Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 212332
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
632 PM CDT Sat Oct 21 2017

.UPDATE...
For 00Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 245 PM CDT Sat Oct 21 2017

Mainly high cloud cover is in place over northwestern Alabama east
into Madison county at this time. Further east, mainly clear skies
prevail. A strong cold front extending from the western Great Lakes
southward into eastern Oklahoma continues to move closer to the area
tonight. This should increase winds over the area tonight, as the
surface high over the Mid-Atlantic is slow to move east. These winds
between 5 and 10 mph in lower elevations (10-20 mph above 900 feet)
will continue much of the night. This will make for warm low
temperatures overnight in the mid 50s to lower 60s. Cloud cover
should remain the thickest and most prevalent west of I-65. More
clearing could occur further east. This should allow a decent
gradient in low temperatures despite the winds, with lows in the east
dropping into the mid 50s.

.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 00Z TAFS through 00Z Sunday evening)
Issued at 632 PM CDT Sat Oct 21 2017

VFR conditions are expected through the period at the KMSL and KHSV
terminals. Winds will remain from the southeast and increase to
around 12 kts after 15Z, with some gusts possible through the
afternoon hours. A cold front will approach the area from the west
late tomorrow, with showers and isolated thunderstorms developing
ahead of the front. Based on latest guidance, this activity should
reach the KMSL terminal around 21Z and then reach KHSV after 00Z.
While thunderstorms are possible, confidence is low so will include a
VCTS for KMSL with this issuance.

&&

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

$$

NEAR TERM...KTW
SHORT TERM...KTW
LONG TERM...17
AVIATION...73


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at weather.gov/huntsville.



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