Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 242333

Area Forecast Discussion
National Weather Service Louisville KY
733 PM EDT SAT SEP 24 2016

...Updated Aviation Discussion...

.SHORT TERM (Now through Sunday Night)...
Issued at 300 PM EDT Sat Sep 24 2016

High pressure at the surface and aloft will continue to strongly
influence our weather pattern for the next 30 hours or so.  This
will lead to more warm weather as we push through the remainder of
the weekend...but big changes are on the way in the extended.

In the meantime, sensible weather for the rest of the afternoon and
evening will be generally sunny and very warm for late September
standards.  Afternoon highs will top out in the upper 80s to the
lower 90s.  Lows tonight cool into the middle 60s in most locations
with the urban areas staying in the upper 60s.  Another warm day is
expected on Sunday with afternoon highs topping out again in the
upper 80s to the lower 90s.

Changes will begin late Sunday night as a strong upper trough and
cold front move towards the region.  Clouds will be on the increase
late with a chance of showers and storms across our far NW late in
the period.  Lows will generally be in the middle 60s.

.LONG TERM (Monday through Saturday)...
Issued at 323 PM EDT Sat Sep 24 2016

...Fall Returns To The Ohio Valley...

Monday through Wednesday...

A deep upper level low over the inter-mountain west will move
eastward into the Plains and Midwest early in the forecast period.
This feature will then plow eastward through the Great Lakes
bringing a surging cold front through the Ohio Valley on Monday. For
the last week, the models have hemmed and hawed over the speed and
arrival of this feature.  It appears that strong forecast
convergence has developed with almost all the guidance pointing to
Monday for the frontal passage.  Moisture return ahead of this
feature looks modest at best while stronger dynamics and kinematics
remain further north with the upper low.  Nonetheless, slab forced
ascent along the frontal boundary should lead to a band of showers
and thunderstorms passing through the region on Monday followed by a
drop in temperatures in the late afternoon and evening.  Cooler and
drier weather will then return to the region by late Monday night
into Tuesday.

Highs Monday will likely be the warmest in the morning with mid-upper
70s in the west and shot of lower 80s in the east.  Temperatures
will then fall through the late afternoon/evening in the wake of the
front.  Lows Monday night will likely cool into the upper 40s to the
lower 50s.  We could see slightly cooler temperatures if the
boundary layer decouples a bit more and we do not have much in the
way of cold air stratocu that follows the front.

Tuesday and Wednesday will feature dry and cool days across the
region.  A strong pressure gradient will be in place so breezy
conditions during the period look likely with NW winds of 15-20 MPH
with gusts to 25 MPH at times.  Highs Tuesday will top out in the
very low 70s across the region, though some upper 60s may be the
limit across the a far north.  Lows Tuesday night will likely drop
back into the 40s.  Highs Wednesday will bounce back into the very
low 70s once again.

Wednesday Night through Saturday...

Forecast confidence begins to tank as we head into the later half of
the work week as significant differences continue to show up within
the global model solutions.

By Wednesday night, blocking over the NE US/SE Canada will force the
upper low over the Great Lakes to surge southward into the western
Appalachians.  As this occurs, the ridge axis in the lee of the
Rockies begins to quickly strengthen and migrate eastward into the
Plains.  This leads to a situation where we could see a deep cut off
low lingering across the eastern US.

The GFS and GEM are in general agreement with each other dropping
the upper low in the western Appalachians but then allowing the
system to work off to the east while the aforementioned ridge out
west builds eastward.  The Euro remains steadfast in its position
keeping a closed low over the eastern US.  Should this solution
verify, then our the current forecast may be too optimistic on temps
and dry weather as we get into the Thu/Fri time frame.

For now, have gone with the ensemble compromise, but incorporating
the Euro`s tendencies over the last two runs.  Thus, the forecast is
a little more conservative on keep temps on the lower side of the
guidance envelope for late week.  Am not ready to bite on the
showers just yet, but if that upper low hangs around, then we`ll
likely need to add some PoPs in later forecasts.

Highs Thu/Fri will be largely dependent on cloud cover.  For now
plan on going with mid 70s for highs and overnight lows in the upper
40s to the lower 50s.  Will still allow a little moderation in temps
by Saturday with highs warming back into the upper 70s.


.AVIATION (00Z TAF Update)...
Issued at 735 PM EDT Sat Sep 24 2016

A broad surface high pressure ridge extended south from Michigan
into western Kentucky at 23Z, with light northeast winds at all of
the TAF sites.  Flat fair weather cu have all but dissipated over
most of central KY as of 23Z. Satellite shows a bit more persistence
in a patch south of LEX, so will leave FEW060 in there.

By far the biggest challenge this forecast is the possibility of
radiational fog thanks to the clear skies and light winds. BWG was
socked in with LIFR cigs and vsby this morning while SDF got by with
MVFR vsby and LEX had no restrictions.  This afternoon`s crossover
temperatures showed no change at BWG, but SDF has gone drier, with
the 21Z Td dropping all the way to 58, while the LEX crossover was a
few degrees higher than yesterday.  Given all other conditions
nearly the same, will opt for MVFR vsbys at BWG and LEX - with tempo
to IFR cigs and vsby at BWG, but only drop SDF to 6SM.


.LMK Watches/Warnings/Advisories...


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