textproduct: Great Falls

KEY MESSAGES

- One more day of warm, dry, and windy conditions on Friday with elevated to critical fire weather conditions. A Red Flag Warning has been issued for portions of central and southern Montana.

- A significant pattern change is expected this weekend with a series of cold fronts bringing significantly cooler air into the region.

- A developing weather system coinciding with this push of cooler air is forecast to deliver the season's first widespread snowfall to the region. Confidence in wet snowfall impacts is highest in the higher elevations of central and west-central MT, including at pass level.

DISCUSSION

- Meteorological Overview: A ridge of high pressure centered over the Pacific coast continues to break down tonight and Friday, with increasing westerly winds ahead of an approaching cold front leading to another day of warm, dry, and windy conditions over the region for Friday. Wind gusts during the day are forecast to be in the 30-40 mph range, although the usual locations nearby mountain ranges could see gusts of 50+ mph. A Red Flag Warning has been issued for areas with the most favorable fuel conditions; see the below FIRE WEATHER section for details.

This cold front is associated with a Gulf of Alaska upper level low ejecting south from otherwise strong zonal flow over Canada. This initial push of cooler air Friday evening into the overnight brings light precipitation mainly to the Rocky Mountain Front and immediately adjacent areas with snow expected above about 6000 feet.

As the upper low pinches south late Friday and Saturday, it elongates in a positive orientation, extending from the California coast into Central Canada...leaving Montana squarely under broad southwest flow and in a broad baroclinic zone. Models continue to favor the dominant vorticity maxima associated with the upper level trough splitting from the northern stream, allowing high pressure to build into Alberta...providing a reliable feed of cooler, drier air from the north...and also providing enhanced convergence and developing FGEN over central and west-central MT. This combination of crashing temperatures and increasing coverage of precipitation is expected to bring snow levels to valley floors Saturday evening with locally intense snowfall from mesoscale snow banding Saturday night through Sunday morning. The northerly flow regime north of the I-90 corridor also favors upsloping into the southern RMF and Little Belt Mountains, adding to confidence in impacts in this part of the forecast area.

For snowfall totals...the most likely scenario predicts 6-12 inches for mountainous areas, although the most favored upslope areas and areas under a mesoscale band have potential for 18+ inches through Sunday. The lower elevations are significantly trickier; warm surface temperatures upon onset will limit accumulation to some degree despite crashing temperatures, however by Sunday conditions are expected to be cool enough for mostly all snow to fall at all elevations.

- Forecast Confidence & Scenarios: As is typically the case with shoulder-season snowfall events, the wet nature of the snow is a concern. Many trees still have leaves, and as wet snow accumulates and temperatures continue to decrease there is concern for damage to trees and power lines especially as wind gusts increase during the Saturday night/early Sunday timeframe. There remains considerable uncertainty regarding the rain-snow transition, especially along and south of the I-90 corridor as the influence of tropical remnants threatens to enter the picture. In any case, this upcoming holiday weekend will be much cooler and damp, even if not seeing significant snowfall. Outdoor activities including hunting, camping, etc will be significantly impacted by these conditions.

AVIATION

08/18Z TAF Period.

Predominantly VFR conditions throughout the period with gusty winds up to 25KT to 30 KT out of the south to southwest for most terminals beginning tomorrow morning. There is decent potential for low level wind shear to occur in the KCTB area and towards the Rocky Mountain Front from 13Z to 16Z tomorrow morning. Other potential hazards will be mountain wave turbulence over and up to 25 miles downwind of prominent ranges. - Akins

FIRE WEATHER

A Red Flag Warning has been issued for portions of North Central and Southwest MT for Friday because of low humidity, gusty winds and warm temperatures. A cold front will move southward through the CWA on Friday night, which will bring in cooler air and higher humidity. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 52 81 41 50 / 0 0 30 90 CTB 51 72 32 43 / 0 0 30 40 HLN 47 82 44 53 / 0 0 20 80 BZN 43 82 44 61 / 0 0 10 80 WYS 31 70 38 58 / 0 0 10 80 DLN 41 81 46 62 / 0 0 0 70 HVR 47 84 36 53 / 0 0 10 50 LWT 51 83 41 54 / 0 0 0 70

TFX WATCHES/WARNINGS/ADVISORIES

Winter Storm Watch from Saturday afternoon through Sunday afternoon for Gates of the Mountains-Southern High Plains- Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Storm Watch from Saturday evening through Sunday afternoon for Elkhorn and Boulder Mountains-Helena Valley-Little Belt and Highwood Mountains.

Winter Storm Watch from Saturday evening through late Sunday night for Big Belt, Bridger and Castle Mountains-Gallatin and Madison County Mountains and Centennial Mountains.

Red Flag Warning from noon to 9 PM MDT Friday for Central and Eastern Lewis and Clark National Forest Areas-Helena and Townsend Ranger Districts of the Helena National Forest.