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FXUS63 KLMK 071041 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 641 AM EDT Tue May 7 2024

KEY MESSAGES

* Potential for multiple rounds of strong to severe storms today through early Thursday. All severe hazards will be possible.
There is some potential that a significant severe weather event could develop Wednesday afternoon and night.

* Localized flooding potential will exist where repeated rounds of heavy rainfall occur, especially Wednesday night into early Thursday.

* Below normal temperatures return by Friday and persist into the weekend.

SHORT TERM /THROUGH TONIGHT/
Issued at 318 AM EDT Tue May 7 2024

The short term forecast discussion is broken down into two sections that will handle the potential waves/rounds of strong to severe storms today.

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Round 1 Timing: 15z (11am EDT) - 20z (4pm EDT)
Confidence on severe potential: Low to Medium

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The same line of storms that impacted portions of the central and southern Plains yesterday will come racing toward us this morning, but it will be nowhere near as potent. Convective allowing models (CAMs) are in good agreement with a general weakening trend during the morning hours as the line outraces the better shear and instability that helped to sustain it. Most models prog what's left of the line to arrive on our western CWA border close to ~15z.
Guidance begins to diverge on how it may evolve in the afternoon hours, with some CAMs suggesting isolated/scattered redevelopment along the outflow boundary while others keep things fairly quiet.
Environmental parameters by early afternoon would certainly support severe storms should they develop as stronger shear gradually builds in from the west while instability increases from surface heating (low level lapse rates steepening) and moisture advection from 30- 40kt LLJ. Severe storms would be capable of all severe hazards.
Guidance that does lean toward a 'quieter' or drier scenario tend to have a slightly stronger cap around 850-750mb that would impede convective development, so that will be something worth monitoring through the morning hours to see which models are 'capturing' the atmosphere better... and ultimately, give us, the forecaster, more confidence in which scenario may play out.

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Round 2 Timeframe: 20z (4pm EDT) - 09z (5am EDT)
Confidence on severe potential: Medium

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The next potential (and much more concerning) round of strong to severe storms will come ahead of a cold front to our west.
Storms could start firing off by mid afternoon and push eastward into the western CWA border starting around 20-21z. HRRR guidance progs an axis of ~70 degree dewpoints to nudge in from the southwest ahead of the front, and with low level lapse rates at their steepest levels due to peak heating, HRRR MUCAPE generally range from 2,000-3,000 J/KG. Strong deep layer shear of 45-55kts will be co-located with this higher instability axis, which will support organized convection that includes supercells.

Like this first round, there remains some uncertainty with respect to convective initiation. Some models/CAMs keep initiation to a minimum, and within those same models any convection that does attempt to fire off struggles and remains unimpressive looking.
These models, unsurprisingly, have a slightly stronger capping inversion in the 850-750mb layer. Other models/CAMs with weaker capping inversions tend to result in strong convective cores developing with severe potential. Should severe storms develop, all severe hazards would be possible.

Additional re-development along the front overnight as it slides into the region remains possible. Confidence is low on the severe potential with this activity, though, as the earlier afternoon and evening storms (assuming they develop) could leave cold pools or stabilize the atmosphere in their wake. Assuming the storms have access to an untapped or 'recovered' warm sector, all severe hazards would be possible with severe activity.

It's important to note that the severe weather environmental parameters we'll observe this afternoon into the overnight hours are very unusual for this region and something we typically see only a handful of times per year. If we get severe supercellular storms today, some will be capable of producing very large hail in excess of 2 inches. Modest low level shear and helicity in this moderately unstable environment could result in a strong (+EF2) tornado as well.

LONG TERM /WEDNESDAY THROUGH MONDAY/
Issued at 354 AM EDT Tue May 7 2024

==== Significant Severe Weather Outbreak Possible Wednesday afternoon-Wednesday Night ====

==== Local Flooding Possible Wednesday Night ====

Moderate to high confidence remains for a significant regional severe weather event Wednesday afternoon into Wednesday night for parts of the Ohio and Tennessee Valleys.

