Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 171747
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1147 AM CST Tue Jan 17 2017

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 951 AM CST Tue Jan 17 2017

Generally cloudy skies are expected into this afternoon across the
greater Tennessee Valley. A persistent south-southwest lower level
flow will bring warmth and lower level moisture across the forecast
area. 9 AM temperatures across the region once again are very warm
for this time of year; now in the mid/upper 60s. Normal highs this
time of year are only 51. This time of year of course can get very
cold, as record lows of -4F and -3F in Huntsville and Muscle Shoals
attest to back in 1982.

Along with the warmth, scattered to numerous showers were nearing the
forecast area from northeast and eastern Mississippi. This activity
will continue moving to the northeast, and overspread the area during
the afternoon. Given marginal instability present, will keep in a
slight chance of thunder with predominate showers. Despite extensive
clouds and showers, high temperatures should rise into the upper 60s
most spots. Areas south of the main axis of rain could experience a
70 or 71 for a high temperature.

A cold front extending from western Ohio to northeast Mississippi
will continue moving slowly toward the east-southeast It should reach
the greater Huntsville area after sunset. The front will weaken as
it progresses to the southeast tonight, thanks to its parent low
moving further northeast away from the region. Southerly winds should
become northwesterly as the front passes later today.

.SHORT TERM...(Tonight through Thursday)
Issued at 300 AM CST Tue Jan 17 2017

The front slowly shifts S/SE across the area tonight and precip
chances decreasing behind the front. The cold air arriving with the
front will be rather shallow in nature and an inversion should setup
between 2-3kft. With models indicating that a few weak upper level
waves moving over the area tonight/Wed. morning, there might be just
enough lift to support some light drizzle or an isolated shower
through Wednesday morning. Temps both Wednesday morning and during
the day will be well above normal with lows around 50 and highs near
60. Expect a brief break in the precip Wednesday as a quick moving
ridge progresses across the area.

By Wednesday night a low pressure system moves out of the desert SW
and into the TX/OK area. Strong southerly winds ahead of this
feature will lift the cold front that moved through Tues/Wed back
north as a warm front by Thursday morning. A deep southerly fetch
associated with this system will lead to near record PWAT values
streaming into the area on Thursday. Max PWAT climo for this time of
year is around 1.4-1.5 inches and forecast soundings suggest around
1.4 inches over the area on Thursday.

In this almost completely saturated environment instability will be
lacking. But with the very strong upper level forcing ahead of the
incoming system we could see lapse rates steepen a bit more than the
forecast soundings currently suggest. But with multiple shortwaves
ejecting out ahead of the incoming system we may see more widespread
rain ahead of the main system, which would really lower thunderstorm
potential.

Although we have a strong LLJ and upper level jet nudging into the
region surface winds should be high as well. This may limit shear
values some what and hodographs show some turning of the winds with
height but for the most part are fairly unidirectional. This would
suggest that if thunderstorms do develop Thursday that they may be
more in the form of a QLCS type of an event Thursday into Thursday
night.

During the day Thursday through as the stronger jets move into the area,
winds from the surface up through 2,000 feet increase rapidly and
with soundings indicating 20-30kts at 500 feet, there may be a need
for a Wind Advisory Thursday. Bumped winds up a decent amount for
Thursday as models are under-doing speeds with the system.

One concern with the forecast on Thursday though is that as the
upper level system pivots northward and take on a negative tilt a few
pieces of energy eject E/NE out of SE TX and move along the Gulf
Coast. This could generate a MCS that hugs the coast and could limit
instability and the deeper moisture feed over our region. Still far
enough out to really get a good handle on what exactly will happen
but something worth watching over the next couple of model runs.

Even if we miss out on the better instability, the high PWATs and
strong lift will lead to very heavy rainfall and high rainfall rates.
Analyzing volumetric soil moistures via NASA SPoRT we have saturated
some of the shallow layers of the soil but we remain on the dry side
for the deeper soil moistures. This means that soils can still
handle/absorb rain and that even with the increased rainfall rates,
flooding or possibly flash flooding will likely be very isolated and
only occur in low lying areas.

