Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 200833
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
333 AM EST Sat Dec 20 2014

.SHORT TERM (Now through Sunday)...
Issued at 307 AM EST Sat Dec 20 2014

Some light radar returns continue to show up across southern Indiana
and portions of north central Kentucky this morning associated with
a weak PV anomaly. Some light snow may reach the ground under
the strongest echoes, as it did here at the office. However, the
most that is expected is a very light dusting on elevated surfaces.
This activity will continue to push off to the northeast over the
next few hours and should be out of the area by sunrise.

Clouds will stick around for much of the day but should begin to
clear by late afternoon into the evening. High pressure ridging in
from the northeast will keep the area dry through Sunday. Highs
today will range from the upper 30s in the north to the mid 40s in
the south. Tomorrow will be a bit warmer with highs ranging from the
mid 40s to around 50. Lows tonight will be in the mid to upper 20s.

.LONG TERM (Sunday night through Friday)...
Issued at 332 AM EST Sat Dec 20 2014

Sunday night through Tuesday Night...

Medium range deterministic guidance continues to be in good
agreement with the overall pattern as head into the late weekend and
into early next week.  The upper pattern at the beginning of the
period will feature a broad trough over the inter-mountain west with
ridging along both coasts.  This will place the Ohio Valley in a
broad southwesterly flow aloft early in the period.  As we move into
Monday, the multi-model consensus continues to suggest that the
ridge over the eastern Pacific will build northward into western
Canada.  As this occurs, the Pacific jet will pound it`s way into
Canada and then drop into the Plains.  This will lead to the broad
trough over the inter-mountain west to deepen and close off by
Monday afternoon.  The trough is expected to deepen further
resulting in cyclogenesis over Texas.  This surface low will deepen
as well as it heads NE into the Ohio Valley by late Tuesday night.

We`ll see dry conditions initially in our region at the beginning of
the period, but rain chances will quickly ramp up on Monday as the
broad southwesterly flow pushes quite a bit of moisture up into our
region.  Precipitable water amounts of nearly 1.25 inches look to
be in our vicinity by Tuesday afternoon which is +2 standard
deviations above normal.  Thus, some of the rainfall could be heavy
at times from Monday afternoon through Tuesday night.  With us
remaining in the southwest flow, temperatures will be rather mild
through this portion of the forecast period.  Lows Sunday night will
drop into the lower-middle 30s with highs on Monday warming into the
lower 50s in the north with lower-middle 50s across the south.
Temps will cool off just a bit Monday night with lows mainly in the
lower-mid 40s.  Tuesday look like to be the mildest day of the
period with highs pushing into the upper 50s.  Given the deepening
system and broad southwesterly flow, these numbers for Tuesday are
probably too conservative, and we may see some lower 60 degree
readings down across southern KY.  Some colder air will start to
wrap in late Tuesday night with lows crashing into the mid 30s in
the west with mid-upper 40s in the east.

Wednesday through Friday...

...Very Active Period of Weather Likely for Christmas Eve and into
early Christmas Day...

Latest long term model guidance has been trending westward with the
overall track of the surface low.  The Canadian GEM has been leading
the pack on the westward shift, and the Euro and GFS solutions have
now trended more west with the low.  By Wednesday morning, low
pressure should be in east-central Kentucky with a rather large
precipitation shield encompassing much of the region.  The pressure
gradient will rapidly increase during the morning hours with
sustained winds ramping up to 10-20 MPH by the early afternoon
hours.  The surface low is expected to rapidly intensify during the
afternoon as it races from east-central KY into western Ontario.
Based on the multi-model consensus, we could see the system deepen
by 20+ millibars within a 24 hour period.  As the low pulls away
from the region, we will see winds shift to the northwest and remain
quite elevated with sustained winds of 10-20 MPH and gusts up to
35-40 MPH.  Thermal profiles are slightly warmer during the day
suggestive that much of the precipitation behind the system will be
in the form of rain.  Highs early in the day will be in the upper
30s in the west to the lower-mid 40s in the east and then fall
during the afternoon.

By Christmas Eve, we will see temperature profiles crash from west
to east resulting in a quick change over to snow showers.  Very
blustery and cold conditions look increasingly likely for Christmas
eve with widespread snow shower activity in progress across the
region.  Model soundings continue to show very steep low-level lapse
rates with plenty of moisture in the ice crystal producing layer.
Thus, some snow showers may capable of producing moderate snow at
times during the evening.  While temperatures drop to near freezing,
there will be an opportunity for some minor accumulations.  Given
the relatively warm ground and recent rainfall, any snowfall
accumulation will be dependent upon the intensity of the snow
showers.

The models continue to trend a little faster with the moisture pull
off to the northeast early Christmas morning.  Current thinking is
that we`ll still have some snow shower activity past midnight, but
then rapidly diminish by sunrise.  The flow aloft looks to quickly
revert back to zonal flow which will likely push some drier air into
the region resulting clearing skies during the day.  Temperatures
will be challenging depending on how fast skies clear.  The clear
trend in the models has been warmer, so we have adjusted
temperatures slightly upwards with highs in the upper 30s in the
north to the lower-mid 40s across the south.

Generally dry conditions are expected for Friday. However, another
quick moving system will be pushing into the region by the late part
of the period.  Temps on Friday may be quite mild within the return
flow regime with upper 40s to lower 50s being possible.  A period of
rain showers looks increasingly likely for Friday night with colder
conditions returning by Saturday.

&&

.AVIATION (06Z TAF Update)...
Issued at 1145 PM EST Fri Dec 19 2014

The main challenge for this TAF period will be overnight with the
potential for MVFR ceilings. An upper level disturbance will
continue to cross the area tonight. The atmosphere is beginning to
saturate from the west and light returns can be seen on radar.
Ceilings should lower at all sites overnight. However, whether they
will drop below 3000 ft or remain scattered at that level remains a
bit in question. Have kept things optimistic and VFR at this time.
With this system, BWG and SDF may see some light snow/flurries, but
it is not expected to cause visibility restrictions. Winds will be
light through this TAF period. Clouds are expected to start to clear
late in the afternoon into the evening.

&&

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

$$

Short Term........EER
Long Term.........MJ
Aviation..........EER




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