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

- 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

19/00Z TAF Period

VFR conditons will prevail at the KWYS, KGTF, KCTB, and KHVR terminals during the duration of this TAF Period outside of during any thunderstorms. During the duration of this TAF Period showers and thunderstorms will affect the KBZN, KEKS, and KHLN terminals. Showers and thunderstorms will affect the KLWT terminal from 19/06Z through the end of the TAF Period. During any thunderstorms there will be a brief period of mountain obscuration, heavy rainfall, small hail, frequent lightning, and gusty erratic winds. At the KBZN, KLWT, KHLN, and KEKS terminals there will be at least a period of MVFR-level ceilings during this TAF Period. At the KHLN terminal between 19/12Z and 19/16Z rain showers will reduce visibility to at least MVFR-levels with around a 20% chance for visibility to drop to IFR-levels. At the KGTF terminal during the majority of this TAF period there is at least a 20% chance for rain showers. -IG

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

Additionally, the Helena Valley, Canyon Ferry Area, Meagher County Valleys, and Gallatin Valley were added to a Flood Watch from this evening through Saturday afternoon, with the primary concern being the flooding of low-lying and flood-prone locations due to excessive runoff from heavy rain falling across urban areas. Hourly rainfall rates of between 1/4" and 1/2", especially from the mid-evening hours tonight through early morning hours on Saturday, are possible across these areas which could lead to rapid runoff due to surfaces being hydrophobic which would then lead to flooding of low-lying areas. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 52 66 48 73 / 40 30 10 0 CTB 47 67 45 73 / 0 0 0 0 HLN 52 61 48 73 / 100 100 30 0 BZN 48 59 45 67 / 90 90 80 20 WYS 39 54 36 62 / 80 80 50 20 DLN 47 58 44 67 / 100 90 40 0 HVR 49 70 48 74 / 0 0 0 0 LWT 50 58 48 65 / 70 70 80 40

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch from 6 PM MDT this evening through Saturday evening for Big Belt, Bridger and Castle Mountains-Elkhorn and Boulder Mountains-Little Belt and Highwood Mountains.

Flood Watch from 6 PM MDT this evening through Saturday afternoon for Canyon Ferry Area-Gallatin Valley-Helena Valley- Meagher County Valleys.

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