.LONG TERM...(Thursday night through Monday)
Issued at 300 AM CST Tue Jan 17 2017

After a couple of episodes of showers and some thunderstorms during
the early part of this week, anticipating an active period of spring-
like strong to severe thunderstorm weather for the rest of the
forecast. This is mainly due to the breaking of a blocking pattern
that had been in place for the past week, and a series of troughs of
differing amplitudes and trajectories affecting the eastern half of
the country. The effect of a couple of these troughs may cause strong
to severe weather on Thursday evening/night and again on Saturday
evening into Sunday.

Showers and storms may be ongoing especially over the MS River
Valley on Thursday afternoon before low-topped supercells and perhaps
a QLCS move across the TN Valley on Thursday night. Model trends
continue to depict low instability profiles due to the nearly moist
adiabatic profile aloft, antecedent cloud cover, and warm surface
temperatures. However, there is enough instability to produce
convective updrafts which may be tall enough to support lightning. In
addition, the strongest kinematics with this slightly negatively
tilted trough axis over the Ozarks may be displaced to the north and
east of the TN Valley. Nevertheless, curved hodographs and
southwesterly bulk shear values around 40-50 kts give credence to the
the potential for strong to marginally severe damaging winds with
both the discrete cells and any bowing segments that develop. The
timing of this episode is somewhat quicker than previously forecast
and will trend quicker with some caution as the strength of the
downstream ridge may be under forecast. That trend has the QLCS
arriving sometime between 20-00Z (2PM-6PM) on Thursday
afternoon/evening and ending between 06-10Z (midnight-4AM) on Friday
morning. Another potential impact from these storms is heavy
rainfall. PWATs will be running well above normal for this time of
year around 1.3-1.5 inches and have already incorporated a higher
trend in QPF for this time period.

Lingering showers behind the passing trough axis will be possible
mainly on Friday morning before exiting on Friday afternoon.
Otherwise, broken clouds and above normal daytime highs will still
occur due to the lack of a cold airmass behind this trough would
typically accompany a winter trough axis. A brief break in the shower
activity will occur on Friday night in between the what looks to be
nearly inverted trough axis located over the MO and OH River Valleys
and the next trough crossing the desert southwest to the Central
Plains. This next trough looks to move northeast in response to an
upstream ridge amplifying over Ontario/New England States. While this
is occurring, another broad trough moves across the US/Mexico border
region and rapidly amplifies during the Saturday-Sunday time frame.
The primary shortwave trough within what progresses into a longwave
trough axis over the Southern Plains may induce a strong LLJ and
strong wind shear for a High Shear-Low CAPE QLCS event over the
Southeast. Several details are yet to be determined with high
confidence but the med range models and GFSEnsemble have been
consistently showing similar solutions on the placement of this
trough axis. However, there are differences in the strength of the
trough and how deep the surface low is. It is likely due to the
difficulty in the models handling the amplification of the downstream
ridge over the western Atlantic. If the current solution holds, there
could be a couple of episodes of severe weather: 1) on Saturday night
and 2) on Sunday afternoon. Will continue to highlight the threat for
strong to severe weather.

Following the passage of the storms, anticipating cooler near normal
temperatures on Monday into Tuesday but with the highly stacked upper
low sitting atop the Southeast and OH river valley, could still see
some light lingering showers and cloud cover, especially on Monday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1147 AM CST Tue Jan 17 2017

An area of showers continued moving in a SW-NE manner across the
area from Mississippi. More showers to our SW, and model forecasts
for them to continue into the evening will produce mainly MVFR
CIG/VIS values for this TAF issuance. An embedded thunderstorm cannot
be ruled out into the evening; though chances for one are too low
for inclusion in the TAF. A cold front now moving to the SE across
far NW Alabama should reach KMSL in the next couple of hours, and
KHSV after sunset. SW winds in the 10-15kt range this afternoon
should become NW around 10kt after the front passes. Winds should
become light from the NE around daybreak Wed.

Shower activity should wind down after midnight, but MVFR CIG/VIS,
at times reduced to IFR will continue into the late night. Some
improvements should occur after sunrise Wed, but am holding on to
MVFR CIGs through late Wed morning.

&&

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

$$

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


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