Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 230518

Area Forecast Discussion
National Weather Service Huntsville AL
1118 PM CST Wed Feb 22 2017

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 904 PM CST Wed Feb 22 2017

An ill-defined surface high pressure system is prevailing across the
TN Valley in the wake of the upper low that brought the rain/clouds
on Tuesday/part of Wednesday. Air temperatures are currently holding
in the upper 50s to low 60s while dewpoint temperatures hold in the
mid to upper 50s. The low dewpoint depressions are already causing
patchy fog formation over northeast Alabama. Should start to see a
gradual increase in fog formation overnight as air temperatures
continue to drop. The only limitation to the fog formation will be
occasional sustained winds up to 5-8 MPH. However, in the protected
valleys of northeast Alabama the winds may stay calm with fog
formation likely more prevalent there.

Have already noted fog forming near Fort Payne and a gradual drop in
visibility at Scottsboro. As a result, have adjusted the timing of
the fog formation in the oncoming forecast and will monitor for
possible dense fog advisory over the next couple of hours.

.SHORT TERM...(Thursday through Friday)
Issued at 210 PM CST Wed Feb 22 2017

The fog and low clouds should dissipate readily by mid morning
Thursday, evolving into a scattered to broken deck of strato-
cumulus during the afternoon. The weak surface ridge axis will still
be in place across eastern AL which will keep flow southeasterly for
these locations. More robust south-southwest flow is expected to
take shape from the MS river westward into the southern Plains where
850 mb temperatures will be climbing well into the upper teens to
around 20C. Locally, mixing should reach around 875-900mb yielding
highs in the lower to perhaps middle 70s. A capping inversion begins
to develop around 750 mb which should prevent any deep convection.

The cap will continue to hold and grow stronger into Friday as
southwest flow increases and advects warmer air into the region. A
distinct warm nose at 850mb of 13-15C should induce pressure falls
resulting in a pre-frontal surface trough across western TN into MS
during the late afternoon and evening. The cap appears strong enough
to prevent deep convection during the day until cooler air aloft
arrives during the evening. Convective initiation occurs with the
various short range models between 00-06Z along this pre-frontal
trough. The NAM is a bit later at breaking the cap (03-06Z), which
I`m a bit more inclined to follow. Areal coverage is still
questionable, but will continue with a likely POP for srn TN and far
north AL, and a chance POP further south of the TN river. Deep layer
shear forecasts remain marginally sufficient for strong to severe
storms, but 0-2km shear vectors are aligned nearly parallel to the
potential convective band which is far less favorable for damaging
bowing segments. The low level hodograph curves sharply at 1km but
then is fairly uni-directional with 850mb speed divergence occurring
over our area. The cold front and precipitation will exit the area
completely between 09-12Z if not a bit sooner.

.LONG TERM...(Friday night through Tuesday)
Issued at 210 PM CST Wed Feb 22 2017

The cold front and any precip should be out of the forecast area late
Friday night, allowing the weekend to start off on a dry note. A sfc
high will quickly start to build in from the west on Saturday,
decreasing cloud cover and reinforcing CAA. Highs on Saturday will be
seasonable in the mid 50s and radiational cooling will help overnight
lows to dip back down towards the freezing mark. Yep, I said it. It`s
still winter after all! Sunday will also be dry and a few degrees
warmer than Saturday due to plentiful sunshine and southerly flow
returning as the sfc high begins to push eastward across the

Rain is back in the forecast next week as we start off with an
unsettled weather pattern. Disturbances will swing across the area
providing a chance of showers on Monday and Tuesday. A sfc low will
track across the Plains on Tuesday and up into the Great Lakes by
Wednesday. Ample southerly flow ahead of this system will allow for
us to warm back up towards 70 degrees on Tuesday and Wednesday! As
the previously mentioned sfc low`s cold front approaches, coverage
in showers will increase with thunderstorms possible on Wednesday as
the front moves into the area.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1130 PM CST Wed Feb 22 2017

Fog is expected to form during the overnight across the area,
courtesy of clear skies, light winds, and residual moisture from
recent rain. Although some of the fog could become locally dense -
especially late tonight, the lowest I went in the TAF was 3/4SM. VFR
weather should return shortly after daybreak Thu as an area of high
pressure moves east of the region, bringing a SE-S wind in the late
morning and afternoon.





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