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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 211107

Area Forecast Discussion
National Weather Service Huntsville AL
607 AM CDT Wed Aug 21 2019

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 252 AM CDT Wed Aug 21 2019

Satellite imagery depicts clearing skies over the TN Valley early
this morning as the mid level convergence axis, which served as a
focus for storm development yesterday, pushes southward. At 500 mb,
the trough axis that was settled over the area for the past several
days will finally push eastward as the shortwave over the Ohio 
Valley translates into the Northeast. Subsidence will increase over 
the TN Valley today, which should limit thunderstorm development 
later today. A weak surface convergence boundary will be draped 
through Middle Tennessee and may drift southward into our area this 
afternoon, serving as a potential area of thunderstorm development 
this afternoon and evening. 

Soundings indicate steep low level lapse rates around 7-7.5 C/km, 
which leads to the inverted-v signature indicative of a strong wind 
gust threat in any thunderstorms. Additionally, precipitable water 
values remain near 2" due to continued southerly flow transporting
Gulf moisture into the region. Therefore, a heavy rainfall threat 
will be present within any thunderstorms, as well, which may lead to
isolated wet microbursts. Overall, there will be a conditional 
threat for strong storms, because the likelihood of storm development
is limited by the increasing subsidence; that said, any storms that 
do develop may produce strong winds and heavy rainfall.

Otherwise, look for a relatively milder day with high temperatures in
the low to mid 90s. Because of the increased mixing of drier air
aloft, dew points will decrease once again into the upper 60s to low
70s, yielding heat index values between 100 and 105. Although below
heat advisory criteria, this may still prove to be a dangerous heat
day for sensitive groups (asthma, young children, elderly, etc.).

.SHORT TERM...(Tonight through Friday)
Issued at 252 AM CDT Wed Aug 21 2019

Thunderstorms should once again dissipate shortly after sunset with
loss of daytime heating, although isolated thunderstorms may sustain
for a few hours due to outflow boundary interaction. An additional
shortwave trough at 500 mb will rotate into the Great Lakes region
tonight into tomorrow, as a surface low deepens over the 
Northeast/Quebec. A cold front draped behind the surface low and
extending through the Ohio Valley into the Central Plains will
gradually sag southward through the midweek period. Moisture
convergence on the south side of this frontal boundary will lead to
increasing confidence and coverage of showers and thunderstorms each
afternoon and evening for both Thursday and Friday. Some of these
storms may be strong with gusty winds and heavy rainfall. Additional
enhancement of storms is possible if any MCSs from the Central Plains 
or resultant outflow boundaries track into the TN Valley, but most of
these should stay north of the region.

Due to higher shower and thunderstorm coverage each afternoon,
limited diurnal heating chances will limit high temperatures Thursday
to the low 90s, and Friday will be even lower in the upper 80s.

.LONG TERM...(Friday night through Tuesday)
Issued at 252 AM CDT Wed Aug 21 2019

An active pattern will persist into the long term period, as a weak
frontal boundary remains draped across the region. To start the 
period, a broad upper trough extending across much of the eastern 
CONUS will aid in the development of showers and thunderstorms along
the boundary. Ongoing convection during the afternoon hours will 
diminish Friday evening, as a shortwave develops over the Southern 
Plains. This shortwave will amplify as it moves further east on 
Saturday and Saturday night, with flow aloft strengthening from the 
southwest. Meanwhile, the aforementioned frontal boundary will 
generally remain stalled across, or in the vicinity, of the TN Valley
as the upper forcing continues to weaken. 

The shortwave over the Plains will further amplify as it passes 
through the Mississippi Valley on Sunday, and finally push through 
the local area Sunday night into Monday. This will keep scattered to 
numerous showers and thunderstorms in the forecast through much of 
the weekend, continuing through the first part of the upcoming 
workweek. As the trough passes east of the area on Tuesday, rain 
chances will begin to decrease, though scattered thunderstorms are 
still possible during the day. There are still some model 
discrepancies, so pinning down any dry periods during the long term 
is a bit difficult. However, at this point, the best chances for 
thunderstorms will be during the afternoon hours, as instability 
peaks. PW values will exceed 1.9 inches Saturday through Monday, 
which is near the climatological max for this time of year. So, heavy
rainfall is likely over the weekend and we will have to watch for 
any flooding concerns. 

Temperatures will be on the "cool" side, compared to what we've
experienced the last couple of days. As cloud cover, precip and lower
upper heights linger of the region, temps will struggle to rise out
of the mid 80s. Overnight lows will be relatively mild, with temps 
falling into the lower 70s. 


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 607 AM CDT Wed Aug 21 2019

Patchy fog/low stratus will continue in areas west and south of HSV
and MSL, but are not expected to affect terminals this morning. VFR
conditions will prevail through the TAF period. Scattered showers and
thunderstorms will develop this afternoon and continue through the
early evening. Confidence in placement and timing of storms remains
too low to pinpoint specific timing with this TAF package.





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