Synopsis: Closed upper low centered over the northern Plains Wednesday morning will gradually work eastward towards the Upper Mississippi Valley during the day as a weak shortwave ridge builds over the Ohio Valley. At the surface, a quasi-stationary boundary will be located just along the Ohio River as an elongated area of sfc low pressure over the central plains slowly works towards central IN by Thursday morning. As the sfc low works towards the Ohio Valley, the stalled boundary will become a warm front with dew points south of the boundary over southern IN and central KY in the mid/upper 60s to even a few areas to the south near 70. As temperatures warm into the low/mid 80s during the afternoon, the atmosphere will quickly destabilize with SCAPE values ranging between 1500-3000 J/kg. HRRR ensemble soundings indicate 0-6km bulk shear > 50 kt.

The main challenge will be timing and placement of initial convection during the day Wednesday. While the setup for a significant severe weather threat continues over the Ohio Valley as indicated by the SPC enhanced outlook (3/5) over our entire county warning area, there are a few elements that could potentially impact the overall severe threat during the day Wednesday.

Hi-res models are in agreement of remnant boundaries associated with Tuesday nights storms to be located across central TN Wednesday morning. This boundary looks to be the focal point for convection initiation during the day Wednesday. Timing and location of convective activity Wednesday morning into the early afternoon could play a large role in just how strong storms get later in the afternoon and evening over central KY and southern IN. Some of the hi-res convective allowing models want to develop convection over far western KY while others start to develop activity over MO Wednesday morning. Any cloud cover and potential outflow from this activity arriving early in the day before maximum heating would likely limit storm severity towards the afternoon. Another limiting factor with early convection to our south and southwest early Wednesday would be to limit the LLJ which is indicated by the 00z HRRR and to some extent the NAM3K. Finally, placement of the stalled boundary and its northern progression allowing for the CWA to be in the deep warm moist sector behind increasing instability over the area. A lot of the above mentioned factors will be determined by how the storms Tuesday into Tuesday night unfold over the area. With all that being said, all threats are still possible Wednesday afternoon with initial cellular development before becoming more of a QLCS ahead of an approaching cold front Wednesday night into Thursday morning.

Tornado threat: Model sounding continue to indicate low-level curvature on hodographs showing the potential for plenty of streamwise vorticity ingestion by storms, with some eSTP values for tornadoes >=EF2 Wednesday afternoon.

Hail threat: With forecast soundings showing large CAPE in the -10 to -30C hail growth zone. Strong deep layer shear and steep mid level lapse rates keep large hail a possibility.

Wind threat: While we may not have a strong LLJ to work with, high PWAT values leading to precipitation loading along with steep low- level lapse rates increase the threat of strong damaging wind gusts.

Flash Flood threat: With PWAT values above 1.50" and numerous clusters of storms Wednesday afternoon along with a line of storms Wednesday night into Thursday moving through the area. Add to that already wet ground from rains over the last couple of days will increase the threat and concerned for localized flash flooding.
Especially in areas that will see heavy rain on Tuesday then again on Wednesday.

With the above mentioned limiting factors, confidence is higher for linear severe storms associated with the approaching cold front than the development of supercells during the afternoon and early evening on Wednesday.

**Like any forecast, there are sources for potential bust. Any convection that might get going Wednesday morning could decrease instability for afternoon redevelopment, as would cloudiness being more widespread, or stronger capping in our position well within the warm sector. While a significant severe weather event is still a possibility, this isn't a done deal. Stay tuned to the latest forecast information between now and Wednesday afternoon to keep abreast of changes.**

Thursday - Monday:

Amplified pattern develops Thursday through the remainder of the forecast period with a large ridge building to the west and deep trough across the eastern US. This will keep occasional shower/storms chances in the forecast thanks to a series of shortwave disturbances moving through the northwest flow aloft.
Temperatures during this period will be below normal for early May with highs in the 70s and even 60s on Friday.

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/
Issued at 640 AM EDT Tue May 7 2024

Wide range of cigs/vis across the region this morning, with the worst (LIFR) conditions generally near the I-75 corridor while areas farther to the west are mostly VFR. The stratus/fog should gradually lift to VFR levels by mid to late morning with surface heating.

Multiple rounds of showers and storms are anticipated today. The timing of these rounds is somewhat reflected in the TAFs, though there could be showers/storms outside of the highlighted periods where confidence in development and/or coverage is not very high.
Any TAF site that experiences a shower or storm today could see brief gusty winds as well as reduced cigs/vis.

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

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