Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 270738
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
338 AM EDT Mon Oct 27 2014

.SHORT TERM (Now through Tuesday)...
Issued at 329 AM EDT Mon Oct 27 2014

Many challenges are present in this short term.

At 07Z a warm front extended from low pressure over western Iowa
southeastward through central Kentucky to the southern Appalachians.
This front will move north today and envelop the entire LMK CWA in
very warm air on gusty SSW winds.

The first concern is any fog around sunrise.  Haven`t seen much fog
yet this morning, but some patchy fog will still be possible for a
few hours in southern Kentucky.  However, widespread dense fog like
what we saw yesterday won`t occur.  If anything we should just see
some patchy light fog.

The next concern is dew points this afternoon.  Looking at NAM12
sounding progs, it seems like dew points should be able to fall into
the lower 50s.  Turning to MOS guidance, that`s about 1-2 categories
cooler than what the guidance shows, which lends some confidence
since MOS usually doesn`t take dew points down far enough on mixy,
warm afternoons.  So, will aim for lower and middle 50s for dew
points this afternoon.

Highs in the lower and middle 80s still look good today.  Louisville
stands the best shot at a record temperature (forecast 84, record 84
set in 1940).  Bowling Green may come close (forecast 85, record 87
in 1940).

Tonight will be warm and breezy as the cold front draws near.
Guidance min temps are almost always too cool in this sort of
scenario, so will go warm in the forecast with lows generally in the
lower and middle 60s.  Record warm minimum temperatures are not in
jeopardy for October 28 since we will drop well below the records
before midnight Tuesday evening behind the front.

That cold front will cross southern Indiana and central Kentucky
during the day Tuesday.  Showers and thunderstorms will accompany
the front, with pretty much everybody getting some rain.  Surface
instability will be weak, mid-level lapse rates shallow, and the
front itself will be weakening as it moves through.  The parent low
will be well to our north in Ontario.  As a result, widespread
severe weather looks unlikely.  The best chance for anything strong
looks to be east of I-65, particularly in the Blue Grass (Lexington)
region, closer to the entrance region of the upper jet and where
instability will have the greatest chance to maximize ahead of the
front.  Gusty thunderstorm winds will be the main threat.  High
temperatures Tuesday are still progged in the 70s.

.LONG TERM (Tuesday Night through Sunday)...
Issued at 320 AM EDT Mon Oct 27 2014

Forecast focus in the longer term is on the precipitation and
frontal passage Tuesday/Tuesday night, then a significant cool down
for this upcoming weekend.

Showers with scattered thunderstorms will be ongoing across the
region Tuesday evening. Compared to previous cycles, the 27.00z
models came in slower, especially the ECMWF, which suggests showers
could linger even through the overnight period and into early
Wednesday morning across the southern forecast area. POPs were
bumped up as overall confidence in this system continues to increase
and QPF adjusted up slightly Tuesday night as well. PWATs look to
vary from 1.5 to 1.75 inches, which is in the upper ranges for
climatology so with some efficient rain showers and scattered
thunderstorms, another 0.3 to 0.5 inch Tuesday night could push
totals for the event to 1.0 inch in places. Showers will end
northwest to southeast, exiting the area early Wednesday morning.

In the wake of the frontal passage Tuesday night, the forecast focus
is on several upper level shortwaves that look to move through the
broad longwave trough across Great Lakes region into the
mid-Atlantic. The first arrives Wednesday afternoon, dry with no
sensible weather impact. It will be a cold air advection day, with
850 mb temps falling down to the +1 to +3C range across the region.
Plan on below normal highs a few degrees on either side of 60,
coolest across southern Indiana and northern Kentucky.

For Thursday, another seasonably cool day with increasing clouds as
another shortwave trough begins to advance southeastward toward the
region late in the day. Compared to previous forecast runs, the
27.00z model cycle is faster, suggesting the main upper level energy
arrives Thursday night into Friday morning. The GFS/GEM are the
faster solutions, while the ECMWF is a bit slower. The main
saturation and lift resides in the 700-500mb layer, which looks to
be enough to produce some light precipitation. Kept chances on the
lower end /20-30 percent/ but adjust the timing to include a mention
for Thursday night.

In the wake of that shortwave trough, strong Canadian high pressure
will nose dive southeast for the late week and weekend time frame,
opening the door for a fresh blast of cool autumn air. Compared to
previous cycles though, some of the 27.00z guidance has trended
warmer, keeping the core of the coldest air mass to the north and
east closer to the main upper level trough. Still some disparity
between the deterministic models too, for example 850 mb temps vary
from +6C /GEM/ to -5C /ECMWF/ at 12Z Saturday. So although there is
above average confidence in experiencing a significant cool down for
the region later this week and this weekend, how much so remains a
bit uncertain. Regardless, plan on some crisp dry autumn days with
the coolest being Saturday as highs could only top out in the upper
40s across southern Indiana and northern KY to lower 50s across
south-central KY.

Surface high pressure overhead Friday and Saturday nights should
provide excellent radiational cooling conditions, so overnight lows
are expected to bottom out in the lower 30s. Some of the sheltered,
cooler areas of the region may dip further into the upper 20s,
especially on Sunday morning when winds look to be the lightest. The
threat for a widespread frost remains in place.

As the surface high shifts east of the forecast area late in the
weekend, air mass moderation and a developing southerly flow should
help temperatures rebound slowly into the 50s Sunday and lower 60s
next Monday.

&&

.AVIATION (06Z TAF Update)...
Updated at 123 AM EDT Mon Oct 27 2014

Model data have backed off on the idea of fog or low stratus this
morning at the TAF sites, and at 05Z skies were still clear and vsby
P6SM in the Ohio and lower Tennessee Valleys.  However, still can`t
rule out some low clouds at BWG around sunrise, so will keep the
LIFR cig there for a few hours, along with MVFR vsby. LLWS speeds
look borderline, but with a sharp inversion and 20-30 degrees of
directional shear, will go ahead and include it at SDF and BWG,
which are closer to the main low level jet stretching from Texas to
Illinois, during the overnight hours.

The daylight hours will be VFR with mostly clear skies and SSW winds
gusting to around 22 knots by afternoon as we sit between high
pressure over the mid Atlantic and a cold front approaching from the
west.

LLWS will return tonight as a low level jet revs up from the mid
Mississippi Valley to the lower Great Lakes.  The inversion tonight
won`t be quite as sharp or as low as this morning, and there will be
less directional shear, but speeds will increase quickly in the
lower 2000 feet so will put it in the TAFs.

&&

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

$$

Short Term........13
Long Term.........ZBT
Aviation..........13




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