Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231132 AAA
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
632 AM CDT Sun Jul 23 2017

.UPDATE...
For 12Z TAFS.

&&

.NEAR TERM...(Today)
Issued at 330 AM CDT Sun Jul 23 2017

A slightly complicated forecast in the near term, mainly with respect
to storm coverage and cloud cover -- and the impact it will have on
how hot it will get later on this afternoon. For the predawn hours
through the mid-morning, a high pressure ridge situated across the
Mid-South is helping to promote mostly clear conditions across the
southern Tennessee Valley. This combined, with the calm winds should
provide a good set-up for radiation fog early this morning in areas
that received rainfall yesterday. This is true across portions of NE
Alabama and locations in/near the Bankhead National Forest. Have a
mention of patchy fog in these areas through 14z this morning. One
other thing of note is that dewpoints are in the low to mid 70s
across the region. This should help set the stage for another
oppressively hot and humid day later on (despite the progged storm
coverage). As of 3 AM, KHSV is still 79/77, with similar obs across
the Huntsville Metro.

Speaking for convection, that is the big question for the forecast
today as it will largely determine how high temperatures and heat
indices get this afternoon. The aforementioned ridge of high pressure
should begin to retrograde west towards the Ozarks as an upper-trough
and cold front slips south into Kentucky and Tennessee in it`s
weakness. IR-Satellite shows some robust convection along this
boundary, still located along the Ohio River as of 08z this morning.
With a slightly weaker cap in place, some scattered convection is
possible once again, with models again favoring the terrain of NE
Alabama and locations south of the Tennessee River. This activity
should get going by the late morning and linger through the early
evening. However, the latest mesoscale guidance, including the HRRR,
NAM-3, NSSL-WRF are showing less coverage during the day given,
perhaps, the still somewhat stronger effects of the upper-ridge.

Should cloud cover and storm coverage be on the low-end of guidance,
more heating will occur -- which should get highs into the low to
mid 90s (at least) across the area. A glance at model soundings at
KHSV and KMSL shows that (like Saturday), we may not mix out a whole
lot. That means that dewpoints that are currently well into the mid
70s in many spots will not change a whole lot. Thus, a progged high
temperature in the 92-95 degree range would give Heat Index values
easily in Heat Advisory criteria with a dewpoint around 75 degrees.
Think we will see these values across much of Northwest and North
Central Alabama, and portions of Southern Middle Tennessee this
afternoon and have issued another Heat Advisory for the same areas
as yesterday from 16-00z.

One other note about the convection. Model soundings do show the
potential for some strong to marginally severe storms once again.
Some higher DCAPE values this afternoon (around 1000 J/kg), indicate
a slightly higher wet microburst potential. With PWATs around two
inches, locally heavy rainfall and flooding will also be a concern.

.SHORT TERM...(Tonight through Tuesday)
Issued at 330 AM CDT Sun Jul 23 2017

The aforementioned frontal boundary will continue to progress south
from Kentucky into Tennessee overnight. All models want to develop
an area of stronger storms along a pre-frontal trough/wave along
I-40 in Middle Tennessee by 23-00z this evening. Latest mesoscale
guidance suggests that what is left of it could approach the HUN CWA
in the north by 03z, before dissipating it quickly by 06z as it
drops south toward the Tennessee River. Depending on the exact timing
of this feature, we may see a second round of organized stronger
storms (should it arrive closer to 00z). However, with instability
dropping off quickly overnight, am expecting any marginal severe
threat from this development to be displaced to our north across
Middle Tennessee.

By Monday, the frontal boundary will drop south into Northern
Alabama and will serve as a focus for scattered to numerous showers
and thunderstorms throughout the day (some of which could be strong).
The main impact of this boundary will be the limited relief from
oppressive heat and alleviate the need for another Heat Advisory.
Highs should struggle to reach the 90 degree mark in most spots due
to the cloud cover and rainfall. The boundary will drop south of the
region Monday night into Tuesday, creating lower chances for
precipitation on Tuesday -- and allowing for some breaks in the
clouds. Thus, bumped highs up a degree or two with the high pressure
ridge temporarily trying to build back in.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM CDT Sun Jul 23 2017

The main forcing should be in southern Alabama/Georgia by late
Tuesday afternoon into Tuesday evening. However, models continue to
show much weaker energy aloft still over northern Alabama and
upstream of the area (rotating around the eastern edge of the
upper ridge).  Thus kept isolated to widely scattered showers and
thunderstorms in the forecast. Some slightly drier air will push
into the region, but not much. Lows on Wednesday morning still look
to be in the lower 70s.

