Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 130247
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
947 PM EST Tue Dec 12 2017

.Forecast Update...
Issued at 947 PM EST Tue Dec 12 2017

...Gusty Winds Expected Wednesday...

The forecast is on track for the overnight hours, and no changes
planned other than a recent update to reflect the latest T/Td hourly
trends.

Took a look at the expected winds for tomorrow as a clipper system
dives through the southern Great Lakes region, and decided to issue
a Special Weather Statement for gusts in the 30 to 40 mph range. As
we move through the daylight hours, the 925 mb jet steadily
strengthens to around 40 knots. Looking at forecast soundings during
that time, low level lapse rates appear to allow mixing up into that
level. We should be able to realize a good portion of that momentum
down to the surface for a several hour period between 1 and 6 PM
EST. Winds will still be a bit gusty a few hours before and after
that time frame, but the worst should be during peak "heating". The
winds were already mentioned in the HWO, so no further updates to
that product are needed at this time.

&&

.Short Term...(This evening through Wednesday night)
Issued at 330 PM EST Tue Dec 12 2017

Influx of drier air has commenced across southern Indiana and
portions of central KY this afternoon on the heels of gusty
northwest winds.  Combination of dry air and good mixdown has
resulted in single digit dewpoint readings with KSDF going as low as
2F for a time this afternoon.  Temperatures have fallen throughout
the day and readings remain in the lower 30s across much of the
region.  Some upper 20s were noted across far NE KY.  For the
remainder of the afternoon and evening, quiet weather is expected
with dry conditions.  Winds will gradually die down this evening.
Temperatures this evening will fall into the lower 20s.   Wind chill
readings will likely drop into the single digits making it feel
colder than the actual air temperature.

For the overnight period, dry and cold weather is expected.
Overnight lows will drop into the upper teens east of I-65 with
mainly lower 20s west of the I-65 corridor.

Another clipper will quickly drop into the lower Great Lakes region
on Wednesday and then head into the northeastern US by Wednesday
night.  The clipper will not have a ton of moisture to work with,
but we will feel its power as it passes by in the form of gusty
winds.  We`re likely to see sustained winds on Wednesday in the 15-
25 MPH range with gusts to 40-45 MPH at times.  These strong
gradient winds will carry over into Wednesday night before
diminishing.  A few snow flurries will be possible Wednesday night
across our NE sections of southern IN and into the northern
Bluegrass.  Highs Wednesday will likely exhibit a gradient across
the region.  Highs in the upper 30s to the lower 40s will be seen
across southeastern IN and into the Bluegrass region of central
Kentucky.  Mid-upper 40s will be possible in areas west of the I-65
corridor.  Lows Wednesday night will drop into the mid-upper 20s.

.Long Term...(Thursday through Tuesday)
Issued at 334 PM EST Tue Dec 12 2017

Thursday through Tuesday...

A dry period of weather looks likely Thursday as we`ll be in between
clipper systems.  Temperatures will remain rather cool with highs in
the 35-40 degree range.  Lows Thursday night look to cool into the
lower-middle 20s.   Another fast moving, but rather flat clipper
system will slide north of the region on Friday.  This may bring a
round of snow flurries to the region.  Highs on Friday look to again
warm into the 35-40 degree range with lows Friday night cooling back
into the lower-mid 20s.

By Saturday, we`ll see some ridging build into the region as mid-
level heights fall in the lee of the Rockies.  The ridging, combined
with an increasing southwest flow will result in a milder weather
pattern setting up for the Ohio Valley.  We should see temperatures
warm into the the mid-upper 40s across southern Indiana and into the
Bluegrass region.  Some lower 50s will be possible mainly south of
the WK/BG Parkways.  Lows Saturday night will be a bit warmer with
readings mainly in the lower 30s.

The falling heights in the lee of the Rockies will move east and an
area of surface low pressure will develop over the Red River Valley
and then move northeast across the TN and Ohio Valleys Sunday and
Monday.  Temperature profiles will be mild enough for plain rain for
Sunday and into Monday.  Some of this rainfall could be heavy Sunday
night into Monday.  Temperatures on Sunday should be the warmest
with highs in the lower 50s.  Mild weather looks to hold into Monday
and Tuesday with highs mainly in the upper 40s to near 50 with
overnight lows in the lower to mid 30s.

Beyond Wednesday into Week 2...

A generally milder than normal pattern looks to hold from the middle
of next week into the beginning of the Christmas holiday weekend.
Recent signal analysis shows another weather system affecting the
Ohio Valley around Thursday or Friday of next week.  Timing issues
will be worked out over the next few days.  However, thermal
profiles would suggest rain with this system.

As we move into the Christmas holiday weekend, the operational and
ensemble models are suggest that strong ridging may develop over AK
which would favor a downstream trough over the western US and a
corresponding ridge down over the eastern US and western Atlantic.
The Ohio Valley looks to be dead in the middle of the storm track
and several systems may affect the region during the Christmas
holiday period.

Recent and past signal analysis has been strongly suggestive that
the Christmas holiday week could feature numerous signal crossings
which may bring periods of inclement weather to the area.  The main
question is how will the thermal field evolve during the period.
Current ensemble means and operational runs from the Euro and GFS
are holding much of the cold air to the west, keeping the Ohio
Valley more on the warmer side.

We should note that we`re not entirely sold on that idea as of yet.
This is because the GFS and Euro`s MJO projection of going into
phase space 7 and 8 would argue for a more colder pattern by
Christmas.  We suspect that the dynamical models may just be holding
back the trough a little too much to the west.  So a correction to
the east would not be all that surprising in future runs.

&&

.Aviation...(00Z TAF Issuance)
Issued at 646 PM EST Tue Dec 12 2017

VFR will prevail through this forecast cycle, with little more than
a few mid and upper clouds later tonight and through the day
Wednesday. The only real concern will be wind direction/speeds,
especially on Wednesday.

A clipper system will drop out of Canada and through the southern
Great Lakes by Wednesday evening. Ahead of this feature, the
pressure gradient will tighten dramatically on Wednesday. Our
initial light WNW winds this evening will drop off to light SW
overnight, and then strengthen through the day on Wednesday in
response to the the clipper. Expect SW winds 15 to 25 mph gusting up
around 35 or 40 mph by Wednesday afternoon.


&&

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

$$

Update...BJS
Short Term...MJ
Long Term....MJ
Aviation...BJS



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