Area Forecast Discussion
Issued by NWS Huntsville, AL

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923
FXUS64 KHUN 250003
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
603 PM CST Sat Feb 24 2018

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 220 PM CST Sat Feb 24 2018

Despite widespread cloud cover, there has been enough insolation to
warm temperatures into the mid to upper 70s across much of the TN
Valley. The exception is across our southeastern counties, where
light rain has been observed on and off. Given the current
temperatures, instability has risen quite a bit across the area,
with estimated CAPE values around 1000 j/kg, if not higher. There are
some isolated showers beginning to develop across northern
Mississippi, however this activity looks to move northwest of the
area. With that said, there is still a small potential for isolated
to scattered thunderstorms to develop this afternoon and evening. As
stated in a previous discussions, wind shear values are a bit too
low for organized convection across the TN Valley this afternoon, so
storms should remain below severe limits. Can`t rule out a strong
storm, though, with gusty winds and small hail the main threats.

The threat for severe thunderstorms will increase later tonight as a
QLCS to our west moves eastward. This QLCS has developed along a cold
front that extends from the parent surface low over northwest
Arkansas, southwestward through east Texas. The associated upper
trough will move northeastward this afternoon and evening, helping to
push the cold front into the TN Valley late tonight. Timing of the
QLCS has not changed to much over the last few forecast updates, with
the line of storms expected to enter northwest Alabama by midnight
and then exit our eastern counties by mid/late morning on Sunday. Once
again, there may be some isolated to scattered convection ahead of
this main line this evening, but this will be dependent on if any
mesoscale boundaries are able to initiate the isolated convection.

In terms of the actual severe threat, not much has changed. As
stated above, ample surface based instability has been realized
across much of the area, so it`s a matter of how much of that
instability will be maintained after sunset. With southerly flow
continuing at the surface and through the vertical column, we aren`t
likely to cool down too much after sunset, so we should be able to
hang on to some of that instability. Forecast soundings do show some
cooling in the lower levels, which will help steepen lapse rates and
mitigate any of the surface cooling this evening. The low level jet
is expected to push eastward this evening and clip northwest Alabama
before it begins to race off to the northeast as the upper trough
swings into the Great Lakes region. As the jet pushes eastward, bulk
shear will increase and give us the necessary shear profiles to
maintain at least some of the organization along the line as it moves
into the northwest Alabama. The line is expected to weaken, however,
as it moves eastward and into a more stable atmosphere across our
eastern counties. In addition to slightly more stable air, the jet
will move north of the area by the time the line enters northeast
Alabama, leading to the further decaying of the line. So, with all of
that said, will maintain the main threat of damaging straight line
winds associated with any bowing segments along the line, with a lower
end threat for a weak tornado. The higher chances, of course, will
along and west of I-65, and especially as you get closer to the
Mississippi border. The threat will decrease rapidly early Sunday
morning and generally come to an end by daybreak as the cold front
moves through the area. Outside of thunderstorms, gradient winds will
be fairly high this evening, with a few gusts up to 35 mph possible.

.SHORT TERM...(Sunday)
Issued at 220 PM CST Sat Feb 24 2018

The line of storms may still be lingering over our far southeastern
counties through mid to late morning on Sunday, with scattered
showers possible behind this line. The cold front will be pushing out
of the TN Valley around daybreak Sunday and push completely south of
the area by noon. Light rain may linger just behind the front, so
will keep low chance for this light precip through the day. Models
are fairly consistent on stalling the cold front just to our south,
and this will keep clouds in the forecast through the day. Given the
cold air advection behind the front, and extensive cloud cover,
temperatures will only warm into the lower to mid 60s. So, many
locations will likely observe their daytime highs during the predawn
hours.

.LONG TERM...(Sunday night through Friday)
Issued at 220 PM CST Sat Feb 24 2018

Models continue to shift the front into southern Alabama northeast
to near the Atlanta area later Sunday afternoon. With a 1020 high
quickly on its heels, there looks to be some much drier air and
subsidence pushing into northwestern Alabama and locations north of
the Tennessee River. There still looks to be some light to moderate
rain behind the front south of the Tennessee River though. Models
then depict some fairly strong upper level forcing developing in
Mississippi and Lousiana moving northeast along the front. At this
point this helps to slow the front`s movement further southeast and
keeps more widespread rainfall in the area (especially south of the
Tennessee River) through Sunday night. As this occurs, overunning
increases north of the front, before a 70 to 80 knot 500 jet pushes
through the area with a longwave trough axis, bringing a temporary
end to additional rainfall. Before the longwave trough axis moves
through, an additional half an inch to three quarters of an inch of
additional rainfall looks possible, maybe some locally higher amounts
than that south of the Tennessee River. This could cause some
rivers/streams to rise near to to flood stage and could not rule out
some isolated localized flooding.

A period of dry weather returns Monday afternoon through Tuesday, as
a stronger area of high pressure moves in behind the re-enforcing
longwave trough axis. Some high clouds may linger through noon or so,
but most areas should see quite a bit of sunshine in the afternoon.
Thus, despite some cold air advection, highs should still be in the
lower to mid 60s. A cool night compared to the recent low
temperature spread, with lows in the lower 40s (maybe even upper 30s
in Southern Middle TN). Southeast boundary layer flow returns on
Tuesday, which should allow for an even warmer day on Tuesday. Highs
in the upper 60s to around 70 degrees looks reasonable, especially
with good low level mixing and 925 mb temperatures in the 11 to 14
degree range.

Unfortunately, this only looks like a very brief break from an more
unsettled and rainy weather pattern. By Tuesday afternoon/evening
models show fairly strong upper level energy moving northeast from
Mississippi and Louisiana toward northwestern Alabama, as the surface
high quickly begins to shift east into the Atlantic. PWATS climb to
between 1.0 and 1.5 inches. Very strong forcing both at low and mid
levels are shown by models with this energy. Models do show this
boundary concentrating upper level forcing over the area through
Thursday, when a stronger cold front pushes through the area. Ahead
of this front an additional round of heavy rain and stronger storms
are possible. In all at least 2 to 3 inches of rainfall (possibly a
good bit more) is possible from middle toward the end of next week.
Couldn`t rule out some strong to severe storms on Wednesday, but this
still is very uncertain. The biggest threat right now looks to be
flash flooding and river flooding potential during this period.

Dry, but much cooler (compared to what we have seen over the last
few weeks) is in store with highs dropping back into the mid to upper
50s and lows in the 30s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 603 PM CST Sat Feb 24 2018

Although isolated prefrontal shra/tsra will be possible this
evening, we expect VFR conditions to prevail at HSV/MSL, with sct-bkn
stratocu in the 2500-4000 ft layer beneath bkn altostratus. A lower
stratus deck will likely develop from W-to-E btwn 03-05Z, as a cold
front and associated squall line of tsra begin to approach the region
from the MS Valley. At this time, it appears as if the frontal
squall will impact MSL/06-08Z and HSV/08-10Z, with IFR cigs/vsby psbl
in +TSRA. Gusty SSW flow in the 15-25 knot range will veer quickly
to the NW and diminish in the wake of the squall line, with a 4-6 hr
period of -RA expected along with MVFR cigs/vsby reductions. Threat
for lingering rain should shift to the S/E of the terminals by
14-16Z, with winds bcmg lgt/vrbl as a weak sfc ridge builds into the
TN Valley from the north. Although low-mid level clouds should become
more sct through the late morning/aftn, an overcast cirrostratus
deck will persist through end of the TAF period.

&&

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

$$

NEAR TERM...73
SHORT TERM...73
LONG TERM...KTW
AVIATION...70/DD


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