textproduct: Great Falls

KEY MESSAGES

- Severe storm clusters and a broken squall line will sweep Central Montana and the Hi-Line Wednesday afternoon bringing winds over 60 mph.

- High Wind Warning remains in effect for Madison River Valley where non-thunderstorm winds could gust upwards of 60 mph.

- |Strong winds across Southwest Montana with minimum relative humidity 20-30% will cause critical fire weather conditions.

DISCUSSION

/Issued 1212 AM MDT Wed Sep 2 2026/

- Meteorological Overview:

Wednesday Severe Convective Threat:

An unusually energetic mid-level shortwave impulse and accompanying jet streak will eject northeast across Montana Wednesday afternoon. Extreme Forecast Index guidance (EFI) highlights an anomalous wind event, with values well above climatology and a high Shift of Tails over Central Montana into the Hi-Line.

Poleward monsoonal moisture will yield high surface dewpoints in the upper 40s to low 50s across the plains. Diurnal heating will foster robust destabilization across Central Montana and the Hi-Line, featuring widespread SBCAPE values between 1500 and 2000 J/kg and deep-layer shear near 35kts. Cloud bases will remain high, largely above 8kft, above the superadiabatic boundary layer with steep mid and low-level lapse rates upwards of 9 C/km.

Regarding timing, convection will initiate over SW Montana in the early Afternoon (17z-19z). Storm mode looks to largely favor individual cell clusters initiating off terrain and tracking northeast. As the storms crest the ridge line, continued development is likely as the system begins to grow more upscale into a broken squall line moving into the Plains and Hi-Line. Storm motion will be fast, assisting the storms out of the CWA by late afternoon into evening (23z-01z). Strong sub-cloud evaporation will generate high DCAPE values over 1200 J/kg, which when coupled with efficient momentum transfer from the 40-60kt mid-level jet, will drive a high- confidence threat for severe wind gusts exceeding 60 to 75 mph across the Plain and Hi-Line.

Southwest Montana Synoptic High Wind & Fire Weather:

An anomalous 700mb southerly jet streak of 30-40kts will persist over southwest Montana Wednesday afternoon through Thursday evening. Deep boundary layer mixing and topographic funneling down North- South oriented valleys will support strong synoptic gusts.

NBM probabilities for gusts exceeding 55 mph are highest across the Madison River Valley nearing 45% with localized gusts over 60 mph possible, supporting High Wind Warning criteria. Adjacent valleys, Beaverhead and Blacktail will experience strong winds but confidence is too low (<20%) in winds exceeding 55 mph. That being said, 45-50 mph gusts are certainly possible. Poor overnight decoupling Wednesday night will keep the threat elevated as we move into Thursday evening, though confidence decreases in warning criteria winds. Additionally, deep mixing will drop minimum RH as low as 15% with widespread 20-30% values present across the entirety of southwest Montana. Those factors combined with ongoing fires and cured fuels will lead to critical fire weather conditions both days.

Extended Outlook:

Persistent southwest flow remains anchored in the wake of Wednesdays wave. A broader mid-level shortwave and upper jet streak will traverse the region late Friday into Saturday. While confidence is low in the exact progression of events, its worth noting the potential return of synoptic ascent and thus the potential for gusty winds and scattered convection.

The cut off pacific low responsible for the barrage of shortwaves and resulting active period looks to dislodge late weekend into next week. Confidence is low at the immediate impacts but there is increasing chances of a regime change to broader synoptic forcing.

- Forecast Confidence & Scenarios:

Wednesday Convective Winds: High confidence (>80%) in thunderstorms that will pose the risk of severe gusts in excess of 60mph given strong multi-ensemble agreement. Soundings area wide support efficient deep layer shear and strong instability for storm maintenance.

SW Montana High Winds: Medium Confidence (60%) in meeting High Wind Warning criteria in terrain-aligned valleys on Wednesday evening as the peak of the 700mb jet is overhead. Confidence decreases into Thursday as the jet streak exits north and east.

Fire Weather: Medium Confidence (50%) in critical conditions on Wednesday and Thursday. Winds are the highest source of confidence, humidity is our lowest with values borderline. However, deep mixing advertised by the models does support values approaching critical values.

Extended Regime: Low Confidence (30%) in the overall changes in the upper-level pattern. Cut off lows are notoriously difficult to pin point in the long range as are the resulting impacts.

- JR [Thnks to Eric A & Evan T for preparing me to write my 1st AFD]

AVIATION

02/12Z TAF Period

VFR condtions will predominately prevail throughout the duration of the TAF period; however, a line of thunderstorms moving northeast from 02/18z through 03/03z may briefly reduce conditions to IFR/MVFR, especially where heavy rain falls. More concerning during this timeframe will be the potential for wind gusts to exceed 50kts at the KHLN, KGTF, KHVR, and KCTB terminals as the thunderstorms move northeast. Mountain obscuration will be limited to where precipitation occurs. - Moldan

For KGTF: Winds will quickly pick up out of the south/southwest by the early afternoon. There is a 50% chance for exceeding a 15kt crosswind between 02/18Z and 02/20Z. After 02/20Z there is a chance for thunderstorms which will produce strong/gusty winds and lightning. Recent models have tried to push the timing of the thunderstorms earlier but the current window of confidence is still between 02/22Z and 02/24Z. Right now there is very good agreement among the CAMs for wind gusts in excess of 50kts with gusts up to 60kts well within the range of possibilities. Additionally there is an expected wind shift with this line from a more south/southwest direction to the west behind the line of thunderstorms. Gusty winds will continue through at least 03/04Z before slowly tapering off as the night continues. The threat for crosswinds will be low but the headwinds may be above 15kts through at least 03/03Z. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 84 50 77 49 / 50 10 0 10 CTB 78 45 68 44 / 90 40 0 30 HLN 80 49 77 50 / 80 10 0 10 BZN 80 48 77 48 / 10 0 0 0 WYS 65 34 64 35 / 10 10 20 0 DLN 75 45 73 45 / 30 10 0 10 HVR 88 49 81 50 / 40 50 0 0 LWT 84 49 79 51 / 30 10 0 0

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Warning from noon today to 9 PM MDT this evening for Madison River Valley.

High Wind Watch from this evening through Thursday evening for Madison River Valley.