Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 281118 AAA

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
618 AM CDT Wed Jun 28 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 302 AM CDT Wed Jun 28 2017

Benign weather pattern continues early this morning, with high
pressure built in at the surface and a broad upper-trough exiting the
region to the east. The drier air that filtered into the region has
made for a mostly clear sky early this morning, save for a few high
clouds passing overhead. There may be enough boundary layer moisture
near rivers/lakes to get some light patchy fog this morning, but
generally think we`ll stay free of it in most locations. With the
ridge generally dominating things today, think we`ll generally see
mostly sunny conditions throughout the day given the subsidence
taking place. However, winds will veer to the southeast and
eventually to the south by early evening. This will set the stage for
a good return flow of Gulf moisture heading back into the area in
the coming days. With increased thickness values/temperatures aloft,
highs will run anywhere from 3-5 degrees higher than yesterday
(generally in the mid to upper 80s).

.SHORT TERM...(Tonight through Friday Night)
Issued at 302 AM CDT Wed Jun 28 2017

A return of a muggy, humid air mass is expected late this week --
along with increasing chances for showers and thunderstorms. As
mentioned above, a good return flow from the Gulf of Mexico will
set-up by tonight as winds increase from the southwest. A weak area
of low pressure along the MS/LA Gulf coast may help to generate some
shower activity across lower Alabama later tonight that could
potentially drift into our far southern zones by Thursday morning.
The main generator of precipitation on Thursday, however, will be
daytime heating. This activity will favor the higher terrain of
northeast Alabama and will be outflow driven once it gets going.
Model soundings indicate some modest instability and low shear values,
so just think general thunder in the form of pulse cells is the most
likely storm mode. Feel confident enough to go likely PoPs across
areas east of I-65 (favoring the terrain of Sand Mountain) between
18-00z, with solid chance PoPs elsewhere.

A deepening upper-trough across the Great Lakes will send a cold
front SE toward the Mississippi Valley Friday into Friday night.
Though this boundary will remain NW of the area, it will serve as a
focus for showers and storms -- some of which may approach the region
from the west late Friday night. As for the day on Friday, the
aforementioned high will shift to the east, resulting in lower
thickness values and a weaker CAP in place. Though there is no true
forcing mechanism in place, diurnally driven convection will
redevelop across the area by early afternoon and persist well into
the evening. Instability parameters look at little bit better (with
ML CAPEs around 1200 J/kg) which could yield a strong storm or two
capable of some gusty winds and locally heavy rainfall given the very
high PWATs that will be close to 2 inches. Model solutions diverge a
bit for Friday night, with the GFS far more bullish on higher PoPs
and QPF late into the night (especially toward the west near the
front). For now, have gone with a blend to handle this (slightly
favoring the drier NAM).

One other note: do have fairly good confidence that cloud cover for
this period will be a little bit higher than previously thought
earlier this week. Thus, this may keep high temperatures a degree or
two cooler -- closer to the mid 80s.

.LONG TERM...(Saturday through Tuesday)
Issued at 302 AM CDT Wed Jun 28 2017

This weekend it looks like we will be caught in the middle of a
trough moving across the Great Lakes and a subtropical ridge
centered over the eastern Gulf of Mexico. The result of which is a
few disturbances moving between these features and across the TN
Valley. The primary forcing should remain well to the north but
there will be enough lift with the weak disturbances to generate
numerous thunderstorms on Saturday. CAPE on the order of 1,000-2,000
J/kg and shear albeit weak (~20kts) should be more than enough for a
few strong storms. The only question for Saturday is that models are
showing what is likely to be a complex of storms developing over
OK/TX late Friday night and moving towards the area during the
overnight period. These may keep skies cloudy and could limit
afternoon heating but those kinds of details are a bit hard to
discern this far out.

The ridge never really goes anywhere and for the most part the same
can be said for the trough, which should allow for a continued
progression of weak waves across the area into early next week. The
weak forcing tied to those waves and the generally warm and humid
summer-time airmass should allow for scattered thunderstorms to
develop each afternoon Sunday and Monday. The GFS and to some extent
the ECMWF are showing significant drying of the lower levels Sunday
and Monday afternoon with a very pronounced inverted-v on the latest
GFS sounding both days. That seems rather overdone at this point and
will wait another few runs before trying to pin down some of the
storm threats. With highs in the upper 80s to around 90 degrees and
dewpoints in the lower 70s, heat index values should be in the mid
to upper 90s.

Beyond Monday, model and ensemble spread increases with some
guidance attempting to build a ridge out west and trapping a
low/shortwave over the TN Valley and others showing the ridge over
the central CONUS. In either case it could be a stormy Fourth of
July around the TN Valley. Due to the differences and spread in the
guidance have gone with a blend of guidance, which puts temps near
normal for this time of year (upper 80s - lower 90s).


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 618 AM CDT Wed Jun 28 2017

VFR conditions will prevail at each terminal through the period, with
light southerly winds and FEW/SCT Cu developing by late morning. Very
late in the period, some denser low clouds could move into the
region from the south, but did not have enough confidence they would
reach either TAF site by 12z.




LONG TERM...Stumpf

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