Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 201708
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1208 PM EST Sat Dec 20 2014

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1025 AM EST Sat Dec 20 2014

Main forcing that brought light snow to the area this morning has
moved off into Ohio, so the remainder of the day is expected to be
dry. The afternoon forecast is largely on track, though the
challenge is cloud cover. Models are not handling the low/mid level
moisture very well at all as there is a large, although thin, deck
of clouds upstream. The HRRR seems to have a somewhat decent handle
on the current situation, so trended the forecast in that direction,
but went more pessimistic given current satellite and low sun angle
this time of year. This gives more of an cloudy/overcast afternoon
to the area, though there could be some breaks along/south of the KY
parkways. As a result of the increased clouds, lowered high
temperatures a degree or two.

Issued at 815 AM EST Sat Dec 20 2014

Radar returns, automated observations and spotter reports confirm
that the light snow continues across the Louisville metro area and
points north and east. With temperatures remaining in the upper 20s
to around 30, slick spots on untreated roads, particularly bridges
and overpasses could develop. Adjusted forecast grids and continued
SPS for portions of the area.

The main forcing for this activity is an upper level shortwave and
this is rapidly moving out of the area, so I`m expecting this
activity to end west to east over the next few hours.

Issued at 542 AM EST Sat Dec 20 2014

Light snow continues across portions of southern IN and north
central KY this morning. Some light accumulations have been reported
on cars and area roadways this morning for this activity. Updated
the grids to reflect current radar trends and issued an SPS talking
about slick spots. Still think most of this activity should move out
of the area by around sunrise.

.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 (18Z TAF Update)...
Issued at 1204 PM EST Sat Dec 20 2014

Much of the area around SDF/LEX/BWG remains VFR at this hour with
2000-2500 ft MVFR ceilings across southern Indiana and southwest
Ohio. Expecting VFR clouds to persist through the period with
light/variable winds this evening becoming more northeasterly to
easterly overnight. There are some indications that clouds could
scatter out overnight into Sunday morning, especially at BWG. Less
confident at SDF/LEX. If clouds do scatter out, the baggy wind field
and residual low-level moisture could result in light fog.

&&

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

$$

Update.........ZBT
Short Term.....EER
Long Term......MJ
Aviation.......ZBT





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