textproduct: Great Falls

KEY MESSAGES

- Light isolated to scattered snow showers across the region Tuesday.

- Widespread, longer duration, strong winds possible late Wednesday into Thursday across the region.

- Heavy mountain snowfall will impact the Continental Divide during the second half of the week, with lower elevation snowfall possible Friday and Saturday.

DISCUSSION

- Meteorological Overview:

Zonal flow aloft this week will keep streaming in moisture and Pacific waves through the weekend and into the beginning of next week. This will keep persistent snow along the Continental Divide through the forecast period. better synoptic forcing will allow for better chances for snow to set up late this week along the mountains and lower elevations. For the near term forecast for today, a mini shortwave trough will move through the region Tuesday. This will produce isolated to scattered brief, snow showers across the region throughout the day.

Wednesday, westerly flow aloft will keep orographic snow over the Continental Divide throughout the day. However, strong 700mb winds and a low pressure system coming from the West Coast will begin a period of more impactful weather. Starting with widespread strong winds late Wednesday through Thursday. Mountain wave activity will be possible overnight Wednesday into Thursday morning, with a strong pressure gradient developing. Diurnal mixing kicking in late Thursday morning will allow for the stronger winds aloft to mix down to the surface. The strong winds will also bring an elevated risk for wildfire spread if one does start.

The low pressure system moving in late Wednesday and Thursday will also bring heavy snow to the Continental Divide. A front pushing south into MT and stalling across North-Central MT will help bring snow down to lower elevations. This front wavering/oscillating through the weekend and continuous warm moist air over running colder air will continue to bring periods of impactful snow to the mountains/North Central MT lower elevations Friday and Saturday. Strong winds late Wednesday night and into Thursday will also bring the concern for whiteout/blizzard conditions in the mountains and the Rocky Mountain Front foothills due to blowing snow. One final push of the front south Saturday will wind down the impactful snow.

Next week, an upper level ridge builds in over the Western U.S., keeping MT on the eastern side. Temperatures look to warm up to well above normal midweek. There will be additional chances for precipitation at times due to the northwest flow aloft pattern.

- Forecast Confidence & Scenarios:

Snow Tuesday:

Weak upper-level forcing will keep snow lighter over Marias Pass (up to a few inches) and lower elevations. Snowfall accumulations from snow showers across lower elevations will be more localized, and above freezing afternoon temperatures will make snow struggle to accumulate.

Winds Wednesday Into Thursday:

There is a concern for another longer duration and widespread wind event late Wednesday into Thursday across the region. ECMWF EFI (Extreme Forecast Index) values range from 0.8-0.95 for most of the region (the Hi-line has lesser values) for Thursday. The 700mb jet peaks to 75-90kts Thursday morning. Probabilities for 58 mph exceed 60-70% chances across the region, except the Hi-line. The persistent strong pressure gradient throughout the event brings a concern for persistent sustained winds. Vegetation weeekend from last Sunday's wind storm will be impacted in this upcoming event. One uncertainty that remains is how much blowing dust occurs Thursday. Recent snowfall across the lower elevations should help the dust not be as severe as Sunday's wind event. However, with the higher intensity of the winds, I can't rule out some blowing dust.

Impactful Snowfall Wednesday Evening Through Saturday:

There remains the concern for impactful heavy snowfall along the Northern Continental Divide Wednesday through Saturday. Remnant moisture from the atmospheric river along the Pacific U.S. coast will bring high amounts of QPF to the Northern Continental Divide. ECMWF EFI (Extreme Forecast Index) has values reaching 0.70-0.90 and shifted tails contours are between 1-2 over the Continental Divide Wednesday evening through Friday evening. Friday over North-Central MT, PWAT values reach to 0.40-0.50", which just about exceeds the 90th percentile for this time of year. This combined with the front stalling out, looks to be a sufficient set up for impactful snow at lower elevations Friday and Saturday. Over a 72 hour period through Saturday afternoon, there's a 50-70% chance for 6" of snow across the North-Central MT plains.

Models also show a little bit of instability Thursday, which will bring periods of moderate to heavy snow along the Continental Divide. This combined with strong winds along the Rocky Mountain Front foothills, brings a concern for blizzard conditions to set up late Wednesday into Thursday. The Day 3 Winter Storm Severity Index is showing Moderate impacts for Marias Pass and Major Impacts for U.S. 2 for East Glacier to Browning for blowing Snow impacts late Wednesday through Thursday. The long duration of the snow will increase stress on the infrastructure throughout the event. The heavy snow and strong winds Thursday will also bring the risk for downed trees and power lines. Traveling in the mountains/Marias Pass during this time frame will be dangerous to impossible.

South of the MT Highway 200/Little Belts/Snowys corridor looks to see a less of an impact from this system due to the region being south of the stalled front. Impacts across lower elevations will ultimately be determined based on where that front stalls Thursday through Saturday, which will have a little variability this far our still. -Wilson

AVIATION

10/06z TAF Period

Low-VFR/VFR conditions will prevail throughout the 1006/1106 TAF period, with scatted snow showers developing and spreading east over much of Montana between 18-24z Tuesday. Mountain obscuration will increase beyond 09z Tuesday, with most mountain tops remaining obscured through the remainder of the TAF period. - Moldan

PRELIMINARY POINT TEMPS/POPS

GTF 19 38 21 45 / 20 20 20 10 CTB 12 33 17 38 / 20 30 20 20 HLN 22 38 20 46 / 10 30 20 30 BZN 19 38 14 46 / 10 30 20 10 WYS 17 32 4 35 / 20 50 20 10 DLN 20 35 16 45 / 0 20 20 10 HVR 17 40 19 45 / 20 20 20 10 LWT 19 36 16 41 / 20 40 20 10

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from Wednesday evening through Thursday afternoon for Beaverhead and Western Madison below 6000ft-Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Cascade County below 5000ft-East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Elkhorn and Boulder Mountains- Gallatin Valley-Gallatin and Madison County Mountains and Centennial Mountains-Gates of the Mountains-Helena Valley-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains- Madison River Valley-Meagher County Valleys-Missouri Headwaters- Northern High Plains-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains-Southern High Plains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.

Winter Storm Watch from Wednesday evening through Friday afternoon for East Glacier Park Region-Southern Rocky Mountain Front.

High Wind Watch from late Wednesday night through Thursday afternoon for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Hill County-Northern Blaine County-Snowy and Judith Mountains.