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123
FXUS63 KLMK 231939
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
339 PM EDT Mon Sep 23 2024

.KEY MESSAGES...

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*  Waves of scattered showers and storms will continue this
   afternoon into tonight. A few strong storms will be possible,
   particularly along and south of the Western KY/Bluegrass Parkways.

*  Strong to severe storms look increasingly likely Tuesday,
   especially along and east of I-65. Gusty winds will be the main
   severe threat, with small hail a secondary threat.

*  Drought-easing rainfall in the forecast as the remnants of a
   tropical system move into the region Friday. Stay tuned to latest
   updates from NHC on the forecast path/strength of this system.
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&&

.SHORT TERM /THROUGH TUESDAY/...
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Issued at 339 PM EDT Mon Sep 23 2024

This afternoon and Tonight...

The remnants of a cold front remain draped across the region this
afternoon, with waves of showers and storms moving from west to east
along the boundary. Widespread coverage of clouds and precipitation
along and north of I-64 so far today has created a gradient in
heating and instability which splits the FA in two. North of this
gradient, stable conditions should suppress convection at least in
the near term, while greater instability across south central and
southeastern KY (up to 2000 J/kg MLCAPE) will promote additional
convective initiation over the next several hours. A few strong
storms have already developed, and a continued risk for gusty winds
and small hail will continue into the evening hours, primarily along
and south of the western KY/Bluegrass Parkways.

This evening into tonight, the 500 mb trough over the central Plains
will move toward the Mississippi Valley and amplify, strengthening
deep-layer flow over the Ohio and Tennessee valleys. In response to
the upper trough amplification, a LLJ core is expected to develop,
with 35-40 kt SW winds progged over western KY late tonight into
tomorrow morning. This SW LLJ should condition the atmosphere for
additional waves of showers and thunderstorms as PW values maintain
at around 1.6-1.9". Hi-res sounding progs show a stable layer near
the sfc with around 1000 J/kg MUCAPE aloft, so while the risk for
strong to severe storms will be low, thunderstorms with heavy
rainfall and lightning will continue to be possible overnight. With
the rich moist air mass in place, diurnal temperature ranges will
continue to be suppressed, with lows only expected to fall into the
mid 60s to around 70 tonight.

Tomorrow...

Tomorrow, the upper trough will continue to shift eastward, with jet
dynamics becoming more supportive for broad low-level rising motion
as an area of upper diffluence moves into the Ohio Valley. As the
coupled sfc/upper disturbance strengthens, a sfc low will develop
across MO, moving northeast just north of the Wabash Valley during
the day on Tuesday. The sfc front that is overhead today should be
well north of the area by late tomorrow morning, placing the area
within the warm unstable sector. At the same time, strong flow aloft
for this time of year will allow for 35-40 kt of deep-layer shear,
which will be supportive of organized clusters of convection. The
main uncertainty lies in how unstable we can get tomorrow,
especially if we have multiple waves of convection that move through
late tonight into tomorrow morning. As an example of this, 12Z HREF
mean SBCAPE tomorrow afternoon is around 1500 J/kg along and east of
I-65. However, 10th percentile SBCAPE is less than 200 J/kg, while
90th percentile SBCAPE is around 2500 J/kg. As a result, tomorrow`s
severe risk definitely has a significant bust potential, but also
has a relatively high ceiling as well. By the window of peak
instability tomorrow afternoon, surface winds should be fairly
veered across much of the area, so the main severe threat with any
strong storms would be gusty or damaging straight-line winds, with a
secondary threat being small hail. 12Z HRRR neural network
probabilities indicate the greatest threat for damaging winds east
of I-65 into eastern KY, with the peak timing between 18-22Z Tuesday
(2-6 pm EDT). Multiple waves of convection are possible, continuing
until sfc FROPA Tuesday evening. While antecedent conditions will
allow us to receive quite a bit of water without significant issues,
HREF localized probability-matched mean swaths of QPF greater than
2" do open up the possibility for localized flooding issues tomorrow
afternoon and evening.
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&&

.LONG TERM /TUESDAY NIGHT THROUGH MONDAY/...
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Issued at 339 PM EDT Mon Sep 23 2024

Busy week ahead as we deal with first a cutoff upper low
meandering across the mid-MS Valley and then what becomes of the
remnants of a well-modelled and still not named tropical system!

