Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 150514

1214 AM EST Mon Dec 15 2014

...Updated Aviation Discussion...

...Forecast Update...
Issued at 945 PM EST Sun Dec 14 2014

Cloud shield eroded quite a bit over Tennessee around sunset, but
has held on pretty solidly over Kentucky. Given the absence of any
strong boundary-layer flow, expect the clouds to stay with us
through the night and at least a couple hours beyond daybreak. Will
probably be mid-morning before the gradient tightens enough to start
mixing out the inversion.

This and most of the larger-scale trends are handled well in the
current forecast, but will send a quick update to better refine the
timing of precip spreading into the Ohio Valley Monday afternoon.

.SHORT TERM (Now through Monday Night)...
Issued at 241 PM EST Sun Dec 14 2014

High pressure over Kentucky will be slow to drift eastward tonight.
Low clouds and light fog have plagued southern Indiana and central
Kentucky today in the stagnant atmosphere associated with this high.
Interestingly, though, there has been a fair amount of mixing
breaking up the clouds to our north, south, and west, which the
models have not handled particularly well (nor did they do an
especially good job with today`s ceilings and visibilities).  This
all makes for a very challenging cloud forecast for tonight.  For
now, will go with the thinking that as the mixing shuts off with the
lowering sun this evening, and the high still in place, that clouds
will continue to hang around through much of the night in the LMK

By the pre-dawn hours, though, the surface pressure pattern will
begin to tighten as the high does finally push far enough east and a
storm system begins to encroach from the Plains.  The low clouds, if
they are still around, should scour out quickly from south to north
towards sunrise.

The tricky cloud forecast leads to a tricky low temperature
forecast.  Lows last night stayed quite warm under the blanket of
clouds.  Those clouds are currently forecast to hang tough for most
of the night, suggesting warmer than guidance once again.  However,
if we do get some clearing, that would allow temperatures to
drop...especially in the Dale Hollow and Lake Cumberland regions
closer to the high.  Then again, areas west of I-65 will begin to
get return flow (admittedly very light) by sunrise that could keep
temperatures from falling as much as they otherwise might.  Will go
for lower and middle 40s most places, except middle and upper 30s in
the southeast where there is the best chance of clearer skies and
lighter winds closest to the surface high.

On Monday a stacked low will cross northern Missouri with a band of
showers ahead of the upper trof from the lower Mississippi Valley to
the Great Lakes.  Right now it looks like the morning hours will be
dry, with showers a good bet in the afternoon.  A few rumbles of
elevated thunder will be possible as well. High temperatures will be
in the 50s.

Monday night the low will advance to Chicago and swing its cold
front through our region.  Showers will be most prevalent in the
evening, becoming less widespread from west to east after midnight.
Low temperatures will be in the 40s.

.LONG TERM (Tuesday through Sunday)...
Issued at 339 PM EST Sun Dec 14 2014

Tuesday through Thursday night...

Upper trough axis will shift through the region on Tuesday and lift
into the northeastern US by Wednesday.  The flow aloft will become
briefly zonal on Wednesday as the next trough axis develops in the
lee of the Rockies.  The latest trends in the model data suggest
that a fairly sheared out upper wave will quickly eject from the
Plains and through the Ohio Valley late Wednesday and during the day
on Thursday.  Sensible weather conditions will include a showery
period of weather continuing on Tuesday with drier conditions
arriving Tuesday night and lasting through the day on Wednesday.
Aforementioned upper level wave will likely induce some light
precipitation late Wednesday night and into the day on Thursday.
Wintry precipitation late Wednesday night looks likely with
soundings showing a relatively cold profile.  Some warming of the
boundary layer may commence on Thursday resulting in more of a mixed
precipitation event...especially over southern KY.

So with this forecast, will continue the chance PoPs for the early
part of this forecast period with the upper trough moving through.
We`ll also then introduce PoPs for late Wednesday and for much of
Thursday.  As for temperatures, the warmest readings will be ahead
of trough passage on Tuesday with early day highs in the upper 40s
to near 50.  Temps will drop into the lower 30s for Tuesday night
and then only warm into the upper 30s on Wednesday.  Lows Wednesday
night will likely drop into the upper 20s to around 30 with highs in
the middle-upper 30s on Thursday (which still may be a bit too warm,
but the SuperBlend guidance is trending colder).  Lows Thursday
night are expected to drop into the middle to upper 20s.

