textproduct: Louisville

KEY MESSAGES

* There is a 30-50% chance for thunderstorms this evening into tonight. The strongest storms this evening could generate locally damaging wind gusts, along with frequent cloud to ground lightning.

* Repeated rounds of heavy rainfall in thunderstorms tonight into early Friday could result in flash flooding. There is a level 2 of 4 (slight) risk for excessive rainfall.

* A few additional storms (20-40% chance) may occur Friday into Friday night across areas mainly east of a roughly Jasper, Indiana to Richmond, Kentucky line. Confidence is low on whether a severe weather or flooding risk may develop.

* Outside of storms, heat indices are likely (60-80% chance) to surpass 100 degrees west of I-75 both today and tomorrow. The heat and humidity peaks Saturday and Sunday, when HeatRisk is forecast to reach the Major category area-wide.

SHORT TERM /THROUGH TONIGHT/

Issued at 216 AM EDT Thu Aug 13 2026

***** STRONG STORM AND HEAVY RAINFALL RISK THROUGH TONIGHT *****

At 2am EDT, central Kentucky and southern Indiana finds itself on the northeast periphery of an expansive ridge of high pressure centered across the Great Plains. KLVX's vad profiler revealed a weak westerly LLJ around 20-25 kt, which is slowly bringing some elevated instability back into our area atop the residual cold pool from evening storms. RAP mesoanalysis suggests a rather sharp gradient in this unstable layer rooted around 500-750 m off the surface, with anywhere from 100 J/kg MUCAPE across areas east of I- 75 to nearly 3000 J/kg across western Kentucky. Along the edge of this richer theta-e airmass, a few thunderstorms are sinking southeastward across northern and eastern portions of the CWA. Given the elevated nature of these storms, which various CAMs suggest may linger through the pre-dawn hours, the risk of severe weather appears low, though we'll need to keep an eye on the hydro potential if any backbuilding occurs in locations that already saw heavy rainfall yesterday morning.

Of greater concern will be how the atmosphere recovers this afternoon ahead of an organized thunderstorm cluster now moving across Nebraska. If we get enough of a break in convective activity later this morning into the afternoon, the RAP and NAM suggest we could have a corridor of 2500-4000 (highest west) J/kg MLCAPE set up across the area - more than sufficient to fuel strong to severe thunderstorms, despite only modest (20-30 kt) deep layer shear. HREF 20km neighborhood probabilities for more than 40 dBZ reflectivity are only 20-50% across the area this afternoon and evening, suggesting confidence is low to medium at best in storms at all, but the volatile environment will support a severe weather risk with any parcels lifted through the shallow capping inversion to the LFC. The primary hazard will once again be damaging wind gusts from both precip-laden collapsing cores and especially with any bowing multicell linear segments that manage to form and propagate towards the southeast. Heavy rainfall is a concern with the strongest storms as well, given a deep (>12kft) warm cloud layer and PWATs > 1.8 inches; however, storm motions of 20-35 mph (fastest north) suggest it'll take training of storms over the same areas for that to happen.

The hydro concern will linger during the overnight period tonight into early tomorrow morning as a 25-35 kt LLJ feeds into the effective boundary. The CAMs suggest this front should stay off to our north, as evidenced by the maximum in 90-10th percentile difference in HREF theta-E, along with 6h precip LPMM. However, if this should shift southward at all, training convection overnight into Friday morning could result in flooding across a section of our area - the highest risk being north of a roughly French Lick, IN to Richmond, KY line.

Outside of storms, it'll be a warm and muggy day with heat indices topping 100 degrees across many locations west of roughly I-75. The highest confidence (60-80% chance) from both the HREF and REFS in hitting advisory criteria (105 degrees) is along and west of a roughly Allen to Hancock County line, where a Heat Advisory is in effect from 10am to 8pm CDT.

LONG TERM /FRIDAY THROUGH WEDNESDAY/

Issued at 216 AM EDT Thu Aug 13 2026

***** STORM CHANCES WANE, HEAT BUILDS INTO THE WEEKEND *****

The effective boundary is forecast to drop southward into Kentucky on Friday, possibly (30-50% chance) serving as a focus for additional pop up storms during the afternoon mainly east of the I- 65 corridor. However, the trend should be for ridging to expand eastward heading into the weekend, suggesting chances for showers and storms will taper to less than 20% across the local area Saturday into Sunday. Meanwhile, conditions will grow increasingly hot and humid, with HeatRisk climbing solidly into the Major and possibly (20-40% chance) into the Extreme Category by Sunday. We remain close enough to the periphery of the ridge that if an MCS takes a dive southward, it could (10-20% chance) wreck the temperature forecast both Saturday and Sunday, but if that doesn't happen (80-90% chance) then heat indices are almost certain to exceed 100 degrees across most/all of the area. We suspect NBM's forecast high temperatures might be a degree or two overdone given evapotranspiration (ET) after recent rain is sometimes underforecast by its inputs (global ensembles). However, the oppressive humidity will combine with the heat to make conditions dangerous to vulnerable populations such as the elderly, chronically ill, outdoor workers, and those without proper cooling. In addition, it will likely prevent overnight low temperatures from falling below the mid 70s (upper 70s in urban corridors).

***** LESS HOT, STORMS RETURN EARLY NEXT WORK WEEK *****

A shortwave trough should flatten the ridge and drop a frontal boundary over the area sometime early next work week, potentially offering some relief from the heat for at least a portion of the area and serving as a trigger for showers and storms. It's too early to tell whether a severe weather risk may materialize with this activity, but the joint probability of > 25 kt 0-6km bulk shear and > 2000 J/kg MUCAPE from the LREF is less than 20%, which, at least at this point in time, suggests the risk should be relatively low. Given the richly moist and unstable airmass ahead of the trough and attendant front, heavy rain and localized microbursts in water-laden collapsing cores would seem to be the primary hazards (if any).

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 647 AM EDT Thu Aug 13 2026

VFR conditions should be predominant throughout the forecast period, though scattered storms through around 14-15z could bring visibility reductions in moderate rain. A break in convective activity should occur late morning into early evening, when additional storms may (30-40% chance) move in from the northwest. Winds will be light and variable through around 14z, then predominantly from the southwest at 6-8 kt during the day.

LMK WATCHES/WARNINGS/ADVISORIES

KY...Heat Advisory from 10 AM this morning to 8 PM CDT this evening for KYZ023-026-061-070>073. IN...None.