textproduct: Great Falls

KEY MESSAGES

- Very warm, mostly dry, and breezy to locally windy conditions develop late Sunday through mid- week.

- Potential for record high temperatures, especially on Wednesday as most locations rise into the mid-80 to near 90 degrees.

- Shower and thunderstorm activity returns Wednesday afternoon and persists through the remainder of the work week and upcoming weekend.

UPDATE

Slight changes were made to the near-term precipitation chances to reflect trending obs and the latest model guidance. But otherwise no major changes were made to the going forecast. -thor

DISCUSSION

/Issued 542 PM MDT Sat May 9 2026/

- Meteorological Overview:

Transient ridging aloft quickly replaces the departing wave and brings much warmer and drier conditions by Sunday afternoon. Surface winds will begin increasing late Sunday through Monday in response to a another shortwave moving eastward along the Canadian border. The primary concern with the winds will be at least a brief period elevated fire weather conditions, particularly along the Hi-Line and down in the southwest where antecedent ground conditions are driest. The passing shortwave will have little impact on temperatures with afternoon highs continuing to run mostly in the 70s and 80s for lower elevation locations.

A closed mid-level trough moving onto the Pacific coast will bring a more unstable southeasterly flow aloft for the Wednesday through Friday period. This will introduce daily rounds of shower and thunderstorm activity, some of which maybe on the stronger side with CAPE values approaching 1,000 J/kg on some days. Windy conditions look to develop when the trough and attendant cold front approaches either Thursday or Friday. Ensemble guidance continues to display disagreements with the timing of the trough and this will have a significant impact on the timing of thunderstorm activity and the strongest winds. The overall unsettled weather pattern looks to continue heading into next weekend with more Pacific troughs approaching the Northern Rockies. - RCG

- Forecast Confidence & Scenarios:

Gusty Winds on Monday...

A cold front dropping southeast from Alberta and Saskatchewan late Monday morning will bring a wind shift from the southwest/west to northwest into the early afternoon, with gusty conditions developing in wake of the front, particularly along and north of the US Hwy 2 corridor. NBM probabilities for wind gusts in excess of 35 mph range from range from a 60-90% chance across much of Southwest through North Central Montana, with a 30-70% chance that gusts exceed 45 mph across northern Blaine County.

Hot Temperatures on Wednesday...

While the upcoming work week will see well above normal temperatures each day there is growing consensus that Wednesday will be the warmest day of the week, with the main limiting factor being increasing upper level cloud cover which could hold temperatures back several degrees. NBM probabilities for temperatures to exceed 90 degrees during the day on Wednesday range from a 15-30% chance across most valley and plains locations across Southwest through North Central Montana, with the exception of the Helena Valley and Golden Triangle of the plains of Central and North Central Montana where probabilities are between 50-70%. With the exception of Havre where the record is 92 degrees on Wednesday, any location that exceeds 90 degrees would set a new record high for May 13th. Across these same areas (Helena Valley and Golden Triangle of the plains of Central and North Central Montana) there is even a 10-30% chance that temperatures exceed 95 degrees. - Moldan

Increased shower and thunderstorm activity and winds Wednesday through Friday...

Ensembles continue to agree on a more unstable southwesterly flow developing and bringing multiple rounds of shower and thunderstorm activity during this timeframe; however, there are inconsistencies on which day will have the highest CAPE in the 500 to 1,000 J/kg range. Overall, there is an expectation for daily lightning and gusty wind threats given at least a few hundred joules of CAPE and very warm and dry sub cloud layers. The timing of the trough and associated shortwaves will ultimately determine which day will see the most robust convective activity and severe hail/wind threat.

Around a quarter of the ensembles were highlighting the trough ejecting eastward into the Northern Rockies as early as late Wednesday into Thursday and bringing widespread strong winds with it. The newer deterministic guidance seems to be backing off of this solution, though anomaly indices, including the ECMWF Extreme Forecast Index still have a weak signal for anomalously strong winds and NBM probabilistic guidance still paints a daily 20 to 30% chance for gusts over 55 mph for much of the forecast area Wednesday through Friday. Both the thunderstorm activity and wind situations will be monitored for this timeframe in the coming days. - RCG

AVIATION

10/00Z TAF Period

Isolated showers continue across North-Central MT through the evening. However, VFR conditions prevail with showers. Clouds will clear once showers wind down later tonight. Sunday afternoon, breezy west to southwest winds begin to pick up. -Wilson

FIRE WEATHER

Well above, to near record, high temperatures are expected across the Northern Rockies for the better part of the upcoming work week, with the hottest day occurring on Wednesday as temperatures soar into the mid-80s to near 90 degrees. Low afternoon relative humidity values will be found each day as values fall to around 15% to 25%, with dry conditions expected through Wednesday afternoon before showers and thunderstorms make a return for the second half of the work week. Breezy and gusty northwest winds develop in wake of a cold front diving south from Canada on Monday over the plains of Central and North Central Montana, with gusts of generally 30 to 45 mph occurring along and east of the US Hwy 87 corridor from Havre to Great Falls. Hot-Dry-Windy Index (HDWI) on Monday for those Fire Weather Zones east of the aforementioned region, mainly 113 and 115, only rise to within the 50-75Pct; which tends to support an elevated fire weather day for those areas that have not sufficiently green-up over the last several weeks. By Wednesday south to west surface winds increase across all of Southwest through North Central Montana, particularly along and south of the I-90 corridor in Southwest Montana and west of the I-15 corridor across Central and North Central Montana. These winds will help to push multiple GEFS members of the HDWI to between the 75-95Pct, which will need to be monitored for the potential need of fire weather highlights. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 41 81 49 76 / 10 0 10 0 CTB 40 77 50 71 / 20 10 10 0 HLN 37 80 46 78 / 10 0 10 0 BZN 33 78 43 79 / 10 0 10 0 WYS 27 70 36 76 / 10 0 0 0 DLN 35 79 43 81 / 0 0 0 0 HVR 39 81 51 75 / 20 0 10 0 LWT 37 76 46 72 / 20 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.