Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 201149

Area Forecast Discussion
National Weather Service Huntsville AL
649 AM CDT Wed Sep 20 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 517 AM CDT Wed Sep 20 2017

Water vapor satellite data confirms that the axis of a mid-
tropospheric trough continues to slowly cross the region this
morning. Near term guidance is in agreement in forecasting this
feature to advance southeastward across the southern Appalachians and
into the western Carolinas by late afternoon, as a high amplitude
ridge builds northeastward into the eastern Great Lakes region.

At the surface, calm winds persist across the forecast area this
morning due to our proximity to a surface ridge positioned along the
spine of the southern Appalachians. The combination of light winds
and mostly clear skies has allowed fog to form once again, with the
fog most widespread/dense in locations that received significant
rainfall yesterday. A Dense Fog Advisory will continue for portions
of north central/northeast AL through 15Z.

For the remainder of the day, we expect warm and humid conditions
uncharacteristic for mid/late September to continue. Although a few
showers and storms may develop across portions of both northwest and
northeast AL this afternoon, lack of deep-layer ascent and lower PW
values (1.2-1.4 inch range) suggest that most of the area will remain
dry. With less clouds and precipitation compared to yesterday,
afternoon temps should warm back into the u80s/l90s with heat indices
in the 95-100 degree range across the western half of the region.

.SHORT TERM...(Tonight through Friday night)
Issued at 517 AM CDT Wed Sep 20 2017

During the period from tonight through Friday night, a mid-level
ridge extending from the Arklatex region northeastward into the
eastern Great Lakes/western New England will gradually strengthen
while the trough over the Carolinas drifts further to the southeast
and expands westward into the central Gulf coast. Winds aloft across
our region are initially predicted to veer to the northeast and
strengthen as this occurs, but should diminish and shift to more of
an easterly direction by Friday/Friday night as a low cuts off across
the northeast Gulf coast. This regime will maintain the
southwestward advection of a dry mid-level airmass, with PW values
expected to remain in the 1.3-1.5 inch range through this period. A
few weak disturbances embedded within the gradient flow aloft should
cross the region with little impact on sensible weather.

At the surface, light/variable winds and mostly clear skies will
support the development of fog once again tonight -- although the
coverage should be quite a bit less based on the presumption of less
rainfall today. It appears as if a subtle windshift to the northeast
will push across the region on Thursday, as a reinforcing dome of
high pressure builds into southern Ontario and northern New England.
Lift along this feature may be strong enough to initiate scattered
showers and thunderstorms based on forecast soundings indicating
sufficient moisture/instability, and a 30 POP was included for this
reason on Thursday. A slightly drier low-level airmass will be
advected into the region as flow veers to the northeast, and this
should result in slightly cooler minimum temps on Thursday/Friday
nights along with a lower spatial coverage of fog. Highs will still
reach the m/u 80s on Thursday/Friday, but lower dewpoints will keep
heat indices in the l90s.

.LONG TERM...(Saturday through Tuesday)
Issued at 517 AM CDT Wed Sep 20 2017

The upper level pattern will be dominated by a strong upper ridge
over the eastern CONUS, with an amplified trough over the western
CONUS at the start of the long term period. TC Jose and TC Maria will
be meandering off the east coast, remaining over the open waters.
Meanwhile, a weakness in the upper ridge over the northern Gulf will
translate into the development of a cut off low that will move
eastward just south of the TN Valley on Saturday. Given the proximity
to this enhanced synoptic lift, along with moisture advecting in
southeast flow through the lower levels, isolated to scattered
thunderstorms will be possible on Saturday. The higher chance will be
across the southern portion of the forecast area, closer to the
upper low. This low will remain over the region through Sunday,
though moisture will be a bit lacking, so kept a 15 percent chance
for thunderstorms to account for diurnally driven thunderstorms.

A drying trend will begin on Monday, as the upper low weakens and
becomes absorbed in the strong upper ridge over much of the eastern
CONUS. Meanwhile, high pressure will build in from the northwest,
with a light northwest surface flow bringing in slightly drier air.
Thus, no rain is forecast for the TN Valley for at least the first
half of the next work week. Temperatures through the period will
generally remain on the warm side, given the higher upper level
heights over the area. Afternoon highs will warm into the mid to
upper 80s, with overnight lows falling into the 60s.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 649 AM CDT Wed Sep 20 2017

Fog will continue to impact the HSV/MSL terminals this morning, with
vsby reductions potentially as low as 2SM expected thru 20/14Z. VFR
conditions are anticipated after the fog dissipates arnd 15Z, with a
surface ridge maintaining lgt/vrbl winds. Although strong boundary
layer heating will force the development of sct cu by 16-17Z, there
is little to focus the development of aftn shra/tsra and we will
include no pcpn in the forecast attm. The afternoon cu field will
dissipate quickly arnd sunset, with only sct high-lvl clouds expected
overnight. Conditions will be favorable for the development of
patchy fog once again -- especially in river valleys -- and a tempo
group for 5SM/BR has been introduced btwn 21/08-12Z.


AL...Dense Fog Advisory until 10 AM CDT this morning for ALZ006>010-016.




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