textproduct: Great Falls

KEY MESSAGES

- Another cooler day with showers and thunderstorms.

- Some thunderstorms will be capable of heavy rain today.

- Trending warmer this weekend, though afternoon and evening showers and thunderstorms will remain.

DISCUSSION

/Issued 509 AM MDT Fri Aug 14 2026/

- Meteorological Overview:

A southwesterly flow aloft will persist across the region over the upcoming week. This flow aloft is initially cyclonic in nature, which will aid in lots of showers and thunderstorms, even during the overnight, through tonight. Precipitable water values are still somewhat elevated, which will allow for a few stronger thunderstorms to produce heavy rainfall rates, at least briefly. Should one of these stronger thunderstorms move over an urban area or a burn scar, there would be flash flooding concerns.

Heading into the weekend the southwesterly flow becomes more anticyclonic in nature, with rising heights aloft. Typically this would suppress convection somewhat, but given the forecast lingering moisture near the surface across Southwest and Central Montana and rising temperatures, another round of showers and thunderstorms looks to develop Saturday afternoon and evening. Precipitable water looks to decrease at least somewhat, which would favor a lower probability for excessive rainfall.

The same story plays out Sunday, though with another day of drying, overall coverage of showers and thunderstorms becomes more questionable. Continued southwest flow aloft with a few embedded waves will foster additional round of scattered showers and thunderstorms into early next week as temperatures trend to near average. Confidence in specifics wanes after Tuesday, though the main takeaway continues to be for additional scattered showers and thunderstorms. -AM

- Forecast Confidence & Scenarios:

Flooding risk today:

Precipitable water values fall ever so slightly today, 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. A few high resolution members take a shower or thunderstorm through this area early this morning, but the probability of this occurring is too low to warrant having the watch begin early this morning. Trends will continue to be monitored for changes in confidence in this area this morning.

As of now the concern level at Horse Gulch Burn Scar is slightly lower. Should this concern level rise even slightly, an additional flash flood watch would become necessary. -AM

AVIATION

14/18Z TAF Period

Scattered showers and thunderstorms will continue through this evening, resulting in lower ceilings and visibilities throughout the beginning of this TAF period. Ceilings will begin to rise for KBZN, KEKS, KWYS and KHLN around 14/05Z, while the other terminals will continue to see MVFR ceilings throughout the rest of this TAF period. KGTF and KLWT will have VLIFR conditions due to fog starting around 15/07Z. The fog is expected to lift around 15/11Z for KGTF, while it will continue in KLWT until around 15/15Z. Mountain obscuration will occur across North-Central, Central and Southwestern Montana throughout this TAF period.

For KGTF: Low ceilings and visibilities will dominate the beginning of this TAF period due to passing showers through the evening. The showers are expected to dissipate by 15/02Z, and while unlikely, we cannot rule out an isolated thunderstorm this evening. While visibilities will increase later this evening, MVFR ceilings will remain throughout the TAF period, with the exception of a period of IFR conditions 15/07Z - 15/11Z due to fog. Light winds will create a near zero probability of a 15 kt cross-wind. Icing remains a concern for the afternoon, as forecast soundings continue to feature at least a shallow layer of clouds in the 0C to -15C range until the early evening. -Dzomba

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 66 50 76 50 / 70 50 10 10 CTB 59 44 71 45 / 70 20 10 10 HLN 70 50 77 51 / 70 40 30 30 BZN 73 47 75 46 / 80 60 30 50 WYS 66 35 70 34 / 60 70 40 60 DLN 72 45 73 44 / 70 40 70 50 HVR 68 48 76 48 / 50 50 10 20 LWT 65 47 72 48 / 70 60 30 50

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch until 9 PM MDT this evening for Northwest Beaverhead County.