textproduct: Indianapolis

KEY MESSAGES

- Severe storms possible this afternoon and again Tuesday afternoon and evening with damaging winds the primary threat

- Heavy rain and some flooding are possible into the evening and again Tuesday

- Highs in the 80s Tuesday

- Storms will end Wednesday as the front shifts south, but unsettled weather likely returns late this week into the holiday weekend

MESOSCALE UPDATE

Issued at 1244 PM EDT Mon May 18 2026

Thunderstorms are developing along the Indiana Illinois state line. These storms have been loosely organized so far, but an outflow boundary from convection further upstream is impinging from the west. This outflow boundary may allow storms to consolidate into a more coherent line as they progress deeper into Indiana.

A recent sounding from Purdue University shows modest lapse rates between 6-7 C/Km, with a moist profile to about 500 mb. Additionally, modest shear is also present (20-25 knots) with messy hodographs centered around the mean storm motion. As such, little in the way of storm relative upper-level outflow appears likely...storms should behave more like single cells to multicell clusters until they become influenced by the outflow boundary or their own outflow.

Once storms consolidate on the outflow boundary, they should still be relatively 'pulsey' in nature but more organized than in the open warm sector. The primary threat today appears to be strong wind gusts, as enough moisture loading within mature updrafts is possible for wet microbursts. Additionally, cold pool dynamics such as rear- inflow jet surges are also possible which may produce strong to damaging wind gusts. Given a mean shear vector towards the northeast, cold pools that orient in a NW to SE direction pose the greatest threat for severe wind gusts.

Hail is possible but given only modest lapse rates, high moisture content, and the lack of long straight hodographs it does not appear to be all that likely. Tornado potential also appears low, given the outflow dominant nature of storms today. A brief QLCS spin up cannot be ruled out, however.

Another threat, likely replacing wind as the primary hazard later this evening, is the potential for flooding and flash flooding. High- resolution guidance shows the cold pool settling in a SW to NE fashion across central Indiana. This may allow for repeated rounds of storms into tonight. As mentioned above, moisture content in the profile is high and freezing levels are near 15,000 feet. With a warm cloud layer over 10,000 feet in thickness, warm rain processes should lead to efficient rainfall production today. As such, various CAMs are showing pockets of very heavy rainfall totals between 3 to 5 inches. HREF ensemble mean shows a large area of 1 to 2 inches. Because of this potential, a Flood Watch has been issued for for western portions of the area. As cold pools become more defined, this may need to be trimmed or expanded.

DISCUSSION (This evening through Monday)

Issued at 246 PM EDT Mon May 18 2026

Rest of This Afternoon and Tonight...

Please see the latest Mesoscale Discussion section for details on the current convective situation.

Thunderstorms will continue to move east this afternoon, with faster movement across the northern forecast area. To the south, the cold pool is having a harder time moving south, and showers and storms continue to develop as the low level jet impinges upon it.

Later this afternoon into early evening, some additional upper energy will produce additional showers and thunderstorms. Will continue high PoPs this afternoon into early evening, especially southwest where the regeneration of storms will continue along the cold pool boundary.

This regeneration of showers and storms in the southwest may lead to flooding, so will continue the Flood Watch. May have to adjust the location of the watch if the area of regeneration shifts.

The main threat for severe will be out ahead of the initial line of storms into the afternoon. Damaging winds will be the primary threat.

Coverage of rain will diminish this evening into the overnight as most forcing exits. However, some weak upper energy may keep the threat for a few showers around northwest.

Went cooler than guidance for lows given the rain cooled air.

Tuesday...

Uncertainty remains on how convection will develop on Tuesday with the surface cold front moving in. There may be cloud cover in the morning from upstream convection which would help limit instability. Some CAMs limit coverage to the southeast forecast area, while larger scale models bring rain to all areas Tuesday into Tuesday night.

Given the strength of the front and the plentiful moisture, went likely PoPs all areas at some point Tuesday afternoon and evening. The severe threat will depend on how well the instability is able to build. Damaging winds will be the primary severe threat. Given today's rainfall and the plentiful moisture Tuesday, heavy rain and flooding will remain a threat for Tuesday as well.

Highs on Tuesday could reach the middle and upper 80s if morning clouds are thin enough and rain holds off long enough.

Wednesday and Thursday...

Some rain may linger Wednesday morning in the south, so kept some low PoPs there. There remains some uncertainty on how far south the front will then settle and thus how fast it returns north. GFS seems aggressive with returning rain on Thursday, so will keep most of the area dry through the daytime Thursday. Rain chances begin to spread north Thursday night.

Much cooler air will return for this period with highs in the upper 60s to around 70 and lows in the middle 40s to lower 50s.

Friday through Memorial Day...

A northern stream upper trough will move into the central USA while a southern stream smaller upper wave moves into the area. These will help bring the surface front northward back into the area into Saturday. Decent moisture will move back north with it, so will have likely or higher PoPs Friday into Friday night.

Upper energy may interact with the lingering front to produce some rain on Saturday, so will keep PoPs around.

A couple of upper troughs will bring more rain chances Sunday into Memorial Day. There remains some decent uncertainty on timing and strength, so will broadbrush PoPs and keep them under the likely category until uncertainty improves.

With the surface front having lifted north, warmer air will return, with perhaps 80s again by the latter half of the holiday weekend. However, confidence isn't high in these given the uncertainty in rain coverage Sunday into Memorial Day.

AVIATION (00Z TAF Issuance)

Issued at 757 PM EDT Mon May 18 2026

Impacts:

- Showers at times into tonight

- Peak wind gusts Tuesday afternoon at 25 to 30kts

- Showers and strorms possible Tuesday afternoon and evening

Discussion:

Rain and thunderstorms have decreased in intensity across central Indiana. However, lingering showers are expected to be present to some extent through about 06z. Brief reductions in visibility are possible with shower activity.

Showers and thunderstorms are possible again Tuesday afternoon, but uncertainty is a bit higher than usual since this activity is due to storms currently far to our west over Kansas. How they evolve will determine timing and likelihood of rain tomorrow over central Indiana.

Southwesterly flow continues with marginal LLWS possible (30-35kt). Shear decreases after sunrise Tuesday with surface winds out of the SW picking up to between 10-20kt gusting 25-30kt at times.

IND WATCHES/WARNINGS/ADVISORIES

Flood Watch until 11 PM EDT this evening for INZ043>047-051>055- 060>063-067.