204
FXUS63 KLMK 031415
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1015 AM EDT Fri May 3 2024

.KEY MESSAGES...

*   Widespread showers with isolated embedded thunderstorms this
    morning. Scattered thunderstorms possible this afternoon.

*   Probability of thunder is 15-25% in the morning, increasing to
    40-50% this afternoon even as rain chances decrease.

*   Showers and thunderstorms will be more scattered on Saturday,
    though forecast confidence in exact timing and coverage remains
    low.

*   Increasing potential for strong to severe thunderstorms and
    flooding next Tuesday through Thursday, though confidence is
    still low.

&&

.UPDATE...
Issued at 1014 AM EDT Fri May 3 2024

Regional radar mosaic shows the early morning widespread precip
shield is shifting off to our east as expected, with some scattered
showers in the wake. Precip coverage will diminish for the midday
hours, with only some scattered showers expected across the area as
the cold front continues to approach from the west-northwest. Around
or shortly after 18z, we should begin to see additional scattered
showers and storms fire up in a marginally unstable but moist
environment. Soundings continue to support slow moving and
unorganized convection this afternoon with skinny CAPE profiles and
weak shear. However, PWATs are already 1.5" across the forecast area
this morning, and those will persist for the rest of today. As a
result, heavy rain and lightning will be the main threats with
scattered storms this afternoon. Probability of thunder increases to
40-50% after 18z and will diminish this evening as peak heating is
lost. Forecast is in great shape and no changes are planned at this
time.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 326 AM EDT Fri May 3 2024

Warm advection/isentropic lift have spawned a band of light to
moderate showers ahead of the incoming cold front. Precip shield is
just starting to expand east of the I-65 corridor, but very few
lightning strikes have been observed as instability is lacking thus
far.

Expect this band to push through most of the area by mid-morning,
with a minimal break before the weak cold front comes in from the W-
NW in the afternoon. Not looking for much of an opportunity to warm
up, but in a muggy air mass with PWATs near 1.5 inches, sfc temps in
the mid 70s are all that`s needed for some marginal instability.
Wind speeds are generally 25 kt or less in fairly deep SW flow, so
not enough shear to support storm organization. Long story short,
there`s a slight (15-20%) chance for embedded thunder in the band of
showers this morning, with scattered (~30% coverage) thunderstorm
development possible in the afternoon. We are not expecting an all-
day rain, but brief moderate rainfall is possible in any of the
heavier showers or thunderstorms.

Precip chances taper down this evening and overnight given the lack
of heating or identifiable sources of lift. However, in the
increasingly moist environment it won`t take much, so will hold at
least a slight chance through the night.  Better chances will be
east of I-65 and south of the Bluegrass Parkway into south-central
Kentucky, where we could see a weak impulse lifting NE from the Deep
South.

Forecast confidence is high that we`ll see showers at some point. At
best medium confidence in thunder, as well as any specific timing.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 326 AM EDT Fri May 3 2024

Saturday and Saturday Night...

Saturday`s forecast continues to look tricky as far as precipitation
chances are concerned, with forecast confidence in timing and
coverage of showers and thunderstorms remaining low. The synoptic
scale pattern for Saturday and much of the weekend will not be
dissimilar to a typical mid-late summer pattern, with positive
height anomalies stretching across much of the eastern one-third of
the CONUS and better dynamic support from faster flow aloft
remaining well to the northwest of the Ohio Valley.

The front which will bring increased chances for rain later today
will dissipate before clearing the region, leaving a moist unstable
airmass in place for the day on Saturday. 00Z HREF mean sfc dewpoint
progs are in the mid 60s Saturday afternoon, with mean PW values
ranging from 1.25-1.5", with slightly lower values in southern IN. A
combination of hi-res and global model sounding progs show anywhere
from 1000-2000 J/kg SBCAPE, and forecast highs on Saturday are
expected to exceed convective temperatures.

