643
FXUS63 KLMK 220148
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
948 PM EDT Sat Sep 21 2024

.KEY MESSAGES...

*  Some patchy fog tonight.

*  Multiple waves of scattered showers and storms are expected
   Sunday afternoon through Tuesday night. There is a low, but
   non- zero chance for strong to severe storms Monday and
   Tuesday.

&&

.UPDATE...
Issued at 947 PM EDT Sat Sep 21 2024

Had a few isolated storms this evening that were able to grab on to
the limited shear and modest instability over the region. At this
time, most showers and storms have dissipated, except for a cluster
of storms over the Bowling Green area. This cluster is mostly
stationary and producing heavy rain. This cluster will dissipate
within the hour. The remainder of the overnight will feature
thinning skies and light winds. With recent precipitation, some fog
development in river valleys and the southeastern half of the
region. Fog will quickly dissipate as the sun rises. Low
temperatures are expected to be in the 60s over the Bluegrass and
upper 60s and low 70s elsewhere.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 333 PM EDT Sat Sep 21 2024

Things are pretty quiet across the CWA at this hour as morning
convection has pushed off to the south and east of the area. A
healthy cumulus field has developed in a moist and unstable
atmosphere (ML CAPE values 1500-2000 J/KG), however there isn`t much
in the way of a trigger for the remainder of the PM hours. As a
result, expecting a mostly quiet afternoon with hot temps peaking in
the upper 80s and low 90s.

We will have to watch our SW CWA around sunset as some hi-res
guidance suggests that some convection could redevelop. It appears
this would occur on outflow/differential heating coming upstream
from the MO Bootheel complex. Given that we need an additional
trigger this could be it, but only have low confidence of it
actually occurring. Will linger a 20 Pop down in that area to
account for the potential and continue to monitor. Should anything
develop, heavy rain and locally gusty winds would be a concern.
PWATs are in the 1.6 to 1.7 range, and DCAPE are quite high as well.
Add in weak steering flow, and locally heavy amounts can`t be ruled
out.

As we move into the overnight, the only real concern is fog
potential given the moist low level airmass and otherwise light/calm
winds. Data shows a decent amount of upper sky cover, although with
some breaks likely. As a result, could see some fog development,
especially across our S and SE where the surface front never quite
made it. Will keep the upper sky cover as the limiting factor for
more widespread/dense concerns, but can`t rule it out at this stage.

The upper ridge axis slides east tomorrow as the mid level flow
flattens in response to an ejecting shortwave and associated surface
low. Expecting the morning to be mostly dry, however we`ll likely be
staring down an upstream convective complex poised to enter our NW
CWA by late morning midday timeframe. This complex is expected to be
in a weakening state, and tough to tell exactly how far east the
better coverage of showers and storms will make it before
dissipation. Right, the highest pops are along and west of I-65, and
along and north of I-64, or in other words our NW CWA quadrant. The
best time frame for some showers and storms will be from mid to late
morning until mid afternoon. From there, the degree of re-
destabilization is in question, along with south and eastward extent
of precipitation, if much at all. Overall, highs will be cooler
across our north, with low 90s still more likely across southern KY.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Issued at 333 PM EDT Sat Sep 21 2024

Sunday Night through Tuesday Night...

There is high confidence in unsettled weather for the first half of
the upcoming week with multiple waves of showers and thunderstorms
likely Sunday night through Tuesday night. A disorganized frontal
boundary will stretch from central TX northeast to southern ON
Sunday evening, separating a cooler and drier air mass to the
northwest from a warm and muggy air mass which will be over our
region. An upper level shortwave trough over the central High Plains
on Sunday will eject eastward Monday into Tuesday, helping to
develop a wave along the remnant front and pushing the boundary
farther to the east by the middle of the week.

While the main area of forcing associated with the approaching mid-
and upper trough will still be a little west of the region on
Monday, a moderately unstable environment will be present, with some
medium-range guidance showing 1000-1500 J/kg SBCAPE Monday
afternoon. The amount of instability realized will be dependent on
earlier convection and cloud cover, so there is considerable
uncertainty in this amount of destabilization. If we are able to get
marginally unstable Monday afternoon, modest mid-level flow would
result in deep-layer shear that would be supportive for a few
strong storms. Gusty winds and small hail would be the primary
hazards with any stronger storm, in addition to heavy rain
and lightning. By Tuesday, the mid-level trough should be on our
doorstep, with stronger mid-level flow present across the area,
leading to greater deep-layer shear. However, uncertainty in
our ability to destabilize is even greater on Tuesday, as
differences in where the sfc front will be located further
complicate the forecast. If we are able to realize sufficient
instability, the potential for strong or severe storms may even be
slightly higher on Tuesday, though we will first have to see how
things evolve over the next few days. For what it`s worth, Colorado
State machine learning guidance does have a low end severe threat in
our region both Monday and Tuesday, while the ECMWF Extreme Forecast
Index shows somewhat unusual values of CAPE and CAPE-Shear parameter
during the same time period.

The exact temperature forecast remains uncertain for the early week
time period, though there is fairly good agreement that temperatures
will gradually trend downward as the sfc front creeps southeast.
Greater amounts of cloud cover and showers and storms will also keep
daytime temperatures suppressed, though breaks in
precipitation/clouds would allow for quick warming.

Wednesday through the rest of next week...

By Tuesday night into Wednesday morning, mid-level trough passage
should help push the sfc cold front through the region, bringing
drier air in from the west and north. However, it is looking
increasingly likely that the region will remain close to the
boundary into the mid- and late-week time period, with the humid air
mass located just south and east of the region. As a result, slight
changes in the evolution of the synoptic pattern will lead to
drastic changes in expected sensible weather, particularly as it
relates to precipitation chances from Wednesday onward.

There are still several camps of solutions in medium-to-long range
ensemble guidance which have roughly equal representation, so
forecast confidence still looks low for the middle and end of the
upcoming week. With a tropical system coming into the Gulf of Mexico
around this time period, interactions between the mid-latitude flow
pattern and this system could lead to impactful weather across the
region by late this week into next weekend, though confidence in
this is very low. Unfortunately, we remain in a "wait and see"
position at this time until models begin to converge on a particular
solution.

&&

.AVIATION /00Z TAFS THROUGH 06Z MONDAY/...
Issued at 812 PM EDT Sat Sep 21 2024

Showers and storms have nearly dissipated over the region. The
remainder of the evening and overnight will feature thinning skies
and light winds. In the early morning, some light fog development is
expected over BWG/RGA/LEX. Tomorrow morning, a line of showers is
expected to move into the region and weaken. Have PROB30 groups over
HNB in the morning and SDF/BWG in the early afternoon.

&&

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

&&

$$

UPDATE...SRM
SHORT TERM...BJS
LONG TERM...CSG
AVIATION...SRM