textproduct: Great Falls

KEY MESSAGES

- Daily chances for showers and thunderstorms, with higher rainfall amounts through the weekend, with the potential for some minor mountain snow at ridge tops on Sunday.

- Breezy and gusty winds on Sunday, especially along the Rocky Mountain Front and across the Montana Hwy 200 corridor.

- Well below normal temperatures on Sunday.

DISCUSSION

- Meteorological Overview:

An active and unsettled pattern will bring daily chances for precipitation, with the exception of Monday, to most locations over the course of the next week. Over the course of the next week temperatures will fluctuate between near to below normal, with the coldest day of the upcoming week arriving on Sunday as a large upper level trough moves overhead. As this upper level trough and closed low moves overhead on Sunday snow levels will fall to as low as 7000ft across the Glacier National Park region, which could lead to some light snow accumulations for the peaks of the park. Additionally on Sunday, increasing cross barrier flow will contribute to breezy and gusty west winds for most locations across Southwest through North Central Montana, especially over the plains of Central and North Central Montana. Prior to the trough moving overhead on Sunday moist and unstable southwest flow aloft combined with sufficient shear will help to support the potential for a few strong thunderstorms across Southwest Montana today, generally along and south of the I-90 corridor. The main threat from any storm that does become strong or severe will be strong winds, especially along the immediate Idaho and Wyoming borders.

Beyond the weekend broad troughing continues to be supported across the Western CONUS, especially from Tuesday through next Saturday, with moist southwest flow over the Northern Rockies helping to support the aforementioned daily chances for showers and thunderstorms. - MOldan

- Forecast Confidence & Scenarios:

Gusty Winds on Sunday...

ECMWF EFIs continue to support the potential for a climatologically unusual, with respect to the final week of August, wind event along the Rocky Mountain and across the Montana Hwy 200 corridor on Sunday as cross barrier flow increases to between 35-45kts; which is generally 1 to 1.5 standard deviations above normal per NAEFS analysis. BUFKIT soundings across the aforementioned areas depict deep mixing to between 4-6kft during the afternoon hours on Sunday, which will coincide with the passage off the upper level trough axis and associated subsidence. This gives increased confidence that a period of strong and gusty winds will occur, especially across wind prone areas of Central and North Central Montana, during the afternoon and into the early evening hours. While High Wind highlights do not look necessary those traveling along the Rocky Mountain Front in high profile vehicles should be prepared for periods of strong cross winds.

Listed below are the NBM5.0 probabilities for wind gusts to exceed 35, 45, and 55 mph for select locations across North Central, Central, and Southwest Montana.

Sunday's Wind Gusts Probabilities LOCATION > 35 mph | > 45 mph | > 55 mph Babb 100% | 60% | 10% East Glacier 100% | 70% | 5% Browning 100% | 70% | 10% Cut Bank 100% | 75% | 10% Shelby 90% | 20% | <5% Havre 90% | 40% | <5% Heart Butte 100% | 85% | 15% Choteau 80% | 15% | <5% Augusta 85% | 20% | <5% Great Falls 90% | 55% | 5% Lewistown 65% | 10% | 0% Helena 85% | 25% | <5% Bozeman 55% | 20% | <5% Dillon 75% | 10% | 0% Ennis 60% | 10% | 0% West Yellowstone 85% | 10% | 0%

Weekend Precipitation...

Multiple rounds of showers and isolated thunderstorms will bring beneficial rainfall to most of Southwest through North Central Montana, with the exception of the the East Glacier Park Region and Northern High Plains zones where the probability for wetting (0.1") rainfall is largely less than a 50% chance, over the next 48 hours (12z Saturday through 12z Monday). Outside of these areas most other locations have a 70%+ chance of a wetting rainfall through Sunday night, with a corridor of 40-80% chance probabilities for a 0.5" of rainfall for areas along and east of a Lewistown, to Great Falls, to Townsend, to Bozeman, to Ennis, to Monida Pass line.

Listed below are the NBM5.0 probabilities for rainfall amounts to exceed 0.25", 0.50", and 1.00" for select locations across North Central, Central, and Southwest Montana.

Precipitation Probabilities thru Sunday Night LOCATION > 0.25" | > 0.50" | >1.00" Browning 10% | 5% | <5% Cut Bank 40% | 10% | 5% Havre 60% | 20% | 10% Great Falls 85% | 50% | 10% Lewistown 90% | 45% | 10% Helena 80% | 40% | 5% White Sulphur Springs 85% | 45% | 10% Bozeman 85% | 60% | 15% Dillon 35% | 10% | 5% Ennis 90% | 65% | 20% West Yellowstone 95% | 90% | 45%

- Moldan

AVIATION

29/06Z TAF Period

Rain showers will begin in Southwest MT around 08Z, moving northeast through KWYS, KEKS, KBZN and KLWT. Thunderstorms will begin to develop in Southwest MT around 16Z, also moving in a northeast fashion in the hours to follow. The main threat with these thunderstorms is gusty, erratic winds and frequent lightning, although any thunderstorm that develops will be capable of producing small hail and brief, heavy downpours.

As the thunderstorms dissipate in Central MT, rain showers will develop over KHLN and KGTF around 03Z. VFR conditions will generally prevail throughout this TAF period except for during thunderstorms, in which MVFR conditions are possible. Due to passing rain showers and thunderstorms there will be mountain obscuration at times.

For KGTF: Rain showers will begin to develop around KGTF starting near 15z, in which visibility may lower to MVFR conditions. Thunderstorms will begin to develop around 21Z, before dissipating around 02Z. Rain showers will follow this thunderstorm activity. With the wind direction being predominately southwest, there is a less than 10% chance of a cross-wind exceeding 15 kts outside of a thunderstorm. -Dzomba

PRELIMINARY POINT TEMPS/POPS

GTF 80 50 64 45 / 70 80 70 0 CTB 75 47 61 44 / 60 60 40 10 HLN 78 48 66 44 / 80 90 50 0 BZN 76 46 67 38 / 60 80 80 0 WYS 63 36 61 29 / 90 90 90 0 DLN 74 45 68 37 / 70 70 50 0 HVR 83 52 67 48 / 20 80 60 10 LWT 76 49 63 42 / 60 70 80 10

TFX WATCHES/WARNINGS/ADVISORIES

None.