textproduct: Louisville

KEY MESSAGES

* Several waves of showers and thunderstorms are expected today. Strong to severe storms will be possible this afternoon and evening, especially east of I-65. Damaging winds are the main threat, and a few spin-up tornadoes are possible.

* Isolated rainfall amounts of 2-4+" could lead to flash flooding today. The greatest probability of heavier rainfall amounts will be east of I-65.

* More scattered shower/storm activity possible through the middle of next week, mainly in the afternoon and evening hours.

SHORT TERM /THROUGH TONIGHT/

Issued at 247 AM EDT Sat Aug 1 2026

Early this morning, a robust line of showers and thunderstorms is moving into western KY and southwest IN. Along and just ahead of this corridor of heavy rain, a strengthening 850 mb jet is priming the atmosphere from west to east across the area this morning, with a corridor of 2+" PW values expected to move across the area during the morning hours. All of the moisture advection is occurring within the warm conveyor belt of an approaching low pressure system which is nearly vertically stacked over eastern IA as of 06Z. As this low pressure system drifts to the southeast later today, multiple bands of showers and thunderstorms are expected to move across the area from west to east.

The first wave of showers and storms to move through the area this morning is expected to have a very low severe threat with it as lingering stability near the surface should keep convection mainly elevated through mid-morning. Still, with an axis of 1000+ J/kg MUCAPE expected to make it to the I-65 corridor by around sunrise, embedded thunderstorms are likely across most if not all of the area with the first wave of precipitation. Some 6-hr HREF/REFS localized prob-matched mean swaths of 1-3" are associated with the morning convection, so a low, but non-zero flooding threat will also be possible.

Later this morning into the afternoon hours, a 45-50 kt 500 mb jet will swing around the base of the upper low across western KY. This will place our area into a region of diffluence aloft and broad rising motion in the low-to-mid levels. Combine this with an anomalously moist environment (PW 2-2.25") and building instability with daytime heating, and several ingredients for additional rounds of showers and thunderstorms will be in place this afternoon and evening. The last of those three ingredients may be the most uncertain, as some 00Z high-res model guidance shows fairly persistent convective development through the late morning and early afternoon hours. If this scenario plays out, it would lessen the destabilization this afternoon (at least for a portion of the area), which would lessen the potential for strong to severe thunderstorms.

Nevertheless, if there are breaks in the clouds/precipitation during the late morning and afternoon hours, this will allow pockets of 1000-2000 J/kg MLCAPE to develop, with the largest probabilities of higher instability (>1500 J/kg) expected between I-65 and I-75 and south of I-64. Along and north of I-64, there is more uncertainty whether instability will be sufficient for strong updrafts to develop. While weakening mid- and upper-level flow will keep bulk shear values around 25-30 knots, a significant component of the bulk shear vector should be perpendicular to the low-level forcing boundary (i.e., the sfc cold front), increasing the likelihood of cellular convection initially. Low-level shear is abnormally strong for August, with hodographs showing a decent amount of curvature in the lowest 1-2 km ahead of the cold front. As a result, while damaging winds will be the most likely severe threat this afternoon, a few spin-up tornadoes will be possible. 00Z HREF guidance does show a couple swaths of 2-5 km Updraft Helicity in excess of 75 m2/s2 mainly east of I-65, highlighting the potential for rotating updrafts. It should be noted that the parameter space is much less supportive for significant severe weather than on July 21st, so long track tornadoes and higher-end damaging wind gusts (70+ mph) are very unlikely.

In addition to the severe weather potential, any training of thunderstorms this afternoon and evening could lead to a localized flash flooding threat. As mentioned above, the atmosphere will be quite saturated ahead of the sfc cold front, and deep warm cloud depths should support some increase in the rainfall efficiency. The 00Z HREF/REFS both show a few swaths of 2-4+" in 6 hours, with a stronger signal along and east of US 127. However, given the relatively isolated coverage of these heavier swaths and a general progressive nature to the storms, decided to hold off on any flood watch at this time.

The main timing for the severe threat looks to be between 18Z and 01Z, though a few stronger storms could linger east of US 127 as late as 02-03Z. After sunset, the loss of diurnal heating should lead to a downward trend in convective intensity, with isolated showers expected after midnight tonight. For the late evening and early morning hours on Sunday, isolated showers should continue, with the best chances expected across southern IN and northwest KY, which will be closer to the center of the low. Temperatures should fall into the upper 60s and low 70s Sunday morning.

LONG TERM /SUNDAY THROUGH FRIDAY/

Issued at 247 AM EDT Sat Aug 1 2026

Sunday - Tuesday Night...

From Sunday through the first half of the upcoming week, an upper low will continue to spin over the Ohio Valley. Most ensemble 500 mb progs have the low starting to pull out of the region Tuesday evening into Wednesday morning as it gets absorbed into an approaching synoptic trough over central Canada. During the time period that the low is still overhead, above normal PW values will persist, with LREF mean PW values gradually stepping down from 1.75- 1.9" Sunday to 1.5-1.7" by Tuesday. The elevated moisture levels should lead to greater cloud cover and continued scattered shower and thunderstorm chances mainly in the afternoon and early evening hours. Weak mid-level flow in the vicinity of the trough/low will result in deep-layer shear of only 10-15 knots, so any storms which form should be disorganized with strong/severe storms unlikely. Temperatures should be below normal for early August due to the elevated moisture/cloud coverage, with highs in the upper 70s and low-to-mid 80s and lows in the mid-to-upper 60s.

Wednesday - Friday Night...

As the upper low pulls out of the area mid-week, PW values should return to more seasonable levels, and without a defined triggering mechanism, precipitation coverage/chances should decrease Wednesday into Thursday. For the second half of the week, a transition to a more zonal pattern is favored. Positive upper height anomalies over the western US should spread into the southern Plains, with broad troughing spreading across the northern CONUS and Canada. While we'll have to watch for any smaller-scale waves within the broader zonal/NW flow pattern that could bring enhanced chances for showers and storms, at this time, most guidance keeps the better rain chances to the north and west of the region. Temperatures are expected to creep up toward climo normals by the end of the week, with highs returning to the upper 80s and low 90s.

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 644 AM EDT Sat Aug 1 2026

A round of showers and a few storms currently moving through the area. Cigs are falling to MVFR/IFR due to lowered ceilings with these initial showers. Ceilings will begin to drop more widespread below 3kft in BWG around 12z and continue to build in eastward into all sites. A secondary line of showers and storms will move through this afternoon and evening. Gusty winds of at least 20kts are likely to accompany these storms. Lowered vis can also be expected throughout the forecast period as storms move through. Ceilings will potentially improve to VFR for LEX/BWG/SDF after the rain moves through, however another low cloud deck may build back in late tonight. PROB30s were put in for SDF/HNB/LEX/RGA to cover the possibility for lingering rain showers tomorrow morning.

LMK WATCHES/WARNINGS/ADVISORIES

KY...None. IN...None.