Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 210941

Area Forecast Discussion
National Weather Service Huntsville AL
441 AM CDT Thu Sep 21 2017

.NEAR TERM...(Today)
Issued at 441 AM CDT Thu Sep 21 2017

Patchy fog, dense in some places, continues to develop across
portions of the TN Valley, as winds become calm and dewpoint
depressions remain in the 0 to 2 degree range. As the early morning
hours continue to progress, expect the fog to increase in area, with
further reductions in visibilities expected. At this point, will hold
off on a dense fog advisory, as locations experiencing visibilities
below 1/4 mile should be confided to valley locations and near bodies
of water. However, caution should be used if traveling early this
morning and during the morning commute.

Fog should dissipate by 9 am across the area, leaving partly cloudy
skies for the remainder of the day. The TN Valley will be situated
between a strong upper ridge over the central CONUS, and an upper
trough over the Atlantic coast. A shortwave is expected to dig
southward along the western periphery of the upper trough through
the day. This increase in forcing will combine with a fairly moist
environment to produce isolated to scattered thunderstorms across the
area. Hires guidance is fairly consistent on bringing showers and
thunderstorms southward just before daybreak, but due to a lack in
forcing, will hold POPs off until 15Z. There may be slightly higher
coverage during the afternoon, given the potential for outflow
boundaries, but for now will continue the chance POPs for the area.
Despite cloud cover and thunderstorm potential, ample heating is
forecast across the TN Valley today, with highs expected to approach
and exceed 90 degrees in some places. Given surface dewpoints in the
upper 60s to lower 70s, this will translate into instability
approaching 1500 j/kg. So, a few storms could produce gusty winds up
to 40 to 50 mph.

.SHORT TERM...(Tonight through Saturday night)
Issued at 441 AM CDT Thu Sep 21 2017

Thunderstorms will quickly come to an end by sunset, with the loss
of daytime heating and as the mid level forcing exits the area.
Skies will clear once again during the late evening hours and winds
will generally remain calm. Given decent radiational cooling
conditions, temperatures will cool into the upper 60s, close to
forecast dewpoints. Patchy fog will be possible early Friday morning,
and may be a little more dense in some areas, depending on how much
rain occurs.

Similar conditions are expected on Friday, as the upper ridge tries
to slowly build eastward on Friday. Meanwhile surface high pressure
along the southeast coast keeps our winds in an easterly direction.
Weaknesses in the upper ridge will move across the region, and may
provide some aid to diurnally driven convection during the afternoon
hours. Coverage may be slightly lower on Friday due to slightly less
upper forcing, and will be mostly dependent on mesoscale boundaries.
Temperatures will rise into the upper 80s to lower 90s once again.
Thunderstorms will diminish by sunrise and overnight lows will fall
into the mid to upper 60s once again Friday night.

A weak area of low pressure will develop along the northeast Gulf of
Mexico on Saturday and lift north-northeastward through the day.
Meanwhile, winds will shift to a more southeasterly direction,
filtering in slightly more moisture across the TN Valley. This will
lead to another day of isolated showers and thunderstorms, mainly
during the afternoon. ALthough coverage may be a bit higher than
currently forecast, especially south of the TN River, the influence
of the upper ridge to our north may limit the coverage. Therefore
will cap POPS at 20 percent. Cloud cover may be a bit higher on
Saturday, and with height falls, temperatures may be a couple of
degrees cooler than those observed the previous days. Even still,
temps will rise into the mid to upper 80s once again. Overnight lows
on Saturday night will fall into the lower to mid 60s.

.LONG TERM...(Sunday through Wednesday)
Issued at 441 AM CDT Thu Sep 21 2017

Guidance from the various global models is in agreement on the mid-
level height pattern at the beginning of the extended period, with a
strong 500-mb high expected to be centered across the lower Great
Lakes and a weak inverted trough forecast to be along the central
Gulf coast. During the period from Sunday-Monday, the inverted trough
will lift slowly north-northwestward around the western periphery of
the weakening ridge as it translates eastward into New England. With
the trough expected to eventually become centered across AL/MS, an
isolated shower or thunderstorm will be possible both Sunday and
Monday afternoons. However, based on the weak nature of the trough
and no evidence of synoptic scale low-level convergence, the coverage
of this activity will be quite sparse. Due to the persistent nature
of this regime, high temps were increased and low temps were
decreased a bit above/below blended guidance. Most locations within
the forecast area should reach well into the m/u 80s during the
afternoon, and fall into the l/m 60s at night.

Beginning Monday night, the trough across our region is expected to
weaken and accelerate southeastward across northern FL and into the
Atlantic, eventually becoming absorbed in the broader circulation
around TC Maria. This will occur as mid-tropospheric ridging builds
from the lower MS Valley northeastward into the TN Valley downstream
from an amplifying trough over the Rockies. Thus, we anticipate dry
conditions from Monday night-Wednesday, along with even warmer
afternoon temps in the mid 80s-around 90. Fortunately, dewpoints in
the l/m 60s will mitigate the risk for high heat indices, and should
also allow lows to fall to similar values in the l/m 60s each night.
Although not explicitly mentioned in the weather grids, at least
patchy fog will be possible in river valleys and near large bodies of
water each night during the extended period.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1223 AM CDT Thu Sep 21 2017

VFR conditions at the start of the TAF should become MVFR with 3-5
miles surface visibility from fog forming. Reductions to IFR 1SM at
KMSL to 2SM at KHSV are possible before daybreak Thu. Fog that forms
should dissipate a couple of hours after sunrise, with VFR weather
returning. Strong late summer heating, available moisture, and an
approaching upper level system could bring isolated to scattered
afternoon convection. As such, have added a VCTS for both airports
for the mid/late afternoon. Light southerly winds overnight and Thu
AM should become northerly around 5kt in the afternoon. Winds again
should become light and variable after sunset.





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