Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 250509

Area Forecast Discussion
National Weather Service Huntsville AL
1109 PM CST Sat Feb 24 2018

For 06Z TAFS.


.NEAR TERM...(Rest of tonight)
Issued at 945 PM CST Sat Feb 24 2018

A mid-level shortwave trough currently over the northern Plains is
forecast to deepen and pivot northeastward into the upper MS
Valley/western Great Lakes overnight. Near term guidance suggests
that unamplified southwest flow aloft will be maintained across our
region between the trough and a subtropical ridge extending from the
FL peninsula west-southwestward into the Bay of Campeche.

As the intensifying surface low related to the northern stream
trough lifts north-northeastward and away from our region, the
trailing cold front will advance southeastward--likely entering
northwest AL shortly before Midnight and exiting the southeastern
counties just before sunrise. Showers and a few embedded
thunderstorms will continue to spread eastward across the CWFA as the
cold front approaches, and although the threat for storms will end
with passage of the front, a 2-4 hour period of light-moderate rain
will occur in the wake of the frontal wind shift to the northwest.

With boundary layer dewpoints now in the lower 60s beneath a 50-60
knot southwesterly low-level jet, any deeper convective updrafts in
our region will have the capability of developing mesocyclones with
an attendant threat for locally damaging winds and brief tornadoes.
However, with the strongest synoptic scale ascent related to the mid-
level trough lifting northeastward with time, lapse rates will
continue to weaken--limiting overall instability and the coverage of
thunderstorms. Based on this reasoning, we will continue to mention
brief tornadoes/damaging winds as threats, but will gradually scale
back the potential coverage of severe storms. Aside from hazards
directly associated with convection, gradient flow along the higher
terrain of the Cumberland Plateau in southern TN may still approach
Wind Advisory criteria late this evening, and persistent moderate-
locally heavy rainfall may result in some minor flooding by tomorrow

.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

.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 06Z TAFS through 06Z Sunday night)
Issued at 1109 PM CST Sat Feb 24 2018

No changes from previous aviation forecast reasoning. Although VFR
conditions are currently observed at MSL/HSV, a lower stratus deck
will likely develop from W-to-E by 06Z, as a cold front and broken
squall line of tsra begin to approach the region from the west. 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 brief period of lgt- mod rain
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. Low-mid level clouds should become more sct through the late
morning/aftn, although an overcast cirrostratus deck will persist
through the end of the TAF period. Lgt rain is expected to redevelop
north of the front once again tomorrow evening, with -RA included in
both TAFs beginning at 26/02Z.





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