textproduct: Great Falls

KEY MESSAGES

- Cooler temperatures persist across the region through the end of the week with showers and thunderstorms around at times.

- A few thunderstorms will be capable of producing heavy rain, which would be impactful if they were to move over a city or burn scar.

- Temperatures trend closer to normal by the end of the weekend, with continued opportunities for afternoon and evening showers and thunderstorms.

DISCUSSION

/Issued 610 PM MDT Thu Aug 13 2026/

- Meteorological Overview:

A southwesterly flow aloft will more or less persist across the Northern Rockies through the weekend into early next week. This flow is initially more cyclonic in nature, which will foster plenty of showers and thunderstorms through Friday evening across the region. Persistent surface winds from the east and northeast will continue to usher in cooler temperatures. Precipitable water values will tick a touch higher today, resulting in increased precipitation efficiency with showers and thunderstorms that develop. The most robust thunderstorms that develop over the next few days will be capable of producing at least briefly heavy rainfall. Should one of these stronger storms move over urban areas or burn scars, some localized flash flooding cannot be ruled out. Lightning and gusty winds will also be a concern with stronger thunderstorms.

Heading into the weekend the nature of the southwesterly flow aloft becomes a bit more anti-cyclonic. That said, moisture working in from the southwest will combine with temperatures trending closer to normal to result in scattered afternoon showers and thunderstorms.

Greater uncertainty in the pattern develops toward the middle of next week. At this point the main takeaway will be for scattered showers and thunderstorms most days with temperatures near average. -AM

- Forecast Confidence & Scenarios:

Heavy Rainfall through Saturday morning...

Hi-Res Ensemble Forecast (HREF) guidance remains, and has remarkably been so for the past several days, consistent with showers and thunderstorms developing each day across the Northern Rockies as monsoon moisture streams northeast and over low level north-to-east flow in place across Southwest through North Central Montana. This mornings 12z RAOB from Great Falls measured PWATs at 0.97", which was accurately prog'd in the NAEFS since Tuesday. This consistent run-to-run model guidance gives increased confidence in rainfall amounts and locations through the end of the work week, with the heaviest precipitation expected to fall along and south of the Montana Hwy 200 over this timeframe, most notably between the I-90 and US Hwy 12 corridors.

Included below are tables listing 24 hour rainfall probabilities from the NBM5.0 for 0.25", 0.5" and 1" for select locations across North Central, Central, and Southwest Montana. The first table reflects the 24 hour period from 12PM Thursday through 12PM Friday, with the second table reflecting the timeframe from 12PM Friday through 12PM Saturday.

24hr Rainfall Probabilities thru 12PM Friday LOCATION > 0.25" | > 0.50" | > 1.00" Browning 65% | 40% | 15% Cut Bank 50% | 25% | 5% Havre 65% | 30% | 5% Great Falls 60% | 35% | 10% Lewistown 55% | 30% | 10% Helena 60% | 40% | 10% White Sulphur Springs 60% | 50% | 15% Bozeman 65% | 45% | 15% Dillon 80% | 50% | 10% Ennis 65% | 40% | 10% West Yellowstone 70% | 55% | 15%

24hr Rainfall Probabilities thru 12PM Saturday LOCATION > 0.25" | > 0.50" | > 1.00" Browning 20% | 10% | <5% Cut Bank 15% | 10% | 5% Havre 20% | 10% | <5% Great Falls 30% | 5% | <5% Lewistown 50% | 30% | 10% Helena 40% | 15% | 5% White Sulphur Springs 70% | 50% | 10% Bozeman 75% | 50% | 15% Dillon 75% | 35% | 10% Ennis 85% | 50% | 5% West Yellowstone 55% | 30% | <5%

Rainfall amounts exceeding 2" can not be ruled through Saturday morning within the corridor between the I-90 and US Hwy 12 corridors, with probabilities near a 10% chance here. - Moldan

Risk for strong thunderstorms:

While inverted-V soundings will be around across SW MT the next two days, these inverted-Vs are rather shallow all things considered. This will help reduce the risk for a strong to severe gust. At this time the main concern will continue to be lightning, aside from briefly heavy rainfall. -AM

AVIATION

14/06Z TAF Period

Low clouds with scattered showers and thunderstorms will be the main concern this TAF period. Recent precipitation in the region is largely limiting smoke to areas near active fires. Mountain obscuration will continue this TAF period.

For KGTF: Light winds are forecast this TAF period, which will yield a nearly zero probability of a 15 kt cross wind. Aside from the low clouds and intermittent showers or isolated thunderstorm, the main concern will be for icing. Most forecast soundings feature at least at shallow layer of clouds in the 0C to -15C range through the early afternoon Friday. -AM

HYDROLOGY

Precipitable water values fall ever so slightly Friday, but still look to be high enough to result in some heavier rainfall rates. Storm motions look to be slightly faster, which also would favor lesser of a risk for excessive rainfall leading to flash flooding. Given the threshold for flash flooding is so low over the active fires in the Pioneers, and that they have already received a wetting rain over the previous 24 hours, another flash flood watch was issued from 1 pm to 9 pm for today. HREF probabilities for a quarter inch of rain over an hour in the neighborhood of the active fires is roughly 50%. This gave enough confidence to warrant the flash flood watch. Trends will continue to be monitored further north for the Horse Gulch Burn Scar for an additional flash flood watch.

PRELIMINARY POINT TEMPS/POPS

GTF 50 64 49 77 / 90 60 50 10 CTB 43 59 44 73 / 70 50 40 20 HLN 52 66 51 77 / 80 80 70 10 BZN 50 70 47 76 / 80 90 80 20 WYS 39 65 35 70 / 80 60 70 60 DLN 48 70 45 74 / 90 90 60 40 HVR 48 69 48 76 / 50 40 40 30 LWT 48 64 46 72 / 80 80 80 50

TFX WATCHES/WARNINGS/ADVISORIES

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