Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 132054

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

.NEAR TERM...(Tonight)
Issued at 254 PM CST Wed Dec 13 2017

Latest water vapor imagery shows a fairly broad longwave trough
encompassing much of the eastern CONUS, with an amplified ridge
building over the west coast. At the surface, high pressure along
the western Gulf of Mexico was keeping southerly flow across the
local air, while an area of low pressure was shifting to the
northeast and into the Great Lakes region. With the tightening
pressure gradient, winds have increased to between 15 and 20 mph.
The combination of the strong southerly flow and ample sunshine has
allowed temperatures to warm into the lower 50s across much of the

A weak shortwave is expected to shift southeastward within the broad
upper trough. Meanwhile, the surface low will continue to shift to
the east-northeast, trailing a strong cold front across the region.
This front will quickly move through the TN Valley during the
overnight hours, with winds shifting to the northwest by daybreak.
Extremely dry conditions will be in place ahead of this front, so no
precip is anticipated. Given the southerly flow for most of the
overnight hours, temperatures will be a few degrees warmer than
those observed this morning, with lows in the lower to mid 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 18Z TAFS through 18Z Thursday afternoon)
Issued at 1137 AM CST Wed Dec 13 2017

VFR conditions should continue across the Tennessee Valley for this
TAF period. S-SW winds of 5-10kt in the late morning, should become
SW and increase into the 15-25kt range for much of the afternoon.
Winds will decrease to under 10kt from the SW after sunset. A dry
cold front is poised to move across the area around daybreak Thursday
morning. Winds will become northerly around 5kt after the front





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