266
FXUS63 KLMK 041201
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
801 AM EDT Sat May 4 2024

.KEY MESSAGES...

*   Isolated to scattered showers and thunderstorms possible this
    afternoon. Slow moving storm motion and moderate to heavy rain
    expected with any storm.

*   Isolated showers and thunderstorms are expected on Sunday before
    widespread rain chances return Sunday night into Monday.

*   Increasing potential for multiple rounds of strong to severe
    thunderstorms next Tuesday through Thursday, though confidence
    in details is still low.

*   Elevated flooding potential will exist where multiple rounds
    of heavy rain from strong storms occurs over the next week.

&&

.UPDATE...
Issued at 801 AM EDT Sat May 4 2024

Hiding under the mask of high clouds moving across the region, we
have some lower-topped storms developing in a couple of patches this
region, one nearing the KCVG terminal and another across south
central KY.  Have bumped up thunder chances and pops in general this
morning to account for current radar trends. SPC mesoanalysis does
show a corridor of MUCAPE running from south central KY NE to the
Bluegrass. This axis has been weakening over the past few hours, so
would not be surprised to see thunder chances take a brief pause
before returning later this afternoon with some heating. Updated
forecast already out.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 331 AM EDT Sat May 4 2024

========== This Morning ==========

A weak, stalled frontal boundary remains draped across portions of
the region this morning. Temperature/Moisture spreads on either side
of the boundary remain diffuse, but weak low level convergence has
aided in sporadic shower development early this morning. ACARS from
SDF have indicated poor low/mid level lapse rates, so thunderstorms
are highly unlikely in most of this activity.

While it`s been slow to develop, stratus is expected to become
gradually widespread toward dawn along with some patchy fog. These
clouds will gradually lift after sunrise and transition to more of a
strato-cu by the afternoon. Should the stratus not be as widespread
as anticipated by dawn... dense fog would then become an issue this
morning in areas of clearing as winds will be light and soils/ground
very moist from recent rains. Will continue to evaluate the
cloud/model trends going through the morning hours but something to
monitor closely.


========== This Afternoon ==========

Surface heating will begin to erode a weak low level cap so that by
early afternoon we should be CIN-free across the region. Atmosphere
will turn more unstable as models indicate 1,000-1,500 J/KG of
MLCAPE to support thunderstorm potential, though deep layer shear
will be very weak so storms that do develop today shouldn`t pose
much of a severe/organized threat. Coverage of storms looks to be
mainly limited by a lack of a defined lifting mechanism as the
frontal boundary currently in place this morning will become diffuse
and `washed` out by the afternoon. Convection that develops today
will generally be along mesoscale/differential heating boundaries,
and most of the CAMs point to those being east of the I-65 corridor.
As such, generally carrying 20-30% chances for rain/storms west of
the I-65 corridor, with 30-50% chances east of the I-65 corridor.

Given weak deep layer shear, storm motions will be on the slower
side today (~10mph), and forecast soundings and environmental
parameters support heavy rain in thunderstorms today. Areas that see
training (repeated) storms over a short timeframe could have some
localized flood issues.

========== Tonight ==========

Showers/Storms from the afternoon should steadily decrease overnight
as a weak nocturnal inversion builds in. Most rain chances drop
considerably after midnight. Should be another mild night as
dewpoints remain elevated and lows by Sunday morning hover in the
low/mid 60s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Issued at 331 AM EDT Sat May 4 2024

Synopsis...

At the beginning of the extended forecast period on Sunday, two
areas of upper level troughing are expected across North America.
First, a lower amplitude trough axis extending from Hudson Bay down
into the upper Great Lakes will quickly eject eastward toward the
Canadian Maritimes during the first half of next week. A sfc cold
front will approach the Ohio Valley from the west on Sunday but is
expected to dissipate as it loses support from the progressive upper
trough. In the wake of the first cold front/upper trough, a subtle
mid-level disturbance is expected to develop across the southern
Plains on Sunday before moving northeast into the mid-Mississippi
and lower Ohio Valleys Sunday night into Monday. Ahead of this
feature, low-mid level moisture return is expected across the Ohio
and TN Valleys, supporting enhanced chances for rainfall as this
wave crosses the region late Sunday night into Monday.

The more impactful large-scale feature in the long term will be an
amplified upper trough which is expected to eject eastward from the
intermountain west into the central and eastern CONUS during the
middle of next week. With the trough progged to broaden considerably
in medium-range ensemble guidance, its eastward transition will be
fairly slow, leaving our region in deep-layer SW flow as embedded
shortwaves transit from SW to NE over much of next week. This will
allow for multiple chances of showers and thunderstorms, as support
from upper-level shortwaves will combine with a moist, unstable air
mass south and east of a broad baroclinic zone. By the end of next
week into early next weekend, the large-scale trough is expected to
settle across the eastern half of North America, bringing a
reduction in instability, and by extension, shower and thunderstorm
chances across our region.

