Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 260508
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1208 AM EST Wed Nov 26 2014

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1007 PM EST Tue Nov 25 2014

We will continue to enjoy quiet weather tonight.  The forecast is in
good shape and no major changes are needed.

Taking a quick peek ahead to tomorrow night`s clipper...  Over its
last five runs the GFS has trended slower with onset in the west,
and over its past three runs has suppressed the main precipitation
farther south into southern Kentucky.  The entire region will still
probably see some precipitation, but the best chances appear to be
concentrating over the southern half of Kentucky.  The system will
be weakening as it enters the region.

The precipitation will begin as light rain in the evening before
transitioning to light snow for the overnight hours.  The ground
will be warm (and possibly wet) after having experienced afternoon
air temperatures peaking in the middle 40s.  Also, near-surface
temperatures through the night will mostly be in the middle 30s,
possibly dipping into the lower 30s by morning.  So, even areas that
do get several hours of light snow should see minimal impact.
Overall most locations should see less than an inch and mostly in
grassy areas.  Of course, if a narrow band develops and trains from
northwest to southeast, then locally higher amounts could be
realized.

The most likely timing for any accumulating snow will be between
midnight and dawn, which is another factor leading to the thinking
that impact from this system will not be great.

Of course, winter weather can change quickly, so be sure to keep up
with the latest forecasts, especially if you`re planning to travel
Wednesday night.

.SHORT TERM (Now through Wednesday Night)...
Issued at 302 PM EST TUE Nov 25 2014

...Wintry mix possible late Wednesday into Wednesday night...

The synoptic pattern this afternoon features broad troughing across
much of the CONUS, making for temperatures continuing to run below
normal.  A clipper system will dive southeast through the trough
aloft tomorrow into tomorrow night, bringing the potential for a
wintry mix to portions of the region.

Conditions will remain tranquil tonight as high pressure settles
into the region.  High clouds passing across portions of eastern KY
will likely help keep temperatures up a bit there, otherwise most
other locations should dip into the mid and upper 20s.

Wednesday will start off dry as high pressure slowly drifts east of
the region.  However, attention will quickly turn to the northwest
as a clipper system dives southeast through portions of the Midwest
and into the Ohio Valley.  The left-exit region of the upper jet
will help induce a tightening mid-level thermal gradient, which
should lead to a frontogenetically induced band of precipitation.

The main forecast challenge will be both with the exact track of
this narrow precip band, along with the thermal profiles beneath
it.  The latest guidance continues its trend of dipping the bulk of
the precipitation further south into central and southern KY.  This
is not a huge surprise given that this northward placement is a
known bias in most numerical guidance.  With better convergence of
the model solutions, have gone ahead and upped pops across portions
of southern KY, while dropping them slightly for portions of
southern IN/northern KY.  The hardest challenge with this event will
be low-level thermal profiles.  The good news is that it appears
surface wet bulb temperatures will remain above zero through the
event.  However, if we get heavy enough rates, it could wet bulb the
column down just enough to set up a very narrow band of moderate
snow.  Looking at the forcing, it appears to be weakening as it
enters the Ohio Valley, given a weakening upper-level jet structure
and the fgen response.  Therefore, while a band of moderate
precipitation will likely still move across portions of
central/southern KY, not sold it will be long-lived enough to wet
bulb the column and give widespread snow.  Given the surface low
will also be weakening as it moves into the region, surface cold air
advection will likely not play too much of a role either.

Bottom line is that this event will start as rain and remain mostly
as rain.  However, rates may become just enough to wet bulb the
column and allow for some wet snow to mix in.  If this does occur, a
moderate burst of snow could drop a few tenths of an inch of
accumulation briefly, before melting again once rates decrease.
All-in-all, not much, if any, impacts are expected from this event.

Ahead of the system, highs will climb into the low to mid 40s
tomorrow (warming ground/road temps ahead of this event).  Behind
this system, overnight lows tomorrow night will dip into the upper
20s and lower 30s.

.LONG TERM (Thursday through Tuesday)...
Issued at 300 PM EST Tue Nov 25 2014

Thanksgiving Day may just be the coldest day of the long term.
Models had been divergent on a cold front coming into the region for
the weekend, but now they stall the front to our north Saturday and
Sunday before slowly pushing it through at some point early next
work week. That will mean an above normal forecast for temperatures
this weekend, but also a better chance for rains starting Saturday
night. The front will be to our north, but warm/moist air riding
near that front would promote showers and storms over an extended
period of time, at least into Monday. A lot will depend on the
location of that front Monday and Tuesday, so will lean towards
climatology and start decreasing pops by Day 7 (Tuesday).

&&

.AVIATION (06Z TAF Update)...
Updated at 1207 AM EST Wed Nov 26 2014

VFR conditions are expected overnight as we will remain in between
weather systems.  High ceilings are expected along and east of I-65
overnight as well advertised east coast storm gets organized and
moves northeast.  These high clouds will move out toward morning.
Winds overnight are expected to be light and variable at KBWG with
a light northeasterly wind at KSDF and KLEX.  Next weather system
will be a fast moving clipper that will dive into the region from
the west later today.  This will bring clouds to the region by
evening with rain showers moving into areas mainly west of I-65 by
evening.  A relatively warm boundary layer will keep precipitation
in the form of rain at the start of the event, but colder air
flowing in behind the system will result in change over to snow by
Thursday morning.  More details on tonight`s system will be
forthcoming in the 26/12Z TAF package later this morning.

&&

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

$$

Update.........13
Short Term.....KJD
Long Term......RJS
Aviation.......MJ





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