Sunrise 6:42 AM   Sunset 8:38 PM Moonrise 3:47 AM   Moonset 3:51 PM

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FXUS63 KLMK 040145 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 945 PM EDT Fri May 3 2024

KEY MESSAGES

* Isolated shower and thunderstorm possible this afternoon. Slow moving storm motion and moderate to heavy rain expected with any storm.

* 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 in details is still low.

UPDATE
Issued at 944 PM EDT Fri May 3 2024

As of 0130z, WSR-88D continues to show a few lingering isolated showers and thunderstorms, mainly over the Bluegrass. Weak sfc low and associated inverted sfc trough will continue to push east. Given the rain from this evening, high PWAT values around 1.50" and dew points in the low/mid 60s some patchy fog as well as drizzle associated with low stratus is possible during the overnight. Only made a few minor adjustments to the grids and the forecast to account for the patchy fog and updated PoP grids to account for some isolated light rain and or even a possible brief rumble of thunder.

SHORT TERM /THROUGH SATURDAY/
Issued at 328 PM EDT Fri May 3 2024

========== This Afternoon ==========

The cold front is located just north of the Ohio River this afternoon, with our area within the warm sector of the system.
Mostly cloudy sky cover has kept temps cooler than in previous days, with highs so far in the low 70s. However, the airmass is muggy enough to develop some marginal instability, with ACARS data from SDF showing over 500 J/kg of MLCAPE. Convective temps are close to the mid 70s, so if we can reach another degree or two we could see isolated convection fire up along the front or in the warm sector. We are already seeing some showers redevelop down towards Bowling Green, as well as along the front closer to Huntingburg, IN. Probability of thunder for this afternoon peaks around 50%, so coverage will be mainly isolated, and not everyone will see a thunderstorm. For those that do see storm activity in the next few hours, moderate to heavy downpours will be expected thanks to the PWATs around 1.5 inches.
Additionally, very weak flow and shear will promote unorganized and very slow storm motions in addition to the lightning threat this afternoon.

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

Around sunset and into tonight, precip coverage will begin to diminish as our daytime heating is lost. The cold front appears to wash out tonight, leaving no real source of lift in place. However, those high PWATs will linger, so will need to keep at least a slight chance through the night. However, better chances for additional showers come in the morning hours.

Temps will be mild tonight, with lows only reaching the low to mid 60s. With the muggy airmass in place, and saturated grounds from recent rains, there should be a good chance for low stratus and fog tomorrow morning. After sunrise, stratus and fog will burn off.

========== Saturday ==========

Another weak mid-level impulse should lift northeast from the south into the region tomorrow morning. This will bring additional rounds of isolated to scattered showers in the morning, with the greatest chances to the east of I-65.

The forecast for tomorrow afternoon remains complicated in regard to shower and storm activity without much of a trigger mechanism in place. As mentioned earlier, the cold front will have washed out, leaving behind a moist and unstable airmass across the region. There will likely be some weakening mid-level vorticity left over from the morning impulse from the south, but overall the greater forcing will be up towards the Great Lakes later in the day.

It'll be another muggy day, with dewpoints possibly reaching the upper 60s, with 12z HRRR favoring 75-80% chance for dewpoints over 65F. PWATs don't appear to be as high as today, but still between 1.25-1.4" possible. However, sfc temperatures will be much warmer with highs hitting the upper 70s and possibly even the low 80s in some locations. This should lead to more potential instability during the afternoon, with forecast soundings suggesting between 1000-2000 J/kg of SBCAPE possible. Convection initiation tomorrow afternoon will probably be driven by differential heating boundaries or other mesoscale features present at the time. Due to this continued uncertainty, will continue with broad brush PoPs for the entire region. Severe thunderstorms are not expected due to another day of weak flow aloft. Storms that do form will be fairly slow movers, with storm motions generally less than 20 mph expected.

LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/
Issued at 328 PM EDT Fri May 3 2024

Synopsis...The medium-range forecast starts on Sunday by featuring an expansive mid-level ridge over the central and eastern US as mid- level shortwave energy lifts from Upper Midwest to Canada. Further west, a strong mid- to upper-level trough will move onshore from the Pacific eventually amplifying across the western two-thirds of the country by the middle of next week. The ensuing broad cyclonic circulation will allow the upper jet to stretch closer to the Ohio Valley while periodic shortwave energy rotate around the trough axis. As a result, there is a chance of organized, strong to severe storms for several days next week across the Lower Ohio Valley as the main trough meanders around the Plains.

Model Evaluation/Confidence...Confidence in synoptic weather pattern has continue to increase as CMC/ECMWF/GFS deterministic models agree fairly well on the position and evolution of the main features (trough/ridge). On the other hand, confidence decreases in the timing and intensity of individual shortwave troughs embedded within the larger trough circulation impacting the ability to indicate the exact times of the severe weather risks. Nonetheless, the CSU and NCAR machine learning algorithms have been consistent at depicting, at least, low probabilities of severe weather for the Tuesday- Thursday timeframe.

Sat Night - Sunday...A slight chance of showers and storms will continue Saturday night into sunrise on Sunday as a weak frontal boundary approaches the area overnight. The second half of Sunday morning and early afternoon looks drier as the diffuse boundary stalls close to the Ohio River. Another low chance of showers and storms might be in place late Sunday afternoon and early evening ahead and along the frontal wave provided that cloud coverage diminishes and solar radiation destabilizes the BL to yield CI once convective temperatures are met. Given the weak steering flow, mesoscale boundary interactions could play an important role for convective initiation and maintenance.

Sun Night - Monday...An increase in shower intensity and coverage can be expected Sunday night into Monday morning as a southern- stream shortwave trough moves from the southern Plains to the Ohio Valley. GFS forecast soundings show increasing moisture profile with overall decreasing instability and warming mid-level temperatures.
Therefore, there could be a chance of brief moderate to heavy rainfall with low thunder probabilities. This vorticity wave will push the stalled frontal boundary further north, so the shower coverage should decrease towards the afternoon and evening given the lack of any substantial forcing mechanism.

Tuesday - Friday...As mentioned above, chances of strong to severe storms will be on the rise during the Tuesday-Thursday timeframe owing to the increasing shear associated to the incoming upper jet, the presence of moderately high instability values, and sufficient forcing provided by the periodic mid-level shortwave troughs.
Although confidence in timing and details on type of severe risk are low at the moment, available medium-range CAM guidance (GFDL-SHIELD)
hint at a multi-day overnight severe threat on Tue and Wed night with probabilities displaced further south for Thu night. A flooding risk cannot be ruled out either given the convective nature and duration of the weather events.

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/
Issued at 714 PM EDT Fri May 3 2024

Weak area of low pressure was located just west of KFTK this hour and will continue to move east-northeast this evening. Cold front remains northwest of the KSDF/KBWG/KLEX/KRGA terminals and will slowly move SE this evening. Cigs across the region have remained generally VFR with some tempo drops to MVFR in passing showers.
Later this evening and into the overnight period, the cold front will slowly washout over the region leaving light winds and relatively moist boundary layer. Much of the guidance suggests that low clouds/fog/mist will likely develop with cigs falling through the MVFR range and then into the IFR range overnight. Vsbys look to remain mostly MVFR but IFR/LIFR vsbys may develop at KBWG late tonight. Cigs/vsbys should come back up in the 04/13-14Z range with skies eventually returning to VFR by late morning. For tomorrow afternoon, a bit more heating will take place and scattered showers and a few storms will be possible across the region. So will continue the VCSH mention in the new TAF package.

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

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