Sunrise 6:39 AM   Sunset 8:40 PM Moonrise 4:37 AM   Moonset 6:15 PM

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FXUS63 KLMK 061051 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 651 AM EDT Mon May 6 2024

Updated Aviation Discussion

KEY MESSAGES

* Multiple rounds of showers and thunderstorms will move across the region today. One or two storms may produce gusty winds and/or small hail. Localized flooding is also possible.

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

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

* Cooler temperatures return by Friday and persist into the weekend.

SHORT TERM /THROUGH TONIGHT/
Issued at 333 AM EDT Mon May 6 2024

Now through sunrise...

A NW-SE oriented band of showers and thunderstorms is lifting to the north across the CWA at this hour. So far this morning, this band has put down anywhere from 0.25-0.75" of rainfall, with 20-30 mph wind gusts being observed along the leading edge. Would expect shower/storm intensities to trend downward over the next hour or two as convection is outrunning the area of >500 J/kg MUCAPE. This band should lift through the area by just after sunrise, with an area of showers and thunderstorms currently over Western KY/TN moving in behind this leading band.

Today...

A mid-level shortwave trough currently analyzed extending from the Ozark Plateau southeastward into the mid- and lower-Mississippi Valley will lift northeastward into the Ohio and Tennessee Valleys.
A modest low-mid level SW jet will assist in advecting an anomalously moist air mass into the region, with 00Z HRRR PWAT values ranging from 1.5-1.75" this afternoon. For reference, the 90th percentile PWAT climatology from BNA is ~1.4", with the daily maximum ~1.7". The combination of ample moisture, large-scale ascent thanks to mid-level CVA, and 1000-1500 J/kg MLCAPE will allow for multiple rounds of showers and thunderstorms across the region.
Timing of individual waves of showers and storms will be difficult if not impossible to anticipate more than a few hours in advance given the primed environment and the lack of a well-defined forcing boundary, although thunderstorms should be more likely this afternoon thanks to diurnal destabilization. Any stronger storms this afternoon could produce gusty winds and small hail, but widespread severe convection is not expected given limited values of DCAPE and relatively high freezing heights. HRRR LPMM QPF values indicate that localized swaths of 1-2+" of rain will be possible where multiple heavy precipitation cores set up, so a localized flooding threat will also be in place. Showers and storms should subside after sunset tonight as instability decreases and the mid- level trough moves off to the east.

Tonight...

There should be a relative lull in showers and storms overnight tonight before our next round of storms arrives Tuesday morning. One or two showers/storms may develop as a warm front/theta-E surge lifts northward through the region. The potential exists for patchy fog Tuesday morning, especially in locations which see heavier amounts of rainfall today. Southerly winds will begin to increase Tuesday morning, which will both limit overall fog potential as well as keep temperatures mild, with lows only expected in the low-to-mid 60s Tuesday morning.

LONG TERM /TUESDAY THROUGH SUNDAY/
Issued at 333 AM EDT Mon May 6 2024

============== Tuesday & Tuesday Night ==============

By Tuesday morning, showers and storms that formed in the Plains and Midwest along a cold front will quickly race toward our region. This activity will out-pace the surface cold front and begin to lose some strength initially as it approaches our region... but low level moisture advection out of south/southwest and surface heating will lead to steepening low level lapse rates that will aid in some destabilization. Models differ on how the convection will evolve once it enters our region, and some of this will be impacted on the time of day it arrives (later arrival would favor strong/severe potential).

Environmental parameters favoring severe storms will begin to reallly increase Tuesday afternoon into the overnight hours as the surface cold front inches closer toward the region. Models prog an axis of ~70 degree dewpoints to funnel in from the southwest that will lead to moderately high levels of instability (2,000 J/KG MUCAPE) in a weakly capped environment. Deep layer shear will also increase as the front approaches. Two big questions that will impact severe storm potential Tuesday are... (1) when will the first wave of convection arrive (as well as depart), and (2) will subsequent convection in the late afternoon/overnight period be able to overcome a weak/subtle cap? Models don't provide a clear, consistent picture on this, unfortunately. For the late afternoon/overnight convection, we will have some 'triggers' to get convective development going (approaching cold front, mesoscale boundaries left behind from previous storms), and the weak/subtle capping should result in said convection being largely discrete in nature. Assuming storms do initiate and sustain themselves, given the shear profiles, supercells would be the most likely storm mode overnight. All severe weather hazards would be possible (tornadoes, large hail, damaging straight line winds). The general hodograph shape is straight-line through much of the troposphere, which would result in splitting supercells that wouldn't favor one specific type of mover (left/right). With that being said, the hodographs do show some low level clockwise curvature in the lowest 1 kilometer, so any right- mover supercells that persist for some time would need to be watched very closely as environmental Sig-Tor is fairly high (+3).

