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Forecast Discussion for Indianapolis, IN
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932 FXUS63 KIND 190135 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 935 PM EDT Sat Oct 18 2025 .KEY MESSAGES... - Showers and thunderstorms becoming widespread this evening and tonight with locally heavy rainfall possible - A few strong to severe storms possible into tonight with damaging winds as the primary threat - Cooler with rain and wind gusts peaking at 35 to 40 mph Sunday - Seasonably cool temperatures next week && .MESOSCALE UPDATE... Issued at 935 PM EDT Sat Oct 18 2025 The second round of thunderstorms will likely reach central Indiana in within the next few hours and continue through 07Z tonight. This will be associated with a convergent boundary out ahead of an occluding surface low. The main concern with this setup will be a robust LLJ between 55-65MPH, of which could lead to 35-45MPH non- thunderstorm wind gusts along with creating a highly sheared environment within any convection. Severe Threat: As stated, shear will be abundant tonight along the convergent convective line. The main limiting factor will be the lack of low level instability, and what is likely going to be a relatively weak cold pool. This will lead to a wide variety of potential outcomes. Currently, this looks most likely to be a volatile convective line with minimal lightning, of which would produce isolated 50mph wind gusts along and south of I-70. However, there is still a chance for a stronger QLCS to form if this convective line is able to tap into some limited instability within some weak moisture return. If this does occur, severe wind gusts and embedded weak tornadoes cannot be ruled out. There will also be some convective showers ahead of the line, of which could tap into the aforementioned LLJ and produce isolated strong wind gusts. Flood Threat: Luckily, this round of thunderstorms should not have as deep of a convective core, resulting in less efficient rainfall. Also, training is unlikely given shear vectors and the robust dynamics of this low. That said, consistent stratiform rain across NW portions of Central Indiana could lead to a fairly wide corridor of 2+ inches of rain overnight, of which could create minor street flooding. && .SHORT TERM (This evening through Sunday)... Issued at 210 PM EDT Sat Oct 18 2025 Rest of This Afternoon... The initial round of forcing and area of showers and thunderstorms will push east into central Indiana soon and then it will continue to move east and northeast. Greatest coverage is expected to be along and west of I-69. Given the instability that has been able to build ahead of it along with some shear, a strong wind gust is possible with these. Locally heavy rain will be a threat as well. Tonight... A strengthening upper trough will approach the area while a quickly deepening surface low moves into lower Michigan. The low level jet will increase to over 50kt. This will aid in transporting moisture into the area. The strong forcing and plentiful moisture will bring widespread rain to central Indiana, so will go high PoPs all areas. (The initial area of rain looks to move north early tonight as the surface low deepens, leaving only scattered convection southeast.) A couple of embedded lines of convection will move through as higher resolution guidance shows a prefrontal line and another with the cold front itself. Instability will be weak, but what`s there with some shear could cause isolated damaging wind gusts or a brief tornado from mesovorticies along the line. Still think 04-08Z timeframe looks best for this threat. Another item to consider is some strong wind gusts (40 mph) ahead of the line across the eastern half of the area. The low level jet will strengthen, and stronger winds will get closer to the surface. Some scattered showers ahead of the main area of convection could bring down some of the stronger winds as they fall through an initially dry near surface layer. Will have to keep an eye on this. The plentiful moisture will lead to the threat of locally heavy rain, especially closer to the path of the surface low across the northwest forecast area. This area needs rain, so this will mitigate flooding concerns some. Lows will be in the 50s. Sunday... There may be a brief period of lower coverage of rain behind the cold front, but forcing from the upper trough will bring more widespread rain Sunday. Timing is a little slower than previously thought, so rain looks to continue into the afternoon before ending across much of the area from west to east. Will have high PoPs all areas to start the day, with PoPs gradually diminishing from west to east. Relatively strong winds will continue not too far aloft on Sunday, and some of these will mix down to produce wind gusts in the 30 to 40 mph range at times. Temperatures will be a far cry from Saturday`s, with readings in the 50s. The wind and rain will help give a chill to the air. && .LONG TERM (Sunday night through Saturday)... Issued at 210 PM EDT Sat Oct 18 2025 Sunday night through Monday... Some rain may linger in the far east early Sunday. Otherwise, high pressure will keep the weather dry during this period. As the high builds in Sunday night, winds will diminish. However, as a cold front approaches by Monday afternoon, the tightening pressure gradient will bring winds up with some gusts around 25mph possible. Temperatures will be seasonable with lows in the upper 30s and highs in the 60s. Monday night into Wednesday... An upper low will develop and move into the lower Great Lakes area. At the surface, a cold front will move through Monday night. There isn`t a lot of moisture to work with, so will go no higher than chance category PoPs, with highest PoPs north closer to better forcing. The upper low may have some spokes of energy with it on Tuesday and Wednesday, and these may be enough for an isolated shower, mainly north. At the moment confidence is not high enough to include any PoPs, but they may have to be added later. Cooler air will move in, with highs in the 50s to lower 60s. Lows will be in the 30s and 40s. Thursday and beyond... Uncertainty rises in this part of the long term as questions remain on the timing and path of a southern stream upper low. It could bring chances for rain around Friday. Temperatures will be seasonable, and some frost is possible in parts of the area Thursday and Friday mornings. && .AVIATION (00Z TAF Issuance)... Issued at 752 PM EDT Sat Oct 18 2025 Impacts: - Wind gusts up to 25kt this afternoon - Widespread rain this evening into Sunday - Outside of convection, worsening conditions this evening - Winds gusting up to 30kts predawn Sunday into the day Discussion: Rain will be widespread overnight and flying conditions will worsen to low MVFR. A brief break in the rain may occur near 10- 12Z then scattered to numerous rain showers will return through Sunday afternoon. Improvement to VFR will likely occur after 18Z Sunday. Gusty winds expected at times tonight and then continuously tomorrow. A wind shift will occur overnight with a cold front. Strong winds aloft will create near non-convective LLWS conditions tomorrow night. && .IND WATCHES/WARNINGS/ADVISORIES... None. && $$ MESOSCALE...Updike SHORT TERM...50 LONG TERM...50 AVIATION...Updike |
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