245
FXUS63 KLMK 202351
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
751 PM EDT Mon May 20 2024

...Updated Aviation Discussion...

.KEY MESSAGES...

*   Dry weather and above normal temperatures continue on Tuesday.

*   Unsettled weather returns Wednesday, with on and off chances for
    showers and storms through the weekend.

*   Strong to severe storms possible Wednesday afternoon into
    Wednesday night, with an increased risk for localized flooding
    Thursday through Saturday.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 327 PM EDT Mon May 20 2024

This afternoon, the surface high pressure axis is moving east of the
Appalachian Mountains, allowing weak anticyclonic S/SW flow to
emerge across central KY and southern IN. Another warm day is
underway, as temperatures have already risen into the mid 80s in
most locations, with highs expected to range from the mid 80s to
around 90 later this afternoon. Reasonable amounts of low-level
moisture have supported the development of scattered cumulus clouds,
with development generally being suppressed by warm air around 700
mb. Localized convergence, possibly driven by terrain effects, has
allowed for a few taller cu to develop into small showers over
middle and eastern TN. While a very isolated shower cannot be ruled
out across in the Lake Cumberland area this afternoon, most if not
all should remain dry, and we`ll continue with a dry forecast for
now.

Another dry night is expected tonight across the region, with the
diurnal cumulus field expected to dissipate during the hours around
sunset. While additional mid-level clouds are expected to move
across southern IN during the evening and early overnight hours,
there should still be enough clearing for reasonably good
radiational cooling tonight. It is worth noting that some high
resolution guidance (e.g., the 12Z HRRR and RRFS) do show a couple
light showers developing late tonight on the leading edge of a 925
mb theta-E surge; however, would expect this to be overdone given
how well-mixed the sfc-850 mb layer will be this afternoon and
evening. Lows should be a few degrees warmer Tuesday morning
compared to today, with almost all falling into the 60s.

Tomorrow`s setup looks pretty similar to today with another mid-
level shortwave ejecting into the central Plains as ridging
amplifies from the lower Mississippi Valley into the Tennessee and
lower Ohio Valleys. Falling heights to the north and west of the
region will increase wind speeds tomorrow, with 10-15 mph S/SW winds
expected during the late morning and afternoon hours as the sfc
pressure gradient strengthens. Once again, hi-res guidance tries to
develop a diurnal cu field into a few showers and storms across the
Bluegrass region during the afternoon hours; however, convection
would be expected to struggle given poor mid-level lapse rates. At
this time, will advertise a 10% chance for a shower or storm
tomorrow afternoon, though almost all should remain dry.

Late Tuesday night, ongoing convection across the mid-Mississippi
Valley will begin to approach northwestern portions of the area as
the mid- and upper-level shortwave and associated sfc cold front
moves closer to the region. Convection will outrun the best
instability and shear by sunrise Wednesday morning, with storms
generally weakening as they approach southern IN. For most
locations, Tuesday night will remain dry, with temperatures
remaining mild in the presence of increasing clouds and
southwesterly winds.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 327 PM EDT Mon May 20 2024

Wednesday through Wednesday Night...

A deep, nearly vertically stacked low pressure system will rotate
from northern MN into southern ON Wednesday, with a trailing cold
front sagging southeast into the Ohio Valley. A warm, moist airmass
will be in place to the south of this boundary in southern IN and
central KY, characterized by sfc dewpoints in the mid/upper 60s and
PW values near 1.5 inches. HREF members indicate we could have
weakening showers and perhaps a few storms ongoing at the start of
the period Wednesday morning, but this activity would most likely
continue to dissipate.

A (relatively) much greater chance for renewed convection exists
Wednesday afternoon into the overnight hours. Moderate
destabilization appears likely by the afternoon with SBCAPE
exceeding ~1000-1500 J/kg. 3-6 km lapse rates will approach 7 C/km
over southern IN and northern KY. And with a moist environment and
steep low-level lapse rates during peak heating, there is at least
some risk for isolated damaging winds/wet microbursts. The strongest
updrafts may produce small to marginally severe hail, but the hail
potential will be kept in check by the moist profile and marginal
(30-35 kt) deep-layer shear.

Rain and thunderstorm chances will gradually ramp up Wednesday
afternoon into the overnight hours. Exact timing/evolution remains
somewhat unclear, but unsettled weather is expected along with warm,
humid conditions. Highs will be in the 80s with lows in the
mid/upper 60s.

Thursday through Sunday...

Unfortunately, it still looks like Wednesday`s frontal boundary is
likely to stall in the vicinity of the Lower Ohio Valley on Thursday
as it parallels the W/SW flow aloft. Multiple mid-level shortwave
disturbances, some convectively-enhanced, rippling through broad SW
flow will keep a chance of showers and storms in the forecast
Thursday into the weekend.

Some stronger storms will be possible at times, but it is impossible
to try to time individual waves of convection at this point.
Mesoscale evolution each day will be dependent on the prior day`s
and night`s activity. Moderate destabilization of a warm, moist
airmass does look possible each day, but in general, deep-layer
shear weakens late in the week. Depending on day-to-day evolution,
think there will be an increasing risk for flooding Thursday into
the weekend. Slower-moving storms and any cell training will boost
the flash flood potential. Each day will not be a total washout, but
there will be an increased risk for localized flooding.

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 751 PM EDT Mon May 20 2024

VFR conditions are expected to continue through the current forecast
period. SCT-BKN cirrus will spread east at the start of the TAF
period, with mainly clear skies by the early morning hours of
Tuesday. Very light southerly to calm winds expected overnight.

A strengthening low pressure system is forecast to lift northeast
from Kansas to Minnesota on Tuesday, which will tighten the pressure
gradient and increase SSW winds to ~10 kts with gusts up to 15-20
kts from late morning through the afternoon hours. Expect another
healthy cu field Tuesday afternoon. There is a 5-10% chance for an
isolated shower in the Bluegrass Region Tue afternoon, but of course
will not mention precip in the TAF given the low probability.

&&

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

&&

$$

SHORT TERM...CSG
LONG TERM...EBW
AVIATION...EBW