Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 171129

Area Forecast Discussion
National Weather Service Louisville KY
629 AM EST Sun Dec 17 2017

...Updated Aviation Discussion...

.Short Term...(Today through Monday)
Issued at 256 AM EST Sun Dec 17 2017

Currently quiet across the CWA at this hour as mid to high level
clouds stream in from the southwest ahead of a shortwave in the
southern Plains. Regional radars show widespread rain showers across
the lower Mississippi Valley region, with all of that activity
pushing off to the northeast. The rain will overspread western and
south-central Kentucky this morning and attempt to spread northward
this afternoon. However, this system will be moving fast and weaken
quite a bit by the time it arrives to the lower Ohio Valley, and
some areas across northern KY and southern IN may not even see
measurable precipitation today. Even across southern KY where the
best chances for precipitation exist, overall amounts will generally
be under a tenth of an inch.

Precipitation will begin tapering off this afternoon, but low clouds
will linger through the day. Low level moisture advection will
increase this evening, and forecast soundings show a saturated
column between the surface and ~850mb. As a result, we may see some
drizzle developing after midnight and persisting into Monday
morning, though the NAM is a bit more aggressive with the moisture
and keeps the drizzle going into Tuesday morning. Regardless of what
solution plays out, it does appear that we`ll be socked in with
stratus or low clouds for the short-term forecast period.
Temperatures during this period will not fluctuate much as well,
with diurnal ranges generally limited to ~10 degrees.

.Long Term...(Monday night through Saturday)
Issued at 304 AM EST Sun Dec 17 2017

The stratus and perhaps even some drizzle will be ongoing Monday
evening, though there are some signs that it will begin to break up
Tuesday across northern KY and southern IN, especially after a cold
front moves through Tuesday evening. Rain showers will be ongoing
near and south of the TN border ahead of an upper level wave, so
south-central KY will stay locked in with clouds and perhaps a few
showers Tuesday and Wednesday until that shortwave pushes eastward
and drier air behind the front advects in.

A deep, strong upper level trough will begin developing over the
central CONUS during the Thursday-Friday timeframe, while a stout
upper level ridge begins to amplify just off the eastern CONUS. At
the surface, cyclogenesis off the Rockies will lead to a deepening
low pressure system with a strong trailing cold front. A warm front
will lift north of the region into the Great Lakes region Thursday,
keeping the lower Ohio Valley well within the warm sector. As a
result, highs Thursday and Friday will be warm by December standards
(50s to near 60).

The cold front will approach late Friday evening with a line of
showers and perhaps even some isolated thunderstorms. Low level
shear and deep layer shear for this event will be strong, and with
some models showing minor amounts of instability, will have to watch
this one closely in the coming days for severe weather potential.
The front will be moving quickly, and should be east of the region
by early Saturday morning.

By the time we reach the weekend, operational models and their
respective ensembles begin to vary on solutions regarding
temperature profiles and moisture. There is some potential that we
could see wintry precipitation during the Christmas weekend and
early week timeframe, but forecaster confidence is low given the
model discrepancies. The best thing anyone can do at this point is
to continue to monitor the weather forecasts in the coming days,
especially if you plan to travel.


.Aviation...(12Z TAF Issuance)
Issued at 625 AM EST Sun Dec 17 2017

VFR conditions are expected for the next 6-9 hrs.  A weakening
southern stream shortwave will move northeast into the Ohio Valley
today bringing light rain by mid to late morning.  Light on and off
rain showers will continue through the day and diminish by this
evening.  However, plentiful low level moisture will become trapped
under an inversion this evening and during the overnight hours
resulting in flight conditions lowering to at least low end MVFR or
IFR with patchy drizzle as well. Winds will remain southerly at or
below 7 kts throughout the TAF period.




Short Term...DM
Long Term...DM
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