============== Wednesday & Thursday ==============

General model consensus (with some notable exceptions, more on that in the next paragraph) have storms pushing out of the region Wednesday morning while our frontal boundary will begins to lift northward as a warm front. This would allow a period of drying/clearing that would lead to destabilization again. A surface low and cold front will begin approaching from the west ahead of an upper level trough, and severe weather parameters ahead of these features look even more concerning vs what we'll deal with Tuesday into Tuesday night. Additionally, stronger forcing will be in place to develop more widespread storms (with both linear and discrete storm modes possible). Severe storms will be capable of large hail, damaging straight line winds, and tornadoes. Localized flood issues may also arise given we'll be dealing with multiple rounds of heavy rain over a short timeframe. Models do have some minor differences in the timing of convective development, but strong/severe storms could start as early as Wednesday afternoon... with higher threats Wednesday evening into the early morning hours Thursday as the front approaches. This has the potential to be a significant severe weather event.

As alluded to earlier, there are some model 'outliers' that show things evolving a bit differently on Wednesday. For example, convection from Tuesday night storms could linger into Wednesday and limit overall destabilization ahead of the next wave. Or, mesoscale boundaries from previous convection may initiate showers/storms early in the day on Wednesday and once again limit destabilization.
Or perhaps a combo of the two could unfold. These possible scenarios limit confidence on a potentially significant severe weather episode to some degree.

With all that being said, the Wednesday into Thursday morning setup bears very close watching because of how impactful and significant it could be. Should discrete supercells develop Wednesday evening into early Thursday morning, environmental shear profiles strongly favor right movers. Hodographs are quite scary looking and show large, clockwise curvature in the lowest 1km with high amounts of shear and helicity. Given other environmental parameters favoring tornadic conditions, there is very concerning potential for a strong (+EF2) tornado if we end up with an untapped warm sector ahead of the front.

Those living within the region will want to keep up to date with the latest forecasts and make sure they have multiple ways to get warnings, especially since these severe storms could come through in the middle of the night when many are asleep.

============== Friday into the Weekend ==============

Much cooler conditions will filter in behind a cold front Friday into the weekend. Doesn't look like we stay entirely dry during this timeframe as shortwaves pass through the region, but it shouldn't be a washout by any means either. There are some differences in how the long range models handle upper level features, so for now will ride with NBM PoPs for rain chances.

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/
Issued at 651 AM EDT Mon May 6 2024

A first wave of showers and thunderstorms is clearing LEX/RGA at this hour, with another band of showers and storms moving into BWG at this time. This area of rain will continue northeast through the morning hours and would be expected to bring IFR/MVFR visibilities if it maintains its current intensity. Model guidance suggests the highest potential for reduced VIS from this area at rain at SDF, so will go ahead and mention this in a TEMPO, though others may be needed at LEX/RGA/HNB. Current observations show an area of MVFR ceilings within and just behind this area of rain across western KY and TN. Would expect these lower ceilings to track along with the band of rain, with a period of MVFR ceilings likely at all forecast sites later this morning. Ceilings are expected to lift back to VFR levels this afternoon, though lower CIGs would still be likely inside of showers/storms.

Beyond this morning's band of rain, more waves of showers and storms are expected to develop this afternoon, with confidence in timing and location remaining low. It still looks like short-fuse amendments will be needed later today to handle subsequent impacts from these showers and storms. Precipitation is expected to taper after sunset this evening, with medium-high confidence in VFR conditions at all sites until around 06Z Tuesday. Tomorrow morning, there is a potential for fog to develop, with the strongest signal at this time at LEX and RGA.

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

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