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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

                            
468 
FXUS64 KHUN 150005
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
705 PM CDT Sat Jul 14 2018

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 216 PM CDT Sat Jul 14 2018

Another very hot and humid afternoon across the Tennessee Valley,
with temperatures in the upper 80s to lower 90s as of 19z -- and 
dewpoints in the mid 70s. Thus, Heat Index values have climbed above
100 degrees across most of the region -- with portions of Northwest 
and North Central Alabama above 105 degrees. A Heat Advisory will 
remain in effect for the aforementioned areas through 6 PM CDT late 
this afternoon.

Despite a ridge of high pressure sitting almost directly overhead,
daytime heating and abundant moisture in the atmosphere has helped 
to develop scattered thunderstorms across the Tennessee Valley -- 
mainly west of interstate 65 so far. A current mesoanalysis 
indicates some fairly high CAPE values across the region (SBCAPE 
3000-4000 J/Kg and MLCAPE values up 2000-2500 J/Kg. The result has 
been a few strong storms across the Shoals that are putting out a lot
of lightning and some heavy rainfall. The higher instability will 
allow for some stronger updrafts and likely some gusty downdraft 
winds of 40-50 mph. The lack of any appreciable shear/flow will make
this activity very pulse-like and slow moving. The storms will 
persist until early this evening when the atmosphere will stabilize 
with the loss of heating. 

With the waning convection, a mostly clear to partly cloudy sky is
expected overnight as high pressure will remain over the region. As
has been the case the past several nights, some patchy fog will
develop late tonight into early Sunday morning due to the moist
boundary layer. Some of this fog could be locally dense in areas that
get rainfall this afternoon, and along rivers and lakes.

.SHORT TERM...(Sunday through Monday Night)
Issued at 216 PM CDT Sat Jul 14 2018

The upper-ridge will gradually sink south on Sunday into 
Central/South Alabama as a weak upper trough moves east from the 
Southern Plains into the Mid-Mississippi Valley. Despite this, the 
heat and humidity will remain the story on Sunday with Heat Advisory 
conditions possible again over Northwest Alabama -- and the rest of 
the area coming very close. Models have underdone dewpoint recoveries
at night and the morning hours in this very moist environment. 
Reliable observations have shown readings in the mid 70s to even 
upper 70s at times for several days now, and am not expecting this to
change tomorrow. Even with additional cloud cover and perhaps some 
more storm coverage, it's entirely possible another Heat Advisory may
be needed again tomorrow for Northwest Alabama. 

With a weaker cap and less subsidence in place, storm coverage should
be greater Sunday. Some breaks in the cloud cover during the morning
hours should allow for some rapid destabilization by early afternoon
and MLCAPE values up to 2500 J/Kg per the latest HiRes guidance. 
Thus, like today, a few strong, pulse storms will be possible again 
on Sunday. The aforementioned upper-trough will gradually move into 
the lower Ohio Valley Sunday night and eventually the Southern 
Appalachians on Monday. This feature will likely force a weak front 
into or close to the area sometime on Monday -- which would serve as 
a focus for additional showers and storms. Based on this, have 
tweaked PoPs up slightly for Monday to low-end likely values. A 
slightly cooling trend is also expected early next week due to this 
presence of this boundary and the expected convection. Therefore, 
despite the high RH values, it does not appear there would be a need 
for additional Heat Advisory products beyond Sunday. 

.LONG TERM...(Tuesday through Friday)
Issued at 216 PM CDT Sat Jul 14 2018

A rather fluid and mildly unsettled pattern will dominate a few days 
of the long term forecast as we move into next weekend. At the start
of the period, a weak cold front will be tracking south through 
Middle TN and will shift through the TN Valley during the day. While 
there is limited mid/upper level support for stronger, more organized
convection on Tuesday, the the front itself will be able to tap into
the low level moisture to produce some scattered afternoon storms, 
with more numerous showers and storms mainly south of the TN River. 

As the front sags south by Tues night/Wed morning, high pressure 
will building back over the region from the north and  dry will 
advect across the region. While temps will only drop a couple 
degrees behind the front, dew points will be falling into the low 
70s by Wednesday and mid 60s by Thursday. Given the "dry" air mass 
and weak subsidence inversion in place, rain chances will be limited 
across the region. Additionally, we'll see a temporary break from 
the 100 degree+ heat indices as values are only forecast to top out 
in mid/upper 90s on Wednesday/Thursday. 

By Friday, expect a slight surge in moisture as low level flow gains 
a more southerly component ahead of another weak front. Scattered 
showers and thunderstorm, with more humid conditions, return to the 
forecast.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 705 PM CDT Sat Jul 14 2018

VFR conditions are currently observed at the HSV/MSL terminals, with
just a few lingering stratocu beneath sct-bkn high-lvl convective 
debris clouds. Although trends in IR satellite data indicate that the
coverage of debris clouds will gradually increase this evening and 
overnight, light flow and abundant boundary layer moisture will 
contribute to the development of br/fg which could lead to MVFR vsby 
reductions btwn 05-12Z. Isolated-widely scattered shra and tsra will 
continue early this evening within a broad axis of low-level 
convergence draped across the TN Valley, with redevelopment of 
convection possible just before sunrise tomorrow. However, due to 
uncertainty regarding timing/coverage we will not mention this in the
TAFs at this point. The threat for storms will increase more 
substantially by mid-late morning across NW AL, with convection 
expected to spread ewd thru the aftn. Due to higher confidence, a 
PROB30 group was included for each terminal. After assuming a SSE 
component before sunrise, sfc winds will veer to SW and increase to 
arnd 5 knots after 12Z tomorrow.

&&

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

$$

NEAR TERM...AMP.24
SHORT TERM...AMP.24
LONG TERM...Barron
AVIATION...70/DD


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at weather.gov/huntsville.