textproduct: Great Falls

KEY MESSAGES

- Strong to severe thunderstorms today and Friday, with strong winds, hail ,and flash flooding possible.

- Dry conditions on Sunday before daily chances for showers and thunderstorms return next week.

DISCUSSION

- Meteorological Overview:

A closed upper-level low along the Pacific Northwest coast keeps us in a southwest flow aloft pattern through the rest of the week. This pattern bringing a warm and unstable airmass will continue daily chances for showers and thunderstorms through Saturday. Strong to severe thunderstorms will be possible each afternoon and evening through Saturday.

For the rest of today, isolated showers and thunderstorms have developed over Southwest MT and the higher terrain. Instability approaching 2,000 J/kg of CAPE with 30-50kts of bulk effective wind shear, storms will be capable of becoming severe through the evening. Large hail is the primary threat with lots of elevated instability but severe wind gusts are also possible due to deep vertical mixing and 1,000+ DCAPE. Storms will continue to move off of the higher terrain and into the lower elevations over the next few hours and become more scattered in coverage. PWATs nearing an inch will also bring a flash flood threat for any storm producing heavy rainfall, mainly for the Horse Gulch Burn Scar and urban areas.

Friday's storm threat is not too different, but storms converging into clusters/a broken line will bring a primary wind threat, but isolated large hail will be possible. Similar high PWATs also bring another threat for flash flooding with heavy rainfall to the Horse Gulch Burn Scar and urban areas. Saturday's storm threat will be more isolated, with best chances in Southwest MT.

Sunday keeps trending dry with that closed low moving to the north, with hotter temperatures reaching into the 90s across lower elevations. However, dry conditions don't look to stay long as another "dirty ridge" builds back in for the week, keeping daily chances for showers and thunderstorms. -Wilson

AVIATION

16/18Z TAF Period

Showers and thunderstorms will quickly develop across the mountains of Southwest Montana and along the Continental Divide between 19-21z this afternoon, with this activity then lifting north and east over the adjacent plains and valleys through 03-06z Friday. While most terminals have a chance of seeing thunderstorms over this time frame the KHLN, KBZN, and KEKS terminals have the highest chance of seeing strong to severe thunderstorms, especially between 16/19z and 17/01z. Primary threats with any severe thunderstorms will be wind gusts in excess of 50kts and hail larger than 1" in diameter. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 61 95 61 95 / 20 30 70 10 CTB 59 89 59 91 / 50 40 50 10 HLN 62 93 62 93 / 40 50 70 20 BZN 58 93 57 92 / 30 40 60 10 WYS 49 85 49 85 / 20 50 80 20 DLN 55 88 55 89 / 50 40 50 10 HVR 62 97 62 97 / 40 10 60 10 LWT 56 89 56 90 / 40 10 70 10

TFX WATCHES/WARNINGS/ADVISORIES

Flash Flood Watch until midnight MDT tonight for Big Belt, Bridger and Castle Mountains.