Sunday through Monday Night...

As mentioned above, a decaying cold front will approach on Sunday,
which will primarily be identified by a low-level moisture gradient,
as drier air tries to push in behind the front from the north and
west. With the front expected to hang up/wash out over our area, low-
level moisture will still remain elevated, with HREF mean dewpoints
Sunday afternoon in the mid 60s. This will allow for 1000-1500 J/kg
SBCAPE to develop (HREF 25th-75th percentile), which when combined
with the subtle forcing from the front, should be enough to initiate
isolated to scattered showers and storms Sunday afternoon. Current
PoPs range from 30-50%, but this may need to be adjusted downward if
subsequent hi-res model runs increases confidence in more isolated
convection. Coverage of showers and storms should decrease initially
after sunset Sunday as instability decreases.

Sunday night into Monday, increasing southerly flow and moisture
advection downstream of the approaching mid-level shortwave will
support increasing coverage of showers and a few embedded
thunderstorms. There is still some disagreement in the models on the
exact track of Monday`s system, although enhanced rain chances from
most solutions, with variations in track mainly affecting
precipitation totals. The severe potential on Monday looks to be
limited, as a deep warm and moist layer creates sounding profiles
which more closely resemble the "tall, skinny CAPE" paradigm, which
is generally unsupportive of severe convection. By Monday night,
precipitation should begin to clear the region to the east, assisted
along as the deeper trough over the western CONUS pushes its way
into the central CONUS. Monday night into Tuesday morning should see
a relative lull in precipitation coverage and intensity as brief
ridging aloft moves across the Ohio/TN Valleys.

Tuesday through Late Next Week...

Confidence continues to increase in the potential for multiple waves
of strong to severe thunderstorms and flooding during the middle of
next week. While there is relatively good agreement on the large-
scale features during the middle of next week, there is lower
confidence in timing and expected intensity of individual waves of
showers and thunderstorms, which will be driven by transient
shortwaves ejecting along the southeastern flank of the larger
trough. Of particular note is continued elevated values of the CAPE
and CAPE-Shear fields within the ECMWF Extreme Forecast Index, with
values exceeding 0.8 in both fields from Tuesday through Thursday.
This suggests that there is good agreement within the ECMWF ensemble
members in elevated values of CAPE and wind shear during this time
period. Colorado State Machine Learning severe weather probabilities
show an area of greater than 30% chances of severe weather across
the mid-MS, OH, and TN valleys during this time period. In fact,
there is relatively good agreement in extended rage severe
convection proxies of multiple kinds (Neural Network/ML, analogs,
ensemble-derived) during the middle of next week.

While this whole Tuesday-Thursday time period exhibits elevated
concern, the expected combination of kinematics and thermodynamics
appears most concerning next Wednesday into Wednesday night.
Ensemble guidance generally shows a sfc low passing north of the I-
70 corridor, with destabilization taking place across central KY and
southern IN within the warm sector Wednesday afternoon. There are
certainly failure modes out there (residual effects from antecedent
convection, to name one), but taking model sounding progs at face
value, there would be ample instability and shear for storms to work
with, in addition to a well-defined forcing boundary in the form of
a descending sfc cold front. The combination of multiple rounds of
showers and thunderstorms over the next 3-6 days will mean that
river levels as well as areal flooding will need to be monitored,
especially the further we head into next week.

By the end of next week, the large-scale trough is favored to sink
southward across the eastern CONUS, with low-level NW flow helping
to clear out the moist and unstable air mass. However, exactly how
quickly this transition takes place is a source of lingering model
disagreement, so we`ll continue to leave PoPs in the forecast into
Friday. On the whole, a quieter period of weather is expected across
the region next weekend, as the post-frontal environment will have
limited instability and cooler temperatures.

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 629 AM EDT Sat May 4 2024

Complicated and low confidence forecast for the TAF period. MVFR to
IFR cigs with localized LIFR conditions have developed this morning,
along with some patchy fog. While cigs will bounce around some
through the remainder morning hours, expect a gradual improvement
trend where by the early afternoon, all sites should be at VFR
cigs with 4-5kft strato-cu decks.

Isolated to scattered storms be possible in the afternoon, with the
highest chances for precipitation generally east of the I-65
corridor. For now, have VCTS mention at KLEX/KRGA, but PoPs
elsewhere are not high enough to warrant thunder mention. Forecast
will be monitored closely in the near term and thunder chances for
KSDF/KHNB/KBWG may be introduced in subsequent forecast updates if
confidence increases.

&&

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

&&

$$

UPDATE...RJS
SHORT TERM...DM
LONG TERM...CSG
AVIATION...DM