862
FXUS63 KLMK 210141
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
941 PM EDT Fri Sep 20 2024

.KEY MESSAGES...

*  Above normal temperatures continue this weekend

*  Slight chance (20 percent) for showers and storms this evening
   and overnight for southern Indiana and the Ohio River vicinity

*  Slight chance continues for central Kentucky during the day
   Saturday as a weak frontal boundary stalls over that region

*  More widespread precipitation chances arrive early next week
   ahead of a cold front.

&&

.UPDATE...
Issued at 941 PM EDT Fri Sep 20 2024

Weak frontal boundary to our north continues to serve as a focus for
convective development this evening.  Much of the activity has
remained well north of the the CWA.  Have been monitoring a couple
of clusters of storms.  The first is in the Bloomington, IN area
with a secondary cluster out west of Vincennes, IN.  This activity
is moving southeast and may hold together over the next few hours
and eventually get into perhaps portions of Dubois/Orange county
after midnight.

The boundary to the north is forecast to drop southeastward tonight
and may produce some additional convection across southern IN and
into portions of north-central KY after midnight.  Latest HRRR runs
continue to suggest this scenario, though overall coverage still
looks to be 20% or less.  Severe weather is not expected with this
activity as the boundary layer will continue to stabilize.  However,
PWATs ware up above 1.5 inches to around 1.75 inches so locally
heavy rainfall will be the main weather hazard.  Given the dry
conditions, this should not cause any hydro issues, in fact, this
would be quite welcome rainfall.  Current forecast has all of this
well in hand.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 323 PM EDT Fri Sep 20 2024

Weak cold front is generating shower/storm activity over northern
Indiana this hour, with a notable gap of surface-based cloud cover
over central Indiana. This front will accelerate some southward this
evening, as storm-generated cooler air pushes it. However, as those
storms weaken approaching the Ohio Valley, that push will end and
expect a stalled boundary to set up somewhere along the Ohio River
Valley late tonight and through Saturday. This boundary should serve
as the focus for isolated to widely scattered convection.
Precipitable waters will be well above normal, in the 1.5-1.7" range
in this vicinity. Given potential for slower motions along the
boundary, heavy rain is a concern, but falling over short-range
drought stricken areas will alleviate that concern somewhat.

Added cloud cover should help out with temperatures tomorrow, but
still expecting above normal readings, both for morning lows thanks
to that moist airmass and afternoon highs, still in the upper 80s to
around 90. Given the added moisture, heat indices actually may end
up a few degrees warmer than today...so something to be mindful of
with any outdoor weekend activities.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 323 PM EDT Fri Sep 20 2024

Saturday Night - Tuesday...

As we head into the second half of this weekend, a pattern shift
will begin to take place that will bring more active weather to the
Ohio Valley for the first half of next week. A northern stream upper
trough ejecting from Manitoba toward Hudson Bay will bring a surface
cold front into the upper Midwest, with the front expected to hang
north and west of the region at least through the day on Sunday.
Increasing low-level moisture ahead of the front will aid in the
development of showers and thunderstorms Saturday night into Sunday
over central IL and IN, with some of this activity spilling into our
area on Sunday. Confidence in temperatures will decrease as the
front approaches, as uncertainty related to the existence of rain
and clouds leads to increased spread in guidance. Expect a general
northwest-southeast gradient in temperatures on Sunday, as more
cloud cover/rain will limit warming along and north of the Ohio
River. Across eastern and south central KY, another day of well-
above normal temperatures is likely, with highs in the upper 80s and
low 90s.

For the beginning of next week, a southern stream open-wave trough
over the central Rockies is expected to eject eastward toward the
Mississippi Valley. As the system pushes eastward, another sfc wave
is expected to develop along an elongated baroclinic zone located
just northwest of the region. This second wave will bring more
widespread shower and storm chances Monday into Tuesday as the front
and upper wave gradually push east. Low-level moisture just ahead of
the front will increase instability, with 12Z GFS showing 1000-1500
J/kg SBCAPE across southern IN and north central KY Monday
afternoon. Mid-level flow will be modestly strong, allowing for wind
shear that will be marginally supportive for organized convection.
Can`t rule out a few strong storms Monday afternoon and evening,
though machine learning and other severe proxy guidance are not
particularly concerning at this time.

The unsettled pattern will continue into Tuesday as the front
lingers in the area. Temperatures should continue a downward trend,
though ensemble members and NBM probabilistic guidance continue to
show higher levels of spread, likely due to uncertainty in
prevalence of clouds and precipitation.

Mid-to-Late Next Week...

Model divergence in the synoptic pattern increases considerably from
Tuesday through the end of next week, leading to a wide range in
potential impacts. WPC ensemble cluster analysis shows two general
families of solutions late next week.

The first solution, represented in general by the 00Z (and to a
lesser extent, the 12Z) operational ECMWF, has a farther eastward
progression of the aforementioned upper trough by the middle of next
week, with the trough axis centered near the lee of the Appalachians
for the second half of next week. This solution brings the drier
post-frontal air mass all the way into the region, leading to
seasonable temperatures and relatively uneventful weather for mid-
late next week.

The second solution, represented by the 12Z operational GFS, has the
upper trough hang back to the west of the Mississippi Valley,
keeping our area in a humid and unsettled air mass on the east side
of the trough. This solution would lead to continued rain chances
through most if not all of next week, and would be much more
beneficial for ameliorating local drought concerns. Unfortunately,
this solution would increase the potential for tropical activity as
a disturbance which is expected to develop near the Yucatan
Peninsula early next week gets pulled into the central Gulf of
Mexico (and potentially farther north toward our area).

These two solutions have roughly equal percentages of ensemble
support (the first solution is slightly greater) in the 00Z ensemble
guidance, and forecast confidence in any one solution remains low,
with the current forecast sort of splitting the difference between
the two. Stay tuned as significant changes to the forecast for next
week may be required as subsequent model guidance comes in later
this weekend.

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 714 PM EDT Fri Sep 20 2024

VFR conditions are expected through the upcoming TAF period.  Look
for southwest to west-southwest winds this evening and these should
slacken off after sunset.  Will continue to watch storms to the
northwest of the region to see if they can make it down into
KHNB/KSDF by late evening.  Current indications suggest this is a
low probability scenario, so will continue to leave TAF`s dry with
this package.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...Air Quality Alert until midnight EDT tonight for KYZ029>031.
IN...Air Quality Alert until midnight EDT tonight for INZ077>079-
     090>092.

&&

$$

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