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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 191135

Area Forecast Discussion
National Weather Service Huntsville AL
635 AM CDT Sun Aug 19 2018

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 412 AM CDT Sun Aug 19 2018

The night time microphysics view from GOES-16 indicated broken to
overcast conditions developing over the region. This was sort of
being confirmed by isentropic upglide in the 305K (~800-900 mb) 
region. Cloud bases across most of the area were for the most part
ranging between MVFR (1000-3000' AGL) and IFR (500-1000' AGL). Along
with the increase in clouds, light showers were in progress across 
parts of the region, mainly over southern middle Tennessee, moving 
ESE around 10 mph.

Short-term guidance has the upglide continuing, but weakening
somewhat into the afternoon. Precipitable water amounts over the
Tennessee Valley should decrease somewhat this morning into the
1.3-1.5" range, then begin increasing towards the evening. Even 
though moisture levels will be lower, the upglide should keep more 
clouds than sun in the forecast, along with scattered showers and 
t-storms. With slightly less moisture, highs should warm this 
afternoon into the mid and upper 80s. 

.SHORT TERM...(Tonight through Tuesday)
Issued at 412 AM CDT Sun Aug 19 2018

An upper system currently moving to the east across NE Wyoming will 
result in the formation of a surface low central High Plains this
evening. This new surface low will deepen as it moves to the NE,
reaching the Great Lakes region before daybreak Tuesday. A cold 
front trailing SW of the surface will move towards the Tennessee 
Valley, reaching the region Tuesday night.

Precipitable water values should increase tonight and on Monday,
reaching the 2" range for much of the day. This increase in moisture
and instability will bring widespread showers and thunderstorms to 
the region Monday afternoon and night. Some of the storms during 
Monday afternoon/evening could become strong to marginally severe, 
with strong winds the main threat. The high amounts of water will 
obviously result in locally heavy rainfall, as well as a potential of
an increase in lightning activity in the stronger storms. Despite 
the clouds, temperatures before rain commences should rise into the 
mid and upper 80s.

Even though the front will arrive until after this period, overall
atmospheric moisture values should go on a downward trend during
Tuesday. Will keep scattered showers and thunderstorms in the
forecast on Tuesday, with highs again in the mid/upper 80s. 

.LONG TERM...(Tuesday night through Saturday)
Issued at 412 AM CDT Sun Aug 19 2018

Lingering showers and thunderstorms will begin to taper of towards
the SE in the wake of a frontal passage across the cntrl TN Valley
early in the day Tue. Even with the diminishing rainfall, iso 
showers/tstms will still have some impact on locations along/east of 
the I-65 corridor Tue night into Wed, especially with ample amounts 
of buoyant energy and modest deep layer/effective shear values near 
35-40kt. This should translate into a continuing line/cluster of 
tstms affecting the wrn portions of the local area thru Wed, before 
lingering convection finally weakens and moves off to the SE. Given 
the low level dynamics in place coupled with adequate amounts of 
instability in the 1-2K J/kg range Tue night into early Wed, a few 
strong storms capable of gusty outflow winds, hail and brief heavy 
downpours are all possible.

Rainfall will then begin to taper off Wed night, as an area of low
pressure at the sfc quickly moves ewd into the Midwest/OH Valley
regions and weak upper ridging out of the srn Plains translates ewd
into the wrn/cntrl Gulf regions. Even with cloud cover and rainfall
diminishing, overall temps look to remain near/slightly below normal
for most of next week. Afternoon highs from Tue on predom look to be
mainly in the mid/upper 80s range while overnight lows look to remain
in the 60s for most nights going into the new work week.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 635 AM CDT Sun Aug 19 2018

Overall moisture values in the atmosphere were on the decrease, with
IFR CIGs likely to rise into the MVFR range, then VFR by the early 
afternoon. Despite a slightly drier atmosphere, on-going ascent just
above the surface, and still sufficient moisture will keep VCSH 
going through the late morning. Daytime heating and a more unstable 
environment will be more conducive for t-storms in the afternoon and
early evening. Uncertain with storm timing precludes adding it to 
the TAF at this time. Shower/t-storm chances are possible late at 
KMSL, where greater instability preceding a frontal boundary will be 





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