Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 132230
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
530 PM EST Wed Dec 13 2017

.Forecast Update...
Issued at 530 PM EST Wed Dec 13 2017

Have cancelled the Wind Advisory early this evening as winds are no
longer a threat to gust over 40 mph. The loss of daytime heating
combined with transient upper sky cover has limited mixing up into
the 40-45 knot 925 mb jet. Did replace the Wind Advisory and ongoing
SPS with a new SPS highlighting gusty winds a bit deeper into the
evening. A look at upstream obs behind the secondary cold front that
will move through shows some gusts in the 30 to 40 mph range. Don`t
expect any new Wind Advisories to be hoisted, but will have to
monitor gusts with this secondary feature later this evening. Will
update the remaining products to remove Wind Advisory wording
shortly.

&&

.Short Term...(This evening through Thursday night)
Issued at 300 PM EST Wed Dec 13 2017

Fast moving clipper system will continue to move through the
southern Great Lakes this afternoon and will drag a cold front
through the region this evening.  Winds have been a bit slower to
pick up today.  However, we`ve had gusts at or near 35 MPH out west
of the I-65 corridor.  The gradient is still tightening up, so a few
gusts to 40 MPH still look likely this afternoon, west of I-65.  A
few gusts to 35 MPH still look likely in areas east of I-65 this
evening.  Therefore, we plan on leaving the Wind Advisory in place
through 700 PM EST.  Temperatures this afternoon have exhibited a
gradient across the state.  Readings were in the lower 50s over far
western KY, near 40 in the I-65 corridor, and in the upper 30s over
eastern KY.  Not expecting much change in the temperature field this
afternoon, but readings should fall into the upper 30s this evening.

For tonight, the front to the west will pass through the region.
There is not a whole lot of moisture to work with, so mainly we will
see just an increase in cloudiness as the boundary moves through.
Some of the high resolution models still show some sprinkles or
drizzle behind the front, mainly north of the I-64 corridor.  We
plan on keeping a mention of sprinkles this evening an a few
flurries into the overnight hours as the column cools down.  Lows
tonight look to cool into the upper 20s in the north with lows
around 30 in the south.

For Thursday and Thursday night, weak high pressure will build into
the region resulting in dry weather.  Highs Thursday look to warm
into the 30-35 degree range across our northeastern sections with 35-
40 across much of central and southwestern KY.  A chilly night is
expected Thursday night with lows dropping into the upper teens to
the lower 20s.

.Long Term...(Friday through Wednesday)
Issued at 328 PM EST Wed Dec 13 2017

Friday through Wednesday...

We`ll see another upper level wave cross through the Great Lakes on
Friday.  This feature continues to have a weak surface reflection
and much of the precipitation looks to stay well to our north.  The
pressure gradient is not all that tight with this system, so much
less windy conditions are expected.  Highs on Friday will remain
cool with highs in the mid-upper 30s in the north with highs in the
upper 30s to near 40 across the south.  Lows Friday night look to
cool into the mid-upper 20s.

We still expect to see the southeastern US ridge rise a bit over the
weekend as we will see sharp height falls develop in the lee of the
Rockies.  Saturday looks to be dry and a little milder at this point
with highs in the upper 40s in the north the lower 50s in the south.
Saturday night still looks to be mostly dry, but can`t rule out a
shower across the far southwest.  Lows will range from 32-36.

By Sunday, the upper trough in the lee of the Rockies will head east
and surface cyclogenesis will take place over the southern Red River
Valley.  We`ll see clouds rapidly increase Saturday night with rain
showers overspreading the area from the southwest to the northeast
during the day on Sunday.  Have bumped up PoPs a bit for Sunday
given the latest data.  Highs will remain mild with readings in the
upper 40s to around 50.  Lows Sunday night will cool into the mid
30s.  A secondary trough axis will move through the region on Monday
brining additional rain showers to the region.  Highs look to be in
the upper 40s with lows cooling back into the mid 30s Monday night.

By Tuesday we`ll see a long wave trough axis centered just to our
east and it is forecast to push slowly eastward by Wednesday.  This
should result in dry weather for our area.  Highs on Tuesday look to
remain in the upper 40s to around 50s with lows in the lower 30s.
Highs Wednesday look to be a bit cooler with readings generally in
the upper 30s to the lower 40s and overnight lows dropping back into
the upper 20s.

Beyond Wednesday and into Christmas Week...

Overall, not much change from our analysis from yesterday.  The
latest data continues to point to a potentially active period of
weather as we head into the Christmas Holiday period.  It appears
that we`ll see a weather system impact the region in the
Thursday/Friday time frame (12/21-22).  The current dynamical models
do agree with bringing the system into the region in that time
frame, but differ greatly from Saturday (12/23) onward.  In general,
the GFS and its ensembles push the system off to the east and allow
a much colder airmass to move in from the west.  The current Euro
solutions stall the boundary out and allow another system to push
into the region around the 12/23-24 time frame.

The GFS and its ensembles now show a much better support of its MJO
forecast which take the MJO from phase 6 through phase 7 and then 8
which would result in a cold regime for much of the eastern half of
the US.  While that would bring plenty of cold to the region, it
could result in the main storm track being shifted east and possibly
southeast of our region.  There may be a little bit of the
progressive GFS bias in that solution however.   On the other hand,
the Euro extended panels do not push the cold as far east and keep
the storm track a bit further west which could result in a period of
active weather for the Ohio Valley during the holiday period.  Most
likely, a general solution in-between the Euro and GFS is probably
what will happen.  So our latest thinking is that active weather
looks very likely in the holiday period, but specifics on the
thermal pattern still leave a lot of questions.  However, there is
plenty of time to let the details get worked out over the next week.

&&

.Aviation...(18Z TAF Issuance)
Issued at 1241 PM EST Wed Dec 13 2017

A cold front currently stretched from northwestern IL down through
central MO is pushing towards the TAF sites in southern IN and
central KY. As this front approaches, winds from the south are
increasing to 15-20 mph with gusts to 25-30 mph. Over the TAF
period, as the front passes, winds will move west before becoming
north during the afternoon tomorrow.

Expect conditions to remain VFR today. During the
overnight hours ceilings will drop to low end VFR. Some areas could
see future TAFs drop to MVFR.


&&

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

$$

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



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