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000
FXUS63 KLMK 151729
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
129 PM EDT Mon Apr 15 2024

...Updated Aviation Discussion...

.KEY MESSAGES...

*   Isolated strong storms will be possible this afternoon. Main
    threat would be large hail and gusty winds in the strongest
    storms.

*   Showers and thunderstorms are likely Tuesday night through
    Wednesday night. The severe threat Tuesday night into Wednesday
    morning looks low, with the greater severe threat expected
    Wednesday late morning into the afternoon and evening.

*   Significant cool-down appears increasingly likely next
    weekend.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 415 AM EDT Mon Apr 15 2024

Synopsis…A low-level frontal boundary will slowly sink southward
across the Ohio Valley today as mid-level ridging moves in from the
Central US. As a result, a warm and moist environment will exist for
strong to marginally severe storms to proliferate along and south of
the stalling boundary. Then, mid-level shortwave/upper low
progressing from the Desert Southwest to the Mid-Mississippi Valley
will start to lift the front northward reducing the precipitation
chances overnight.

Model Evaluation/Confidence…Overall, there is high confidence in
isolated to widely scattered thunderstorm chances for late this
afternoon and early evening based on the good consistency and model
agreement hi-res guidance (HREF and RRFS). As for severe weather
chances, the CSU ML algorithm has been highlighting a low chance of
severe hail and wind threat around the position of the front with
similar, if not lower, chances in the Nadocast ML output.
Accordingly, SPC has included most of the forecast area in a
Marginal risk of severe weather.

Today…Current satellite imagery shows clear skies across the lower
Ohio Valley with surface observations and RGB Airmass satellite
product depicting the frontal boundary moving from northern to
central Indiana. During the early morning hours, the front will keep
moving closer to the Ohio Valley while quiet and mostly clear
conditions continue in the forecast area. Rapid boundary layer
warming due to unblocked solar radiation will foster cumulus
development ahead of the front by midmorning with moderate surface-
based instability quickly building amid a 850-700 mb capping
inversion. Dry weather should continue during the first half of the
afternoon as the surface front extends around the Ohio River and mid-
level subsidence starts to develop a second inversion around 500-mb
which will reduce a bit the ongoing impressive mid-level lapse
rates. Even with a scattered cumulus cloud layer, highs might
overachieve given SW low-level winds and positive upper height
anomalies; therefore, raised afternoon temperatures by 2 or 3
degrees.

Then, active weather will set in during the second half of the
afternoon and early evening as convective temperatures are easily
achieved and SW winds enhance moisture convergence along and ahead
the frontal boundary. Other mechanisms for convective initiation
might include outflow boundary interactions and differential heating
boundaries amid a thick upper cloud layer moving in from Arkansas
and Tennessee. Based on CAM models, storm initiation should easily
break through the low-level capping inversion realizing MUCAPE
around 2000 J/kg or higher. Forecast soundings indicate dry mid-
level air entrainment, unusually steep mid-level lapse rates (above
8 degC/km), and enough effective bulk shear (above 25 knots) to
highlight an isolated large hail threat, especially if splitting
supercells develop based on the straight hodograph profile and
marginally favorable upper-level winds. That being said, the main
convective mode should be multicell clusters with sub-severe hail
and gusty winds. It is important to mention that convective coverage
is low confidence at the moment as the position of the front and
resultant storm initiation varies across models.

Tonight…Showers and isolated sub-severe storms will continue for a
few hours after sunset, but overall the marginal severe risk
(intensity and coverage) will greatly diminish shortly after daytime
heating is lost. In addition, the effective frontal boundary will
slowly move northward overnight as a warm front as the mid-level
shortwave/upper low approaches the region..

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 351 AM EDT Mon Apr 15 2024

Tuesday through Wednesday Night...

Precipitation may be ongoing over northern portions of the region
Tuesday morning as a frontal boundary lifts into the upper Ohio
Valley, but dry conditions should persist the remainder of the day
in the wake of that precipitation. The next upper level low/system
to impact us will be on the horizon and we`ll likely get a one-two
punch from it swings into the Midwest.

The first wave of showers/storms from that system will arrive late
Tuesday night. This precipitation will develop first across the
Missouri Valley into the Mid-South ahead of a cold front and
gradually slide eastward. These storms will initiate in a plume of
rich low level moisture and modest instability in a sheared
environment supportive of organized convection, but as it
transitions eastward, it will out-pace the better low level moisture
and instability (while still remaining in a highly sheared
environment). Most guidance points to the storms gradually weakening
with eastward extent across western and central Kentucky/Indiana due
to the loss of instability, and this should keep the severe threat
low/isolated for our region and mainly west of I-65.

The cold front over the Plains/Midwest will begin to approach our
region on Wednesday, and its along and ahead of it that we`ll see
our second (and more concerning) wave of showers/storms develop. By
Wednesday late morning into the afternoon, the aforementioned plume
of low-level moisture will be over southern Indiana and central
Kentucky as our second round of precipitation develops. Shear will
be in place to support organized strong/severe convection. All
severe weather hazards (straight line wind, hail, tornadoes) will be
possible with severe convection. One important factor that will
influence storm intensity/severity is how quickly we clear out and
destabilize in the wake of the first round of showers/storms. Low
level moisture/dewpoint advection ahead of the front will contribute
to some destabilization... but lingering rain and/or clouds would
limit the amount of low/mid level lapse rate steepening that would
aid in higher amounts of instability. For what it`s worth, global
ensemble statistics from a combination of the Euro, GFS, and
Canadian advertise ~50% chance of a plume of +1,000J/KG SBCAPE
across portions of the region Wednesday afternoon. With strong low
and deep layer shear in place, this would easily support a severe
weather episode in the region should these numbers verify.

Storms will clear out from west to east Wednesday night as the cold
front advances eastward.

Thursday into the Weekend...

Weak ridging aloft builds in Thursday before flow transitions to
more of a quasi-zonal pattern for Friday/Saturday. A subtle wave
within the flow will pass through the area on Friday and likely
bring some clouds/rain to the region. Depending on how far north the
surface low tracks, portions of our region could be within its warm
sector so there may be a low-end storm risk. Otherwise, a fairly
strong cold front by mid/late April standards will come crashing
into the area Friday night into early Saturday. Some guidance shows
rain lingering behind the front, so those with outdoor plans may
want to monitor subsequent forecasts to see how this trends.

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 128 PM EDT Mon Apr 15 2024

Impacts/Confidence...
Slightly increased confidence in thunderstorm occurrence and general
timing window this evening at the northern TAF sites. Low confidence
in exact placement of widely scattered storm development.

Discussion...
A cold front was analyzed this afternoon from just
south of STL to just south of CMH. As this front sinks southward
into a moderately unstable atmosphere, widely scattered
thunderstorms will be possible across southern Indiana and north
central Kentucky. Any storms that do form will have the potential to
develop quickly and become severe. Will include VCTS at the northern
TAF sites for now, and AMD as cells begin to develop and a more
precise location/timing determination can be made.

Storms should wane in intensity and coverage towards midnight, with
dry conditions into the daylight hours Tuesday under VFR ceilings.

&&

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

&&

$$

SHORT TERM...ALL
LONG TERM...DM
AVIATION...13