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FXUS63 KLMK 130247

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 

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.




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