Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 172312
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
612 PM EST Wed Dec 17 2014

.SHORT TERM (Now through Thursday Night)...
Issued at 250 PM EST Wed Dec 17 2014

...Light Wintry Precip Late Tonight and Thursday Morning...

A shortwave trough now over southeast NM will advance toward MO by
daybreak Thursday. Current precipitation over OK and northwest AR
will move toward us as well, bringing light precip to the southwest
half of the forecast area after midnight. This precip will have to
overcome some dry air initially at low/mid levels of the atmosphere,
but expect this to happen in the 08-13Z window. Temperatures will be
cold enough for a wintry mix of precipitation, as clearing skies
this afternoon should, despite upper clouds thickening this evening,
allow for several hours of good radiational cooling conditions
through the evening hours. Thus by midnight, many areas getting this
precip should get down to around 30 degrees.

The cold layer aloft may be deep enough to allow for either snow or
sleet, but a few model soundings show enough warm air aloft that
freezing rain cannot be thrown out of the mix yet. In addition,
winds will be light enough and the coldest air remaining over our
southeast counties may be moist enough for some patchy freezing fog
to occur ahead of any precip. Lots to watch for in other words. The
saving grace here is the dry air aloft evaporating a lot of the
precip as well as the short duration of the event, keeping QPF
light. Have an area of half inch accumulations for snow along an
axis from Hartford, KY to roughly Glasgow. Also have a few
hundredths of ice accumulations. These light values warrant issuing
a special weather statement highlighting these threats.

The rest of the day Thursday should remain cloudy and cold, with
highs stuck in the 30s. Diurnal curves will be narrow for
temperatures as cloud cover sticks around Thursday night as well,
leaving lows in the mid to upper 20s.

.LONG TERM (Friday through Wednesday)...
Issued at 302 PM EST Wed Dec 17 2014

The synoptic pattern at the start of the long term period will
feature a ridge across the western CONUS, with a downstream trough
across the south-central Plains.  This trough will deamplify as it
pushes into the Ohio Valley this weekend.  A more substantial,
anomalous trough will then build into the Ohio Valley towards the
end of the period (around Christmas Eve).

The main focus of the long term period continues to be on the
upcoming weekend system, slated to move through the region Friday
night into Saturday.  The latest model guidance has continued to
trend weaker and further south with this system.  The 17/12Z
operational NAM is the most aggressive with QPF into the Ohio
Valley, owing to the fact it is a bit more negatively tilted and
stronger with the mid-level shortwave and associated surface low.
However, the ECMWF/GFS/GEM solutions, along with the 17/15Z SREF
solutions, depict more of an open, positively-tilted shortwave which
keeps the system more suppressed and weaker.  Looking at the
forcing, it appears the better moisture transport and
frontogenetical circulations will be well south of the KY border.  A
coupled jet structure, with a rapidly weakening northern stream
branch, will be the main driver of precipitation this far north,
which means amounts/rates should remain light.  Given the agreement
of these solutions and rather meager forcing progs, have trended the
forecast drier and further south.

What this means for the sensible weather in the Ohio Valley is less
in the way of impact, and more of just a glancing blow of perhaps
some light wintry precip.  Model soundings still differ a bit on
their solutions, with some even suggesting some light sleet
mixing in as this system passes, but will leave that out of the
forecast for now.  Some minor snow accumulations, mainly on grassy
surfaces, still appear possible generally along and southeast of a
line from Leitchfield to Richmond.  Further northwest, some light
snow may fall, but do not expect much in the way of accumulations
given the light QPF and poor rates to overcome warm grounds/roads.

Once this system passes, high pressure will build into the region
Sunday into Monday.  However, by Monday night, an anomalous trough
will be diving into portions of the central CONUS, with a surface
low expected to rapidly deepen somewhere over the eastern United
States.  Ensembles and long range deterministic guidance have been
hinting at this system for days, so confidence is high in a
significant storm somewhere over the eastern CONUS.  However,
details continue to remain murky on whether this system will impact
the Ohio Valley in the form of rain, snow, or a wintry mix.  For
now, taking an ensemble approach, it appears this system will be
mostly rain on the front side Monday night into Tuesday, changing to
snow as cold air quickly filters in Tuesday night into Wednesday.
Given the impact on the Christmas holiday, this is certainly a
system we will continue to monitor closely.

&&

.AVIATION (00Z TAF Update)...
Issued at 600 PM EST Wed Dec 17 2014

A southern Plains storm system will slide across southern KY
tonight, bringing some mixed precipitation to BWG. At this point,
precipitation looks to be light enough to cause minimal impacts both
from a visibility and accumulation standpoint. Brief periods of MVFR
Vis/ceilings and very light accumulations cannot be ruled out from 2
AM CDT through 9 AM CDT. The bulk of the precipitation should end by
mid to late morning with only mention of VCSH through the rest of
the forecast cycle, along with MVFR ceilings.

Elsewhere, SDF/LEX should stay out of the measurable precipitation
although a few flurries may fall at times. Biggest concern at SDF is
whether the MVFR ceilings will hold as it is currently on the
southern fringes of the deck. Do think they will hang on until
around Midnight or after so will go with that and amend as
necessary. VFR at SDF/LEX otherwise with light and variable winds
generally out of the NW to NE range.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
IN...NONE.
&&

$$

Short Term........RJS
Long Term.........KJD
Aviation..........BJS




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