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672
FXUS63 KLMK 041415
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1015 AM EDT Tue Jun 4 2024

.KEY MESSAGES...

*   Scattered storms will be possible today and tomorrow. A few
    strong to severe storms could produce heavy downpours and gusty
    winds.

*   Drier weather and cooler temperatures are favored late week into
    the first half of this coming weekend.

&&

.UPDATE...
-- Changed Discussion --
Issued at 1015 AM EDT Tue Jun 4 2024

Morning satellite imagery reveals mostly sunny skies over east-
central and eastern KY with mostly cloudy skies out in area west of
I-65.  Temperatures were in the upper 70s in the sunny areas with
lower 70s in the cloudy areas.  Area radars show a band of
convection lifting north-northeast across the Pennyrile area of KY.
This activity looks to be driven by isentropic lift and weak low-
level jetting, though a mid-level shortwave trough axis is lifting
toward the region.  This activity should continue north-northeast
through the morning and affect our far northwestern counties.

As the afternoon wears on, additional convection looks to fire in
the I-65 corridor and points east where stronger insolation will
take place allowing a marginally unstable atmosphere to develop.
MLCAPE values this afternoon will likely be in the 750-1500 J/kg
range, though bulk shear values will generally be 20kts or less.  As
alluded in the earlier discussion, lapse rates above 500 hPa look to
struggle, so the convective character this afternoon will likely be
low-centroid pulse type convection posing a gusty wind risk and
heavy rainfall risk.  The slow movement may result in some isolated
hydro issues.
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&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 334 AM EDT Tue Jun 4 2024

Things are quiet across the CWA at this hour. Convection from
earlier this evening has diminished, and the only thing worth
watching toward dawn would be whether any patchy fog is able to
develop. Variable upper cloudiness should help mitigate this for the
most part, but will continue to monitor through the early morning.

Focus shifts to an upstream disturbance from sunrise onward as
shower and storm chances are expected to increase ahead of the
slowly approaching feature. The upper ridge axis that kept us mostly
dry yesterday has now shifted east in response to a shortwave
approaching from the mid Mississippi River Valley. Forecast
soundings continue to show the atmosphere becoming modestly unstable
with increasing deep moisture (PWATs jumping to around 1.75" through
the column). HREF probabilities have around a 40-50% chance of ML
CAPE values exceeding 1000 J/KG, so think it is reasonable expect
peak values somewhere around the 750-1250 J/KG range this afternoon
and early evening. Forecast soundings show most of that positive
area between 850-500 mb where mid level lapse rates are steepest.
Above H5, the lapse rates are not impressive, so fully expect that
updrafts/cores aloft may begin to struggle a bit between 20 and 30
thousand feet. Another limiting factor to storm strength will be the
lack of deep layer speed shear. 0-6km shear values will only top
out around 20 knots, below what is needed for more organized
updrafts. As a result, mainly expecting poorly organized pulse
storm modes, or at best a few brief multicell clusters. Given
the strong positive area in the lowest half of the atmosphere,
do expect that an isolated wet microburst or two could develop
with the strongest storms. SPC Marginal Risk mainly along and
west of I-65 handles this pretty well, although not sure about
the 5% hail given the moist sounding and relatively poor lapse
rates above H5.

The more concerning threat from any showers and storms this
afternoon may end up being the potential for some isolated heavy
rain, or even a few instances of localized flash flooding. Given the
weak atmospheric flow, storm movement will not be all that fast. In
addition, moist soundings will promote efficient rainfall rates.
Soils are still pretty moist from recent rains, so FFG values are a
bit compromised in spots. The HREF 48 hour Max and PMM values aren`t
all that concerning, but do like the Marginal Risk for Excessive
Rainfall offered by WPC. Seems reasonable given that we have already
had a couple instances of isolated Flash Flooding the past two
evenings when it was less likely.

Heading into tonight, there is some uncertainty into whether
convection will persist, or die off with the loss of heating. Hi-res
models mostly kill off the convection after sunset like we saw this
past evening, however the lower resolution deterministic models
suggest that convection could continue overnight. This is likely in
response to some subtle low level jetting around 30 knots. Could
honestly see it going either way, but tending to lean toward the
less active scenario for now, which is why we have lowered pops to
just 20-30% for much of the overnight.

