Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 202339

Area Forecast Discussion
National Weather Service Huntsville AL
539 PM CST Mon Feb 20 2017

For 00Z TAFS.


.NEAR TERM...(Tonight)
Issued at 325 PM CST Mon Feb 20 2017

Veils of high clouds will continue to overspread the area, at times
opaque this evening, and most likely opaque after midnight. The warm
conveyor belt (WCB) will advance east and enter our western counties
late tonight. There will be a sharp gradient of rain-no rain, so
have scaled the POP in this fashion. Temperatures may dip into the
40s in our sheltered eastern valleys, while staying rather mild in
the 50s west.

.SHORT TERM...(Tuesday through Wednesday)
Issued at 325 PM CST Mon Feb 20 2017

The short range models have come into good agreement with a WCB rain
event on Tuesday into Tuesday Night. Precipitation should be
convective in nature, although thunderstorms look to be too low of a
threat to include at this time with no surface based convective
instability, and limited elevated instability noted in soundings.
Moderate rain at times can be expected on Tuesday as PW`s increase
into the 1.25-1.5 inch range with deep southerly flow developing
within the WCB with a steady isentropic lift occurring. The best
differential PVA will occur across the deep south, where instability
will be higher, and thunderstorms are more likely to occur. With the
clouds and rainfall, temperatures will be held back in the lower to
middle 60s for highs (upper 50s atop the Cumberland Plateau). As the
upper low dips south into the eastern Gulf of Mexico on Wednesday,
moist east-southeast low level flow will maintain cloud cover and
perhaps some light shower activity. Will hold off on drizzle for now,
but a slow west to east clearing will take place late Wednesday into
Wednesday night. Went with lower 70s for highs in our western
counties where some afternoon clearing may be just enough.

.LONG TERM...(Thursday through Sunday)
Issued at 325 PM CST Mon Feb 20 2017

Increasing low level southerly flow develops on Thursday, as the
pressure gradient between the surface high off the Atlantic coast and
a developing low over Texas/Oklahoma. Southerly gusts around 15 mph
are possible. Although not much forcing is expected, enough low level
moisture may be in place to keep a low cloud deck in place through
Thursday night. Models forecast 925 mb temperatures to climb to
between 15 and 17 degrees, thus kept highs above normal in the mid to
upper 70s in most locations.

This will likely continue through Friday, as the surface low deepens
significantly to around 998 mb and moves into the western Great
Lakes region. A weaker front or pre-frontal convergence area is
forecast by models extending southwest from the low into western
Tennessee and central Arkansas. However, models still show a very
strong CAP in place around 700 mb. This should keep any convective
initiation very limited. Dropping chance of showers and storms to 20
percent on Friday, despite some low level/elevated instability in the
afternoon. If the CAP can break late in the afternoon, some of these
isolated storms could be strong to severe. Large hail and marginally
severe winds look to be the biggest threats. At this time, low level
helicity does not look strong at all, so tornadoes are not expected.
With strong 925 mb (~30 knots) and 850 mb (30-40 knots) winds
developing ahead of the front, gusts to between 25 and 30 mph seem
reasonable, especially later in the afternoon into the evening hours.

Models do bring strong vorticity through extreme northern Alabama
and into Tennessee Friday night along and ahead of the main cold
front. If this forcing is far enough south, some strong to severe
storms may develop. Main threats would be marginally severe winds and
large hail. At this point kept 30 to 50 pop in the evening and
overnight. The best chance of thunderstorms though will be before
midnight. Models continue to show very warm 925 mb temperatures ahead
of the front on Friday as well (between 15 and 18 degrees Celsius).
This should allow highs to climb into the upper 70s to lower 80s.

Much colder and dry weather (especially given the very warm/possible
record heat a few days before) will move in behind the front on
Saturday. Highs in the 50s look reasonable and this may be a bit


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 539 PM CST Mon Feb 20 2017

VFR weather should continue across the area through the late night.
Conditions afterward will deteriorate from west to east, as a weak
storm system advancing eastward from the Southern Plains brings
deeper moisture, and increased chances of showers across the
Tennessee Valley, beginning after daybreak Tuesday. VIS/CIG values
should be reduced into the MVFR range during Tue morning due to rain
showers/fog. Reductions to IFR from low CIGS and/or VIS values look
probable Tue afternoon. SE winds 5-10kt tonight will increase into
the 10-15kt range for much of Tue.





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