textproduct: Great Falls

KEY MESSAGES

- Very warm to hot temperatures continue for much of the workweek,especially over the plains.

- There will also be daily rounds of showers and thunderstorms, some will be strong to severe and may contain localized heavy downpours.

- Cooler temperatures and an opportunity for more widespread precipitation moves in this weekend.

DISCUSSION

/Issued 527 PM MDT Tue May 26 2026/

- Meteorological Overview:

A blocking pattern will maintain troughing over the western conus for the next week or so, with a mid -level closed low drifting southeastward from the Pacific NW into the Great Basin and remaining there through the workweek. This will bring in an increasingly unstable south to southeasterly flow aloft over the Northern Rockies for daily rounds of showers and thunderstorms. A shortwave will provide bulk shear over 30 kts today while ML CAPE stays mostly in the 200 to 600 J/kg range, highest along the Rocky Mountain Front. The primary threat for this afternoon and evening will be localized gusty winds with inverted V forecast soundings, but some instances of near or severe hail can't be ruled out with H500 temperatures as low as -15C.

The flow aloft becomes weakly diffluent on Wednesday and Thursday and shear levels drop to negligible levels due to a general lack of shortwave energy; however, ML CAPE ramps up to over 1000 J/kg and PWATS begin to approach and even exceed the one inch mark, highest over central and north-central MT. The lack of forcing will result in lower storm coverage away from terrain, but localized downpours will become a concern in addition to a continued wind and hail threat.

The closed low and associated elongated trough over the Pacific NW will undergo some shearing and transformation before it swings northward into the Northern Rockies heading towards the weekend. This looks to bring a round of stronger to severe thunderstorms sometime between late Thursday through early Saturday; followed by a period of cooling temperatures and more widespread areas of rain and embedded thunderstorms later Saturday into Sunday. A southwesterly flow aloft then develops in the wake of the departing system and brings warming temperatures and more rounds of showers and thunderstorms next week. - RCG

- Forecast Confidence & Scenarios:

Daily rounds of showers and thunderstorms though Friday followed by more widespread precipitation this weekend...

Showers are already lifting northward into western and southwestern MT early this afternoon. Cloud cover is hampering CAPE and DCAPE over the southwest, but this should change as this activity moves over the warm and dry surface farther north. Forecast soundings indicate that strong wind gusts will be the primary hazard to deal with and this is corroborated by the latest suite of hires guidance. Convective wind gusts will generally range somewhere between 35 and 60 mph, strongest along the Rocky Mountain Front and over central/north-central MT, especially along and southwest of a Sweetgrass Hills to Lewistown line. Storm motion will be south/southeast to north/northwest, a little different than the typical southwest to northeast motion we typically see. This added easterly component may also increase the risk for an isolated land spout, although the chances for such an event are less than 2% within 25 miles of any point. The strongest and most widespread shower and thunderstorm activity will generally occur between 3 and 9 pm tonight, with a more isolated brand of showers continuing into the overnight hours.

The wind shear necessary for the maintenance of stronger showers and thunderstorms drops off on Wednesday and Thursday with lack of shortwave energy and the forcing from the main low still far to our southwest. Some of this void will be fulfilled by ML CAPE levels increasing to over 1000 J/kg and PWATs nearing the one inch mark, but storm coverage should generally be slightly less than today for areas away from higher terrain. The slower storm motion and higher moisture levels will increase the concern for localized flooding for higher prone areas such as burn scars, especially for mountain areas and lower elevations west of I15. There will also be continued instances of thunderstorm winds and hail.

Most ensembles keep the main closed low pressure system over the Great Basin area through the workweek before swinging back northward towards the Northern Rockies this weekend. If this pans out as forecast, there will be a period of stronger thunderstorms sometime between late Thursday and early Saturday followed by more widespread rains and embedded thunderstorms for the second half of the weekend. Although, there are some uncertainties to iron out, the heaviest precipitation is projected to occur along the Continental Divide and over central and southwest areas extending from the highway 12 corridor northward to the central island ranges. The latest NBM probabilistic guidance currently gives a 50 to 70% chance for rainfall amounts exceeding one inch for the 48 hour period ending 6 am Monday morning. - RCG

AVIATION

27/06Z TAF Period

For locations that received rain earlier today and are able to clear out, there is a chance for fog developing overnight. At this point in time, confidence was too low to include it in the TAF but there were some hints in the models that it could develop at KWYS, KBZN, and KHLN.

For Wednesday afternoon, mid-level clouds are expected to stick around through much of the day with another chance for showers and thunderstorms in the afternoon and evening. The biggest impact to terminal operations will be gusty and erratic winds up to 45kts beneath and near thunderstorm activity. MVFR to IFR conditions are possible under heavier showers with mountain obscuration expected to remain a concern through much of the period. -thor

PRELIMINARY POINT TEMPS/POPS

GTF 55 85 56 91 / 60 40 30 20 CTB 54 82 52 88 / 60 50 50 30 HLN 51 84 52 90 / 70 50 30 20 BZN 45 83 46 90 / 50 40 30 20 WYS 37 79 39 83 / 50 40 10 20 DLN 43 79 46 87 / 60 40 20 20 HVR 61 95 55 97 / 30 30 30 20 LWT 53 85 52 89 / 40 40 30 30

TFX WATCHES/WARNINGS/ADVISORIES

Heat Advisory until 9 PM MDT Wednesday for Hill County-Northern Blaine County.