Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240935
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
335 AM CST Sat Feb 24 2018

.NEAR TERM...(Today and Tonight)
Issued at 335 AM CST Sat Feb 24 2018

A progressive shortwave trough moving across the Intermountain West
towards the Southern Plains will act to compress a subtropical jet
while a ridge centered over the western Atlantic weakens. This
shortwave trough will gain a negative tilt later this morning as it
shifts northwest on the western flank of the ridge. The merging of
the polar and subtropical jet, the NE to SW orientation of the jet
streak, the strengthening of the LLJ, and the resulting
kinematic/thermodynamic environment are just a few of the ingredients
for this potential severe weather event later this evening into the
overnight hours. Meanwhile a frontal boundary has been stalled
across the MS/OH River Valleys.

First though, for today, broken to overcast low to mid level cloud
cover will prevail with a strengthening southerly surface flow
keeping unseasonably warm air and dewpoint temperatures in place.
Made only modest adjustments to the daytime highs with the potential
for cloud cover keeping them lower (in mid 70s). With steepening
lapse rates aloft and convective temperatures, surface based
instability will rise to between 1000-2000 J/kg later this afternoon.
The highest amounts of instability actually resides to the southwest
and west closer to the moisture tongue stretching from the TX Gulf
Coast to the OH River Valley. Supercells are expected to develop
ahead of the slow moving cold front and pre-frontal QLCS over the
Ozarks area to northeast TX. These supercells should merge with the
QLCS as the QLCS accelerates across MS River Valley. For the TN
Valley, there may be scattered showers and thunderstorms during the
day today, and some of these could be strong with small hail and
gusty winds the main threat during the afternoon. The main limiting
factor will be cloud cover and challenges in timing of mesoscale
embedded undulations within the subtropical jet which may be just
enough to lift surface based parcels to the LFC.

Before the arrival of this QLCS later this evening, deep layer bulk
shear will be increasing to 50-60 kts from the SW along with a
decrease in instability with mixed layer CAPE between 500-1000 J/kg
as the LLJ strengthens. Surface dewpoints will be in the mid 60s. The
timing of this QLCS arrival is a bit uncertain given changes in the
mesoscale/cold pool speed. However, the latest consensus of hi-res
and short range model guidance is between 03Z-07Z arrival over NW AL.
With the quick movement northeast of the shortwave trough, occluding
complement surface parent low over WI/MI, and lower
shear/instability environment further east, anticipate the QLCS to
begin decaying upon arrival. Damaging winds along bowing
segments/LEWPs of the QLCS are possible, especially where the these
bowing segments are normal to the SW bulk shear vectors. In
addition, curved hodographs and veering of shear profile also support
a tornado threat (though diminishing as it arrives) along the apex
of any of these bowing segments. As is the case with most QLCS
tornado scenarios, the tornadoes will be of a quick spin-up nature
and may not have any mesocyclones. The QLCS should be exiting
northeast Alabama by early Sunday morning bringing an end to the
event.

.SHORT TERM...(Sunday and Sunday night)
Issued at 335 AM CST Sat Feb 24 2018

The front is then expected to pass to the southeast by late Sunday
morning into the early afternoon. Models are showing that it could
stall along the I-20 to I-85 corridors before another embedded
shortwave trough (in the subtropical jet oriented across the region)
could bring additional showers northward towards north Alabama on
Sunday afternoon and evening. Ongoing POPs/Wx only needed modest
adjustments to account for the latest model trends. One note about
the front: the extent of the cool air behind the front that arrives
here is modified and will only cause a modest decrease in the
daytime highs (compared to early cold front passages) to the low 60s.
The more substantial temperature change will be noticed on the
overnight lows (in the 40s) as drier air arrives.

.LONG TERM...(Monday through Friday)
Issued at 335 AM CST Sat Feb 24 2018

Decided to keep a very small chance of rain in on Monday morning for
portions of NE AL. Models disagree on how quickly the rain will be
out of the forecast area as a shortwave moves into the TN Valley. The
front that passed through on Sunday will be located to our south and
will be where the axis of heavier rainfall will be. The ECMWF is the
wetter and slower solution so tended to lean more towards the GFS
and NAM for this system.

A sfc high building into the region on Monday will push the front
down to the coast and filter in "cooler" and drier air. Using cooler
in quotes as temperatures will still remain above normal. Dry
conditions will persist into Tuesday. Monday will see highs in the
low/mid 60s with overnight lows in the 40s, while Tuesday will see
the mid/upper 60s and lows in the 50s.

Models agree that an upper low will swing out of the Southwest U.S
on Wednesday. They diverge from there on how quickly it moves east
and if it phases with a northern stream of energy or not. The GFS
does phase and develops a deeper sfc low which lifts into the Great
Lakes region. The ECMWF tries to phase but ends up developing a
secondary sfc low in the MS Valley, weakening the sfc low heading
into the Great Lakes. This solution is much slower and brings more
rainfall. Between the Canadian clearing out the fastest and the ECMWF
so slow, will again follow close to guidance in the extended. So
overall, we are looking at rain chances increasing Tuesday night as a
warm front lifts into the area, with ample moisture return in the
warm sector on Wednesday. Kept in thunder for Wednesday and Wednesday
night as there is a little bit of instability in the soundings.
Soundings also show a very good wind field Wednesday night into
Thursday ahead of the cold front, so will keep an eye on additional
model runs. Precip will taper off Thursday night with drier
conditions expected on Friday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1135 PM CST Fri Feb 23 2018

Light/mod ra/shra continue to spread ewd across parts of NW/N cntrl
AL this late Fri evening. Even with the precip, VFR conds prevail at
the two main terminals with predom just some mid level cloud cover
streaming into the area with the shra activity. Cigs are xpcted to
lower closer to 3-4K ft overnight, as S winds increase near 10KT.
Cigs may briefly lower into the MVFR cat/2-3K ft early Sat morning,
as additional ra/shra move across the area. There may actually be a
break in the precip during the day Sat, thereby allowing the cigs to
climb above 3K ft during the early afternoon hrs, before additional
precip develops/moves into the area late in the TAF period. Sfc winds
out of the SSW are also xpcted to increase near 12-14KT with higher
gusts Sat morning, and persist for much of the day.


&&

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

$$

NEAR TERM...SL.77
SHORT TERM...SL.77
LONG TERM...JMS
AVIATION...09


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