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FXUS63 KLMK 031113 AFDLMK

Area Forecast Discussion National Weather Service Louisville KY 713 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 30-40% 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.

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 HRRR 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.

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