Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 291124

Area Forecast Discussion
National Weather Service Huntsville AL
624 AM CDT Sat Apr 29 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 330 AM CDT Sat Apr 29 2017

Early morning analysis of multi-spectral satellite imagery shows an
area of low clouds spreading primarily across the western half of the
HUN forecast area this morning. These clouds are in association with
moisture advection and lift ahead of a short wave trough moving
northward from the Gulf along the eastern flank of the broad wave
trough. Low cloud development will likely translate slowly eastward
this morning as the upr wave likewise has an easterly component to
its motion. Nevertheless, the cloud deck will probably be broken to
scattered in coverage through the morning. Breezy and very mild
conditions will be prevalent this morning and today. At 3 AM,
temperatures were in the low to mid 70s at nearly every reporting
station in the area. Southerly winds were around 10 to 15 mph, with
some gusts up to 25-30 mph some concern to the
ongoing wind forecast and the possibility for a Wind Advisory later
today. Falling pressures along the southern flank of an extensive
band of deep moist convection situated from the Southern Plains to
the Ohio Valley regions is enhancing the regional pressure gradient,
leading to the breezy conditions this morning. The pressure gradient
may fluctuate in intensity today as the low drifts northeastward
through the Southern Plains and deep convection modifies the pressure
field. Winds may tend to increase a little with the onset of deeper
mixing later this morning, but overall winds are expected to remain
below the 25 mph sustained/35 mph gust criteria. This may bear
watching though.

Otherwise, the afore-mentioned upr wave will translate across the
area today, and may help to instigate isolated shower/thunderstorm
development this afternoon. Forecast soundings indicate sufficient
heating to break the capping inversion perhaps by ~18-20Z today.
Locations in central and eastern portions of the area may be favored
due to the location of the upr wave. But, soundings do indicate the
potential for substantial CAPE (~3000 J/Kg). Deep layer shear will be
rather weak today, so storm organization/longevity does not appear
to be a significant threat. However, due to the CAPE alone, any cells
that may happen to develop could be quite strong. Due to the lack of
significant forcing though, only a 20% POP has been retained in the
forecast. High temperatures will be quite warm today, with some
locations reaching 90 degrees, but most locations in the mid to upper

.SHORT TERM...(Tonight through Monday)
Issued at 345 AM CDT Sat Apr 29 2017

Focus of attention in the short term will then shift to Sunday and
especially late Sunday afternoon and night as a broadening upper
trough moves from the Southern Plains into the Midwest. A cold front
associated with the trough will develop and approach the TN Valley
region during the day on Sunday. Strong forcing for ascent associated
with the sfc front and coupled with favorable upper level jet
dynamics will produce a line of strong to perhaps severe
thunderstorms just along and ahead of the front. Thunderstorms are
likely to begin impacting northwestern Alabama during the late
afternoon and move slowly across the area during the evening and into
the overnight period. A low/mid level capping inversion will tend to
limit development well out ahead of the line of storms most of the
day Sunday, but a few isolated cells may develop immediately ahead of
the line and merge with the line causing some enhanced local
vorticity with stronger updrafts. So, there will be a slight tornado
threat with cells especially late in the afternoon and early in the
evening over northwestern Alabama. This threat is expected to
gradually weaken as the line moves eastward and overall large scale
instability weakens with loss of daytime heating. The main threat
with this line of showers/storms will probably be damaging winds with
an enhanced low-lvl jet containing wind speeds perhaps ~60-70 kts.
Also, high PW values, deep layer ascent and some cell training may
give rise to a localized flash flood threat. Our relatively high
antecedent soil moisture values already suggest perhaps a longer-term
areal and/or stream/river flooding threat which will have to be
watched immediately following the heavy rain event, which should
produce around 1.5 to 2 inches of rainfall across the area.

The storms will gradually come to an end on Monday night from west to
east with the last of the rain/storms exiting eastern parts of the
area on Monday morning. A drier airmass will filter into the region
on Monday, with dew point temperatures falling into the 40s during
the day and highs on Monday only forecast to reach the low 70s at
most locations.

.LONG TERM...(Monday night through Friday)
Issued at 330 AM CDT Sat Apr 29 2017

The cold front should be well to our east Monday night with a
general zonal flow aloft. A mid level ridge will start to build in
Monday night and into Tuesday with weak high pressure over the area,
leading to the quick return of southwesterly flow. Lower dew points,
in the 40s, will return along with a brief temperature relief. Lows
Monday night will be around 50 degrees which surprisingly is
slightly below normal.

Relief is short lived as warm advection returns as the mid level
ridge strengthens with sunny skies and high temperatures in the
upper 70s for Tuesday. The ridge starts to break down on Wednesday
as the next upper system starts to sharply dig into the Texas
Panhandle. Highs will remain in the upper 70s. As the trough digs
east, surface cyclogenesis will occur somewhere in the AR/LA/MS area
with differences noted between the GFS/ECMWF. Timing wise, both are
consistent in bringing in the precip between 18-00z Wednesday. The
ECMWF quickly lifts the surface low northeast with the rainfall out
of the area by 00z Friday. GFS however has a much deeper upper
trough, stalls the frontal boundary with waves of low pressure
riding along it and much higher QPF. Much of the higher values are
to the south, along the coast, but multiple waves of rainfall
continue through almost 12z Saturday. The rainfall after Friday
morning would be associated with strong wrap around upper lift. The
SuperBlend pops therefore are quite high given the GFS forecast and
have not made any changes given how far this is and the major
differences. Overall instability and shear is weak with this system
so embedded thunderstorms is more likely with a low threat of
severe. Heavy rain and training of cells seems to be more of a

It will get cooler Thursday/Friday with 850mb temps around 0-2C
leading to highs in the mid to upper 60s with lows in the mid to
upper 40s.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 620 AM CDT Sat Apr 29 2017

A low cloud deck moving into the area is beginning to disperse
already, but current predom MVFR ceilings may continue for another
hour or two at KMSL. Otherwise, ceilings and vis are likely to
remain predom VFR for most of the period at KHSV and KMSL. Southeast
winds will veer more southerly and will quickly become gusty with
the onset of mixing this morning. Gusts are likely to exceed 20 kts
by 17-18Z and possibly sooner. A few thunderstorms may develop in
northern AL today, but expected coverage is low and will not warrant
mention in the TAFs at this time.





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