textproduct: Louisville

KEY MESSAGES

* Heat Advisory extended through Friday for SW Indiana and portions of central Kentucky. Unseasonably hot temperatures continue with highs in the mid to upper 90s and heat index values up to 105 degrees.

* Weak front brings low end (20-40%) shower or storm chances on Friday.

* Better chances of rain return early to mid next week, however temperatures gradually settle back to near normal for this time of year.

SHORT TERM /THROUGH FRIDAY NIGHT/

Issued at 243 PM EDT Thu Sep 17 2026

Main story today continues to be the ongoing heat and humidity. Temperatures have worked into the low and mid 90s along with dew points in the upper 60s to around 70. This is yielding max heat indices in the 100 to 105F range for locations in the current Heat Advisory. The NE third of the advisory expires tonight at 8 PM EDT, however the SW 2/3 of the current headline has been extended through Friday. More on that below.

Outside of the continuing heat and humidity, things are mostly quiet across the CWA at this hour, although a few showers and storms have popped with the heat of the day. Modest ML CAPE values around 1000- 1500 J/KG have developed, however no real deep layer shear exists. Focusing on one area across SE KY, and another area of isolated development along and just south of a a remnant outflow boundary across southern IN. DCAPE values are pretty high (1000-1200 J/KG) given the dry lower levels so any briefly stronger pulse storm could put down a very localized 40+ MPH gust.

Do think that the convection that is initiating just north of our CWA could put out additional outflow later this evening into the overnight, and wherever this feature ends up could be a new focus for isolated to widely scattered showers (perhaps a storm) by late morning into the early afternoon on Friday. Right now, hi-res models suggest this could be just south of I-64 but will have to monitor progression overnight. As we get deeper into the afternoon and evening, additional widely scattered showers and storms could develop along the main synoptic front for areas along and north of I- 64.

Given that some convectively reinforced outflow and a weak surface boundary are expected to affect the NE half or so of our CWA tomorrow, decided to only extend a portion of the Heat Advisory for areas along and SW of a Jasper, IN to Elizabethtown, KY, to Lake Cumberland, KY line. Looks like a fairly steady NE surface flow, additional cloud cover, and some widely scattered precip chances could keep the rest of the CWA in slightly better shape with respect to max HI values than previous days.

LONG TERM /SATURDAY THROUGH WEDNESDAY/

Issued at 243 PM EDT Thu Sep 17 2026

Saturday - Sunday...

The center of the upper ridge retreats back toward the Red River Valley Saturday and Sunday, and this will leave the Ohio River Valley more in between the upper ridge and the stronger belt of zonal westerlies across the NE CONUS. This will continue to be a mostly dry and hot pattern for us, however we'll likely lose enough of the mid level subsidence inversion, and have a weak frontal boundary for a trigger (at least on Saturday) for some isolated to widely scattered diurnally driven shower or storm chances. Focus should mainly be across the NE half of the CWA for these time periods, farther away from the upper ridge center.

Still looking at temperatures in the low to mid 90s on Saturday with dew points also continuing to be in the upper 60s and low 70s so max afternoon heat indices in the upper 90s to around 100 will be possible. For Sunday, temperatures will be more solidly into the mid 90s (perhaps even a few upper 90s across the SW), however dew points look to drop off more into the 60s by this time which will provide a little relief from the triple digit max heat index values.

Monday - Tuesday...

We'll remain in a similar upper pattern going into early next week with the upper ridge centered back toward the Rio Grande, and a swath of zonal westerlies stretching from the Corn Belt eastward through the mid Atlantic. Some instability will still build over our region on the NE periphery of that upper ridge, however we'll also be in play for some weak shortwave activity passing through that corridor of slightly enhanced upper flow mentioned above. The end result will be increased chances for mainly diurnally driven showers and storms each day, but could get a little extra boost from any passing shortwave activity as well.

In addition to the increased shower and storm chances, it looks like we also see a decent shot at some cooler air with the upper ridge losing influence. ECM Ensemble temperature guidance still has some notable spread in the data for Monday and Tuesday (10-15 degrees between the hi and low members!). For now, the latest NBM data leans more toward the higher end of the guidance for Monday with most of our temps ranging in the upper 80s and low 90s. The NE CWA will likely be notably cooler in the low to mid 80s. By Tuesday, the NBM leans more toward the cooler end of the guidance envelope with highs in the upper 70s and low 80s for most. The combination of increased shower and storm activity combined with the loss of the upper ridge seems to be the driving factor to these changes.

Wednesday - Thursday...

Upper ridging tries to build back to the NE a bit through mid week. Lower confidence on how much if any the upper ridge gets established back over our area, and the latest NBM temp data suggests that will be delayed a bit. Will call for an overall drier forecast by this time with highs peaking in the upper 70s and low 80s. These temperatures will be values more close to normal for this time of year.

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 156 PM EDT Thu Sep 17 2026

VFR conditions are expected to prevail this forecast cycle with a steady WNW to W wind today along with some higher based Sct-Bkn cumulus clouds. Winds slacken tonight and go more SW. Look for light and variable winds tomorrow with some remnant outflow and a weak frontal boundary stalling near the area. Could see some afternoon and evening showers or a storm along and north of I-64 and put a Prob30 at KSDF to account for this in the 24-30 hour period.

LMK WATCHES/WARNINGS/ADVISORIES

KY...Heat Advisory until 8 PM EDT /7 PM CDT/ this evening for KYZ029>034-038-045-046-054-065-077. Heat Advisory until 8 PM EDT /7 PM CDT/ Friday for KYZ023>028- 053-061>064-070>076-081. IN...Heat Advisory until 8 PM EDT this evening for INZ076>079- 090>092. Heat Advisory until 8 PM EDT /7 PM CDT/ Friday for INZ083-084- 089.