458
FXUS63 KLMK 271042
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
642 AM EDT Fri Sep 27 2024

.KEY MESSAGES...

*  Widespread moderate to heavy rain and windy conditions today as
   the remnants of Helene move through the Lower Ohio Valley.

*  Potential for 3 to 5 inches of rain through early Saturday
   morning. Minor river flooding may develop in the lower Green
   River.

*  Wind gusts of 45 to 55 mph are possible this afternoon with
   higher gusts possible over eastern Kentucky and southern Indiana.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 350 AM EDT Fri Sep 27 2024

Synopsis...TC Helen has moved inland and it is currently moving to
the northeast across the Southeast US. Binary interaction with the
broader cut-off upper low located over the western section of the
Tennessee Valley will drag the decaying tropical system counter-
clockwise over the region and towards the Lower Ohio Valley. The
rapid translational speed of the low pressure center and its
potential track will allow the strong dynamics of the right
semicircle to transition across the forecast area today, enhancing
moisture advection and windy conditions.

Today...Persistent moisture influx from TC Helen rainbands
stretching along the retrograding frontal boundary is supporting
widespread moderate rainfall this morning and will most likely
continue to do so the rest of the period. Although PW values will
surge above 2 inches and depth of warm cloud layer will exceed
10kft, latest HREF solution (27/0Z) shows heavy rainfall evenly
distributed across the region and very low probability of 1-hr QPF
greater than 1 inch at any given moment. As a result, still
expecting minor flooding issues, especially in low-laying areas and
poorly-drained areas with slight risk of flash flooding. IN
addition, Flood Watch still in effect until Saturday morning with
slight spatial modification as another tier of counties was added to
the north after collaboration with surrounding offices. As for river
flooding, the points along the Green River Basin (Rochester and
Woodbury Upper) have around a 20 percent chance of reaching minor
flood stage on Saturday.

Biggest change with this forecast update is the issuance of a High
Wind Warning (HWW) for portions of the Bluegrass, northern Kentucky
and southern Indiana. Latest model runs (ECMWF, GFS, HREF) have
indicated greater than 60 percent of attaining wind gusts of 58 mph
or more late this morning and particularly during the afternoon.
ECWMF and HREF have these kind of winds extending farther south
while the strongest winds in the GFS ensemble member runs are
more constrained to the north of the CWA. The difference can be
probably explained by differences in the low pressure track and how
the associated wind field reacts to the binary interaction. That
being said, model data and further collaboration with surrounding
offices support issuing a HWW from late this morning and until late
around 28/0Z. Last but not least, the forecast took into account the
presence of moist adiabatic lapse rates and persistent cloud cover,
but seems to be the case that 50-knot low level momentum could mix
down during the afternoon with a drying trend noticed in the PW and
cloud fields in both the ECMWF and HREF models.

Tonight...It will remain rainy and windy but strongest wind gusts
will fall below High Wind Warning and even Wind Advisory criteria.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 350 AM EDT Fri Sep 27 2024

Saturday - Tuesday...

The remnants of Helene are expected to wobble across the Ohio River
Valley through the weekend, gradually weakening as it does so. As a
result, we`ll keep periods of rain showers (categorical pops) in the
forecast Sat/Sun, and then taper to lingering chances on Monday, and
a slight chance east of I-65 by Tuesday. Saturday will carry an
additional .5" to 1.25" of rainfall, with Sunday amounts dropping to
between a tenth and a quarter inch. Negligible amounts less than a
tenth of an inch are then expected for spotty areas Monday and
especially Tuesday.

Only real concern for any lingering or additional flooding concerns
will be on Saturday for locations that get another inch or so. FFG
values and overall saturated grounds likely won`t want to handle
much more rainfall by then. Much will depend how much falls on
Friday, and how well we handle that rainfall to determine whether
any lingering flood threat carries into Saturday. May have to
contend with a river basin or two getting into action or minor flood
should the overall rainfall amounts of widespread 4-5" verify for an
event total. If so, The Green River basin would be the most likely
spot for some river points to get into action or Minor Flood.

Small diurnal ranges due to heavy cloud cover and periodic
precipitation will also be a continuing theme through the weekend
with highs on Saturday only around 70. Sunday highs are slightly
milder in the low and mid 70s, and then mid to upper 70s by Monday.
Lows in the low and mid 60s expected each night.

Tuesday Night - Thursday...

Surface high pressure and dry zonal flow aloft take hold through mid
week. May even start to get some SE CONUS upper ridging building in
toward late week. As a result, expect a dry stretch of weather with
highs back up into the upper 70s and around 80 degrees.

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 639 AM EDT Fri Sep 27 2024

Impacts/Confidence:
- High confidence in persistent IFR ceilings and visibilities
throughout the period.
- High confidence in gusty winds today with 35+ knots gusts this
afternoon at LEX, RGA, SDF, and HNB.

Discussion...Persistent rain from the periphery of TC Helen has
moved in from the south bringing with it IFR CIGS and decreasing
VIS. Current forecast reasoning is that IFR ceilings will
predominate for most, if not all, the TAF period while IFR/MVFR
visibility will eventually set in as the heavier shower activity
impacts the terminals. Surface observations have show winds rapidly
increasing this morning (especially at LEX). Peak wind gusts will
occur late this morning and during the afternoon. Strongest wind
gusts (above 40+ knots) are likely along the northern terminals
(including LEX and SDF) with widespread values between 25 to 35
knots elsewhere. By tonight, winds will diminish in response to
substantial weakening of the remnants of Helen but still sporadic
gusts of 20 to 30 knots are possible before midnight. In addition,
winds will shift from the east to the south given the low pressure
track across southern Kentucky.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...High Wind Warning from 10 AM EDT /9 AM CDT/ this morning to 8
    PM EDT /7 PM CDT/ this evening for KYZ029>043-045>049-
     054>057-065>067-077-078.
     Flood Watch through Saturday morning for KYZ023>029-038>041-
     045>049-053>057-061>067-070>078-081-082.
     Wind Advisory until 2 AM EDT /1 AM CDT/ Saturday for
     KYZ023>028-053-061>064-070>076-081-082.
IN...High Wind Warning from 10 AM this morning to 8 PM EDT this
     evening for INZ076>079-083-084-090>092.
     Flood Watch through Saturday morning for INZ089.
     Wind Advisory until 1 AM CDT Saturday for INZ089.

&&

$$

SHORT TERM...ALL
LONG TERM...BJS
AVIATION...ALL