Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 240542

Area Forecast Discussion
National Weather Service Huntsville AL
1242 AM CDT Sun Sep 24 2017

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 848 PM CDT Sat Sep 23 2017

Fairly quiet evening across the Tennessee valley, save for a few
rogue thunderstorms that refuse to die despite the loss of solar
heating. This activity is tied to lingering outflow boundaries from
earlier convection, modest elevated instability, and the presence of
boundary layer moisture (evident by dewpoints in the upper 60s). Do
expect this very isolated thunder threat will linger until about
midnight, but the general weakening trend will continue. Main impacts
from this activity will continue to be lightning, as well as some
small hail, and perhaps a brief 30 mph wind gust.

With a mostly clear sky in place, temperatures have dropped off
quickly in most areas into the low to mid 70s. As mentioned, boundary
layer moisture is quite high, with dewpoint depressions in the 0-2
range in many sites already. As a result, am expecting lows to be
quite mild once again -- ranging from the mid 60s along the Plateau
east of I-65 to near 70 degrees across the HSV/DCU/MSL metros
further west. This boundary layer moisture, along with light winds
will allow for patchy fog to develop once again late tonight,
especially in areas near bodies of water, sheltered valleys, and
where rainfall occurred.

.SHORT TERM...(Sunday through Monday night)
Issued at 307 PM CDT Sat Sep 23 2017

Patchy fog will dissipate shortly after sunrise. Daytime highs will
continue to be warmer than normal as we head into next week. An
upper low anchored in the northern Gulf will lift northwest into
central Mississippi on Sunday, with weak height falls across the TN
Valley. This will allow temps to be a degree or two cooler than the
past few days, mid 80s, and a few showers/storms Sunday afternoon,
mainly in the western half of the forecast area. Monday on the other
hand will be dry as a ridge to our northeast builds further south
thanks to Hurricane Maria.

.LONG TERM...(Tuesday through Friday)
Issued at 307 PM CDT Sat Sep 23 2017

The TN Valley will remain under the influence of an upper level
ridge and surface high pressure at the start of the long term
period. Meanwhile, an upper trough will slowly eject east of the
Rockies and push in the Central Plains on Tuesday, while TC Maria
continues its slow northward progression off the Atlantic Coast.
Given the high in place, dry conditions will continue through the
day on Tuesday, with the continuation of above normal temperatures
as highs warm into the upper 80s across the area.

As the upper trough continues to move east on Wednesday, an upper
ridge will build from the central Gulf Coast and into the TN Valley.
Meanwhile, high pressure to the north will shift winds to a more
northerly direction during the day. Temperatures will be hot once
again due to the increase in thicknesses, however with northerly
flow returning, dewpoints will be a couple of degrees cooler. The
upper trough will essentially flatten as it moves into the area,
leaving near zonal mid level flow across much of the region on
Thursday and continuing through the weekend.

A reinforcing cold front will swing across the area late Wednesday
night, bringing a nice change across the area beginning as early as
Thursday. As the stronger surface high builds in from southern
Canada, cooler air will filter southward. Latest guidance shows very
little in the way of precip with this front, as we get cut off from
the moisture ahead of it and the forcing associated with the actual
front remains weak. Therefore, have removed pops from the forecast
on Thursday. Temperatures will be the noticeable difference behind
the front, with highs on Thursday remaining near seasonal norms in
the lower 80s. As the high continues to move southward, even cooler
temperatures are forecast, with highs on Friday and Saturday
struggling to make it out of the 70s. Dewpoints will also cool, and
help overnight lows drop into the 50s for the latter part of the
extended forecast period.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1242 AM CDT Sun Sep 24 2017

VFR conditions should continue into most of the night. However with
clear skies, light winds, and moisture (some from recent rains),
patchy fog formation is possible during the predawn. Have MVFR 3-5SM
VIS values at both sites beginning between 08-10Z, with TEMPO IFR
VIS 1-2SM around sunrise. Conditions should return to VFR by 14Z,
continuing for the remainder of the day. More strong heating and
resulting instability could produce isolated convection in the
afternoon. Some of the models were suggesting better rain chances
across NW AL, but will hold off putting TS in the TAF this issuance,
and hope for better inter-model agreement by the next TAF.





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