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Area Forecast Discussion

FXUS64 KHUN 261057 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
557 AM CDT Mon Sep 26 2016

For 12Z TAFs.


(Issued 321 AM CDT Mon Sep 26 2016)

Dry and cool autumn weather is on the way this week after record
breaking heat this weekend. This cooler weather will occur as a mid-
upper level vortex pushes a surface cold front southeast today and
tonight! The front is currently located from central IN southwest
over the MS river near KMEM to SE TX.

Scattered shower/thunderstorm activity is expected to accompany this
front which will be the highest potential for precipitation (though only
30-45% chance) this week. Further to the south (over the gulf coast)
a weak upper low continues to churn with some moisture making it
northward on the northern flank of this low. A building ridge over
the west along with amplifying ridge over the western Atlantic Ocean
may cause the large vortex over the Great Lakes/OH River valley
regions to become cutoff from the polar jet stream. If this occurs,
anticipating another cold front later this week.



(Issued 321 AM CDT Mon Sep 26 2016)

As mentioned above isolated to scattered showers and thunderstorms
associated with a cold front. This activity looks to be more
anafrontal with showers/storms likely to be of higher potential just
after or along the frontal passage. One of the ingredients that has
been so far slow to respond, due to the far removed upper support
pushing this front, has been moisture advection. Right now despite
the upper low over the gulf coast, the low and mid-levels dewpoint
depressions have been high. However, as the front approaches, some
saturation in the low to mid-levels should be enough to at least
support isolated scattered showers and thunderstorms later this
morning. An increase in areal coverage of storms should occur later
this afternoon, especially over NE AL. On timing of the front and
convective activity: The front should arrive over NW AL by 15Z-16Z,
north central AL/S Middle TN by 18Z-20Z, and finally NE AL by 21Z-
23Z. Some of the convective activity will accompany the front while
another area of thunderstorms is possible behind the front. Due to
the scattered nature of the thunderstorm activity and overall lack of
moisture, made some timing adjustments to the POPs and weather
forecast. Mainly reducing POPs in the morning and incorporating
better timing for this afternoon. Thunderstorms should generally be
ordinary in nature due to low instability and wind shear profiles.
However, if any storms do have sustained updrafts, anticipating them
to be short- lived with gusty winds up to 30 MPH. Meanwhile, the
temperature forecast will be challenging especially with already warm
overnight temperatures this morning in the 70s. For now, have left
mid to upper 80s, but would not be surprised to see slightly higher
temps if the front slows its propagation.

Post-frontal showers and thunderstorms should gradually end from NW
to SE this evening. All lingering activity should end by midnight.
The best news for all of those that love fall weather is that
overnight lows will drop into the upper 50s, and possibly lower 50s
in higher elevations. Sunny conditions should prevail on Tuesday as
the upper flow pattern over the Southeast shifts to a prevailing
trough with NW/W flow. Daytime highs on Tuesday should be in the mid
to upper 70s. Then, one of the coolest nights of the month should
occur Tuesday night as the cool cP airmass settles over the region
with overnight lows in the low 50s!

The evolution of the upper vortex mentioned previously will be the
primary focus for the rest of the forecast. It appears that this
upper vortex may wobble south on Wednesday/Wednesday night acting to
send a reinforcing cold front southward. Moisture convergence along
the front and around the trough axis is fairly limited. However, did
introduce a slight chance pop mainly over NE AL/Southern Middle TN
for Wednesday night as the front moves across the region. This will
also mean continued cool daytime highs on Thursday in the 70s and
overnight lows near 50 on Thursday night.

The weekend`s forecast is a little more uncertain due to the
progression of this upper vortex. The ECMWF/Canadian portray a
stronger western Atlantic ridge acting to keep this vortex in place
across the eastern half of the country. However, the GFS shows a
opening of the cutoff low and merging back with the polar jet about
24 hours sooner. So, for now will keep the forecast dry and
seasonably cool.



For 12Z TAFs: A cold front will arrive by 19Z at KMSL and 21Z at
KHSV. Isolated to scattered showers and thunderstorms will be
possible along and just behind the cold front through this evening.
Winds will shift from west-southwest to northwest behind the front
with speeds of 8-11kt. Eventually, winds will veer to the north
overnight. VFR cigs aoa bkn035 are forecast this evening, although
lower MVFR cigs blo 020agl are psbl in some areas.




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at weather.gov/huntsville.

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