Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 230815

315 AM EST Tue Dec 23 2014

.SHORT TERM (Now through Wednesday)...
Issued at 308 AM EST Tue Dec 23 2014

A wet period is in store for the short term with multiple rounds of
rain expected. This will be due to a couple of different systems
moving through the lower Ohio Valley. The first is the shortwave
moving through early this morning. The main batch of rain from this
system continues to push off to the northeast this morning.

A cold front associated with a low over Iowa will move into the
forecast area from the west this morning. This front will stall out
somewhere around the I-65 corridor as another low begins to approach
from the southwest. This low track continues to trend a bit to the
the northwest. Most models have made a slight trend. However, the
NAM, which was the farthest east solution yesterday is now the
slowest and farthest west. Regardless of which track you choose, we
should see all rain through the short term as the low moves across
Kentucky by mid day Wednesday and continues to quickly pull off to
the northwest in the afternoon.

For today we should see mainly isolated to scattered showers across
the area this morning. Rain chances will increase from the south
this afternoon as the low moves north and a weak disturbance moves
through the southwesterly flow aloft. There could be another lull in
the late evening before the low itself moves through bringing an
increased chance for rain. Have tried to give some delineation in
the forecast regarding these waves. Rain chances will continue on
Wednesday with the main bulk of the precip moving off to the
northeast in the afternoon. Some soundings indicate we could have
weak instability developing late tonight into tomorrow, so have
added in isolated thunderstorm chances.

The other concern for Wednesday will be the winds. Surface winds
will increase in the afternoon to 15-20 mph. Winds aloft will
strengthen as well. By late afternoon gusts will increase to 30-35
mph with gusts up to 40 mph certainly possible. As with other
features of this system, there is some spread in the models as to
how strong the winds aloft will become and how much of these winds
will mix down. However, a Wind Advisory may be needed at some point.
In addition, if there are any thunderstorms that develop in the late
afternoon they could mix down stronger winds.

Temperatures will remain quite warm for this time of year. Highs
today will range from the mid 50s to around 60. Lows tonight will
not drop much, bottoming out in the mid 40s to mid 50s. Temperatures
Wednesday will remain warm for most in the morning but will begin to
drop by mid to late afternoon.

.LONG TERM (Wednesday Night through Monday)...
Issued at 245 AM EST Tue Dec 23 2014

Well...the long term forecast has 2 systems.  The 1st will be
dealing with the wrap around phase of synoptic scale Christmas Eve
Cyclone pulling out to the Srn Great Lakes and another complex
system with varying model solutions for the weekend.

Christmas Eve Night to Early Christmas Morning...

Will it Snow?

Well...low pressure pulls out rapidly from Lower Wolverine State
towards the St Lawrence River Seaway and moisture is trying to pull
out too. It will be windy (15 to 30 mph through midnight) for folks
going out to church and other festivities. Temperatures will start
out in the upper 30s to lower 40s at 00z Thu, so it will begin as
all liquid initially.

The strong isentropic lift pulls out with the main system but there
is still adequate sfc instability and llvl lapse rates steepen up.
Both NAM and GFS soundings denote brief window of seeder feeder
process from 03 to 09z with -4C to -10C at top of moisture plume
bringing SCT RASN and transitioning to SNSH showers or flurries as
deformation area pulls out. Have pinpointed areas along and north of
the Blue Grass Parkway, especially across Srn IN and the Nrn
Bluegrass region for some snow gelid snowy pcpn. Some areas  might
be receiving Bing Crosby`s White Christmas wish.  However, little or
no accumulations likely as wet/warm surfaces.

For Christmas, plan on clearing skies west to east as a Pacific
airmass high pressure quickly builds in from the southwest.
Temperatures will be around normal in the mid 40s. Friday will be
dry as well. ECM is much warmer than the GFS, bringing almost 60
degrees to the area Fri-Sat.  Not going that high with next system
pushing in, but veering wind profile will certainly allow temps to
climb into the 50s.

The weekend forecast has some uncertainty as the whole upper level
flow remains amplified and active across the CONUS. A sfc low lifts
through the Great Lakes late Friday night, dragging a cold front
through the local area during the day on Saturday. 30-40 percent
rain chances look reasonable at this time. Have changed the fcst on
Fri night and Saturday to all liquid as the soundings and H9 and H8
temperature mandatory levels are too warm.

However on Sat night and Sunday the ECMWF/GFS/GEM models diverge
significantly with ensemble members showing wide spread with pcpn.
Very potent upper jet with this system with 150 kts 250 mb winds
across AL and TN on Sat.

Made no changes beyond Sat and here is the Monday 3 pm discussion ...
This would spread precipitation back across the area. Temperature
profiles this time would support a rain/snow mix for the northwest
half of the forecast area. However, the forecast details, including
any specific storm system/track, beyond Saturday have low confidence
as there are a wide range of solutions in this type of upper level
pattern. Next week looks to feature similar uncertainty as both
ECMWF and GFS show varying storm systems to possibly impact the
area. An active stretch through the end of the year.


.AVIATION (06Z TAF Update)...
Issued at 1209 AM EST Tue Dec 23 2014

Showers will continue off and on through this TAF period. Through
this morning a shortwave will continue to lift off to the northeast. Winds
at 2 kft will continue to increase to around 40 knots. Though it
will be marginal, have decided to keep LLWS in the TAFs for SDF and
BWG through 09-10Z. Winds aloft will decrease thereafter.

The other challenge for this forecast will be ceilings and
visibility. Visibility will likely bounce between VFR and MVFR as
multiple waves of showers cross the TAF sites. Guidance suggests
that ceilings will continue to lower overnight. SDF and particularly
BWG will likely drop to IFR. Some improvement will be possible at
SDF through the day, but BWG looks to remain IFR throughout after it
drops. LEX is more questionable, so have kept cigs more optimistic

Winds will decrease through the morning hours to less than 7 knots.
An increase is then expected again tonight as a low approaches from
the south.




Short Term........EER
Long Term.........JDG
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