textproduct: Great Falls

KEY MESSAGES

- Widespread, long duration strong winds pick up late Wednesday evening into Thursday.

- Heavy mountain snowfall will impact the Continental Divide during Wednesday through Saturday, with moderate to heavy lower elevation snowfall possible Friday and Saturday in North-Central MT.

- Another round of gusty winds for Southwest MT saturday.

- Warmer temperatures, a few rounds of precipitation, and windy conditions are on tap for next week.

DISCUSSION

- Meteorological Overview:

Westerly flow over the Continental Divide will keep bringing in moisture and orographic lift to keep snow showers over the mountains Wednesday morning and afternoon. A low pressure system moving in will bring better forcing for heavy snow to begin over the Continental Divide late Wednesday into Thursday. Westerly flow aloft will bring in a strong mid level jet up to 80-100kts statewide late Wednesday into Thursday as well. Strong winds look to develop region-wide through Thursday despite timing of the peak mid level jet being overnight into Thursday morning. Perhaps this is due to precipitation helping mix down these winds, the pressure gradient tightening during the night, and mountain wave activity. The combination of the strong winds and heavy snowfall along the Continental Divide will bring near whiteout condtions at times, particularly Thursday morning.

A warm front Thursday morning will allow for light snow to develop along the Hi-line before the main snow system comes in. A Canadian cold front will push south into the Hi- line and North- Central MT. This will help undercut the strong winds and bring widespread snow to North-Central MT. The front is forecasted to stall somewhere across North-Central MT Thursday. Westerly flow aloft bringing in remnant moisture from the Pacific atmospheric river will over run the stalled front, and bring periods of moderate to heavy snow at lower elevations Friday and Saturday. The best corridor for heavier snowfall will be where the front stalls out. Periods of snow continues through Saturday/overnight into Sunday before exiting.

Southwest MT being south of the front will not be impacted as much by the snow. There will be periods of light snow to more moderate snow at the mountain peaks during this time period. However, the main concern for Southwest MT during this time frame will be periods of gusty winds.

Next week, an upper-level ridge builds in over the Western U.S. Montana will be on the eastern edge of this ridge. This will continue northwest flow aloft into the beginning of next week, before the ridge flattens out mid next week. This will bring a few waves of rain/snow as well as windy conditions and much above normal temperatures.

- Forecast Confidence & Scenarios:

Winds Late Wednesday Into Thursday:

Model guidance has been pretty consistent in the strength of the 700mb winds. Although the timing of these peak winds is during the nighttime and pre-dawn hours and therefore not utilizing peak day time vertical mixing, deterministic and hi-res ensembles still have these stronger winds mix down to the surface. Mountain wave activity will be possible overnight, and will likely be the main driver for the strongest winds that develop. Snow and a building pressure gradient will also help mix down strong winds during this time period as well, but there remains a slight chance in these peak winds being limited by happening during the overnight hours. This 700mb jet will be persistent for a long period of time, which winds can become longer duration. The main window of concern will be early Thursday morning through about noon time. Travel will be difficult to dangerous for high profile vehicles, and if a longer duration of winds do set up, then tree/power line damage will also be a concern. Strong winds also bring an increase risk of wildfire spread if one does develop. Higher end values exceed 90 mph over the Rocky Mountain Foothills and 80 mph across the North- Central MT plains.

Snowfall Wednesday Through Saturday:

Moderate snow will build back in along the Continental Divide Wednesday morning and will transition to heavy snow Wednesday evening. From Wednesday evening through Saturday, it is currently forecasted for snow accumulations to reach 1-4 feet of snow. Heavy convective snow combined with gusty winds Thursday morning will bring whiteout conditions at times should the winds mix down to the surface. The heavy snow will make travel over Marias Pass extremely difficult. The heavy snow will also weigh down on power lines and trees, which can cause them to fall down.

The cold front pushing through Thursday will usher in colder air into North-Central MT through the weekend. There remains a little variability in where this front stalls out, but general consensus at the time is somewhere around the Central Island ranges through the MT Highway 200 corridor through Roger's Pass. Generally, snow accumulations of 6-10" look plausible for lower elevations in North-Central MT and exceeding a foot in the Central MT mountains and areas that the front stalls out in. Snow will accumulate better at night and morning hours where overnight temperatures drop into the 20s/teens. Marginal temperatures in the afternoon may slightly limit accumulations in areas that don't develop a heavier snow band. The current Winter Weather Watches remain on track. -Wilson

AVIATION

11/00z TAF Period

Winds and scattered snow showers will wind down early this evening. Snow showers will continue to be a "hit or miss" at terminal through 11/01 - 11/03z. The main concern for Wednesday is diurnal driven breezy to gusty winds picking up again across the region mid to late morning, and continuing throughout Wednesday afternoon. -Wilson

PRELIMINARY POINT TEMPS/POPS

GTF 19 44 37 49 / 10 10 40 70 CTB 14 39 27 41 / 10 10 50 60 HLN 20 46 38 51 / 10 20 50 90 BZN 14 46 35 56 / 20 10 20 50 WYS 5 34 27 46 / 20 20 60 60 DLN 15 44 35 52 / 10 10 30 40 HVR 16 45 29 46 / 10 10 70 80 LWT 16 41 31 47 / 30 10 40 90

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 6 PM MDT Wednesday for East Glacier Park Region-Southern Rocky Mountain Front.

Winter Storm Warning from 6 PM Wednesday to 6 PM MDT Saturday for East Glacier Park Region-Southern Rocky Mountain Front.

Winter Storm Watch from Thursday evening through Saturday afternoon for Bears Paw Mountains and Southern Blaine-Cascade County below 5000ft-Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Fergus County below 4500ft-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Northern High Plains-Snowy and Judith Mountains-Southern High Plains-Western and Central Chouteau County.

High Wind Warning from midnight Wednesday night to 6 PM MDT Thursday for Beaverhead and Western Madison below 6000ft-Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Cascade County below 5000ft-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-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains-Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.

High Wind Warning from 6 PM Wednesday to 3 PM MDT Thursday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains- Southern Rocky Mountain Front.

High Wind Warning from 3 AM to 6 PM MDT Thursday for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Hill County-Northern Blaine County-Snowy and Judith Mountains.

Winter Weather Advisory from 6 PM Wednesday to noon MDT Friday for Northwest Beaverhead County.