Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 170803
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
303 AM EST Mon Dec 17 2018

.Short Term...(Today and tonight)
Issued at 255 AM EST Mon Dec 17 2018

Cloud cover has mainly pulled off to the east, but light winds and
trapped moisture have resulted in fog development. Decent coverage
and lowest visibilities are noted over south-central Kentucky
generally east of Mammoth Cave toward Lake Cumberland. Westerly
winds are trying to advect in drier air, so in most other locations
the fog is limited to river valleys and the traditional fog-prone
areas. The biggest question mark is how much the westerly winds
will continue to erode the thicker and more widespread fog even
before daybreak. Will monitor its progress over the next hour or so,
but expect at least a Special Weather Statement for the most
extensive early morning fog.

Once the fog burns off, expect a mostly sunny and mild day under
deep NW flow and increasing heights aloft. It`s worth noting that
the NAM still tries to hang on to low clouds and fog for much of the
day from the Bluegrass region south to Lake Cumberland. We have
tried to reduce the NAM influence on thermal fields as we believe
the drier NW flow will win out by midday, but should be aware that
bust potential exists.

Fog setup tonight may be even more favorable with the sfc ridge axis
in the area and temps expected to cross over afternoon dewpoints yet
again. Will include at least a patchy fog mention, but know that
will likely be ratcheted up as we hone in on the more favored areas.

&&

.Long Term...(Tuesday through Sunday)
Issued at 250 AM EST Mon Dec 17 2018

...Fair and Mild Tue-Wed, Significant Weather System Late Week...

Tuesday and Wednesday...

Shortwave ridging aloft will progress across the OH and TN Valleys
on Tue, shifting east Wed as SW flow aloft develops over the region.
At the surface, high pressure centered over the OH Valley will
control our weather Tue. The high will push to our east Wed, with
developing southerly flow during the day. As a result, expect dry
weather with above normal afternoon highs Tue in the mid 40s to
lower 50s, then ranging through the 50s Wed afternoon. Still chilly
Wed morning with lows upper 20s to lower 30s.

Thursday and Friday...

The weather becomes much more active Thu and Fri. The flow aloft is
forecast to undergo strong amplification as a shortwave progresses
east out of the SW U.S. and additional energy digs SE from the
northern Rockies into the developing trough over the central U.S.
Models agree that this phasing will result in development of a
closed mid-level low over the lower MS Valley Thu night, with S to N
meridional flow ahead of it from the Gulf States through OH Valley.
At the surface, a low pressure system will form over the lower MS
Valley and intensify as it moves NE across the TN Valley and up the
spine of the Appalachians.

Climatologically, this is a type of winter pattern that can bring
heavy snow to some parts of the western/lower OH Valley. However,
this particular system does not look "cold," and is acting on an
initially mild air mass, with moist low-level warm advection ahead
of the low into our region with the warm conveyer belt. This
suggests a rapid northward moisture transport from the Gulf with
showers breaking out over our area during the day Thu, continuing
Thu night into Fri. Models suggest weak elevated instability mainly
south of KY, but still could be some heavier rainfall elements at
times, especially south-central KY.

As the low lifts NE, a deformation zone should set up to its N and
W, somewhere over the mid MS, OH, and wrn TN Valleys. The thermal
structure suggests a TROWAL setup within the deformation zone, again
which can be a snowy setup with the proper thermal gradients and
cold air advection and dynamic cooling aloft on the N and W side of
the system. But in this case, current thermal guidance does not
suggests this that much for our area. Even with sufficient cooling
aloft, boundary layer temps are forecast to be mild enough for snow
aloft to melt to rain as it falls.

It is possible that precip could change to wet snow on the back edge
of the deep moisture and deformation field N and W of our forecast
area. And it is not out of the question there could be a brief
changeover in our area before precip ends (as suggested most by the
ECMWF), but overall the consensus is that by the time the boundary
layer cools sufficiently, the air mass aloft may be drying out
thereby cutting off much additional precip production. Total
rainfall amounts are difficult to predict at this time for our area,
but roughly from 0.5 to 1.5 inches is a reasonable range.

This will be a complex weather system, and models will change to
some degree as the time nears, so we will monitor the system. A
model consensus/ensemble blend is the way to go. Emphasis should not
be placed on any one model at any one particular model run time.

Saturday and Sunday...

After the late week storm departs to our N and E, the flow aloft
will quickly become much more zonal resulting in a more tranquil
weather pattern. Expect mainly dry conditions and seasonal temps
over the weekend, with highs generally in the 40s. A weak weather
disturbance could be isolated showers to parts of the area Sunday
night.

&&

.Aviation...(06Z TAF Issuance)
Updated at 1150 PM EST Sun Dec 16 2018

Dry air has kept HNB/SDF out of the fog so far tonight. Still think
SDF briefly could get down to MVFR around daybreak, but gave a
little smaller window for that to occur based on current trends.
Still have BWG/LEX with worse conditions through the overnight, with
IFR or worse possible until improvement comes just after daybreak
Monday. Afterwards expect a general northwest winds for the day
before they become variable and light Monday evening.


&&

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

$$

Short Term...RAS
Long Term...TWF
Aviation...RJS



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