Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 230304
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1004 PM EST Mon Dec 22 2014

...Forecast Update...
Issued at 1000 PM EST Mon Dec 22 2014

Good area of rain continues to move northeast over west-central to
north-central KY and south-central IN at this time. This is in
response to strong shortwave tilting negatively to the south and
east of main upper low over central Plains. This shortwave trough
over MO will lift rapidly northeast overnight. The trough is
enhancing forcing over the lower Ohio Valley. Looking at model
soundings and Nashville`s 00 UTC observed sounding, there is a bit
of elevated instability. The combination of forcing and less stable
air aloft is leading to good ascent ahead of the shortwave and the
area of rain, some moderate to isolated heavy, in our forecast area.

As the shortwave lifts northeast, so will the rain. Given the weak
instability aloft and given a few specks of 50 dBZ on radar here and
there, plus a few lightning strikes earlier, will include a slight
chance of thunder overnight, also supported by a 40 kt low-level
jet. Will carry categorical POPs with this feature overnight but
rain should then diminish somewhat toward morning, as supported by
latest HRRR run and as suggested by 00z NAM. Nevertheless, scattered
showers still are expected. Temps won`t dip much overnight, perhaps
a couple degrees then steady. Showers should then pick up again
during the day on Tuesday over central KY.

Issued at 650 PM EST Mon Dec 22 2014

Quick update for latest trends. Central KY and south-central IN has
been largely devoid of precip other than a few sprinkles early this
evening. However, better area of showers is developing and moving
northeast over western parts of KY and TN at this time, with some
briefly heavier cells. This is in response to forcing associated
with tail end of shortwave trough entering MO as the trough rotates
around the underside of the upper low over the central Plains.

As this shortwave moves northeast tonight, the area of showers will
lift northeast as well, spreading across the western half of our
forecast area. The Bluegrass region may stay on eastern fringe of
precip with only scattered showers. Surface winds will pick up a bit
this evening as well, especially along the I-65 corridor in response
to the shortwave rotating through to our west and northwest. This
will keep temps pretty much in check this evening.

.SHORT TERM (Now through Tuesday Night)...
Issued at 310 PM EST Mon Dec 22 2014

Deep upper low is just about to close off over the Northern Plains
this afternoon, with deep=layer S-SW flow increasing over the
Mississippi and Ohio Valleys. This pattern will result in mild but
unsettled weather over the next 36 hrs or so, as a series of
disturbances will touch off rounds of showers.

Moisture transport is just getting established, so the main precip
shield associated with the lead disturbance has remained confined
mainly to the Wabash Valley and points west, with very little in the
way of precip over central Kentucky or southern Indiana. A
smattering of showers did develop over northern Alabama, but not
showing much organization as it lifts through Middle Tennessee.

Still expecting precip to blossom tonight as additional upper waves
interact with a 40-45 kt low-level jet, but QPF looks fairly modest.
Therefore will still carry categorical POPs, even though confidence
in getting any substantial rain amounts is decreasing. An isolated
rumble of thunder still can`t be completely ruled out, but continues
to look less likely as this system appears to come at us in pieces.
Therefore will go forward without mention of thunder at this time.

By daybreak the best feed of deep moisture will be shunted to our
north and east, so would expect a break of a few hours in the precip
Tuesday morning. However the next wave will take shape late in the
day, spreading rain back in from south to north late afternoon/early
evening. Deepening surface low still progged to run up between the
I-65/I-75 corridors, with tremendous bust potential depending on
exactly where it tracks. Categorical POPs and steady temps in the
50s along/east of the track, while farther west, precip chances are
barely 50-60% and temps could drop into the lower 40s by Wednesday
morning. There will be a tight gradient in between but the placement
is a little iffy, and for now we will assume that to be somewhere
near I-65.

.LONG TERM (Wednesday through Monday)...
Issued at 300 PM EST Mon Dec 22 2014

An active long term period with the main forecast challenge being
temperatures and precipitation timing/types Christmas Eve through
Christmas morning, followed by more precipitation chances this
weekend.

