Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 220516 AAC
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1216 AM CDT Sun Oct 22 2017

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 831 PM CDT Sat Oct 21 2017

The upper ridge that has brought us dry conditions over the last few
days will continue to move east of the area as an upper trough pushes
eastward through the Plains. Meanwhile, a cold front continues to
push slowly eastward, but will remain west of the Mississippi River
through the near term period. A leading shortwave moving around the
eastern periphery of the trough was generating showers and
thunderstorms over central and southern Mississippi. However, the
atmosphere further east is still fairly dry and with the loss of
daytime heating, that activity has been slowly diminishing over the
last couple of hours. High clouds continue to stream northeastward
across the area as this shortwave ejects to the northeast, but the
area will remain dry through the overnight hours. Low level winds
will remain out of the southeast tonight, keeping a steady flow of
low level moisture and weak warm air advection. Given the cloud cover
and these winds, temperatures will be a few degrees warmer tonight
than in previous nights, with overnight lows dropping into the upper
50s across the east to lower 60s across the west.

.SHORT TERM...(Sunday through Monday)
Issued at 245 PM CDT Sat Oct 21 2017

New guidance seems to push forcing ahead of the front into the area
a bit quicker than previous runs. This will bring scattered to likely
chances of showers and thunderstorms to the regions by the early
afternoon hours near the AL/MS border. At this time, although shear
is decent, helicity is not very impressive and a very strong (50-60
knot) low level jet does not materialize in forecast data. Shear
vectors remain parallel to the orientation of the approaching area
of showers and thunderstorms as well. So main threats with
thunderstorms will be gusty winds to around 45 mph and very heavy
rainfall.

The new model guidance does not slow down the front as much as
previous runs. It does continue to develop a strong upper low on the
southern end of the front. This very strong forcing swings through
the area (although a bit further north than in previous runs as well)
late Sunday night into Monday morning. Very moist air continues to
be advected ahead of this front Sunday night through Monday morning,
driving PWAT values to around 1.9 inches. Given the very strong
forcing associated with the upper low and surface front itself, heavy
rainfall still looks like a good bet. Two to three inches of
rainfall is expected.

A break from the heaviest rainfall is expected Monday afternoon into
Monday evening, as the strong upper level forcing swings northeast
toward the Ohio Valley. Lowered precipitation chances during this
timeframe to 30 to 60 percent (mainly until 7 pm). This should not be
nearly as heavy as drier air begins to work into the area.

.LONG TERM...(Monday night through Friday)
Issued at 245 PM CDT Sat Oct 21 2017

A strong upper level trof/cold pool aloft will be digging southeast
through the TN valley on Tuesday.  Strong divQ is indicated implying
significant subsidence potential. Flow at 850 mb is not terribly
impressive with 25-30kt, but dry adiabatic lapse rates up to 775mb
should be enough to produce gusts that high at times. Temps will
tumble into the 50s by late afternoon, with upper 30s to around 40 by
Wednesday morning. The well mixed boundary layer and wind flow
should prevent frost development. Wednesday will be sunny and cool,
but the more progressive nature to the pattern (trof shifting east)
should allow for low level flow to become more westerly through the
day, inducing warm advection. The 00Z EC and 12Z GFS differ in timing
and evolution of large trof amplification and movement in the
Rockies/Plains/Midwest late next week. That being said, confidence is
increasing in more pronounced warm advection in the TN Valley on
Thursday with 850mb temps soaring into the 10-13C range. So surface
high temps should reach the upper 60s to lower 70s rather easily. If
the faster GFS is correct, a good thermal and moist axis will shift
into region on Friday as another amplification of a trof takes place
over the upper Midwest south through the Ozarks and southern Plains.
A warm conveyor belt of showers would be expected in this zone ahead
of developing cold front. With such uncertainty in timing/evolution,
will just introduce shower chances on Friday per suggested blend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1144 PM CDT Sat Oct 21 2017

VFR conditions will prevail at the HSV/MSL terminals overnight with a
SE flow around 8 kts and few stratocu beneath sct-bkn layers of
altostratus/cirrostratus clouds. Winds will begin to gradually veer
and increase by 22/15Z as a cold front approaches the region from
the west, with sustained speeds arnd 12 kts and gusts up to 20 kts
expected for remainder of the morning and afternoon. Although cloud
coverage will increase during this period, bkn-ovc cigs should only
descend to arnd 8 kft thru late afternoon/early evening. Threat for
sct shra/tsra will begin to increase at MSL/20Z and HSV/22Z. However,
more widespread shra and embedded tsra will likely occur btwn
23/00-06Z as a wave of low pressure forms along the front, forcing it
to decelerate as it moves across the TN Valley. Although not
explicitly noted in TAFs, cigs may fall to arnd 1500 ft at both
airports btwn 04-06Z, and a wind shift to NNW may also occur at MSL
close to 06Z.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...73
SHORT TERM...KTW
LONG TERM...17
AVIATION...70/DD


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