textproduct: Great Falls

KEY MESSAGES

- Thunderstorms, a few of which look to be on the stronger side, develop through the day today, mostly across Central and Southwest Montana.

- Areas of precipitation continue into Saturday, resulting in increased concerns for localized flooding.

- A drier and warmer stretch looks to ensue late weekend through much of next week.

DISCUSSION

/Issued 119 AM MDT Fri Sep 18 2026/

- Meteorological Overview:

A somewhat elongated upper level low across the Pacific NW and vicinity will slowly shift eastward through the day today. Although forcing from this approaching upper system will result in showers and thunderstorms developing rather early in the convective cycle, cool temperatures aloft will still yield sufficiently steep lapse rates for a few stronger thunderstorms. Areas where shear ends up being maximized will have a threat for marginally large hail. Otherwise the main threat will be for slow moving, heavy rain producing thunderstorms through the day today.

Difluent upper level flow on the northeast side of the upper level system look to combine with lingering instability and forcing from the approaching trough to result in an extended stretch of rain, mostly near and south of a line from Helena to Lewistown southward to a line from Dillon to Big Sky tonight through the day Saturday. This prolonged stretch of rain will result in concern for excessive rainfall, particularly over burn scars, but for urban areas as well.

Precipitation slowly exits eastward late Saturday night and Sunday morning as a messy/weak upper level ridge builds in Sunday into Monday. Much of next week looks dry, though weak disturbances cannot be ruled out, which would be capable of producing spotty light precipitation at times. -AM

- Forecast Confidence & Scenarios:

Rain through Saturday:

Confidence is increasing in a concentrated area of moderate rainfall, mainly along and south of a Helena to Lewistown line southward through a Dillon to Big Sky line. Over 48 hours ending Sunday morning the corridor along I-90 east of Butte through Bozeman has around a 90% probability to receive 1 inch of rain. These probabilities slowly decrease as you go further north and south of this corridor.

This rainfall looks to be partially convective driven and partially stratiform in nature. The risk for excessive rainfall leading to flash flooding will be highest while convective processes are dominant, which will be primarily in the afternoon and evening. That said, burn scars will be susceptible to flash flooding at any time given the significantly lower precipitation amount thresholds. -AM

AVIATION

18/06Z TAF Period

Showers tracking northeast of eastern ID may clip the KWYS terminal overnight, bringing some lower clouds. Otherwise, mainly mid- high level cloud-cover continues to stream across the region in south to southwesterly flow aloft with a small risk for low cloud and/or patchy fog development near the KHVR terminal late tonight/early Friday morning. Coverage of showers and thunderstorms increases across SW MT Friday afternoon. Hoenisch

HYDROLOGY

A Flash Flood Watch continues for parts of central and southwest Montana through Saturday, targeting areas near recent (Summer 2026) and vulnerable older (2024 Horse Gulch) burn scars. As noted previously, the peak threat for heavy rainfall rates exceeding 0.50/hr and associated potential runoff and debris flow is expected early this evening. Beyond burn scars, a wider area of moderate to locally heavy rain developing tonight and persisting into Saturday could trigger minor urban and rural drainage concerns. High-resolution ensemble models place the highest risk along and north of the I-90 corridor up to US-12, showing a 6080% chance of 24-hour rainfall totals reaching or exceeding 2 across parts of Jefferson, Broadwater, and Meagher counties. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 73 51 66 47 / 10 40 40 20 CTB 70 46 67 44 / 10 0 0 0 HLN 74 51 64 47 / 60 90 80 30 BZN 72 46 59 42 / 70 90 90 70 WYS 66 37 57 34 / 90 80 70 50 DLN 68 46 60 43 / 80 70 90 60 HVR 76 48 71 45 / 10 0 10 10 LWT 73 48 59 46 / 30 70 80 80

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch from this evening through Saturday evening for Big Belt, Bridger and Castle Mountains-Elkhorn and Boulder Mountains-Gallatin Valley-Little Belt and Highwood Mountains.

Flash Flood Watch from 3 PM MDT this afternoon through Saturday afternoon for Northwest Beaverhead County.