The 22.12z guidance is trending toward a similar solution with the
main surface low track Wednesday morning. Outside of the NAM, which
was an eastern outlier, the ECMWF/GFS/GEM show a strengthening low
lifting along a Nashville to Louisville to Indianapolis track during
the course of the day. A time trend analysis shows this is a slight
shift west and about 3-6 hours later than the previous consensus.
The impacts this has to precipitation timing and type were very
minimal, as there has been good consensus for steady rain to move
across the region during the course of the day. However, the change
does allow for warmer air to move through central/eastern Kentucky
and as a result, high temperatures were increased to the mid/upper
50s from the Cumberland to Bluegrass region, including the Lexington
metro area. For the Louisville area, the tighter baroclinic zone
will be right along the I-65 corridor, making Wednesday morning
temperatures difficult to pinpoint at this time. Nonetheless, the
area should warm into the low 50s before cold advection kicks in and
temperatures crash into the low/mid 40s by afternoon. Western
forecast area should see the coolest readings, likely stuck in the
upper 40s, then falling into the upper 30s by mid afternoon.

The main message/impact for the public will be a wet, unsettled
Christmas Eve with falling temperatures during the afternoon. It`ll
also be windy, both ahead and behind the surface low. Not ideal
holiday travel conditions, even if it`s just all liquid.

Soundings show a potential changeover from rain to snow west of I-65
during the evening hours /after 7 pm/. However, the greater
frontogenetical forcing and deformation precipitation band should be
west of the forecast area and we`ll be dealing with scattered, light
precipitation. Will continue to advertise a rain to rain/snow mix to
light snow during the nighttime hours. Little accumulations likely
as wet/warm surfaces and temperatures in the low/mid 30s will limit
accumulations. 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.

The weekend forecast has some uncertainty as the whole upper level
flow remains amplified and active across the CONUS. At least
initially, there is some consensus that a surface low lifts through
the Great Lakes Friday night, dragging a cold front through the
local area Saturday. 30-40 percent rain chances look reasonable at
this time. Differences arise between the ECMWF/GFS/GEM as the 22.12z
ECMWF/GEM camp develop a secondary low along the Texas gulf coast
Saturday night, lifting it toward eastern Tennessee Sunday. 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 featuer similar uncertainity as both
ECMWF and GFS show varying storm systems to possibly impact the
area. An active stretch through the end of the year.

&&

.AVIATION (00Z TAF Update)...
Issued at 630 PM EST Mon Dec 22 2014

Shortwave trough rotating east across KS and MO at this time will
lift northeast rapidly tonight, which will increase low-level winds
for a time this evening and overnight until shortwave passes by. As
a result, expect surface winds to increase to 10-15 mph this evening
at TAF sites, especially at BWG and SDF. Winds 2-3 kft off the
surface should increase to about 40-45 kts at these locations as
well. As a result, will continue with mention of low-level wind
shear at BWG and SDF late this evening and overnight, with winds
diminishing in the 10-12 UTC time frame. It looks like LEX will be
less affected, still with some increasing winds, but not wind shear
criteria as LEX will be more displaced from influence of shortwave.

Otherwise, area of precip over western KY at this time should expand
northeast later this evening with forcing from the wave. This will
affect SDF and BWG the most. This passes by late tonight, with just
some lighter sct showers by morning before more showers ramp up and
expand in coverage during the day Tuesday.

As for cigs, lowest cigs (low MVFR to possibly IFR) will affect SDF
and BWG late this evening and overnight, continuing on Tuesday,
with LEX cigs a bit higher, displaced from the best low-level
saturation.

Surface winds on Tuesday will be a bit tricky with south to
southeast flow, then possibly backing some during the day or even
becoming variable as a surface trough becomes established roughly in
the BWG to SDF corridor. Winds at LEX should stay south to southeast.

&&

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

$$

Update.........TWF
Short Term.....RAS
Long Term......ZBT
Aviation.......TWF





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