Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 301756
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1256 PM CDT Thu Mar 30 2017

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 1020 AM CDT Thu Mar 30 2017

A complicated forecast for short term, as a powerful system over the
Mid Mississippi Valley region moves to the ENE. This system will
bring a strong cold front across the region tonight, with numerous
showers and thunderstorms - some possibly severe.

First of all, a decaying area of showers noted via radar moving
eastward across portions of north/central Alabama was producing
wake-low type gusty winds across parts of central and northern
Alabama and southern middle Tennessee. Wind gusts into the 25-30 mph
range have been noted at various mesoscale and primary/METAR sites.
Given a generally dry layer between ~900-450mb (over central Alabama)
has resulted in the shower activity dissipating with little or no
rain reaching the ground - instead evaporating and helping to produce
these gusty winds.

Another item of note is a large area of convection from ~100 miles
or so SW of the Mississippi Delta to across southern Mississippi.
Blow off clouds associated with this activity was overspreading the
region to the north, per SSW upper level winds. Both legacy and GOES
16 animation of the various infrared and water vapor bands suggest
that these clouds will thin somewhat during the course of the day.
This could result in high temperatures going higher than what we have
now (in the upper 70s).

Model output from the various short-term mesoscale (RAP, HRRR) and
the 12Z NAM show different solutions regarding the development of
convection later today. The RAP was indicating activity forming
shortly across NW Alabama; while the HRRR and NAM play "a delay of
game" holding off on the heaviest showers/strongest storms until late
this afternoon and this evening. The NAM taking a similar more
ominous look like it had yesterday was showing two distinct areas of
convection - with the more northern one affecting the Tennessee
Valley late this afternoon and evening. Given these discrepancies
and other variances shown by earlier NAM/GFS/ARW runs, the Storm
Prediction Center has at this time lower the odds for severe storms -
moving the Enhanced severe risk a bit north of this area. However,
all modes of strong to severe storms - including tornadoes do in fact
remain possible into tonight.

Going with possible clearing, have raised high temps a degree or two
this afternoon. Given radar and short term radar trends, have made
for a dry forecast into the early afternoon, before moving showers
across the area this afternoon. Kept the severe wording for the later
afternoon as was before.

.SHORT TERM...(Friday through Saturday night)
Issued at 259 AM CDT Thu Mar 30 2017

As the shortwave trough moves off to the northeast and cold front
finally passes over the region a quiet and pleasant weather
environment on Friday is expected. Daytime highs should be in the low
70s/upper 60s with overnight lows in the upper 40s to low 50s on
Saturday morning. Very nice spring conditions should continue through
Saturday into Saturday night as a upper level ridge builds over the
Southeast.

.LONG TERM...(Sunday through Wednesday)
Issued at 259 AM CDT Thu Mar 30 2017

By Sunday, a trough will be exiting the Four Corners into the
southern Plains.  The GFS and ECMWF are showing distinct timing
issues (about 12 hours apart with the 500 mb wave).  However, the
onset of precipitation may not be too far off, with blended PoP
guidance looking good at this point.  The lead shortwave that is
indicated will be ejecting northeast through the lower MS valley on
Sunday. However, this wave will become starved for moisture as it
encounters the southeast U.S. ridge.  Thus, the chance of
precipitation will not begin until Sunday Night as the main wave
tracks east into through MS and AL. The track is far enough south
that our forecast area will remain in the cool sector at the
surface. The best low level inflow will likely be directed into a
vigorous MCS that will track along the Gulf Coast.  Again, timing is
different, but the overall scenario appears similar amongst the GFS
and ECMWF.  Elevated showers and thunderstorms will remain possible
over the TN valley Sunday Night into Monday as an 850 mb jet of 50-
60kt enhances convergence northeast of the 850 mb low track. Then, a
distinct deformation zone will produce rainfall across TN and MS
that pivots across north AL into GA on Monday.

The synoptic pattern will remain quite progressive and active later
next week.  After a brief break late Tuesday, the next trough digs
southeast quickly behind, but with a more northerly track through
the central and eastern corn belt. A cold frontal passage will occur
on Wednesday or Wednesday Night bringing the next shot of
thunderstorms. It`s far too early to say what hazard/impacts we will
see with this one, but initially looks like a high-shear/low CAPE
(HSLC) environment. A noticeable cooling trend should follow on Day 7
and just beyond.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1256 PM CDT Thu Mar 30 2017

Lower clouds have mixed out and/or lifted higher resulting in VFR
weather across the region. A feed of low to high altitude moisture
was moving across the forecast area from the SSW. This moisture and
more heating/increased instability will result in the formation of
scattered to numerous showers/t-storms later this afternoon and this
evening. Some storms could become strong to severe with wind gusts
over 50kt, large hail, and a possible tornado. A very challenging
forecast concerning best rain chances with lots of various model
output. Am keying in from 20-01Z as when the heaviest convection
will affect the terminals (KMSL first, then KHSV). Have TEMPO CIG/VIS
reductions to MVFR, however lower reductions are likely if heavy
convection affects a terminal. The shower activity will taper off
from west to east late tonight, with VFR weather returning before
daybreak Friday. West winds will increase to the 10-20kt range after
sunrise Fri.

&&

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

$$

NEAR TERM...RSB
SHORT TERM...SL.77
LONG TERM...17
AVIATION...RSB


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