Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 071816

Area Forecast Discussion
National Weather Service Huntsville AL
116 PM CDT Fri Aug 7 2020

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 935 AM CDT Fri Aug 7 2020

The TN Valley remains tranquil this morning, with high clouds
beginning to subside from the west, and locally dense fog in river
valleys dissipating as we continue to warm and mix. A stationary
boundary draped across central AL and extending to the northeast
will begin to lazily drift northward into this afternoon, but
confidence on northward extent throughout the day remains low. A
mid-level vorticity maxima currently located in central MS is making
its way westward and may clip the southern edges of our CWA before
breaking up. Despite uncertainty in strength and eventual position of
the above features, it is likely their combined influence will
result in isolated showers and thunderstorms developing on an axis
extending between Cullman and Jackson counties from the late morning
until the boundary layer erodes around sunset. Available instability
is fairly weak, but the strongest storms that form will be capable
of producing gusty winds, lightning, and locally heavy rainfall.
Otherwise, expect highs near 90 degrees across the majority of our

.SHORT TERM...(Tonight through Sunday night)
Issued at 445 AM CDT Fri Aug 7 2020

In similar fashion to the last couple of days, we expect lingering
afternoon convection to dissipate rather quickly across the
southeastern portion of the forecast area as the boundary layer
begins to stabilize around sunset. Aside from a few patches of high-
level cloud cover in the weakening northwest flow aloft, mostly clear
skies will once again support the development of patchy river valley
fog early Saturday morning as temperatures approach predicted lows
in the m-u 60s. The northwest flow regime will continue to subside on
Saturday, as the strengthening subtropical ridge to our southwest
begins to expand eastward into the Mid-South and Lower-MS Valley
regions. Although intense warming of the boundary layer and the
presence of a weak convectively-induced vort max warrant maintaining
a 10-14% POP in the grids Saturday, subsidence aloft in this regime
will in all likelihood result in hot and dry conditions, with
afternoon temperatures ranging from the upper 80s in elevated
terrain to the mid 90s elsewhere.

During the period from Saturday night-Sunday night, the subtropical
ridge will retreat to the west as it becomes suppressed by a strong
mid-latitude trough sweeping eastward across the Canadian Prairie
provinces and northern Plains. Weakening subsidence aloft should
allow for the development of isolated-widely scattered showers and
thunderstorms on Sunday, although it is important to note that the
coverage could be greater than currently indicated if any disturbance
in the northwest flow aloft tracks southeastward across the region.
Lows will rise back into the u60s-l70s on Sunday and Monday mornings
due to an increase in low-level moisture, with highs Sunday reaching
the l-m 90s once again.

.LONG TERM...(Monday through Thursday)
Issued at 445 AM CDT Fri Aug 7 2020

A strong upper ridge will remain centered over New Mexico and Texas
on Monday. A weakness in the ridge further east is shown as well
extending from just off the coast of the Carolinas into the Florida
panhandle or southern Alabama for the most part. In this guidance the
upper ridge builds more over northern Alabama and southern middle
Tennessee. The NAM12 and ECMWF reflect this solution and keep the
boundary in place as the day progresses. This would help to limit
precipitation chances and produce warmer 925 mb temperatures to mix
up to in central and northern Alabama. However, GFS does extends the
boundary from off the east coast into central Alabama very early in
the morning and shifts it northward during the day. This would be a
wetter and not quite as hot of a scenario. For now leaning in between
the two, but a leaning a tad toward the warmer and drier side of
things. Expect highs to reach the 92 to 95 degree range in most
locations with scattered showers and thunderstorms possible (30-40
percent coverage). Strong storms look possible despite no shear being
available. If the warmer solutions are correct, could see some highs
a bit warmer than that and lower precipitation chances. Heat index
values look to climb to 100 to 104 degrees in many locations in the
late morning or afternoon hours. If temperatures end up looking to be
a few degrees warmer, a heat advisory may be needed.

Due to the loss of heating and the weak nature of the boundary,
expect that only isolated convection would linger Monday night. Do
expect warmer low temperatures, as better moisture advection occurs
ahead of a frontal boundary approaching from the west allowing
dewpoints to remain in the lower 70s in most locations. Though valley
areas could cool down to the upper 60s.

Tuesday looks wetter as a pre-frontal area of convergence is shown
pushing into northern Alabama in just about all model guidance. Have
high chance to likely in the forecast even in the morning hours. At
this point, shear is not strong in guidance and instability is very
different in magnitude, but enough for thunderstorms regardless.
Again, some could be strong pulse type storms. This will keep high
temperatures from being excessively hot given some of the 925 mb
temperatures aloft shown over the area. Upper 80s to lower 90s look
to be the rule of thumb. Though a 94 or 95 here or there is possible.
It will remain humid, so heat index values will top out around 101
or 102 in many locations.

Expect very similar conditions Tuesday night into Wednesday, as high
coverage of shower/storms quickly lessens and becomes isolated to
scattered Tuesday night. It will remain warm overnight with lows only
dropping into the lower 70s in most locations. Wednesday looks
similar as well with another pre-frontal tough axis moving east into
the area. Highs again look limited by cloud cover and higher
precipitation chances and again expect them to climb only into the
upper 80s to lower 90s. Again, strong storms remain possible.

This pattern continues through Friday, before the main front finally
pushes into northern Alabama late Friday night into the day on
Saturday. The guidance seems more certain of higher rainfall chances
during this period with scattered showers and storms lingering into
the night. For now, kept Pops in the 40 to 60 percent range during
the day. However, this may need to be raised in future updates. Will
have to watch to see if flooding concerns develop during this period
as well. Strong thunderstorms look possible through this period given
instability shown in models.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 116 PM CDT Fri Aug 7 2020

VFR conditions are expected thru 18Z/08. Patchy MVFR fog may develop
in the 09-12Z timeframe, particularly at KHSV, but confidence is not
high enough to include in the TAFs at this time.





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