Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 220916
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
316 AM CST Mon Jan 22 2018

.NEAR TERM...(Today)
Issued at 315 AM CST Mon Jan 22 2018

Current analysis shows a robust line of showers and storms moving
into the MS River Valley as of 08z. This line is forming along
a pre-frontal trough and a pseudo dryline based off of surface obs.
The true cold front is further back to the W/NW over W OK. A mixed
mode of storm types is occurring along the line with a more QLCS
type north of Little Rock, some break down in the line with more
scatter-shot showers/storms, and then further south near the Gulf
Coast linear segments with likely some embedded supercell structures.
These various storm modes are a the result of a combination of
differences in forcing (stronger north), jet structure/shear, and
instability (better south).

The parent upper level low is back over W KS/NE with a surface low
located in NW MO. Both of these features are forecast to strengthen
through the day as the shift E/NE into the Great Lakes region. As a
result the best upper level forcing will be displaced well to our
north. However, strong WAA as a result of a 45-55 kt low level jet
and the pre-frontal trough mentioned above will be the main sources
of lift today.

In terms of thunderstorm potential, there are a couple of factors
that will likely inhibit convective development. The first being, low
level moisture, winds remain out of the S/SE for much of the day and
looking at upstream obs would suggest that values remain in the 40s
until immediately ahead of the line. Dewpoints quickly increase into
the mid 50s as the frontal feature moves across the area. Next would
be cloud cover, current satellite shows debris cloudiness has
already overspread the area and clouds are forecast to increase
through the day as mid/upper level moisture stream into the area.
This should limit destabilization and it should be noted that the
warmest temps (mid 60s) that we currently have forecast for the highs
will likely occur behind the pre-frontal trough as skies clear out.
Temps ahead of the line are expected to be in the upper 50s to around
60 degrees for much of the day. Finally, as a result of the strong
WAA regime, temps around 850mb may end up creating a weak inversion
as suggested by forecast soundings with the inversion not mixing out
until along the front. While this all points to very limited surface
based instability (<100 J/kg), we may end up with more like 200-300
J/kg of MUCAPE/elevated instability.

Hires guidance is surprisingly in almost complete agreement that as
the line of showers/storms approach NW AL between 12-15z it begins
to weaken due to the more stable and drier air. Might be possible to
see a few embedded storms initially in NW AL but then all guidance
has the line collapsing as it spreads east through the morning hours.
We may end up with just some light to moderate showers for much of
the area due to the collapsing line. The caveat is, models are
suggesting some redevelopment of showers/storms this
afternoon/evening east of a Huntsville/Cullman line. However, as
mentioned we might be dealing with limited instability as the better
moisture (>60F dewpoints) remains to our south over central/southern
AL. Shear on the other hand will remain strong with 60-70 kts of
deep layer shear and 20-30 kts of 0-1km shear. Veering wind
profiles with height this afternoon would support strong to severe
storms with a low end threat of a brief tornado IF anything does
redevelop. But wind profiles quickly become more unidirectional
across the area this evening and would support a more linear storm
type. The strong shear though is more than enough to result in
damaging wind gusts in the stronger showers/storms.

Current SPC outlook has the southern and SE counties in a marginal
risk and this seems reasonable based on what is detailed above.
Going to monitor the line to our west and trends in the HRRR closely
over the next couple of hours to see if we need to change our
forecast. Outside of any storms, the strong pressure gradient will
make for a breezy day and some locations could see gusts up to 30 mph
at times.

.SHORT TERM...(Tonight through Tuesday)
Issued at 315 AM CST Mon Jan 22 2018

A few lingering showers are possible across NE AL but expect these to
be quickly moving out of the area. All precip should be to the east
by midnight. The true cold front won`t arrive until closer to 8-10pm
for much of the area. But expect temps to plummet into the 30s behind
the front with lows Tuesday morning in the mid to upper 30s. Winds
remain elevated overnight and as a result wind chills in the upper
20s are possible Tuesday morning.

As the main trough axis swings through expect cloud cover to increase
with the greatest coverage across the northern half of the area.
The clouds will keep temps even cooler on Tuesday with locations
north of the TN River staying below the 50 degree mark. It`s possible
that there is enough lift associated with the trough axis to squeeze
out a few sprinkles across our TN counties but have kept the forecast
dry.

.LONG TERM...(Tuesday night through Sunday)
Issued at 315 AM CST Mon Jan 22 2018

Zonal flow aloft and high pressure at the surface will promote
tranquil weather Tuesday Night, with lingering clouds behind an
earlier FROPA exiting to the northeast. A clear sky and some
northwest flow at the surface and aloft will bring in a cooler, dry
air mass. Good radiational cooling will allow for temperatures to
drop below the freezing mark for the first time in a few days, with
lows bottoming out in the mid to upper 20s. After some ample
sunshine for a decent warmup Wednesday morning, another shortwave
trough will swing another dry cold front into the Ohio and Tennessee
Valleys during the afternoon. This will bring another shot of
"cooler" air and cap off highs in the upper 40s to lower 50s for the
day, as well as result in chilly conditions in the 20s later that
night.

Quiet weather will continue through the remainder of the work week
on Thursday and Friday as a gradual warming trend takes place. High
pressure will begin to build in from the west on Thursday and become
centered over the region Thursday Night into Friday. This will
result in a wind shift to the south to go along with a mostly clear
sky. Highs will warm into the mid 50s to near 60 degrees each day.
Accompanying these southerly winds will be a gradual moisture return
back into the area. The aforementioned upper-high will slide off to
the SE and become centered over Florida Friday night. Winds will
shift to the SW/WSW Friday night into Saturday as a weak warm front
advancing northeastward accompanying a weak upper-trough moving into
the region. This feature (along with some isentropic lift) will
provide enough lift to generate some scattered showers Friday night.

PoPs will increase Saturday into Saturday night as a deepening upper-
trough swings from the Central Plains into the Ohio Valley. A cold
front will generate a broad line of convective showers during this
window, giving us the best chance for widespread wetting rains in
some time. Given the lack of instability and dewpoints struggling to
reach 50 degrees, have opted to leave a mention of thunder out of
the forecast. Breezy southwest winds will accompany this frontal
system, however, as the pressure gradient tightens ahead of the
FROPA. It should be noted that exact timing of this feature is
uncertain given some subtle differences with the models. As a result,
have been slower to taper PoPs off from west to east Saturday night.
However, think by Sunday morning the front should be moving through
the region, with slightly cooler and much drier air to follow. This
air mass change will bring a swift end to precipitation on Sunday,
along with some highs in the mid 40s to low 50s to kickoff the new
week.


&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1129 PM CST Sun Jan 21 2018

VFR flight weather conditions will continue until early Monday
morning. Ceilings of 020-030agl arrive ~12Z at KMSL and ~14Z at KHSV.
Southeast to south flow will increase to 15-25kt with gusts near 30kt
by 12-14Z. A broken line of showers and thunderstorms will move into
northwest AL including KMSL between 15-18Z and KHSV between 17-20Z.
Wind gusts of over 40kt are possible with this line, but will
continue to hold off on mentioning in TAFs until this becomes more
certain. South winds will shift to the southwest behind this line of
showers and thunderstorms late Monday afternoon and early evening.


&&

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

$$

NEAR TERM...Stumpf
SHORT TERM...Stumpf
LONG TERM...AMP.24
AVIATION...17


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