However, as referenced above, there will be a lack of large-scale
dynamic support, and convective initiation will likely be determined
by the existence/location of differential heating boundaries and
other gradients on the mesoscale and smaller. Due to the inherent
lack of predictability of these factors, the best forecast still
looks to be for a broad area of chance to likely PoPs Saturday
afternoon, with lower probabilities along and north of the Ohio
River, and higher probabilities south of the western KY/Bluegrass
Parkways. The chance for thunderstorms will be closely tied to PoPs,
as the amount of available instability will mean that any area of
convection strong enough to produce rain will have a roughly equal
chance to produce thunder. Severe thunderstorms are not expected on
Saturday, as weak flow aloft results in only 5-15 kt of effective
bulk shear (depending on model), with garden-variety thunderstorms
expected. Finally, any storms which do form will be fairly slow
movers, with cloud layer mean winds supporting storm motions from
WSW to ENE around 15 mph.

A more organized wave of showers and embedded thunderstorms is
expected late Saturday night into Sunday morning as another decaying
mid-level disturbance and sfc cold front falls apart in the vicinity
of the Ohio Valley. Once again, weak flow aloft will curtail any
severe potential, and precipitation coverage along the front will
decrease as it crosses the region from west to east.

Sunday and Next Week...

A prolonged period of active weather looks likely over the next 5-7
days with global ensemble guidance in relatively good agreement on
the large scale pattern across North America. For Sunday into
Monday, the upper level trough over the north central U.S. and
Canada will lift east-northeast into eastern Canada, with upper
level ridging setting up across much of the eastern CONUS in its
place. The trough`s northeastward exit will mean that the humid and
warm air mass over the central and eastern CONUS will continue with
little modification, meaning that diurnally-driven showers and
thunderstorms would be expected to continue. Additionally, transient
mid-level perturbations within large scale SW flow are expected to
swing through the Ohio Valley Sunday into Monday, bringing
temporarily enhanced waves of precipitation. These showers and
storms would largely be sub-severe and disorganized, as flow aloft
will still be too weak to support an organized severe threat.
Temperatures will remain above normal, particularly nighttime lows,
which will be held higher by elevated low-level moisture content.

The severe potential is expected to increase for Tuesday through the
middle portion of next week, as upper troughing over the western
half of the CONUS ejects into the central Plains, increasing large-
scale baroclinicity and, by extension, the strength of the flow
aloft across the Mississippi and Ohio Valleys. At this time,
confidence in specific threats and timing is very low; however, many
ensemble/machine learning/analog severe convection forecasting tools
indicate elevated potential for severe weather across our region
next Tuesday-Thursday. With several rounds of heavy rain/storms
expected over the next week, flooding potential will also need to be
monitored, especially the later we go into next week. At this time,
HEFS/NAEFS guidance indicate most rivers should remain below action
stage; however, these models will not resolve convectively enhanced
rainfall totals well, and they still show rising levels over the
next 5-8 days.

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 713 AM EDT Fri May 3 2024

Look for intermittent rain with occasional restrictions to MVFR
cig/vis this morning.  Heavier showers could result in brief IFR but
uncertainty of timing means we won`t include it in the TAF. The
steadier precip comes to an end midday or just after, but we could
still see scattered convection as the front moves in this afternoon.
Only terminal that we included TS was SDF from 18-22Z, and that`s a
low enough probability to handle with VCTS/CB mentions.

Precip chances are low enough tonight to keep the TAFs free of
precip.  However a stalled boundary and a moist air mass will be
favorable for fog/stratus toward Sat morning. Have included IFR
cig/vis for BWG after 08Z, with MVFR conditions expected at SDF and
LEX for the same time frame.

Forecast confidence is medium at best. Winds could go light/variable
for a substantial portion of the TAF period with the front hung up
across the area.

&&

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

&&

$$

UPDATE...CJP
SHORT TERM...RAS
LONG TERM...CSG
AVIATION...RAS