579
FXUS63 KLMK 270542
AFDLMK

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

...Updated Aviation Discussion...

.KEY MESSAGES...

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

*  Potential for 3 to 5 inches of rain from late Thursday night to
   early Saturday morning. Locally higher amounts are possible.

*  Wind gusts of 45 to 55 mph are possible during the day Friday as
   the center of Helene works into KY and the Ohio Valley.

*  Depending on where the heaviest rain falls, and how much falls,
   minor river flooding may develop. Right now the forecast favors
   southern and central Kentucky, especially the lower Green River,
   as the most likely area for any minor river flooding.

*  The weather next week will be much quieter, with no heavy rain
   expected after Saturday.

&&

.UPDATE...
Issued at 737 PM EDT Thu Sep 26 2024

Now a waiting game for the expansive precip shield ahead of Helene
to pivot westward into central Kentucky. Look for gradually
increasing winds overnight, with the strongest winds still on track
to occur during the day Friday. Some hi-res model output appears to
be overdone on wind gusts, as what we expect to be a long soaking
rain will limit our wind potential. IF the rain can well and truly
shut off a few hrs before sunset, that would be what opens the door
for any 60+ mph wind gusts. The stronger gusts would likely be
limited to the Bluegrass region of Kentucky, but it remains a low
probability for these stronger winds to occur.

Short-term model consensus remains pretty well in line with our
going forecast of 45-55 mph gusts across the northern half of
Kentucky and into southern Indiana. No changes to the segmented Wind
Advisory at this time, but we`ll revisit by approx 10 PM EDT.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 341 PM EDT Thu Sep 26 2024

The focus through short term will be on Helene and the impacts it
will have on the region. The main impacts will be from heavy
rainfall that could lead to potential flooding across central and
southern KY and strong gusty winds during the day tomorrow. As of
writing this discussion, Helene had become a category 3 major
hurricane. It is still anticipated to make landfall along the FL
Panhandle later tonight before quickly tracking northward into the
Tennessee and Ohio River Valleys some time tomorrow morning.

A north-south oriented quasi-stationary boundary just to our east
across eastern KY has been the main focus for light rain and cloud
cover today. The cloud shield has been slow to progress westward and
where we`ve seen mainly clear skies, generally west of I-65, those
temperatures were in the upper 70s to near 80 while locations under
the cloud cover were in the low 70s to upper 60s. Thanks to the
sunshine SBCAPE values were between 500-1000 J/Kg and we`ve seen a
few isolated showers develop across parts of Ohio and Butler
counties. Can`t rule out a few more isolated showers/storms to form
in this area this evening.

As we go overnight and Helene makes landfall sometime around 04z the
clouds and the aforementioned boundary will start to progress north
and westward as Helene races quickly northward through GA overnight
and into the Tennessee Valley by daybreak. As it moves northward,
Helene will gradually weaken bringing with it abundant amounts of
moisture and strong gusty winds into the area for the day on Friday.
Rain coverage is expected to increase from the east-southeast
overnight as the first of the heavy rain bands arrives in the pre-
dawn hours. Rain will then continue through out the day as various
heavy bands work across the state as the center of Helene works into
TN and KY tomorrow morning before slowing and stalling out over the
region. There remains some questions on exactly how far north the
center of Helene will get as it works into the region. There remains
some discrepancies between the forecast models and even the National
Hurricane Center on the placement of the center of circulation. This
matters because depending on how deep the low pressure center is and
where it is located could determine where we see the highest and
strongest swath of gusty winds. Because of this uncertainty, we
decided to go with a Wind Advisory across our entire CWA for the day
tomorrow. Winds will likely be at their peak from late morning to
early afternoon, with the strongest winds working in from the east
across the Bluegrass and then towards north central KY and into
Southern IN. Wind gusts in this area could be between 45 to 55 mph.
This is definitely a high end Wind Advisory and if some of the
forecast trends continue, it is possible this will need to be
upgraded to a High Wind Warning with future packages. Given the
uncertainty of Helene`s strength at landfall and its path into the
Tennessee and Ohio Valleys tomorrow, and after good collaboration
with our surrounding forecast offices decided to go with the Wind
Advisory. To the south and southeast, wind gusts will likely remain
between 35 to 45 mph from late morning into the afternoon, so an
upgrade to a High Wind Warning for this area seems less likely. Keep
in mind these kind of gusts can cause scattered power outages, make
driving difficult especially for high profile vehicles and even blow
over trees or large limbs.

A flood watch has also been issued for central and southern KY for
tonight into Saturday morning. PWAT values of over 2.00" will advect
in ahead of Helene tonight and through the day tomorrow. This will
allow for very efficient rainfall from any bands that work across
the central KY and southern IN. Rainfall totals from tonight through
the weekend range between 3 to around 5 and a half inches of
rainfall with locally higher amounts possible. While there are many
places across central KY that could use the rain, locations further
south have benefited from recent rain and storms and their current
flood potential is higher. We will be under a Slight Risk for
Excessive Rainfall through most of the weekend as Helene meanders
over the region. After collaborating with neighboring forecast
offices, decided to issue the flood watch for counties mainly to our
south and locations where they can handle more rain, they were left
out for the time being.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 341 PM EDT Thu Sep 26 2024

Friday Night - Weekend...

The surface circulation of former Hurricane Helene should be located
just west of Louisville at the start of the Long Term period. The
cyclone will gradually spin down as it wanders around Kentucky
through Sunday as it sits well south of the main upper storm track
across southern Canada. Winds and rainfall amounts will gradually
lessen as well...with wind speeds down to the 10-15 mph by daylight
Saturday. Rainfall amounts Friday night through Sunday should be in
the 1-2" range. Depending on how much rain we get, and where exactly
the heaviest falls on Friday, these weekend rainfall amounts may
lead to minor river flooding, particularly from Mammoth Cave
downstream.

First few days of October...

Proceeding into next week, a weak cold front will pass through the
region on Tuesday as we move into October. Only moderate moisture is
associated with this system, resulting in not much more than widely
scattered light rain showers.

Wednesday and Thursday are forecast to be dry as high pressure
advances from the Midwest and Great Lakes to New England.

Temperatures next week will be seasonable with highs generally in
the 70s...possibly approaching 80 on Tuesday ahead of the cold
front. The coolest night should be Wednesday night with lows in the
upper 40s and lower 50s.

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 140 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.
- Low confidence in LLWS this morning.

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
approaches the area. As mentioned in previous updates, winds will
rapidly increase this morning and peak during the afternoon hours.
Strongest wind gusts (above 40+ knots) are likely along the northern
terminals (including LEX and SDF) with widespread values in the 25-
to 35-knot range. By tonight, winds will diminish some but still
continue gusting about 20 to 30 knots with a marked direction change
to the east and south given the low pressure track across southern
Kentucky.

Low confidence in LLWS as it may, or may not, technically meet
thresholds. However, a certain amount of speed shear is implicit in
the showery weather and increasingly gusty winds ahead of the
incoming LLJ.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...Wind Advisory from 7 AM EDT /6 AM CDT/ this morning to 2 AM
     EDT /1 AM CDT/ Saturday for KYZ023>043-045>049-053>057-
     061>067-070>078-081-082.
     Flood Watch through Saturday morning for KYZ023-024-026>028-
     045>048-053>057-061>067-070>078-081-082.
IN...Wind Advisory from 7 AM EDT /6 AM CDT/ this morning to 2 AM
     EDT /1 AM CDT/ Saturday for INZ076>079-083-084-089>092.

&&

$$

UPDATE...RAS
SHORT TERM...BTN
LONG TERM...13
AVIATION...ALL