Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 250243

Area Forecast Discussion
National Weather Service Huntsville AL
943 PM CDT Mon Jul 24 2017

.NEAR TERM...(Rest of tonight)
Issued at 943 PM CDT Mon Jul 24 2017

Earlier showers and thunderstorms have dissipated across the TN
Valley, with the loss of daytime heating. Given the lack of
instability and little in the way of forcing, redevelopment is not
anticipated through the overnight hours, with quiet conditions
expected through at least daybreak. Temperatures were in the lower to
mid 70s as of 9 pm, and although some spots were close to their
forecast lows, expect temps to rise slightly before midnight and
then slowly fall through daybreak. Meanwhile, as dewpoint depressions
approach 0 and winds remain calm, patchy fog is expected after
midnight, especially across areas that received rainfall earlier.
Only changes made to the previous forecast were to lower min temps by
a degree across the west and update hourly temps/dewpoints.
Otherwise, previous forecast remains on track.

.SHORT TERM...(Tuesday through Wednesday)
Issued at 322 PM CDT Mon Jul 24 2017

On Tuesday, the upper trough that dragged the previous boundary
through the area will continue to press into the northeastern states
with ridging building in across the south. Much drier air aloft
starts to filter in Tuesday and is evidenced on soundings and PW
values dropping to 1.5 inches. An upper level "wave/weakness" over
eastern TX will extend into central MS/AL Tuesday and will provide
another boundary for showers/storms to develop along. Looks like
coverage once again will be the southern and western portions of the
area where lift is greatest. GFS/ECMWF are much more bullish on
Tuesday but the NAM keeps us more at a scattered nature and will
keep that for the POPs. Not confident enough on all the showers
diminishing after sunset Tuesday given the lift so will keep an
isolated chance.

Not much change synoptically for Wednesday with the mid level ridge
building slightly farther east. Even so, a shortwave moves along it
and will be the focus for shower/storm development and once again,
in the southern and western areas. Given the drier air and stronger
high, will keep Wed night dry.

Temperatures will definitely depend on shower coverage and cloud
development each day. Give the recent "drier than the models" trend,
I`m going to hedge drier in my POPs, and therefore warmer, for my
temperatures. Highs for Tuesday are forecast to be slightly higher
compared to Monday`s highs with 850mb temps relatively the same. The
one change will be less cloud cover, especially early in the day.
This should push temps a deg or two higher and have inched the
median guidance upwards. This isn`t much different than the previous
forecast in most areas with highs in the lower to mid 90s. A similar
story on Wednesday with highs about a deg warmer than Tuesday. This
brings both days very close to a 105 heat index in NW AL and around
Huntsville. At this time, not confident enough in the pops/clouds
for an advisory Tuesday and subsequent shifts will have to monitor
the need for Wednesday.

.LONG TERM...(Wednesday night through Sunday)
Issued at 322 PM CDT Mon Jul 24 2017

At the mid and upper levels for the latter half of this week,
ridging west and troughing east setup was becoming established across
the greater North American domain. For the CONUS, and particularly
the southeastern states, this will set the stage for a cooler
temperature trend and what appears to be a "real" frontal passage
towards the end of this week.

Before this occurs, muggy conditions, along with very warm to hot
temperatures will reign across the region on Thursday. The potential
heat will be tempered somewhat, from increasing clouds as a system
approaches from the northwest. This system will bring more clouds,
and scattered convection to the Tennessee Valley. High temperatures
on Thursday will range from the upper 80s east, to low/mid 90s west.
Heat index values could rise into the 101 to 103 range over northwest
Alabama. More clouds rolling in should help trend temperatures down
just below Heat Advisory criteria.

Along with the warmth, chances of rain will continue as moisture and
instability prevails across the region. Precip water amounts in the
atmosphere again rise into the 2" range during Friday, creating a
potential for very heavy rainfall, gusty winds, and very electrically
active storms. A cold front should move across the region during
Saturday, with rain chances ending by Sat night. Given differences in
model output, did not go into too many details exact timing wise
with the front. Along with a true wind shift from the north, cooler
high/low temperatures are expected on Sunday with highs in the mid
80s and lows in the mid/upper 60s.

The front as is usually the case this time of year will not get too
far south. It should return towards the region on Monday, returning
lower end chances of showers and thunderstorms.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 624 PM CDT Mon Jul 24 2017

VFR conditions will prevail through the first 6 hours of the forecast
period at the KMSL and KHSV terminals. Earlier storms have pushed out
of the terminals, with little in the way of precip expected through
the overnight. Visibilities may drop as low as 4 SM after 09Z at
each terminal, however confidence on this occurring is somewhat low.
Visibilities will improve after 14Z, with VFR conditions and light
and variable winds prevailing through the remainder of the period.
Isolated to scattered thunderstorms will be possible again on
Tuesday, generally in the afternoon. However, storm coverage is
expected to be too low to include in the forecast with this issuance.





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