Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 132014

Area Forecast Discussion
National Weather Service Huntsville AL
314 PM CDT Thu Aug 13 2020

.NEAR TERM...(Tonight)
Issued at 230 PM PM CDT Thu Aug 13 2020

For the remainder of the afternoon and into the evening, the region
will lie largely to the east of a mid/upr level wave making its way
eastward through the mid-Miss Valley. Convection will tend to be most
concentrated along a N-S oriented trough and low-lvl convergence axis
to our west. However, we are starting to see outflow-induced
convection moving eastward into the area. This shower and
thunderstorm activity will largely be concentrated in NW portions of
Alabama through the mid-late afternoon, but continuing development
and subsequent propagation of outflow boundaries may lead to further
development downstream in central and eastern portions of the area
late this afternoon and into the early evening. Thermodynamic data
suggest a strong wind gust potential with deeper, more robust cells
into the evening, with high PW values ~2 inch, ML CAPE values ~ 1500
J/kg, and low-lvl lapse rates ~7 C/km. The threat is generally
marginal and only isolated severe activity appears possible at this

The upper trough will continue to slowly move eastward overnight and
may continue to trigger convection due to the presence of broader
dynamic lifting despite the lack of daytime heating. Although,
instability will generally be lower. Fog will likely develop in the
typical narrow valley locations in the east and also where any heavy
precipitation falls this evening.

.SHORT TERM...(Friday through Saturday)
Issued at 230 PM CDT Thu Aug 13 2020

On Friday, the upper low will be moving across the area and enhanced
ascent may additionally occur due to the presence of a subtropical
jet just to our north. The trough axis and the afore-mentioned jet
will be in near prime position on Friday to lead to enhanced coverage
of showers and thunderstorms and the forecast accordingly reflects
this in 80-90 POPs. Thermodynamically, conditions will be fairly
similar to day, indicating another marginal threat for strong to
severe thunderstorms. However, the possibility for abundant cloud
cover may limit the overall threat, which presents a caveat to the
thunderstorm intensity aspect of the forecast.

Drier air aloft will begin to move into the region on Saturday from
the W-NW as the trough axis is aligned mainly across eastern parts
of the area. Thus, POPs are highest in the east (likely) and decrease
to low-end chance in the west for Saturday. So, this will be one
last day for possible deep, organized convection before the trough
exits the area to the east. Thermodynamic conditions in eastern
parts of the area will be similar to today and Friday, but will begin
to wane or weaken gradually weaken as the day progresses. Strong wind
gust potential will still exist mainly in the east for Saturday.

.LONG TERM...(Saturday night through Wednesday)
Issued at 230 PM CDT Thu Aug 13 2020

A fairly significant change in the long wave pattern will begin to
manifest on Sunday, but especially early next week as a high
amplitude ridge builds in the West CONUS, with a trough in the East.
This will lead to the advection of slightly drier/cooler continental
air into the region for next week. There is decent overall agreement
in the long wave pattern amongst the 12Z operational models and
ensemble members. Temperatures will tend to be about 5-10 degrees
below normal for this time of year, and dew point values in the low-
mid 60s are possible early to mid next week...with possibly even some
50s dew points making an appearance during peak afternoon mixing.
There is some remaining uncertainty in the how far south the sfc
front will eventually move and stall to our south, but is does appear
possible if not likely to make it all the way to the coast. This
front may begin to return by the mid to late part of next week as a
short wave moves into the region and stalls perhaps Wed/Thurs. Thus,
chances for showers/storms increase a bit late in the period.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1242 PM CDT Thu Aug 13 2020

An upper trough is poised to the west of the region, which is helping
to instigate more concentrated SHRA/TSRA activity mainly to our
west today. However, a line of SHRA/TSRA has developed along the
leading edge of this trough and may begin to impact KMSL by ~01Z and
KHSV by ~02Z based on latest best timing estimate. The potential
exists for MVFR cigs/vis with this activity at TAF sites, which could
last for about 1 hour. Certainly lower IFR vis could occur if a
strong core directly impacts a TAF site. Although, TAF updates may be
necessary if other activity manages to develop in the unstable air
mass ahead of the evolving line. More concentrated activity expected
tomorrow as the upr trough moves into the area.





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