Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 201141

Area Forecast Discussion
National Weather Service Huntsville AL
541 AM CST Fri Jan 20 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 317 AM CST Fri Jan 20 2017

The rain has finally exited the region--for now. The first of
several potent shortwaves/upper lows is centered over the Missouri
Valley, with a pronounced water vapor plume over the Eastern Seaboard
and dry slot now pushing all the way across the Tennessee Valley. In
its wake, there is quite a bit of scattered-broken low cloudiness,
with areas of fog developing where the clouds have broken up.

Near term issues are fog and light rain. The patchy fog forecast
looks to be in good shape through the early morning hours since it is
entirely dependent on localized pockets of clearing. Satellite and
observations will need to be monitored over the next few hours to see
if anything more organized can develop, but NARR time-lag ensembles
give little reason to think it will. After sunrise, some weak-
moderate isentropic upglide around 295K, coinciding with lingering
moisture, could still produce abundant clouds and some light showers.
The rain should end by afternoon as the moisture shifts east though
the clouds may not completely go away. Highs should reach 70 for much
of the area, and could easily reach low-mid 70s if clouds break up.

.SHORT TERM...(Tonight through Sunday)
Issued at 317 AM CST Fri Jan 20 2017

The dry period will not last long, however, as another round of rain
pushes into the area late tonight in response to the next upper-
level system exiting the base of the western CONUS trough. This
system carries some potential of severe weather with it due to the
increasing wind field and persistent warmth/moisture. While there
will be abundant shear early Saturday morning, instability will be
elevated with a stable boundary layer. That changes around midday
with the approach of a weak warm front, which creates much more
instability but far less low-level shear (though still-impressive
deep-layer shear). As a result, there appears to be a very narrow
window near the front where instability and shear are better
balanced, suggesting a small tornado threat. Once the front passes,
the threat transitions to more of a damaging wind and (particularly)
large hail threat as the deep shear and instability dominate. The 00Z
short term models are in relatively good agreement on this scenario,
but run-to-run consistency has not been that good, so this could
change again quickly.

That severe threat will abate quickly after sunset as the second
shortwave pivots up into the upper Midwest, but the third round
(this time a full-blown low) will arrive quickly on its heels. This
system looks to be more of a rainfall producer. An initial batch of
rain will develop late Saturday night--and looks to be focused more
along the Gulf Coast based on the shortwave track. It gets more
interesting later Sunday as the low moves almost right over
Birmingham, potentially putting us in a favorable deformation-band
setup for heavy rainfall. Model/ensemble spread here is much larger,
though; such a band may well develop but its location remains
uncertain at this point. (This is the type of pattern we`d look for
to produce heavy snow if the temperatures were about 30 degrees
colder.) As it is, the cold core overhead will produce steep lapse
rates and depressed freezing levels, so thunderstorms are a good
possibility, as is the threat of small hail. The low will move east
Sunday night, gradually ending the rain and the prolonged unsettled

.LONG TERM...(Sunday night through Thursday)
Issued at 317 AM CST Fri Jan 20 2017

With the upper and potentially deep surface low departing to the
northeast through the Carolinas on Monday, it will place a strong
pressure gradient across the TN valley. The airmass will rather
homogeneous at this point (i.e. little temp advection), but have
increased wind gusts over suggested blended guidance with 25-30kt
gusts quite possible, especially in higher terrain.  Rain showers
will eventually depart from west to east during the day.  A narrow
high pressure ridge will shift east through the OH/TN valleys on
Tuesday with weak warm advection developing.  Highs should recover
into the u50s/l60s.  A more developed warm/moist conveyor belt
should be in place Tuesday night into Wednesday in advance of next
Pacific cold front.  Low PoPs have been placed in the forecast for
light rain as a result.  However, the medium range models differ on
the frontal bands progression southeast of the area as the
positively tilted upper trof builds southeast into the southern
CONUS.  The ECMWF is drier and faster and the GFS is wetter and
slower with the rain band.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 541 AM CST Fri Jan 20 2017

Occasional stratus in the 1-2kft layer has been enough to keep fog
from forming this morning, and those MVFR ceilings will continue to
be the main issue through the day. They will begin to break up late
this afternoon, allowing for a few hours of VFR conditions before
sunset. However, models are pointing strongly to fog forming after
sunset at KHSV/KMSL. For now, the TAFs will include MVFR vis since
thick mid-level clouds are expected, but lower visibilities are
possible if the clouds take longer to move in. Showers and storms
will move in from the SW at the very end of the TAF period, so they
were not included with this issuance.





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