Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 211704

Area Forecast Discussion
National Weather Service Huntsville AL
1204 PM CDT Thu Sep 21 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 827 AM CDT Thu Sep 21 2017

As of 13z this morning there is still fog in many of the valley
locations of NE AL and southern middle TN with some patchy dense fog
still possible for another hour or so. The one real big change with
this update is to adjust PoPs as storms have rapidly developed near
Tullahoma. Based on satellite the shortwave/vort-max that was
forecast to drop south might be arriving a bit earlier than
previously thought. Looking through hires guidance and the HIRESW
arw/nmm and NSSLWRF appear to be handling the quicker development
better than any other model. So have adjusted PoPs based on those
forecasts. The steering flow is pretty much due south and for the
most part expect storms to drop south through the morning hours. We
might also see some additional development on the outflows from the
storms. Modifying the 12z OHX sounding for what is expected this
afternoon results in 1,000-1,500 J/kg of CAPE but less than 20kts of
shear. Drier air aloft evident in this mornings soundings could allow
for some stronger gusts of 40-50mph but for the most part expect
storms to be on the weaker side.

The only question is what if any this earlier development of storms
might have on temps this afternoon. Thinking that once storms drop
south we may see a brief cool down but then rebound back into the mid
80s. Will watch trends and may have to make some tweaks to the highs
later this morning.

.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 18Z TAFS through 18Z Friday afternoon)
Issued at 1202 PM CDT Thu Sep 21 2017

Isolated to scattered storms are forming around the TN Valley. Have
added a TEMPO group to account for any storms that move over any of
the terminals. Could see brief drops to MVFR/IFR vis with any of the
storms. Winds remain less than 5kts out of the north but gusts of 20
to 30 mph are possible if a storm moves over either HSV or MSL this

Fog is forecast to develop again after 08z tonight with some dense
fog possible in areas that receive heavy rain today and in the valley
locations in NE AL.




NEAR TERM...Stumpf

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