textproduct: Indianapolis

KEY MESSAGES

- Widespread rain likely today with QPF amounts generally ranging from 0.25 to 0.75 inches.

- Wind gusts up to 40 mph possible today and tonight, slightly higher gusts up to 45 mph cannot be ruled out during the afternoon.

- Slick/icy spots likely Friday morning as temperatures drop quickly below freezing.

- Snow showers at times late tonight into the early morning hours on Friday.

- Wind chills into the single digits possible Friday morning.

SHORT TERM (Today and tonight)

Issued at 308 AM EST Thu Dec 18 2025

Strong surface low pressure, roughly 984mb, over North Dakota will move eastward today. Strong southerly flow ahead of the approaching low will drive a potent low-level jet (LLJ) northward. Strong warm air and moisture advection are ongoing, with stratus streaming in from the south as of 2am. Two cold fronts extend south from the low, both of which will move across Indiana with different associated impacts.

Rainfall...

Despite the low passing well to our north, roughly across Lake Superior, strong large-scale forcing will be present from the system's parent trough. Combined with isentropic ascent, widespread shower activity is expected to develop later this morning into the afternoon. The first of the two aforementioned cold fronts will help focus this precipitation into a relatively narrow band. Heavy rainfall rates are likely as this band passes through along with the potential for a rumble or two of thunder. Rainfall totals from 0.25" to about 0.75" are most probable. Flooding is not expected, given the generally modest rainfall totals and fast-moving nature of the heaviest rain band.

Additionally, the band may provide enough downward momentum to bring some of the strong LLJ winds to the surface. A few wind gusts over 40kt would not be out of the question if this were to happen.

Winds...

One of the more challenging part of today's forecast will be surface wind potential this morning and afternoon. With a strong (up to 60kt at 925mb) LLJ just above the surface, any mixing would be enough to bring strong wind gusts to the surface. However, model soundings all show fairly stable low-levels which may limit such mixing. Model soundings destabilize somewhat during the course of the day which may allow mechanical mixing to overcome residual boundary layer stability.

For now, we'll split the difference and raise wind gusts from blended guidance to around 35-40kt. This puts gusts just below advisory thresholds, which may be needed later if observations show a trend towards greater mixing. Under such a scenario, a few wind gusts to 45kt may be possible at times.

Snow and Arctic air...

The first cold front clears the area by around 00z, with cooler air flowing in from the west. The air mass behind the first front is not the arctic air mass, with temperatures falling from the low 50s into the upper 30s. The second front is not far behind, however, arriving across our northwest as early as 03z. Strong cold air advection quickly pushes temperatures below freezing, and then downward into the teens by sunrise Friday. Gusty winds return as well which may result in wind chill values into the single digits Friday morning. Slick/icy spots are likely as any water remaining on roadways quickly freezes.

Guidance shows low-level instability along and within the post- frontal air mass, which may lead to snow showers at times between 06z-12z. Snow showers may be convective in nature and organize into bands, which may lead to a brief reduction in visibility and hazardous travel conditions. Most of the snow shower activity will be light, however. A couple of tenths to a half of an inch of snow is possible in the heavier snow showers.

LONG TERM (Friday through Wednesday)

Issued at 308 AM EST Thu Dec 18 2025

On Friday, a quick shot of cold weather will be pushing across Central Indiana in the wake of the passage of a cold front. The core of the cold air will be over Indiana as we start the day, and warm air advection will begin, but 850mb temps are suggested to only rise to near -6C by the end of the day. Highs will struggle to reach the middle and upper 20s amid the cold air mass. Otherwise, a strong ridge of high pressure will be in place within the lower levels, passing across Indiana through the day. Forecast soundings through Saturday show a dry column, with subsidence on Friday and a slow trend toward saturation on Saturday that never quite gets there late in the day. Thus dry weather will be expected on both Friday and Saturday.

On Saturday night into Sunday, models suggest a quick moving short wave passing within the flow aloft. Best forcing remains well north of Indiana, over Ontario and the northern Great Lakes. This arrives near 00Z, but subsidence quickly arrives later in the evening as the wave aloft and what is expected to be a dry cold front passes fast. Another strong high pressure system is then expected to arrive for Sunday. Thus a dry forecast looks like the way to go here through Sunday Night, although we will keep an eye to the moisture starved front and wave passing to the north for possible changes.

On Monday through Wednesday...weak ridging over the central CONUS will strengthen through Wednesday into a stronger ridge with a ridge axis settling just west of the Mississippi Valley. This will place central Indiana within northwest flow aloft. Little in the way of forcing dynamics is suggested to pass, while high pressure passes through the area at the surface. This will result in dry and mild weather for Monday and Tuesday with above normal temperatures.

By Wednesday within the lower levels a warm front is suggested to set up near the Ohio river and settle over Central Indiana through the day. A strong blocking high pressure system will be found to the south, preventing much in the way of moisture arriving while ridging will be in place aloft. Thus confidence for any precip at this time is low on Wednesday, although the NBM may try to include some low pops.

AVIATION (06Z TAF Issuance)

Issued at 1230 AM EST Thu Dec 18 2025

Impacts:

- MVFR ceilings developing after 06Z, IFR ceilings developing aft 15Z

- Low-level wind shear increasing towards 12Z

- Rain arrives Thursday morning, becoming moderate to heavy during afternoon.

- Gusty surface wind developing towards 12Z, strongest (40 kt) with the band of convective rain showers from 18-21Z.

Discussion:

A potent system will bring a variety of impacts to aviation through the period.

A low level jet will bring non-convective LLWS later this evening and will continue into Thursday morning. Gusty surface winds will develop Thursday, with the most intense gusts during the afternoon in showers along a cold front.

Clouds will gradually thicken and lower, reaching IFR at times on Thursday. There will be some brief improvement late in the period with a dry slot in the system.

Rain will become widespread Thursday. Embedded thunder is possible, but expected coverage will be too low to mention. Snow showers are likely behind the cold front after about 04z Friday, persisting through the end of the TAF period.

IND WATCHES/WARNINGS/ADVISORIES

None.