textproduct: Great Falls

KEY MESSAGES

- Very warm temperatures and breezy to windy conditions continue through at least mid-week, with Wednesday being the hottest day.

- Localized Elevated Fire Weather conditions for portions of Blaine and Fergus Counties this afternoon, and then for much of the forecast area on Wednesday.

- Shower and thunderstorm activity will become more pronounced during the second half of the work week, with some strong to severe storms on Wednesday.

DISCUSSION

/Issued 524 AM MDT Mon May 11 2026/

- Meteorological Overview:

A broad trough and attendant cold front will pass through the Northern Rockies today and bring a few isolated showers/storms, gusty northwesterly winds, and slightly cooler temperatures. The strongest winds and highest concentration of convection will be along the Hi-Line. Despite the increased cloud cover and slightly cooler temperatures, there will be at least a brief period of localized elevated fire weather conditions for Hill, Blaine, and portions of Fergus counties.

Ridging aloft quickly amplifies Tuesday into Wednesday ahead of an approaching Pacific split trough. This will bring the warmest temperatures of the season so far with afternoon highs approaching record levels in the upper 80s and lower 90s on Wednesday. As the trough draws closer, an unstable southwesterly flow will quickly develop during the afternoon hours. This looks to initiate scattered shower and thunderstorm activity in the late afternoon and evening hours. Deep layer shear, ML CAPE in the 300 to 800 J/kg range, and dry sub-cloud layers should be sufficient for a few strong to severe thunderstorms, with localized gusty winds over 50 mph being the primary hazard. Hail and downpours may also be introduce when the storms move northeastward into central/north-central MT where CAPE and moisture levels will be highest, particularly for plains locations east of I15. Before the showers and storms develop, the hot, dry, and breezy to locally windy conditions will bring at least localized elevated fire weather conditions to much of the forecast area (see the fire weather section for more details). Areas along and south of I90 will be most susceptible to non-thunderstorm wind gusts over 50 mph, especially the Dillon area and the Madison Valley.

The split trough moves in Thursday through Friday and brings more scattered shower/thunder activity, breezy to windy conditions, and slightly cooler temperatures. Around two thirds of the ensembles keep the main circulation over the Great Basin area, but the remaining minority bring it over southwest MT for a bit more widespread showers and even some mountain snow on Thursday. Stronger winds aloft move in behind the departing trough Thursday into Friday, but there are still significant timing differences among ensemble members to determine the more precise wind details. The parade of Pacific troughs continue into the weekend for more unsettled conditions. - RCG

- Forecast Confidence & Scenarios:

Winds and thunderstorms Wednesday and Thursday...

Ensemble guidance continue to highlight favorable conditions for scattered showers and thunderstorms capable of at least a localized strong winds in the 50 to 70 mph range. Even if we take the lower end of guidance around 200 to 500 J/kg of CAPE, the 40 to 50 kts H500 flow and hot, dry surfaces will cover the void if CAPE falls short. This activity looks to develop over the higher terrain of west and southwest MT around mid afternoon before racing northeastward and reaching central/nortrh-central MT by the evening hours. Some of the more aggressive ensembles highlight CAPE approaching and even exceeding 1,000 J/kg for plains locations east of I15, which would support hail and downpours in addition to strong wind gusts.

Even without the convective wind gusts, the aforementioned 40 to 50 kt H500 flow may transfer to the surface via robust diurnal heating. This looks most concerning for southwest MT where these winds will funnel through the narrow south to north oriented Valleys. This may increase the risk for non-thunderstrom wind gusts in excess of 55 mph in addition to the thunderstorm wind gusts. The latest NBM probabilistic guidance isn't very impressive with around 50 to 60% chance for 55 mph +wind gusts, mostly over the aforementioned south to north oriented valleys. This may increase with the latest suite of deterministic models beginning to trend more aggressive.

uncertainty increases on Thursday in regards to winds and precipitation. As mentioned in the meteorological section, the more widespread precipitation and even mountain snow will be dependent on the closed circulation moving through the forecast area. Winds will be highly dependent on the timing of the stronger winds aloft. Both of these factors still need some time to resolve. - RCG

AVIATION

11/18Z TAF Period

VFR conditions will largely prevail across the terminals of Southwest through North Central Montana through the 1118/1218 TAF period, but isolated showers and thunderstorms are possible near the KWYS terminal and then along the International Border (i.e. near the KCTB and KHVR terminals) through 02z Tuesday. Main impact from any shower or storm will be gusty and erratic winds up to 40kts, with CIGS falling to low-VFR. Otherwise the main impact to terminals through the day will be breezy and gusty northwest winds in wake of a cold front that will move across the region from the late morning through afternoon hours. - Moldan

FIRE WEATHER

Gusty northwesterly winds will develop in wake of a cold front today, with gusts of between 35 to 45 mph occurring along and east of a Chinook, to Winifred, to Lewistown line. Localized elevated fire weather conditions will exist across eastern portions of Fire Weather Zones 113 and 115 due to the above normal temperatures, gusty northwest winds, and dry conditions; however, widespread critical fire weather conditions are not presently supported with a slight increase in moisture that will prevent minimum RHs from crashing into critical territory on a widespread basis.

Ridging aloft returns Tuesday and brings the warmest temperatures of the season so far for Tuesday and Wednesday while minimum relative humidity values fall to critically low levels. Increasing south to southwest surface winds on Wednesday will push temperatures into the upper 80s to low 90s across all of Southwest through North Central Montana, with the strongest winds occurring along and south of the I- 90 corridor in Southwest Montana and west of the I-15 corridor across Central and North Central Montana. Multiple GEFS members within the HDWI on Wednesday climb to between the 90-95Pct, with ERC values forecasted to climb to within the 80Pct of climatology. There will also be scattered shower and thunderstorm activity that will move in southwest to northeast fashion Wednesday afternoon and evening. Lightning and localized gusty winds in excess of 50 mph will be the primary hazards with this activity.

The one confounding factor that can preclude the heat, winds, and thunderstorms from realizing their potential will be excessive mid- and high level cloudiness during the peak diurnal mixing hours. If clouds increase in the late morning and early afternoon, then there will be less of a fire risk and instances of severe convective gusts and lightning. A Fire Weather Watch may be warranted on Wednesday for lower elevations, but will continue to holdoff for now given the uncertainty on cloud cover and winds. - RCG/Moldan

..RECORD OR NEAR RECORD MAXIMUM TEMPERATURES FORECAST FOR MAY 13

LOCATION FORECAST HIGH RECORD HIGH Boulder 84 83 Bozeman MSU 89 86 Chester 89 86 Choteau 89 85 Conrad 89 87 Cut Bank 86 84 Dillon 88 87 Fort Benton 94 88 Great Falls 91 89 Helena 90 89 Havre 93 92 Lewistown 88 88 Shelby 87 84 Stanford 88 86 West Yellowstone 83 81

PRELIMINARY POINT TEMPS/POPS

GTF 75 43 79 50 / 10 0 0 0 CTB 69 39 76 46 / 20 10 0 0 HLN 77 42 83 49 / 0 0 0 0 BZN 79 39 81 45 / 0 0 0 0 WYS 77 36 80 42 / 20 20 0 0 DLN 80 42 83 49 / 0 0 0 0 HVR 74 39 78 46 / 30 10 0 0 LWT 72 39 74 46 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES