Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 181746 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1146 AM CST Mon Dec 18 2017

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 932 AM CST Mon Dec 18 2017

A moist, stable air mass has taken hold across the Tennessee Valley
in wake of the system that passed through the region yesterday. Model
soundings show a steep inversion, explaining why the fog/drizzle has
held through 15z. Some locally dense fog is still possible for
another hour across portions of Cullman County and Northeast Alabama, so
have extended the Advisory through 16z for these four counties.
Given the moist boundary layer conditions and ceiling heights still
around 500 ft, think that patchy light drizzle/mist continue through
about midday. Thereafter, some slightly drier air should erode light
fog and lower cloud bases enough to put an end to this. Should see a
overcast, gray sky through 18z, but do think some subtle breaks will
develop by the afternoon to warm us a few degrees in the upper 50s.
Otherwise, any rain/showers will hold off well to the south along a
stalled out boundary along the Gulf Coast/South Alabama.

.SHORT TERM...(Tonight through Wednesday)
Issued at 400 AM CST Mon Dec 18 2017

A strong upper level disturbance currently producing precipitation
over the southwestern U.S and northern Mexico will be the next big
player in our sensible weather over the next few days. Models push
this feature into western Texas, mainly in response to energy
pushing into the Great Lakes farther north along the main jetstream.
Over the Tennessee Valley, high pressure will remain in control of
sensible weather though. Could see another good setup for dense fog
again tonight, as winds remain light and some clearing lingers into
the evening hours. Lows in the 40s area expected again tonight.

As the upper low pushes farther east into central Texas on Tuesday,
models are fairly consistent in moving the front to our south
northward into central or northern Alabama. Much of the model
guidance has the actual warm front just to the south of Cullman
county by 6 PM on Tuesday. This forcing and a saturated atmosphere
(with PWATS 1.3 to 1.6 inches) should be able to produce
widespread precipitation over the area. At this point there could be
some moderate rain with this activity. Up to 1.0 inch of rainfall may
occur over areas mainly north of the Tennessee River through midnight
on Tuesday. Also, models continue to show just enough skinny CAPE
for elevated convection to occur on Tuesday. Thus, a chance of
thunderstorms was included on Tuesday/Tuesday evening. Luckily, even
soundings from Cullman county, do not look surface based on
Tuesday/Tuesday evening. This makes sense given that the warm front
might be just to the south of there. Bulk shear is high enough for
organized thunderstorm activity, but the strength of the low level
and mid- level jets do not look overly impressive.

Its important to note that model guidance has been nudging this warm
front further north over the past several runs. So we will need to
watch for potential changes in stronger thunderstorm activity on
Tuesday night if this northward trend with the sfc warm front
continues. Most models kick this upper level energy east-
northeastward late Tuesday night into Wednesday morning. At the same
time fairly strong cyclogenesis is depicted by most models, as a 1012
surface low forms over Lousiana/Arkansas areas. As this occurs, 50
to 70 knots of bulk shear and 200-500 m2/s2 of helicity will push
into the area ahead of this storm system. A heavy band of
precipitation is expected to develop along the warm front somewhere
between the Tennessee River and Nashville, TN. This boundary moves
little as the storm system to our west moves east-northeast and into
northwestern Alabama just after daybreak on Wednesday.

In addition, as this storm system moves into the area Tuesday night
into Wednesday morning an even heavier batch of showers and scattered
thunderstorms area expected along and just ahead of the system.
Another 2.0 to 2.5 inches looks reasonable and there might be even
higher isolated amounts. This could produce some rapid rises in
river levels or some isolated flash flooding. However, given high
flash flood guidance this looks to be a fairly minor concern. We will
need to monitor late Tuesday night into Wednesday morning a bit
closer for possible severe weather. Several models move the surface
warm front northward into our southeastern counties late Tuesday
night into Wednesday morning. As it does so, the low level jet
strengthens to between 45 and 50 knots in most models (50-60 knots in
some) and a stronger 95 knot 500 mb jet develops with this system.
Given very strong forcing all the way through the atmospheric column
(in left exit region of 850 mb jet and right entrance region of 500
mb jet), any areas that realize surface based instability will have
the potential for strong to severe thunderstorms capable of producing
strong/damaging winds. Both GFS and NAM12 show at least up to 500
J/KG of CAPE in our southeastern counties Wednesday morning.

At this point, there is some uncertainty on how far north the warm
front will go, but the trend has been further north.
The worst of the rain and thunderstorm activity should be over by
noon or very shortly after on Wednesday. However, some light rain
could linger through the afternoon hours on Wednesday. It looks like
a dry slot will push in quickly by Wednesday evening, putting and end
to any precipitation. Cloud cover will likely linger though and keep
temperatures from falling off too much behind the front as it pushes
southeast into southern Georgia. Lows in the upper 30s to lower 40s
look reasonable.

.LONG TERM...(Wednesday night through Sunday)
Issued at 400 AM CST Mon Dec 18 2017

Conditions on Thursday are still likely to be dry in the wake of the
upr wave that affects the area on Wednesday. Sfc winds will be
easterly initially, but will gradually shift from the south by late
in the day, heralding the approach of the next trough and low
pressure system to affect the region. High temperatures on Thursday
may reach the mid-60s in some locations, particularly in the west.
Values could be even higher depending on the amount of cloud cover
in the area, which still remains a little uncertain.

By late Thursday night into Friday morning, an elongated upr trough
is expected to extend roughly from the SW CONUS into the Upper
Midwest, with the attendant sfc front located in the Southern and
Central Plains regions. Strengthening and deepening SWRLY upr flow
over the TN and Lower Miss Valley region will lead to warm/moist
advection and isentropic ascent perhaps by daybreak Friday. Better
moisture/ascent will tend to be focused more to the west warranting
slightly higher chances for rain in the forecast in our western
counties overnight Thursday. Elevated instability within the mixed
phase region and above the lifted layers could allow for isolated
thunderstorms, mainly in the west. On Friday, the upr trough is
expected to be progressive, but the region may remain in the warm
sector in deep SWRLY flow for much, if not all, of the day, helping
to push temps well into the 60s perhaps once again. The sfc front
will begin to bear down on the region sometime late Friday into
Friday night, but the suite of operational and ensemble guidance
still varies rather significantly with regards to the timing of the
frontal progression across the area...from around 18Z Friday to
perhaps 12Z Saturday. Nevertheless, the majority of current ensemble
members and operational models bring impacts from a band of
precipitation ahead of the front generally during the Friday night
period. There is the potential for some, albeit weak, surface-based
instability during the evening hours on Friday and dynamic forcing in
assoc/w the trough will be fairly strong, with perhaps a ~50kt low-
lvl jet and 100+kt at 500mb. A line of strong storms with gusty winds
could result. What is becoming increasingly clear with each run (per
operational and ensemble members) is that the bulk of the cold air
is likely to remain well to our west with the initial cold front late
Friday into early Saturday. In fact, 850mb winds only swing around
from the NW perhaps briefly on Saturday before returning from the
W-SW either late Saturday or on Sunday.

As this first trough swings quickly through the east CONUS, another
and more significant trough will likely begin to develop in the west
CONUS, keeping the long wave trough axis to our west and ensuring
the region remains locked in SWRLY flow aloft. The subsequent
movement of the broad upr trough to our west remains uncertain with
a fairly wide spread among ensemble and operational guidance. Thus,
low confidence continues with the the long term portion of the
forecast from Sunday and beyond. The (slightly) larger part of the
suite of guidance would suggest frontogenesis/cyclogenesis somewhere
in the southern Plains and the lower Miss Valley during the period,
with waves of low pressure riding NEWRD across or near the TN Valley.
Most scenarios suggest we would likely remain in the warm sector at
least for Sunday and also into Christmas Day (Monday) with rain
chances continuing. Nevertheless, considering the wide
spatial/temporal spread in the details, the forecast will maintain
generally low chance POPs for now through this period.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1146 AM CST Mon Dec 18 2017

Low IFR ceilings will persist through the mid afternoon as BKN/OVC
decks linger through 21-22z. A few breaks in the clouds may develop
early this evening, but BKN/OVC decks of IFR to low MVFR clouds will
return, especially after midnight. Some light fog will also develop
late tonight as well. After 12z, a warm front will lift north,
generating some -RA for the morning hours. Think mixing and rainfall
will be enough to prevent any really low visibilities, so have gone
conservative and included just vis at MVFR levels for now.





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