Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 191657
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1157 AM EST Fri Dec 19 2014

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1130 AM EST Fri Dec 19 2014

A narrow swath of dry 850mb air is oriented NW-SE across the Ohio
Valley, providing at least a brief window of sunshine for parts of
southern Indiana and central Kentucky. The main short-term challenge
is the southern stream system still taking shape along the Gulf
coast, and just how far north it will be able to produce any precip
tonight.

Still looks like a narrow window for light snow, mainly from 06-12Z
across south central Kentucky. This continues to look like a
low-probability and low-impact event, as POPs are limited to 20-30,
precip type should be all snow, and QPF is quite meager. If
anything, the current forecast is a bit generous, but will not pull
back any further until the rest of the 12Z model runs come in.

.SHORT TERM (Now through Saturday)...
Issued at 300 AM EST Fri Dec 19 2014

Zonal flow at 500mb continues across the southern tier of the CONUS.
An upper wave currently over Arizona will de-amplify as it crosses
the southern plains later today.

Weak high pressure over the Great Lakes has nosed south across the
Commonwealth. This will keep pressure gradients weak and thus bring
very light winds today through Saturday. Cloud cover will be a
challenge today, as satellite confirms model data indicating only a
thin layer of strato-cu at around 850mb. Breaks in these clouds have
already appeared over southern Indiana. Should this trend continue,
partly cloudy skies may develop across southern Indiana and northern
Kentucky by dawn. Eventually, variable cloudiness is expected later
today with some periods of actual sun! Temperatures will peak this
afternoon in the upper 30s.

High and mid level cloudiness will increase overnight as a
disturbance moves across the Gulf States. This shallow amplitude
feature will spread light precipitation as far north as our
Tennessee border early Saturday, but dry air at low levels will not
allow any precipitation to make it as far north as northern
Kentucky. Forecast soundings will be cold enough for snow, and very
light amounts may fall across our far southern counties early
Saturday. However any accumulations will be quite light. Lows early
Saturday will fall into the mid to upper 20s, with highs Saturday
rising into the mid to upper 30s. Mid level moisture will exit later
Saturday morning, but light winds and lingering low level moisture
will probably bring another mostly cloudy day.

.LONG TERM (Saturday Night through Thursday)...
Issued at 321 AM EST Fri Dec 19 2014

===================================
Long Term Synoptic Overview
===================================

Not too much in the way of changes from 24 hours ago in the long
term synoptic overview.  A progressive pattern aloft is expected to
be in place at the beginning of the forecast period.  However, in
the early part of the week, the pattern will start to undergo
significant changes.  Teleconnection indices from the deterministic
and ensemble solutions continue to show the pattern transitioning to
an -AO/-NAO/-EPO pattern aloft.  This results in more significant
ridging building across the western CONUS into western Canada with
downstream troughing developing over the northern Plains.  This
upper trough over the northern Plains will continue to strengthen
and cutoff over the Great Lakes region by mid-week.  The upper low
will likely lift northward back into southern Canada with a cold
flow of air pushing into the eastern part of the US.

===================================
Model Preference/Trends/Confidence
===================================

In general sense, the run to run continuity of the models remain
pretty good, sans some of the usual timing issues.  The GEM and Euro
remain steadfast in their development of the western CONUS ridging
and subsequent development of the strong mid-level wave dropping
into the north-central US.  The OP GFS shows this well, though the
parallel GFS sides a bit more with the GEM/Euro solutions.  While
there are some timing issues, the deterministic runs fit well with
the ensembles.  The 19/00Z Euro OP solution is not as gung ho on
developing a strong deep H5 gyre over central Canada late in the
period as the 18/00Z and 18/12Z runs did.  This may be a
manifestation of the large amount of blocking that it develops over
the north Atlantic, and the shifting of the western CONUS ridge
westward.  Its hard to tell if this is just an off run of the model
or the emergence of some trend.  For now, still plan on sticking to
a blend of the Euro ensemble and NAEFS ensembles for this forecast
which will keep forecast continuity.  Forecast confidence on
temperatures and precipitation for the long term period remain at a
medium level at this time.

===================================
Sensible Weather Impacts
===================================

The weather will start off dry on Sunday across the region with a
weak ridge of high pressure drifting through the region. By Monday,
we`ll start to see the central US trough start to drop southward
into the Plains.  As this occurs, we`ll see a mid-level wave push
through the deep south and off the coast.  This feature looks to
bring mainly rain showers to our region.  Overcast and rainy
conditions look to continue Monday night through Wednesday night as
the strong upper trough approaches the region.  Best precipitation
coverage looks to be late in the day on Tuesday through early
Wednesday.  Thermal profiles remain warm through this period with
rain expected.  An increasing pressure gradient will lead to windy
conditions across the region.  Sustained winds of 15-20 MPH with
gusts up to 30 MPH will be seen at times late Tuesday and into
Wednesday.  Highs Sunday will likely warm into the 40s with upper
40s to the lower 50s expected for Monday and Tuesday.

As the upper low deepens and moves into the southern Great Lakes,
we`ll see strong surface cyclogenesis over eastern KY.  This surface
low will deepen considerably on Wednesday race from east KY into the
eastern Great Lakes.  Widespread rain showers will accompany this
system on Wednesday.  In the wake of the surface low passing, we`ll
see a strong northwest flow come in behind, keeping skies very
cloudy and allowing colder air to filter into the region.  We should
see temperatures fall from the lower 50s into the lower 30s by the
evening hours.  Thermal profiles look to cool sufficiently to see
the widespread rain showers change over to snow showers by Wednesday
evening.  While temperatures are expected to remain in the lower
30s, it will feel colder than that due to the blustery northwesterly
winds.  With surface temps expected to bottom out in the lower 30s
late Wednesday night, we could see some light snow accumulations.
This will be something to keep an eye on over the next several days.

The surface and upper low will continue to lift northward on
Thursday into Canada.  This will result in less windy conditions
along with a drying trend.  Some left over snow showers or flurries
will be possible...mainly in the northeastern sections of our area
early Thursday morning.  Some partial clearing may take place by the
afternoon.  Afternoon highs look to rebound into the upper 30s to
the lower 40s in the north with lower 40s down across the south.

&&

.AVIATION (18Z TAF Update)...
Issued at 1155 AM EST Fri Dec 19 2014

Clear slot that developed this morning is being pinched by a low
strato-cu shield pinwheeling out of Ohio. Fuel-alternate ceiling has
come back into LEX, while the deck has remained scattered at SDF and
BWG. Fairly extensive 4000 foot deck is maintaining a VFR ceiling at
BWG.

Expect the lowest clouds to scatter and/or lift early this
afternoon, putting all 3 terminals back in VFR. As the low gets more
organized along the Gulf coast, expect the ceilings from BWG to
spread north into SDF and eventually LEX. Snow or flurries can`t be
ruled out especially in BWG, but low-level dry air will limit the
impact even if it does happen. If anything, with light winds and an
air mass that has been rather stagnant for a solid week,
visibilities could very well go MVFR in haze/mist for a few hrs
early Saturday.

&&

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

$$

Update.........RAS
Short Term.....JSD
Long Term......13
Aviation.......RAS




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