
instability east of the higher moisture tongue of dew points located
further west. Then, some re-intensification is possible as the line
progresses east and daytime heating occurs. CAPE values are
difficult to project at this point given uncertainties on cloud cover
and how limited the moisture return/dew points will be in our
eastern counties. 6km bulk shear values of generally from 35-45kt
will sustain this line, but 2km bulk shear values are rather modest
around 20kt.

Redevelopment of showers and a few thunderstorms will remain
possible through Saturday Night, and especially on Sunday with an
improving quality of the warm sector. However, with the departing
shortwave and -divQ forcing, the only feature of note is generalize
low level confluence, mainly in a corridor from southern MS through
much of central and eastern AL into eastern TN and GA. 6km shear
values stay in the 30-35kt range within the shortwave ridge position
Sunday, but CAPE values should exceed 1000-1500j/kg in the warm
sector with WBZ heights staying below 8kft. Thus, hail may be the
greatest threat on Sunday from deep convection.

.LONG TERM...(Sunday night through Thursday)
Issued at 302 PM CDT Fri Mar 24 2017

This area of confluence does not move much through Sunday night.
Enough moisture advection night occur along this boundary to produce
some isolated to widely scattered showers and thunderstorms. Some
storms could be strong in nature. These could produce some small
hail and gusty winds around 40 mph. May have to watch for marginally
severe hail, with some models forecasting elevated MUCAPE values
between 1000 and 1500 J/KG. However, weak nature of lift should
limit this potential markedly. Continued southwesterly flow of 5 to
10 mph should only allow temperatures to drop to around 60 degrees.

Much better forcing approaches the Tennessee Valley on Monday. Most
model guidance has a strong CAP in place around 850 mb. This is
likely why most models are moving some decent vorticity into, but not
really producing any precipitation until around 18Z, even as far
west as northwestern Alabama. However, with even stronger forcing
pushing into northwestern Alabama between noon and 3 PM, this will
likely break. Enough bulk shear (~45 KTS) and abundant low level
instability (1200-1800 J/KG) combines with decent low level helicity
(200-300 m2/s2) and wet-bulb values between 7000 and 9000 J/KG to
produce another round of strong to severe thunderstorms. Expect the
severe weather to be more widespread with this system. Large hail and
damaging winds look like the biggest threats (including hail larger
than golfballs). More than enough low level turning of winds
(helicity) exists into the early evening hours to produce tornadoes
as well, if these parameters materialize as models are showing. The
severe threat should end a little before midnight. However, shower
and thunderstorms will remain possible through daybreak on Tuesday.
Models stall this front somewhere between central Alabama and
Tennessee on Tuesday, keeping scattered showers thunderstorms in the
forecast. Very weak dynamics and little shear/instability will make
it hard even for thunderstorm development, but kept them in the
forecast.

Models are more convergent upon even stronger dynamics and
thermodynamics developing with a stronger storm system moving from
Texas towards the Tennessee Valley Wednesday night into
Thursday/Thursday night. These could be severe as well producing
large hail and damaging winds, but this could change as we get closer
to the event.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 121 AM CDT Sat Mar 25 2017

Even with mid/high clouds streaming into the area from the sw, VFR
conds prevail for now at the two main terminals. Cloud bases will
begin to lower below 3K ft after 12Z, as se winds increase into the
12-15KT range with higher gusts and a cold front approaches from the
w. A line of shra/tsra is also xpcted to develop along/ahead of the
approaching cold front during the afternoon hrs, although exact
placement/timing of the front/associated convection remain somewhat
uncertain attm. In any case, the front looks to slow down as it makes
its way into the I-65 corridor, with sfc flow remaining out of the s
near 10KT with higher gusts.

&&

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

$$

NEAR TERM...Barron
SHORT TERM...17
LONG TERM...KTW
AVIATION...09


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