Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 131737

Area Forecast Discussion
National Weather Service Huntsville AL
1137 AM CST Wed Dec 13 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 955 AM CST Wed Dec 13 2017

After a rather cold start across the TN Valley this morning,
temperatures are rising quickly, with temps in the upper 30s. High
pressure will continue to build eastward today, with southerly flow
increasing as the pressure gradient tightens. Sustained winds should
top out around 15 mph, with gusts up to 25 mph this afternoon. This
will help filter in slightly more moisture and warmer temperatures.
The combination of the southerly flow and ample sunshine will help
our highs reach into the lower 50s. Previous forecast was on track so
no major changes were made.

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

After the front arrives, cooler air looks to slowly advect southward
during the day. Light northerly surface winds and weaker cold air
advection appear to be the cause for the lag. With daytime heating
possibly interrupted by low clouds, daytime high temperatures on
Thursday are challenging to forecast. Will keep a blend of the
guidance with some slight adjustments upward in the daytime highs to
account for a cold bias in the guidance. In addition, will keep a
north (`cold`) to south (`warm`) temperature gradient.

Meanwhile, an elongated longwave trough formed from the merging of
sheared mid-level troughs within the southern flank of the polar jet
over the Intermountain West, and another one within the subtropical
jet, will move towards the MS River on Thursday. The arrival of yet
another cold front with this sheared trough over the TN Valley on
Friday will push the stalled/slow moving polar front further south
along with the cold air that has been lagging just to the north. As a
result, Friday`s temperatures will be noticeably cooler (daytime
highs in the 40s) along with mostly cloudy sky conditions during the
morning hours. After sunset, temperatures should drop below freezing
on Friday night.

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

A mild and dry day is expected Saturday with dew points staying in
the lower 30s at most. Southerly flow will pick up substantially
Saturday night across the central Gulf Coast into the lower MS valley
in advance of the shortwave depicted by both medium range models.
This wave ejects from the southern Plains and rapidly through the TN
and lower MS valleys late Saturday night into Sunday morning. In
fact, both the GFS and ECMWF are quite similar with their mass fields
in this regard. They also suggest very dry low levels will likely be
overcome by falling precip as early as late Saturday night, so will
stay the course with PoP increasing Saturday night into the likely
range in northwest AL with chances elsewhere. Suggested blended
guidance brings the PoP into the categorical range on Sunday and
cannot disagree with a large area of showers moving across. Showalter
indices still remain quite positive, so will keep all showers for
now. The warm sector should remain bottled up well to the south with
a better thunderstorm threat. Rainfall amounts of one half to one
inch look quite obtainable, if not a bit heavier in spots.

Then differences continue between the GFS and ECMWF Monday into
Tuesday.  Will side closer to the more progressive ECMWF in this
pattern which leads to a dry Monday and Tuesday.  If model consensus
leans closer to the slower GFS, we may need to adjust later
forecasts. In either case, our airmass will remain temperate with
readings fairly close if not just above seasonal normals early next


.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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