Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 220947

Area Forecast Discussion
National Weather Service Huntsville AL
447 AM CDT Fri Sep 22 2017

.NEAR TERM...(Today)
Issued at 447 AM CDT Fri Sep 22 2017

Tonight is almost a rinse and repeat of last night, with patchy
dense fog developing mainly in valley locations and near bodies of
water. Given clear skies and calm winds, along with dewpoint
depressions approaching 0 degrees, anticipate the fog to be slightly
more widespread tonight, especially given yesterday`s rainfall.
However, some sites have remained mostly fog free and will continue
to monitor conditions for the need of a dense fog advisory. At this
point, it does not appear that one will be needed, as the denser fog
is fairly patchy and somewhat shallow, causing visibilities to vary
at any given location. As is typically the case, fog will quickly
dissipate once the sun rises and boundary layer mixing occurs.

Convection will be a bit more limited today than what we saw
yesterday. To start, model guidance is in fair agreement that an
upper ridge to our north and west will briefly build eastward today,
causing heights to rise across portions of the area. Secondly, with
little in the way of mid or upper level forcing, convection will
generally be mesoscale driven. With that said, moisture will remain
high and heating ample enough to see instability rise above 1000 j/kg
by the afternoon. So we could see isolated thunderstorms develop
along any differential heating boundaries or moisture boundaries
from yesterday. Although the higher likelihood would be across the
southeast given the proximity of the upper ridge, will keep isolated
thunderstorms in for the entire area. Confidence is just not high
enough to go higher than slight chance for this afternoon.
Temperatures will rise into the upper 80s to near 90 degrees, as
much as 10 degrees above normal for this time of year.

.SHORT TERM...(Tonight through Sunday night)
Issued at 447 AM CDT Fri Sep 22 2017

Any thunderstorms that do develop will quickly end by sunset, much
like any other summer day. Skies will clear and winds will become
calm. So patchy fog will be possible once again tonight, especially
along bodies of water and in valley locations. Temperatures will
cool into the mid to upper 60s.

The upper ridge will lift northward on Saturday as a weak upper
trough/cutoff low builds into the area from the Gulf of Mexico. This
low, developing along the base of a more broad trough over the
southeastern CONUS, will lead to slightly cooler temperatures on
Saturday, given lower thickness values. These lower temps will lead
to slightly less instability, though the weak lift associated with
the building trough may counter this to produce isolated
thunderstorms once again Saturday afternoon. Moisture will be
similar to that observed today and yesterday, and with the weak
nature of the trough, not expecting much in the way of widespread
precip. So, while there may be more coverage than currently
forecast, confidence warrants only slight chance POPs for Saturday
during the afternoon.

The upper low will drift to the northwest through the overnight
hours Saturday into Sunday, becoming situated over central
Mississippi by Sunday. As it shifts northwest, moisture will
generally be cut off from the TN Valley, with PW values dropping to
around 1.1 inch, which is just below normal for late Sunday. Given
the lower available moisture and the fact that there will be little
forcing to counter that, will leave precip out of the forecast for
Sunday. Temperatures will be similar to those observed on Saturday,
with highs in the 80s and overnight lows in the 60s.

.LONG TERM...(Monday through Thursday)
Issued at 447 AM CDT Fri Sep 22 2017

Medium range model solutions depict a weak mid-level cyclone across
central MS at the beginning of the extended period, with the forecast
area beneath light southeasterly flow aloft. Guidance suggests that
the low will open into a wave and push slowly eastward on Monday, as
a shortwave trough ejects northeastward out of the central Rockies
and 500-mb heights begin to rise quite substantially across a large
part of the northwestern Gulf coast. The weakening remnants of this
feature are forecast to spread southeastward in the vicinity of the
FL peninsula from Monday night-Tuesday night, as the amplifying ridge
to our west pushes slowly eastward across the lower MS Valley. This
pattern will yield mostly sunny days/mostly clear nights and should
generate very warm temperatures for late September, with highs
solidly in the m/u 80s and perhaps as high as 90-92 in some normally
warmer locations. Based on dewpoints in the m60s and a fairly high
potential for decoupling of the boundary layer each night, lows
should still manage to fall into the m60s, with some u50s/l60s
possible in the east. Strong diabatic warming each day may initiate
isolated convection across the western portion of the CWFA, but
chances are so remote that even a 20 POP is generous.

During the period from Wednesday through Thursday, the mid-level
ridge will become centered across the western Gulf of Mexico but
should deamplify with time. This will allow a stronger zonal flow to
become established between the ridge and a broad longwave trough
encompassing much of central Canada as well as the north central
CONUS. Based on the more consistent solutions from the ECMWF and
Canadian global models, a cold front related to the deepening
longwave trough to our north will push southeastward through the mid-
MS Valley region and into the lower OH Valley Wednesday, before
accelerating further as it enters our region Thursday morning. Aside
from a noticeable drop in dewpoints as low-level flow increases from
the north, the front should cross the region with little fanfare as
convection will be limited by 1) no deep-layer forcing for ascent
within the confluent flow aloft and 2) unfavorable time of day for
fropa. In spite of fairly strong CAA in the wake of this front, highs
on Thursday should still reach the l/m 80s.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1219 AM CDT Fri Sep 22 2017

LIFR fog has developed at the KHSV terminals, with vis down to 1SM.
Expect similar conditions at KMSL within the hour or so due to the
moist atmosphere, clear skies and calm winds. These reduced vis will
persist through at least sunrise, with vis possible dropping below
1SM at times. By 14Z the fog should dissipate, leaving VFR conditions
through the remainder of the forecast period. Although an isolated
thunderstorm is possible this afternoon, coverage is too low to add
mention in the TAF.





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