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000
FXUS63 KLMK 080243
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
943 PM EST Sat Dec 7 2013

...Updated Aviation Discussion...

...Forecast Update...
Issued at 935 PM EST Sat Dec 7 2013

Grid updates this evening have been limited mainly to initialization
tweaks. Winter Weather Advisory is flying starting at 05Z for the
southern 2 or 3 tiers of counties, and then starting at 09Z for the
rest of our Kentucky counties.

Radar returns are expanding into middle Tennessee and western
Kentucky, but those echoes are mainly from mid-level clouds.
Low-level moisture is very shallow and it will take a few hrs to
saturate before precip can make it to the ground. Still well on
track with the onset of precip, but the main impact on Sunday
morning will be from a frontogenetic band that will set up somewhere
over central Kentucky. The 00Z NAM shows the f-gen band near the
Western Kentucky and Bluegrass Parkways late Sunday morning, with
mainly a snow/sleet sounding, while the 18Z GFS is a little faster
and farther north, in more of a pure snow environment.

Either way, the current advisory and forecast look good, and will
leave further refinement up to the oncoming shift when the 00Z GFS
and other new mesoscale model runs come in.

.SHORT TERM (Now through Sunday Night)...
Issued at 324 PM EST Sat Dec 7 2013

...Wintry mix possible overnight into Sunday...

The synoptic pattern this afternoon features continued southwesterly
flow aloft across the Ohio Valley, as a deep trough remains in place
across the western CONUS.  Several upper-level disturbances will
ride through the region through the short-term period, bringing
generally light wintry precip and rainfall to the area.

Clouds will begin to increase once again from the south overnight,
as a PV anomaly approaches the region from the southwest.  High
clouds will increase first, followed by lower clouds as low-level
isentropic ascent develops in response to the passing PV anomaly.
As this occurs, a band of mid-level frontogenesis will develop
across south-central KY in response to the tightening mid-level
thermal gradient.  Upward vertical motion and associated
precipitation will likely break out overnight across southern KY,
spreading into central KY by morning.  This initial band of
precipitation will be falling in cool air at the surface and will
likely fall initially as some snow or light sleet.  As a warm nose
works in aloft, the precipitation will likely change over to more of
a freezing rain/sleet mixture across southern KY by early morning,
with this mixture spreading north toward the Ohio River and the
northern Bluegrass by late Sunday morning/early afternoon.  Warmer
air will work in at the surface across southern KY by late Sunday
morning, changing any wintry precip over to just plain rain. This
warmer air will continue to push northward through Sunday afternoon,
changing most locations (except perhaps along and north of the Ohio
River where the greatest snowpack lies) to plain rain.

The latest guidance shows a bit of spread with the amount/location
of QPF.  Think the 07/12Z NAM is a bit bullish (as it has been in
the past few days) in its QPF depiction.  It seems to be a bit too
robust on the incoming PV anomaly and phases this upper-forcing with
the mid-level frontogenesis, resulting in a deeper vertical
circulation.  Prefer the GFS solution, although think the initial
frontogenetic band of precip will be a bit further north than it has
depicted, more on a line from around Bowling Green to Lexington.  It
is along this line that the warm nose aloft seems to overlay the
cooler surface air the longest, thus think this region has the
highest potential for icing (up to 0.2 inches) with the best
snow/sleet accumulations just north of this line.  The Louisville
metro will be on the edge of the icing potential due to the deeper
cold air and better chances for light sleet/snow initially.
However, enough warm air aloft looks to move in where Louisville
(especially southeast Jefferson County) could a light glaze of ice
before precipitation rates lower and surface temperatures ooze above
freezing.

Have left southern Indiana out of the advisory due to the expected
lighter QPF amounts, and the likelihood that much of this QPF will
fall as light sleet/snow, not freezing rain.  By the time the warmer
air aloft works in to change precip over to freezing rain, the
deeper moisture will be departing.  Will issue an SPS discussing the
light wintry mix potential, but will hold off on any advisory for
now.

Another wave will ride along the front Sunday night.  This will
bring renewed rainfall to southeastern KY, with perhaps some light
freezing rain on the very back edge of the precip.  Temperatures
throughout the next 36 hours will be tricky with the warmer air
moving in.  Lows tonight will range from the teens across southern
IN to the lower 30s in southeastern parts of central KY.  Highs
Sunday will climb near freezing across southern IN and northern KY,
to the lower 40s in southern KY.

.LONG TERM (Monday through Saturday)...
Issued at 248 PM EST Sat Dec 7 2013

Light precipitation will linger into Monday morning behind the
departing weekend system.  A wintry mix is expected, but it should
be quite light and taper off to just some patchy drizzle (south of
the snowpack) or freezing drizzle (on the snow pack) by afternoon.
High temperatures Monday will range from the middle 20s over the
deepest snowfield to the lower 40s around Lake Cumberland.

Monday evening should be dry.  Then late Monday night the next wave
ahead of a slowly advancing upper trof will bring another slug of
precipitation across the southeast U.S.  In our local area this wave
may bring some light snow to the Lake Cumberland and Blue Grass
regions, with flurries elsewhere.  Could see around an inch of
accumulation at Lake Cumberland if this forecast verifies.

The GFS is bringing a streak of light precipitation into the LMK CWA
on Tuesday.  However, of the past eleven runs of the model, this is
the first to bring measurable precip into the region.  Also, the
model is getting very little support of this idea from the ECMWF and
GEM.  At this point little more than flurries/sprinkles are
expected...if anything.

Tuesday night through Friday will be dry as Canadian high pressure
gradually lumbers across the central and eastern United States.  The
next storm system may begin to threaten as early as Saturday but not
enough confidence to throw it in the forecast yet.

There`s no doubt it will be a chilly week.  The coldest temperatures
will arrive Wednesday morning and Thursday morning. Lows will range
from the single digits in snowy southern Indiana to around 20 at
Lake Cumberland.  Temperatures will finally begin to moderate by
Friday/Saturday with afternoon readings ranging from the middle 30s
to middle 40s north to south.

&&

.AVIATION (00Z TAF Update)...
Updated at 635 PM EST Sat Dec 7 2013

Main challenge in this TAF set is P-type and ceilings associated
with the the disturbance crossing the area on Sunday. Small
complication this evening involves the MVFR stratus deck that has
persisted for much of the afternoon, and how much it will scatter
out. Will monitor obs until issuance time and initialize from there.
Potential exists for fuel-alternate conditions to hold in for most
or all of the night at BWG and LEX, while SDF has the best shot to
scatter out and/or lift to higher-end MVFR.

Precip will edge in from the south before daybreak at BWG, and
around sunrise at SDF and LEX. BWG should have enough of a warm nose
to be a freezing rain/sleet mix right from the start, but will take
until late morning to go over to all rain. Will start with a
snow/sleet mix at LEX and especially SDF, given less of a warm nose
and slightly larger dewpoint depressions to overcome. Expect
ceilings to deteriorate to MVFR by mid-morning, but the main
operational impact will be light but fairly persistent icing. These
conditions will last into mid-afternoon before temps rise above
freezing.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...WINTER WEATHER ADVISORY until 3 PM EST /2 PM CST/ Sunday FOR
     KYZ061>067-070>078-081-082.

     WINTER WEATHER ADVISORY from 4 AM EST /3 AM CST/ to 6 PM EST /5
     PM CST/ Sunday FOR KYZ023>043-045>049-053>057.

IN...NONE.
&&

$$

Update.........RAS
Short Term.....KJD
Long Term......13
Aviation.......RAS