Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 040917 CCA
AFDHUN

Area Forecast Discussion...Corrected
National Weather Service Huntsville AL
234 AM CST Sun Dec 4 2016

.NEAR TERM...(Today)
Issued at 234 AM CST Sun Dec 4 2016

A large shield of cold rain covered nearly all of north AL and
southern TN this morning as vast south-southwesterly flow overtops a
cold layer below 775 mb. The BMX sounding at 00Z depicted the depth
of the flow above the inversion at 775 mb, with a northeasterly feed
of cool air beneath it. Thus far, about 1/4-1/2 inch of rain has
fallen across the area.  At the surface, there was an inverted trof
extending northeast from low pressure along the Louisiana Gulf Coast
thru northern GA. At 850 mb, a strong warm front was lifting
northward into central MS and AL. 40-50kt flow was advecting much
greater theta-e air into the TN Valley. As this lifts northeast, the
greatest ascent will occur between the I-59 corridor and the TN
River. This where the greatest QPF is expected today which the GFS
has a good handle on. There is just a sliver of elevated CAPE of
around 100-200 j/kg, so can`t rule out a rumble of thunder, but have
opted to leave out of the forecast attm. Will keep temps at a near
standstill today given the rainfall, just rising a few degrees once
the precip begins to diminish this afternoon.

.SHORT TERM...(Tonight through Tuesday night)
Issued at 234 AM CST Sun Dec 4 2016

With mid-upper level winds becoming west-southwest tonight, and the
surface wave advancing east-northeast through south AL into GA along
warm front, precipitation will diminish/end from west to east this
evening into the overnight bringing an end to about 30 hours of
rainfall. In fact, we may remain dry through the first half of
Monday as well. Then, the next wave of beneficial precipitation will
start to arrive as the upper low now digging into northwest Mexico
will eject northeast as an open negatively tilted wave. This will
provide a rather extensive area of upper level divergence across the
OH/MS/TN valleys down to the Gulf Coast. The models are still in good
agreement on tracking a surface low from the western Gulf of Mexico
early Monday morning northeast across the northwest tip of AL late
Monday Night. The best dPVA and -divQ lifts just north of our area
into TN and KY. However, mid level cooling with this wave will be
sufficient to generate CAPE values above the cool near surface layer
of 300-500 j/kg along and ahead of the occluded front, so will keep
thunderstorms in the forecast. The warm front enhanced by the wedge
should remain to our south along the I-20 corridor per plan view and
sounding profiles. However, a narrow corridor of the warm sector and
60+ dew points may sneak into north AL. Given the amount of deep
layer shear, and low level helicity, we will need to monitor the
quality of the airmass closely. Showers may linger into the early
evening hours in our eastern counties as a weakening canadian cold
front drops southeast behind the departing low pressure. All told,
rain amounts of 1.5-2.5 inches are likely with a few spots seeing
heavier amounts in our southern counties in north AL.

.LONG TERM...(Wednesday through Saturday)
Issued at 234 AM CST Sun Dec 4 2016

The progression of a broad longwave trough and an accompanying
Arctic cold front will be the primary focus for this time period. By
Wednesday, the longwave trough should be moving across the Central
Plains with a 1045 mb surface high pressure system trailing behind
the trough. The weather during the day on Wednesday should be
generally dry, but partly to mostly cloudy conditions should
prevail. Though some discrepancies exist with the med-range models
with respect to strength of moisture advection, another episode of
rain is probably beginning on Wednesday evening into the overnight
hours. Temperatures do look to be slower to drop with the latest
model runs mainly due to the slower progression of the front.
However, could still see a brief mix of rain/snow early Thursday
morning before subsidence and mid-upper level dry air diminish
precipitation, if the GFS/Canadian is correct. However, the ECMWF
continues to show a slower movement of the front with rainfall on
Thursday. At this point, have retained POPs into Thursday, but not
much certainty with this solution.

After the Arctic cold front passes, a very cold airmass will advect
towards the TN valley with a strong anti-cyclone building over the
region on Thursday/Friday. Due to the significant pressure gradient
(20-30 mb) between the high pressure system and the TN valley, winds
will be quite brisk behind the front even during the overnight hours
on Thursday night. NW winds of 10-15 MPH with gusts to 20 MPH are
likely, especially over the higher elevations, as temperature drop
into the teens by early Friday morning. With the increased winds,
wind chills in portions of the forecast area will be in the single
digits.

Some adjustments to the overnight lows/wind chills are possible on
Thursday night if the trough changes trajectory and/or slows.
Nevertheless, strong cold air advection is still expected through
the day on Friday even with the mostly sunny sky conditions. Daytime
highs will be in the upper 30s, which is about 15-20 degrees below
the 30 year climatological normals for this time of year. Record low
maximums look to be safe for Friday [(30 for KMSL (1937) and 30 for
KHSV (1937)]. With the loss of INSOLATION after sunset, will see
temperatures drop into the low 20s on Friday night with little
airmass modification possible on Saturday as the high pressure
system finally moves east over the Appalachians. This will result in
the sfc winds veering to a easterly/southerly direction across the
TN valley and daytime highs rising back into the low to mid 40s.


&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1045 PM CST Sat Dec 3 2016

A large upper low, now over the BAJA peninsula of Mexico, will
continue to pump moisture across the Tn valley overnight and into
Sunday.
This will keep the chc of rain and low clouds over the taf sites for
much of the fcst period. However new NAM model data shows that there
could be a break in the rain beginning arnd 21z and continuing thru
the end of the fcst period. Otherwise expect mainly ifr conditions
for the next 24 hours at both KMSL and KHSV.


&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...17
SHORT TERM...17
LONG TERM...SL.77
AVIATION...007


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