Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 140500

Area Forecast Discussion
National Weather Service Huntsville AL
1100 PM CST Wed Dec 13 2017

For 06Z TAFS.


.NEAR TERM...(Tonight)
Issued at 855 PM CST Wed Dec 13 2017

The current near term grids are in good condition this evening, with
only very minor changes made to adjust hourly grids for real time
observations. Guidance remains fairly consistent in forecasting a
strong mid-level vort max to race east-southeastward across the
southern Great Lakes and into the mid-Atlantic region overnight. A
reinforcing cold front attached to the attendant surface low should
advance southeastward across the CWFA between 08-12Z, veering winds
to the north-northwest. Although lower-middle tropospheric moisture
is far too meager to support any precipitation given the weak nature
of the synoptic scale ascent this far to the south, a gradual
increase in postfrontal low clouds is possible -- especially for the
northeastern half of the region. The combination of increasing
clouds and greater boundary layer mixing will yield warmer low temps
tonight, with most locations only falling into the l-m 30s.

.SHORT TERM...(Thursday through Friday night)
Issued at 254 PM CST Wed Dec 13 2017

A cold front will be pushing south of the area early Thursday
morning, leaving northwest flow across the TN Valley. Zonal flow
will take hold of the upper level pattern, while high pressure
builds at the surface. Mostly clear skies will persist through the
day and may counter the cold air advection to keep afternoon highs
in the upper 40s to lower 50s. The clear skies will persist through
the overnight hours, with winds becoming calm after sunset. This
will create very good radiational cooling conditions across the
area, with overnight lows dropping into the mid to upper 20s.

Another shortwave will dig southward and into the Central Plains
early Friday morning, with the axis of the trough pushing through
during the day. Winds will generally remain from the north and then
begin to back to the west by the afternoon. With this trough, clouds
will increase, with mostly cloudy conditions expected on Friday.
This will keep temperatures from warming too much during the day and
afternoon highs will only top out in the lower 40s. Winds will
continue to back overnight and become southerly by daybreak on
Saturday. However given the light nature of the winds, temperatures
will still cool into the upper 20s Friday night.

.LONG TERM...(Saturday through Tuesday)
Issued at 254 PM CST Wed Dec 13 2017

Saturday will be pleasant with sfc high pressure across the region
and southerly flow providing seasonable temps in the lower 50s.
Through the day, a shortwave will eject from the Gulf of CA, through
the Southern Plains and into the TN Valley by late Sat night/early
Sunday morning. This will force the sfc high eastward and moisture
will return as southerly winds increase overnight.

Models continue to agree that the shortwave will move through Sunday
morning, bringing showers across the forecast area. Instability will
be limited but PWATs will be near the 99th percentile reaching 1.2
inches on Sunday. With dry conditions currently in place and the
system moving fairly quickly, have no flooding concerns. Rainfall
amounts of 0.5 to 1 inch are possible. Models disagree on exactly
when the precip ends, and if another round of precip returns. The
ECMWF is more aggressive and faster with the next shortwave, allowing
dry air and high pressure to clear precip out by Monday
aftn/evening. The GFS however has Gulf Coast convection lifting
northward on Monday and enhanced by a sfc low ahead of the next
shortwave. The shortwave wouldn`t move through until Tuesday
aftn/evening. To accommodate the model differences, held onto POPs a
little longer in the forecast, finally clearing precip out by Tues

Temps will be just above normal Monday and Tuesday with highs in the
upper 50s and lows around 40 degrees.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1100 PM CST Wed Dec 13 2017

No changes to previous aviation forecast reasoning, with VFR
conditions expected to persist at both terminals overnight.
Occasionally scattered mid and high-level clouds will continue in the
prevailing WNW flow aloft, with sfc winds diminishing/veering in
advance of an approaching cold front. Frontal wind shift to NNW
should occur around 09-10Z, and although no precipitation is
associated with the front there is a bit more concern for low stratus
clouds in it`s wake. At this point, we will introduce SCT020 btwn
10-16Z, since it appears as if the more solid deck of overcast
stratus clouds should be displaced to the N/E of both terminals. Any
lingering stratus will begin to lift/scatter by late morning, with
mid/high-lvl clouds returning late in the TAF period.





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