Friday through Sunday...

...Active Weather Period Setting Up for Late Week with a Threat of
Wintry Precipitation This Weekend...

It appears that an active weather period is setting up for the
eastern part of the US as the southern branch of the jet picks up
and the polar jet starts to come back further south.  Overall, this
looks to be the start of a well advertised pattern change that will
bring much colder conditions to the eastern US for the remainder of
the month and into early 2015.

Main weather feature of note for the extended period will be an
upper level trough axis that will be moving through the southern
stream early Friday.  This trough will push through Texas and then
head into the SE US states.  The exact track of this system is still
in question as the amount of blocking over SE Canada will be key to
whether the wave will move more poleward with time or maintain a
more east-northeast track through the northern Gulf coast states.
The model spread continues to be rather large to a certain degree.

With respect to the spread...on the southern edge...The Canadian GEM
continues to hold its course in keeping the southern stream much
more suppressed.  This result in a sheared out system quickly moving
through the SE and then out in the Atlantic.  This result in little
sensible weather impact to our area.

On the northern edge of the spread, we have the GFS.  In its
solutions, it continues to keep the upper low over Canada a bit more
open, but continues to have issues sensing the wave in the southern
stream.  With its less strong SE Canada low, it allows the southern
stream system to gain latitude as it heads eastward.  However, the
model has trended a little more south on this run, and a number of
the ensemble members are trending southward as well.

In the middle...and the most consistent of the bunch...remains the
Euro and its ensemble.  In its solution it keeps a stronger upper
low over SE Canada which doesn`t allow the southern stream wave to
gain much latitude as it heads east.  The stronger upper low
solution that it suggests also would bring a colder boundary layer
much further south.  Taking the Euro solution verbatim, would result
in a winter storm for much of KY and the middle Atlantic.

At this juncture, it remains far too early to judge a potential
track.  In cases like this where there is considerable model spread,
the sensible solution is to lean heavily on the multi-model ensemble
mean while maintaining as much continuity as possible.  With that
said, the previous forecast leaned heavily on the 00Z Euro and
13/12Z Euro ensemble.  Given the consistency of the Euro and its
much better forecast skill, we will trend the forecast closer to the
14/12Z Op Euro and its 14/00Z ensembles.

What results in the forecast here is largely a continuation of the
previous forecast with an increase in the PoPs for Friday night and
Saturday.  The SuperBlend temperature guidance continues to trend
downward, but not as quickly as the operational Euro runs suggest.
Thus, have trended temperatures downward slightly.  This results in
the precipitation forecast being less of a mix and trending more
towards plain snow.  The system will quickly pull off to the east by
Saturday night leaving Sunday potentially dry and cold...depending
on how much snow we see.

With this weekend being a heavy travel weekend, it is suggested that
folks stay abreast of the latest forecasts as this weekends weather
system has the potential bring negative impacts to the region.


.AVIATION (06Z TAF Update)...
Issued at 1205 AM EST Mon Dec 15 2014

Inversion is keeping stratus deck in place at SDF/LEX while BWG lies
on the southern fringe as of 05z. Heights across much of the area
are around 2 kft, plus or minus a few hundred feet. This trend
should continue through mid morning as nighttime cooling combined
with the strong inversion will keep ceilings persistent. At BWG,
light fog has developed where clouds have briefly scattered out.
Thinking that this will be short lived as latest satellite and
observation shows clouds have moved back west over the airfield.

Strengthening low pressure out west will tighten surface pressure
gradient locally by late morning, giving the area a stronger
south/southeast wind component. This will aid to lift/scour the cloud
deck. Based on 15.00z ILN/BNA soundings, moisture resides between
875 and 900 mb, and latest model RH guidance at these levels keep
the clouds in place at BWG until mid-morning and late morning at
SDF/LEX. Trended TAFs in this direction, holding MVFR deck a
couple hours longer at SDF/LEX.

Band of scattered showers will advance eastward during the
afternoon hours, initially reaching BWG/SDF around 21z and not at
LEX until 16.00z. While there is a chance for a rumble or two of
thunder, probabilities are too low to mention in TAF at this
point. Ceilings associated with precipitation should range from
2500 to 3500 ft with the possibility of brief MVFR visibility
restrictions at times. Showers look to continue through the end of
the current TAF period.




Short Term.....13
Long Term......MJ
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