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FXUS63 KLMK 041949 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 349 PM EDT Sat May 4 2024

KEY MESSAGES

* Isolated to scattered showers and thunderstorms are forecast this afternoon. Slow moving storm motion and moderate to heavy rain expected with any storm.

* Isolated showers and thunderstorms are expected on Sunday before widespread rain chances return Sunday night into Monday.

* Potential for multiple rounds of strong to severe storms next Tuesday through Thursday. Although confidence in timing remains low, all severe hazards will be possible.

* Elevated flooding potential will exist where repeated rounds of heavy rainfall occur next week.

SHORT TERM /THROUGH TONIGHT/
Issued at 348 PM EDT Sat May 4 2024

========== Rest of This Afternoon ==========

Fairly moist atmosphere over the area, with precipitable waters around the 90th percentile. Storms have developed in this humid airmass, with best convective development where the MLCAPE per SPC mesoanalysis is over 1000 J/kg. Louisville ACARS most recent sounding shows a couple of small inversions, at ~800 mb and 700 mb, which may be inhibiting deeper development over the city. HRRR sounding forecast represents these inversions well and keep them in place through the afternoon. Thus will keep the best chances for storms east of the i-65 corridor.

========== Tonight ==========

Showers/Storms from the afternoon should steadily decrease this evening. Cannot rule out some isolated development in the moist airmass. Should be another mild night as dewpoints remain elevated and lows by Sunday morning should be in the low 60s. Model progs keep clouds around overnight and thicken/lower them during the morning hours.

========== Sunday ===========

How long those low clouds hang around in the morning and early afternoon hours will have a direct influence on afternoon convection. Have bumped PoPs down a touch, with the better chances south and east of a Russellville to Bardstown to Frankfort line.
Some CAM's have gone dry for the afternoon, but given potential for some breaks in clouds and precipitable waters still above normal, though not as much, feel prudent to keep in some storm chances.
Temperatures should rise into the 80's for most places.

LONG TERM /SUNDAY THROUGH SATURDAY/
Issued at 348 PM EDT Sat May 4 2024

Synopsis...A multi-day severe weather episode will be possible by the middle of the next week as an upper-level trough evolves and amplifies over the central US and eastern CONUS upper ridge axis transitions from the Great Lakes to the Mid Atlantic/Southeast US.
The timing for best chances of severe weather will be tied to the progression of periodic, mid-level shortwave troughs rounding the base of the large through and embedded upper low. Eventually, continuous ejection of strong shortwave troughs will carry the main upper low energy towards the eastern CONUS while deeply amplifying once again due to further reinforcement from upstream Canadian troughing.

Model Evaluation/Confidence...Confidence in large-scale features continue to be medium to high with expected decreasing confidence in timing and intensity of shortwaves ejecting ahead of the main upper low. The timing differences in incoming strong mid-level forcing could lead to great variability in convective intensity, coverage, and maintenance based on a potential onset time around sunset (both Tuesday and Wednesday) and the resultant distribution/amount of available daytime instability. Also, it is too early to pinpoint specifics about meso features that would certainly play an important role in initiation convection, such as the position of the residual outflow boundary on Tuesday afternoon/evening or the effect of previous convection and the interaction with approaching frontal wave on Wednesday. In any case, machine learning derived guidance has been consistently highlighting elevated probabilities of severe weather across the region for Tuesday and Wednesday, with a recent trend for higher probabilities for Wednesday.

Sun Night - Monday...Increased shower activity along with some isolated storms are likely Sun night into Monday morning as a southern-stream shortwave trough moves from the southern Plains to the lower Ohio Valley. A combination of increased moisture advection and potentially slow storm motions/backbuilding could result in isolated accumulations over half an inch or more. On the other hand, warming mid-level temperatures and decreasing instability will limit thunder chances and promote more of a low-centroid type of storms.
The vorticity wave will push a frontal wave stalled across the region to the north, so will keep mentioning reduced PoPs during the afternoon and evening.

Tuesday - Thursday...Daily severe weather chances appear to focus around sunset and overnight every day. For Tuesday, a weakening cold front will move into the forecast area Tuesday morning and early afternoon and although there will be an increase in showers during the morning, it seems that forcing will subside substantially to reinvigorate convection. Nonetheless, the residual outflow boundary will establish somewhere along the Wabash Valley and into central Indiana, which could be the mesoscale focus for convection late in the afternoon and early evening as a surface low (ejecting across the Upper Midwest) drags a cold front to the southeast. Based on current guidance, highest severe probs will be close to the outflow boundary further north with a decreasing trend in convective intensity as it approaches southern Indiana and north-central Kentucky. On the other hand, Wednesday looks more threatening as the front will be located closer to the forecast area and the second mid- level shortwave energy brings a surface low across the Wabash Valley and IN. Therefore, forcing will be significantly higher amid a moderately high unstable warm sector and sufficient deep-layer shear for organized, spinning updrafts. All severe hazards are on the table as well as a risk of flooding given the possibility of repeated rounds of heavy rainfall.

Friday - Sunday...Amplifying trough over the eastern CONUS will push the cold frontal and associated unsettled weather activity south of the forecast area. As a result, rain and storm chances will decrease during the day on Friday with breezy NW winds and persistent low- level cloud cover the rest of the day. Temperatures during the weekend seems to fall near or slightly below normal under light NW breeze and partly to mostly skies. A more stable airmass will indicate dry conditions for Saturday and Sunday.

AVIATION /18Z TAFS THROUGH 00Z MONDAY/
Issued at 348 PM EDT Sat May 4 2024

Plenty of moisture around for early May and with daytime heating, seeing some popup showers/storms. Focus for storm development seems to be south of the I-64 corridor so far, but anticipate KBWG/KLEX/KRGA to see additional storms in their vicinity. Cannot rule out KSDF, but for now will lean on the optimistic side. Storm chances should taper off quickly with loss of heating, but expecting to see some low stratus development overnight and into Sunday morning. Cannot rule out IFR conditions along the I-64 corridor and KRGA, but will go for MVFR thresholds for now, even below 2 kft at the KLEX/KHNB/KRGA terminals.

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

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