During the day on Wednesday, models are fairly divergent concerning
the strength of the upper level ridge over Kentucky/Tennessee.
However, they both produce at least scattered showers and
thunderstorms over northern Alabama and Southern Middle Tennessee
due to either additional upstream energy moving into the area or
energy rotating around the weak upper low over Georgia into the
area.  Given moist low levels, dry air aloft, and SBCAPE values
between 1000 J/KG and 3000 J/KG forecast, we could see some stronger
storms capable of wind gusts to around 50 mph and frequent
lightning. Also on Wednesday, models show decent 925 mb temperatures
between 23 and 26 degrees over the area. Given the expected
scattered nature of convection and cloud cover, included highs in
the the 90 to 94 degree range.  In reality, especially if one or two
models guidance on this is correct, we could see some mid 90s
temperatures again and heat index values between 100 and 105 again.

Shower and thunderstorm chances should drop off a bit with the loss
of daytime heating, but still could see isolated to widely scattered
showers and thunderstorms continuing. Models hint that a weak trough
axis extending from Illinois through western Kentucky that might aid
in moisture convergence over the area as the southern portion of it
moves around the ridge into the area. This should keep lows a bit
warmer in the 70 to 75 degree range.

Models build even warm 925 mb temperatures further east overnight on
Wednesday into Thursday. Thus, included above guidance values for
Thursday`s highs. Again these highs might end up returning briefly
to the mid/upper 90s. The aforementioned trough axis moves further
east on Thursday, but mainly the northern extent of it in the
northeastern states. Over our area, it edges closer into northwestern
Alabama. This would keep dewpoints higher, likely in the lower to
mid 70s, even during the day on Thursday. Thus, we could be very
close to heat advisory criteria again on Thursday. Forcing ahead of
this weak feature could produce some afternoon showers and
thunderstorms on Thursday. Not expecting widespread convection, but
scattered coverage looks reasonable. Again, with moist boundary
layer, ample instability, and the weak convergence in the area,
strong pulse type thunderstorms capable of wind gusts to around 50
mph look possible.

By Thursday night, a stronger longwave trough axis develops over the
Ohio Valley and extends southwest into Texas. Some energy along and
ahead of this feature approaches Southern Middle Tennessee Thursday
night into Friday morning. GFS looks like it develops a pre-frontal
trough axis ahead of the longwave trough axis (front), but ECMWF
does not. Thus, the GFS initiates precipitation/convection earlier
over the area (by the early evening hours).  ECMWF holds off over
the area until after daybreak on Friday to do this. At this point,
stayed a little closer to the GFS, which is not far off from most
blends. Isolated to scattered PoPs were put in the forecast Thursday
night, with the highest scattered PoP in Southern Middle Tennessee.
Although shear does increase slightly, the soundings become very
saturated, so the threat for stronger storms actually is less on
Thursday night.

By Friday, the actual front should be pushing into Tennessee and
perhaps northern Alabama. This should increase rain chances and keep
them high through Friday night over most of the area. Bulk shear
does increase to around 30 knots and SBCAPE is high, but again not
much dry air to work with and not really a good low/mid level jet.
So stronger storms look to produce gusty winds and frequent
lightning, but primarily a low end threat for brief flash flooding
(as PWATS remain around 2 inches). Precipitation and cloud cover
should bring a relief from recent heat, as highs only top off in the
mid 80s to around 90 degrees.

At this point it looks like a good chance of continued showers and
storms will continue through at least a portion of Saturday. Some
decent cold/dry air advection is shown by models later in the day.
This should keep highs in the 80s in most locations.

A break in the rainfall does look possible by Saturday night, as the
front pushes into southern Alabama and Georgia. Even with a decent
amount of sunshine on Sunday, highs should only make it into the
80s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 632 AM CDT Sun Jul 23 2017

VFR conditions will prevail at each terminal this morning, but cloud
cover will be on the increase by midday as scattered TSRA develop.
Given that there may be a little more coverage than Saturday, have
opted to include a TEMPO at each terminal around peak coverage
(19-23z). Storms should diminish with the setting sun, but additional
mid/high clouds will linger (along with a VCSH) thanks to the weak
remnants of a decaying MCS from the north moving in late this
evening and overnight.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory from 11 AM this morning to 7 PM CDT this evening for
     ALZ001>008-016.

TN...Heat Advisory from 11 AM this morning to 7 PM CDT this evening for
     TNZ076-096.

&&

$$

NEAR TERM...AMP.24
SHORT TERM...AMP.24
LONG TERM...KTW
AVIATION...AMP.24


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at weather.gov/huntsville.



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