Tuesday evening, deterministic and ensemble guidance are in
excellent agreement, placing the center of a 567 dm 500-mb low over
northern Missouri. This location keeps our area in a southwesterly
flow aloft at least for that evening, with a moist environment still
in place. CAM`s respond by keeping in widely scattered storms,
mainly south of the I-64 corridor and east of I-65. As we get
through the day Wednesday, the upper low sinks into the AR/MO border
area, bringing some drier air briefly into our region. Cloud cover
that day still should help limit any available instability, so that
day stands the best chance to be low on pops and relatively cool
high temperatures (near normal for late September). By "low" on pops
I should say that still keeps in some likely-range chances (60-70%)
in our southeastern counties, whereas southern IN and northern KY
stay in the 20-30% range.

For Thursday we see a transition, as the upper low continues its
trek southward but interacts with the tropical system moving ashore
somewhere along the NE Gulf Coast. The GEFS spreads for locations of
the upper low are fairly high, whereas the Euro EPS and GEPS have
more confidence. The latter two ensemble solutions allow more
moisture to get pumped into our region, whereas the GEFS keeps more
of it over the Deep South. That obviously has a big impact on how
much precip we can get into our area, especially as we move into
Friday! Latest NBM guidance looks in line with the grand ensemble
data, both for the daytime period Thursday and getting into Thursday
night.

NBM PoP guidance looks pretty good for Friday/Friday night, given
the current NHC forecast for this systems track...again in line with
pops on the grand ensemble. A little concerned by 12Z GFS solution
showing our area getting gapped...with the remnant center hung up in
the Apalachians and the cutoff low much farther southwest, over the
Arklatex. Again, in line with NHC forecast and 00Z EPS/GEPS and 12Z
CMC/Euro solutions, think NBM guidance pops look pretty good. Will
make an adjustment to wind and wind gusts up from NBM, based on Euro
EPS guidance showing median peak gusts in the 30-40 mph range.
Higher values aren`t out of the question, but trending upward seems
a good bet here.

As far as hydro issues with the rains this week...areal average
rainfall totals for this week/weekend based on current forecasts
look to range from 2-4". This much rainfall on the drought grounds
we have currently would barely bring some of our low-flow rivers in
southern KY up to action stage. It would take 5-6" of rainfall to
get them barely into minor flood stage. So it would seem we can
handle (and we need it!) this much rainfall, but will message the
chance for localized flooding possible.

The weekend and start to next week are a bit more up in the air as
far as confidence, as a lot will depend on distance to the merged
cutoff upper low to our area. Still will hang onto fairly cloudy
skies and even solid chances (30-50%) for measurable rainfall each
day.

Given the moist conditions, expect smaller diurnal temperature
ranges each day this period...with high temperatures near normal and
lows above.
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&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 155 PM EDT Mon Sep 23 2024

A cold front continues to be draped across the region this
afternoon, and waves of precipitation are riding along this boundary
from west to east. The main area of precipitation will impact HNB
and SDF over the next few hours, with more scattered showers and
storms expected elsewhere. Confidence beyond this initial wave of
precipitation is quite low, so have only included TSRA mention where
there is moderately good confidence. While VFR conditions are
expected outside of any heavy rain/storms through much of the
forecast period, reduced ceilings and visibility would be possible
Tuesday morning around sunrise, though confidence is also low in
this. Winds will begin to pick up out of the south and southwest
Tuesday morning, with stronger gusts expected Tuesday afternoon.

&&

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

&&

$$

SHORT TERM...CSG
LONG TERM...RJS
AVIATION...CSG