749
FXUS63 KLMK 160517
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
117 AM EDT Tue Jul 16 2024

.KEY MESSAGES...

*   Very warm and muggy conditions overnight provide little relief.

*   Triple-digit heat index values Tuesday.

*   Showers and storms return mid week as a cold front pushes
    through the region.

*   Cooler temps in the 80s for Thursday into Saturday.

&&

.UPDATE...
Issued at 903 PM EDT Mon Jul 15 2024

An exceedingly warm and very muggy night is in store for the Ohio
Valley, as earlier storms over southeast Indiana are dissipating
before they reach the river. Strong to severe convection over
eastern Iowa and northern Illinois will continue organizing into a
MCS and eventually dive southward. At this time, would not expect it
to reach our southern Indiana counties, where we barely have a 20%
chance for precip to cover that possibility. If anything we`re more
likely to see some outflow as far south as Jasper, Salem, and
Madison, but by that point it should be outrunning any convection.

Oppressive heat and humidity will continue on Tuesday, with
different portions of the area teetering on Heat Advisory thresholds
in their own way. In the Bluegrass region, where rainfall deficits
are near 4 inches since the beginning of June, it will be driven
mainly by temperatures in the upper 90s, not that dewpoints near 70
are any picnic. Over in southern Indiana where rain has been more
abundant, it will be driven by the humidity with dewpoints in the
upper 70s to make highs in the lower 90s feel like triple digits.

Much of the area will see heat index values very close to 105
degrees on Tuesday afternoon. Even where we don`t make it, this will
be the third consecutive day of triple digit heat indices with very
limited overnight relief. Therefore we will expand the Heat Advisory
into the remainder of central Kentucky. There is some bust
potential, mainly focused in southern Indiana, if debris cloudiness
from overnight convection to our north is thicker or more
persistent.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 325 PM EDT Mon Jul 15 2024

Scattered cu field across the region this afternoon is a result of
efficient daytime heating. Temps have warmed into the low to mid
90s, with heat index values ranging between 98-104 degrees. Dry
weather continues this afternoon, and should see the cu field
diminish as daytime heating is lost around sunset.

For tonight, dry weather will continue, but will be keeping eyes on
another MCS racing through Chicagoland and then diving
southeastward. A mid-level shortwave passing over the Great Lakes
will be pivoting around the parent upper low over Canada, which will
be the driving force for a cold front pushing the convection into
the Ohio Valley. Ahead of the front, low-level jetting will ramp up
moisture transport well to our north, and WAA even this far south.
Temps overnight will not provide much relief from the daytime heat,
especially in the urban areas. Lows will range in the 70s, but SDF
could remain closer to 80 overnight. The convection will be
weakening as it approaches our forecast area, largely in part due to
the nocturnal inversion still in place and keeping our low levels
stable. Some elevated instability will remain as indicated by model
soundings, so some thunder mention in the forecast will remain. Hi-
res models continue to struggle with the evolution of this MCS, so
confidence on how far south the precip will go remains in question.
Will draw in a 20% PoP to cover the possibility mainly north of the
Ohio River for the morning hours.

Any morning precip will likely lead to more humid dewpoints for the
afternoon. However, any lingering cloud cover from the morning could
slow down temps slightly. Still believe tomorrow will be a hot day,
with temps in the 90s, and dewpoints in the 70s. It appears that
temps might be a few degrees lower than today, but the dewpoints
will be higher. With it being the third day in a row for triple
digit heat indices, and the warm morning lows, the heat advisory
will continue for tomorrow. There`s a chance some afternoon
convective development may spark ahead of the front, which could
linger into the end of the period. Weak/Marginal deep layer shear
will limit any sort of severe threat, but gusty winds in downburst
along with heavy rainfall are likely in the strongest storms.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 325 PM EDT Mon Jul 15 2024

Showers and thunderstorms will be in progress at the start of the
long term as a cold front sweeps through the region. 560dm 5H low
will move E through the Great Lakes while at the surface an
elongated cold front stretching from Quebec into the Central Plains
slowly drops through the OH Valley. Moisture pooling just ahead of
and along the frontal boundary will provide enough fuel for
efficient rainers, with PWATs increasing to above 2 inches which is
reaching daily max values according to SPC sounding climatology.
Storm vectors closely parallel the boundary as well, leading to
training storm potential and flooding concerns. Highest QPF is
forecast to occur from 17/00Z thru 17/18Z with lingering rainfall
continuing into Thursday morning. ECMWF Extreme Forecast Index (EFI)
does highlight an area of higher than normal rainfall amounts among
ensemble members roughly along the OH River, which is coincident
with our highest QPF amounts. Areas north of the parkways can expect
to see an inch to around 2.25", while those south of the parkways
can expect less than an inch. With that said, WPC has included
portions of north central KY and southern Indiana within a Day 2
(16/12Z - 17/12Z) Slight Risk for Excessive Rainfall and a Day 3
(17/12Z - 18/12Z) Marginal Risk.

Environmental parameters regarding severe weather are meager for
Wednesday, likely due to the bulk of the activity passing during a
diurnal minima. CAPE values will range from 500-1500 J/kg by
Wednesday afternoon with most of the impressive shear remaining far
to our north closer to the stronger jet stream dynamics. DCAPE
values do increase during the day Wednesday, which lends to the
potential for stronger gusts associated with convection. As such,
SPC has included our region within a Day 2 (16/12Z - 17/12Z)
Marginal Risk, which is mainly wind driven.

Expect reprieve from the heat in the post-frontal airmass as daily
max temperatures drop into the 80s from Thursday through the
weekend, slightly increasing each day. Worthy of greater rejoicing
is the concurrent drop in dew points. The aforementioned cold front
is expected to stall over the northern Gulf States before meandering
northward into the TN Valley, bringing a return of shower and tstorm
chances back to the southern half of our CWA Saturday and Sunday.

Worth of mention is a shortwave that drops out of central Canada
Friday, which models show undergoing lee cyclogenesis before
becoming detached from upper jet support. This cut-off low will lead
to below normal temperatures across much of the central U.S. as
highlighted in CPC`s 6-10 Day Temperature Outlook.

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 114 AM EDT Tue Jul 16 2024

Very low confidence forecast ahead. A large line of weakening
showers/storms is currently pushing southeastward from central
IL/IN. This line, if it holds its current trajectory, could
impact northern portions of the forecast area shortly after 07-
08z. Models have struggled to capture the extent and area of
this line, so their resulting solutions several hours from now
will likely be incorrect. Though the line has shown a weakening
trend over the last couple of hours, an outflow boundary ahead
of the precipitation could spark a few showers to develop in
northern portions of the forecast area, so have included VCSH
mentions at SDF/LEX.

The rest of the forecast period will be highly dependent on how the
morning activity plays out. Have generally run with dry conditions
after 14/15z as we`ll likely see subsidence aloft in the wake of the
first wave for a while, but can`t rule out some scattered storms
developing by late afternoon hours (coverage doesn`t seem to justify
SHRA/TSRA mention in TAF at this time). The better chances for
showers/storms will come tonight as a frontal boundary sinks into
the region.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...Heat Advisory until 9 PM EDT /8 PM CDT/ this evening for
     KYZ023>043-045>049-053>057-061>067-070>078-081-082.
IN...Heat Advisory until 9 PM EDT /8 PM CDT/ this evening for
     INZ076>079-083-084-089>092.

&&

$$

UPDATE...RAS
SHORT TERM...CJP
LONG TERM...CRG
AVIATION...DM