As far as temperatures go, looking for highs today pretty solidly
into the mid 80s once again, with warmer temps across the northern
half of the CWA, where clouds/precipitation will hold off the
longest. Tonight`s lows look to be mild in the upper 60s and low 70s.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 334 AM EDT Tue Jun 4 2024

Wednesday through Thursday...

A shortwave disturbance on the leading edge of a larger trough
descending across North America is expected to move across the lower
Great Lakes and Ohio Valley Wednesday into Wednesday night. The
trough will deliver somewhat of a glancing blow to the region as it
eventually lifts more northeast toward southern Ontario. An
associated surface cold front moving in from the Mississippi Valley
will weaken as it moves into central KY and southern IN Wednesday
afternoon; however, this front should serve as the main organizing
boundary for showers and thunderstorms Wednesday afternoon and
evening.

GFS/ECMWF/NAM solutions are pretty aggressive on widespread
precipitation in the moist, unstable air mass ahead of the front.
While LREF (GEFS/EPS/GEPS) ensemble mean CAPE values range from 1000-
1500 J/kg Wednesday afternoon, further analysis of sounding profiles
shows relatively unimpressive lapse rates from the sfc up to around
700 mb. This would suggest that precipitation may be more confined
to the vicinity of a triggering boundary (i.e., the cold front), and
might not be as widespread as global models indicate. Evidence of
this can be seen in the 00Z HRRR and NAM 3km runs, which show
limited precipitation coverage away from the frontal zone Wednesday
afternoon and evening. As a result, have lowered PoPs from NBM
starting point, which still leaves 70-80% PoPs across the region.
With the best shear profiles lifting north of the region with the
trough, shear will also be marginal, though it should be sufficient
that a few stronger storms cannot be ruled out. The main threats
with storms would be heavy rainfall (and localized flooding), as
well as strong wind gusts. Behind the first wave/sfc front, medium-
range guidance generally depicts a corridor of deeper dry air which
should help to end precipitation chances relatively quickly
Wednesday night.

For the day on Thursday, medium range guidance is in relatively good
agreement in depicting broad troughing sinking southward across the
eastern CONUS. It will be worth watching to see if any moisture can
overspread the region before this next wave sinks across the region;
if so, this secondary wave could provide enough support (along with
a secondary sfc cold front) to initiate convection Thursday
afternoon. Right now, this appears to be a remote possibility within
ensemble solutions, so we`ll run with a dry forecast on Thursday for
now.

Thursday Night through Saturday Night...

The highest forecast confidence in dry weather is expected Thursday
night through the day on Saturday as both the GEFS and EPS ensemble
families show an anomalously dry air mass settling across the Ohio
and Tennessee valleys during this period. For reference, both the
NAEFS (GEFS/GEPS) and EPS ensemble mean PW values are around the
10th percentile of model climatology during this period. Cooler
temperatures are also expected as NW flow brings a (modified)
Canadian air mass down into the Ohio Valley. Highs in the upper 70s
to around 80 and lows in the mid-to-upper 50s combined with
dewpoints in the 50s should make things feel quite pleasant for June
in central KY and southern IN.

Sunday into early next week...

Forecast confidence decreases for the second half of the upcoming
weekend into the beginning of next week, with model solutions
diverging somewhat with the evolution of the upper trough located
across the eastern CONUS. While both the GEFS and EPS ensemble
families show the trough lingering into early next week, subtle
variations in positioning lead to significant differences in
precipitation chances, particularly on Sunday. The GEFS generally
favors a wetter solution, allowing moisture return into the Ohio
Valley Saturday night into Sunday ahead of another upper level
shortwave and sfc cold front. On the other hand, the EPS leans
toward a more suppressed/drier solution, shoving moisture farther
into the Tennessee Valley and points south while areas along and
north of the Ohio River remain dry. For now, will lean slightly
toward the drier solution, while keeping a 20-30% PoP in for Sunday
and Monday. Severe weather is generally not expected during this
time period, with CSU ML guidance signals remaining unimpressive at
this point.

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 638 AM EDT Tue Jun 4 2024

A mid level disturbance will approach later today, and will bring
scattered showers and thunderstorms with it. BWG will be the first
to have shower and storm chances from now until late afternoon, and
SDF/LEX will be more afternoon into the evening hours. Any shower or
storm will be capable of reducing visibilities briefly below 3 SM.
Outside of that, look for VFR cig/vis outside of any showers/storms
and look for a steady SSW wind around 10 mph.

&&

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

&&

$$

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