Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 231306
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
906 AM EDT Mon Oct 23 2017

.Forecast Update...
Issued at 904 AM EDT Mon Oct 23 2017

Currently, the upper low center is sitting near the TN/KY/AR/MO
confluence with an associated surface low lifting into central
Indiana. The main precipitation concern is with a thin, but
enhanced, band of precipitation where the near-surface front is
intersecting the low level jet. This is creating enhanced
convergence where brief, but intense, rainfall rates are occurring.
A few instances of minor nuisance flooding could occur with this
feature, but the overall eastward motion is resulting in short
duration so don`t expect any major problems.

There is some lighter precip lagging behind the surface front
associated with the warm conveyor belt. The back edge of this
precipitation is directly associated with the sharp gradient with
the tapering 40-50 knot LLJ. As this pushes east through the
morning, expect a relative lull in precipitation coverage. Can`t
go completely dry, but drop Pops into the isolated to scattered
range.

As we move into the afternoon and evening, we`ll watch for better
coverage of showers associated with the TROWAL/deformation zone on
the NW side of the upper low. This feature is very evident over
IL/MO currently on 500-700 mb potential temperature analysis.
Overall, this precipitation will not be as widespread or as heavy as
we saw this morning.

A final round of light rain is then possible overnight as the left
exit region of a 110 knot upper jet noses into the area.

Did update the temps to have them staying steady through the morning
and then falling off into the 50s under a pretty decent cold
advection component. Adding confidence, sky cover should be pretty
much overcast through the day, limiting heating.

Previous Update...
Issued at 550 AM EDT Mon Oct 23 2017

Low-level jet is now nosing into central Kentucky and southern
Indiana, with the heaviest rain now affecting areas mainly along
and west of I-65. Strongest band currently situated from French Lick
to just west of Brandenburg to near Bowling Green. Peak
instantaneous rates are around 1.5 inches per hour, but only
persisting long enough to yield 60-minute rainfall around a half
inch. Briefly reduced visibilities and short-term ponding are the
main impacts so far. Just expect the axis of heaviest precip to
gradually shift east through the morning, finally exiting the
Bluegrass early/mid afternoon.

&&

.Short Term...(Today through Tuesday)
Issued at 300 AM EDT Mon Oct 23 2017

Closed upper low over the Ozarks continues to move quite slowly,
with its low-level jet and associated precip shield still hung up
over western Kentucky. As the morning progresses, expect a digging
shortwave trof in the northern Plains/Upper Midwest to finally start
to kick the closed low eastward, with moderate rain overspreading
southern Indiana and central Kentucky. Look for a good soaking rain
in most locations, with widespread amounts exceeding 1 inch. May see
a few embedded pockets of heavier rain in the shorter time frame,
but given the recent dry conditions, impacts will likely be limited
to ponding or short-duration nuisance flooding.

By early/mid afternoon the warm conveyor belt should be well into
eastern Kentucky, taking the heaviest rain out of the area. However,
POPs will remain in the high chance range for the balance of the day
with the upper low and cold pool overhead. Temperatures will remain
steady or even fall slowly in the afternoon as low-level cold
advection sets up.

Look for plenty of clouds and even a few light showers overnight, as
the upper cold pool is reinforced and we still hold on to decent low-
level moisture. Overnight lows will be close to seasonal normals,
but very little recovery is expected on Tuesday as 850mb temps cool
to near 0C.

.Long Term...(Tuesday night through Sunday)
Issued at 300 AM EDT Mon Oct 23 2017

Tuesday Night - Thursday...

Deep and broad trof over the eastern CONUS will be in control Tue
night into Wed, Latest solutions have the upper low over the Great
Lakes opening up, which means less of a chance for cold pool showers
over southern Indiana and north-central Kentucky. Have gone with a
dry forecast, but that consequently opens the door for potential
frost Wed night/Thu morning. Min temps are in the mid/upper 30s, and
with light winds and dewpoints in the lower 30s, we could see some
frost in the typical cold spots. Will mention patchy frost for now,
and let later shifts decide when and how much to ramp up the
messaging.

Shortwave ridging aloft takes over Wed night/Thu, allowing some
recovery in daytime temps, albeit staying below climo.

Friday - Sunday...

Forecast confidence remains fairly low as model disagreements
persist. ECMWF brings in a cold front on Friday, but hangs it up
across the Ohio Valley for much of the weekend, resulting in
continued high POPs reflecting a raw and chilly rain. The GFS is
much more progressive, which keeps POPs lower as the system even
struggles to tap into a pre-frontal moisture supply, and pushes
through much more cleanly. This solution would give cooler nighttime
temps than the ECMWF. Current approach is to try and play the middle
on temps, keeping it just warm enough at night that the 20-30 POPs
in the forecast do not present a precip type issue. Temps either way
look unseasonably chilly, with daytime highs struggling into the
50s.

&&

.Aviation...(12Z TAF Issuance)
Issued at 635 AM EDT Mon Oct 23 2017

Band of rain with embedded heavier pockets will continue to
overspread all of central Kentucky and southern Indiana this
morning. Expect reduced visibility down to 1 mile at times in the
heaviest rain. In its wake, ceilings will begin to lower as well,
likely reaching IFR at HNB and BWG, and fuel-alternate MVFR at SDF
and LEX. Once the initial front passes, west winds will become gusty
later this morning and this afternoon, with gusts around 20 kts.
Showers will become more scattered this afternoon as the weather
system lifts to the northeast.

However, an upper level system will quickly follow and bring another
chance for scattered showers. This may also keep the lower ceilings
in place at HNB, SDF, and LEX along with gusty west to northwest
winds through the end of the TAF period.


&&

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

$$

Update...BJS
Short Term...RAS
Long Term...RAS
Aviation...ZT


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