Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 230520
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1220 AM EST Fri Jan 23 2015

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1010 PM EST Thu Jan 22 2015

Have tweaked pops based on latest high-res guidance and QPF
forecasts, as well as forecast soundings. Overall totals still look
on track, with a trace to a few tenths of an inch possible along and
east of a BWG/LEX line. The highest amounts look to come in our
eastern counties, Casey/Lincoln/Garrard/Madison, where up to an inch
is possible. There may be two bouts of snow to deal with, the first
coming in the morning over south central Kentucky. This snow will
have trouble accumulating though, given warm ground temperatures.
Road temperatures down there now are reporting in the lower 40s.

With daytime heating we should see a transition to rain as the main
precip type. Though by 22Z dynamic cooling should start winning out
and allowing a transition back to snow over the southeast quarter of
our forecast area. Still preceding rains and warm grounds should
preclude much in the way of accumulations for a few more hours. Will
be issuing a special weather statement to discuss these issues.

.SHORT TERM (Now through Friday Night)...
Issued at 305 PM EST Thu Jan 22 2015

...Minor Snow Accumulations Possible Friday Night...

Model Discussion/QPF Analysis:
------------------------------
The main short term challenge is whether or not we`ll see any light
snow accumulations Friday evening/Friday night.  There is still a
little bit of a spread amongst short range models as to how far
north the precip shield associated with the low pressure system
traversing the southern U.S. will spread into the Ohio Valley.  Some
models indicate only south central KY and the Bluegrass will see
precipitation (like the 12Z NAM/WRF-NMM) while others take the
precip as far north as the Ohio River (like the 12Z GFS/GEM/ECMWF).
Some of the 22/0Z models took precip south of KY all together.  With
all of this data and model trends in mind, nudged the forecast
precipitation area a bit farther south with an area from Bowling
Green to Madison likely to see the best precipitation.  The question
then becomes, what type.

Thermal Profiles/P-Type:
------------------------
Thermal profiles still look to be on the warm side.  Precipitation
arriving in south central KY around sunrise Friday morning could
begin as a rain/snow mix or plain rain, then quickly transition over
to all rain.  We`ll see all rain for the majority of the daylight
hours on Friday as temps rise into the upper 30s and lower 40s.
Expect to see a gradient of temps for highs on Friday with areas
over southern IN/northern KY with little to no precip warming into
the lower 40s and locations with more steady precip wet bulbing
resulting in highs in the upper 30s.  The change over to snow will
likely occur in the moderate precip rates where the boundary layer
cools sufficiently to support some ice crystals reaching the
ground.  Thus, first introduced a mix of -RASN Fri evening over
south central and east central KY where better precip rates are
forecast.  Snow should start to mix in with the rain everywhere
after 8pm due to night time cooling and as colder air starts to
filter in on the back side of this system.  A transition to all snow
doesn`t look likely until around midnight in most areas.  The time
frame for all snow looks less than in previous model runs so that
would bring snow accums down.  Other factors that will work against
much snow sticking to the ground would be antecedent warm grounds
from our Jan thaw and warm grounds due to a steady rain before the
change over to all snow.  Thus, any wet snow accums on the back side
of this system look to remain under 1 inch if any at this time.  The
cold air on the back side of this system will cause low temps to
fall into the upper 20s and lower 30s late Fri night into Sat
morning.  A few slick spots may be possible in areas that reach the
upper 20s toward Sat morning as left over moisture on road surfaces
may freeze.

Tonight:
--------
Sfc high pressure across the region should keep the area dry.  Cloud
cover will be a challenge and affect the temp forecast overnight.
Most models indicate that mostly cloudy skies should give way to
partly cloudy skies or at least a thinner cloud deck over southern
IN/extreme northern KY which would allow for better rad cooling.
Thus, think that this area should drop into the mid 20s tonight
while cloudy areas over central and southern KY should hold in the
upper 20s/lower 30s.

.LONG TERM (Saturday through Thursday)...
Issued at 240 PM EST Thu Jan 22 2015

The synoptic pattern Saturday is expected to feature our main storm
system low pressure off the North Carolina coast while the upper
level trough digs into the deep south. This low tracks further up
the eastern seaboard while broad troughing builds from the Upper
Midwest through the mid-Atlantic and Northeast. Locally,
precipitation will likely be confined to eastern Kentucky during the
early morning hours, possibly in the form of light rain or snow.
Increasing west to northwest flow aloft will bring a drier air mass
into the Ohio Valley and look for clearing skies west to east
throughout the day. Plan on highs in the upper 40s. As low level
lapse rates steepen in the afternoon, west winds could be gusty at
or just above 20 mph.

A quick moving Clipper system then dives down out of the Upper
Midwest Sunday afternoon and night. 22.12z guidance has trended
slower with this system, not bringing the surface low into the area
until late afternoon Sunday. There is still some timing and
placement issues to sort out, but the guidance roughly advertises a
western Wisconsin to northern Ohio low track. This would keep much
of the precipitation north of the area during the afternoon/evening
as well as bring a brief shot of warmer air northward ahead of the
system. Highs Sunday may top 50 degrees. Not until after 06z Monday
that the column cools sufficiently to support a possible changeover
to rain/snow or snow but by that point a lot of the moisture has
exited. There is the potential for a few snowflakes to mix in late
Sunday night or early Monday morning.

In the wake of the Sunday night system, a colder northwest flow
regime sets up across the Upper Midwest / Great Lakes down into
portions of the Ohio Valley. The 22.12z guidance still is struggling
with the depth and southwestward extension of the colder air mass.
850 mb temperatures Monday - Wednesday vary from -8 to -10C off the
GFS/EC to -3C from the GEM. This results in seasonably cool
temperatures with highs in the mid/upper 30s and lows in the 20s
with teens possible in the sheltered, colder locations.

The weather pattern for this period looks dry at this time, though
there will be subtle shortwaves coming through the northwest flow.
One particular wave arrives Tuesday and while models are not
producing any QPF, there is decent saturation in the dendritic
growth zone across northern KY into southern KY. The highest
saturation is further north in Indiana and Ohio but thermal profiles
could support flurries for some areas.

Otherwise, there are signals by mid/late next week of the eastern
CONUS trough shifting offshore as upper level ridging moves
overhead. A northern stream system may pass well north of the area
through the Great Lakes Thursday/Friday, resulting in low/mid level
southwest flow. This would bring a moderating trend in temperatures
and continued dry weather.

&&

.AVIATION (06Z TAF Update)...
Issued at 1215 AM EST Fri Jan 23 2015

Stratus ceilings have persisted across north central Kentucky
through the evening, but only LEX has a high-end MVFR while SDF
remains VFR. Only a mid-level deck at BWG so far. Expect conditions
to deteriorate during the late morning and early afternoon hours,
especially at BWG and LEX as the precip shield from a Deep South low
spreads into Kentucky.

Will bring the precip into BWG by mid-morning, initially as snow but
changing over to rain late morning to around noon. By the time it
spreads into LEX early in the afternoon, low-level temps should be
warm enough to support all rain. Ceilings will drop into fuel-
alternate MVFR and eventually into IFR as the precip becomes more
steady. For now will keep SDF in a high-end MVFR ceiling, with only
VCSH. However, this is probably the lowest-confidence forecast of
any of the TAF sites, as it would only take a slight jog north to
spread in the steadier precip and fuel-alternate or even IFR
conditions.

&&

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

$$

Update.........RJS
Short Term.....AMS
Long Term......ZBT
Aviation.......RAS




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