textproduct: Great Falls

KEY MESSAGES

- Several rounds of showers/thunderstorms today, mainly impacting SW MT.

- Increased shower/thunderstorm coverage tomorrow, with a few capable of producing strong wind gusts, small hail, and heavier showers.

- The potential for periods of heavier precip at times tomorrow and Saturday are causing a minor risk for flash flooding.

DISCUSSION

- Meteorological Overview:

The upper level low, which is currently over the PNW, will continue to bring southerly flow aloft over the rockies today. Due to weaker embedded waves making their way over the region, there will rounds of showers/thunderstorms today into this evening. Drier conditions are expected after 06Z tonight, although an isolated shower/thunderstorm or two can't be ruled out.

As the upper level low approaches the region, there will be an environment that is conducive to developing stronger storms on Friday. However, the threat for severe weather will be dependent on whether there is sufficient shear in the area. A concern associated with a lower shear environment would be the stagnation of the storm movement, which could result in increased risk of excessive rainfall and flash flooding.

As the upper-level low moves eastward and out of the state on Saturday/Saturday night, there will be a shift to drier and warmer conditions. However, showers may continue to linger on Sunday in eastern MT. Monday and Tuesday currently look warm and dry. - ESH

- Forecast Confidence & Scenarios:

Thunderstorms the next few days:

While confidence is high that showers and thunderstorms will develop over the next few days, confidence in just how much instability develops remains on the lower side. Relatively strong forcing will accompany the passage of this system, particularly on Friday. This forcing may ultimately result in a convective initiation that is a bit too early to maximize daytime heating, resulting in ample cloud cover across much of the region by mid-afternoon that limits thunderstorm strength. -AM

AVIATION

17/18Z TAF Period

Showers/thunderstorms continue over SW MT and along the divide this afternoon. Showers/thunderstorms will move north into central MT by this evening. In addition, mountains/passes will be obscured at times due to the showers/thunderstorms. By 06Z tonight, most shower/thunderstorm activity is expected to taper off. After a period of drier conditions, although an isolated storm or two can't be ruled out through the rest of the night, showers/thunderstorms begin developing again in SW MT Friday morning. Shower/thunderstorm coverage will increase and move into central MT as the day progresses. Similar to today, showers/thunderstorms will obscure mountains/passes at times on Friday. - ESH

HYDROLOGY

Precipitable Water (PWATs) values approaching 1", which are 2 to 3 standard deviations above normal, are forecasted to reside over portions of Southwest and Central Montana from Friday morning through Saturday morning as Pacific moisture streams north to northeast and across the Northern Rockies. ECMWF EFIs largely ranging between 0.75 and 0.85 south of a Helena to Lewistown line and north of a Dillon to Bozeman line, with SOTs approaching 1.5 from a Butte to White Sulphur Springs line, support the potential for a very unusual rain event with respect to mid-September climatology thanks to these anomalous PWATs. Developing showers and thunderstorms over this timeframe will have slow storm motions of generally 15-25 mph, which will allow for increased residence time over a given area.

Given all of these indicators (i.e. slow storm motions, anomalous PWATs, and very unusual forecasted precipitation) the need for Flash Flood Watch for portions of Southwest and Central Montana has increased; most notably for areas that are hydrophobic. While the potential for urban flooding can't be ruled, especially in the Helena and Gallatin Valleys, current thinking is that recent burn scars (regardless of size)from the 2026 season and the Horse Gulch area will be the most susceptible for Flash Flooding and debris flows. For this reason a Flash Flood Watch has been issued for Little Belt and Highwood Mountains, Big Belt, Bridger and Castle Mountains, Elkhorn and Boulder Mountains, and Northwest Beaverhead County to account for the recent/past burn scars. Given that hunting season is well underway across the aforementioned areas those recreating near burn scars should be prepared to seek higher ground. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 50 73 52 65 / 10 10 40 50 CTB 47 69 47 67 / 10 0 0 0 HLN 49 75 52 63 / 50 50 100 80 BZN 48 71 48 59 / 10 50 90 100 WYS 39 67 38 57 / 50 80 80 80 DLN 47 68 46 60 / 30 60 80 90 HVR 49 77 50 70 / 10 0 0 20 LWT 50 73 49 59 / 0 20 70 90

TFX WATCHES/WARNINGS/ADVISORIES

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

Flash Flood Watch from Friday afternoon through Saturday afternoon for Northwest Beaverhead County.