textproduct: Great Falls

KEY MESSAGES

- One last day of warm, breezy, and dry conditions will bring elevated to critical fire weather conditions today.

- A weather system Saturday through Monday will bring a big drop in temperatures along with moderate to heavy wet mountain snow and lower elevation rain/snow.

- Cooler and wetter conditions look to continue into mid next week.

DISCUSSION

- Meteorological Overview:

The next incoming trough coming down from Canada will bring an increase in winds aloft today. These stronger winds aloft will mix down to the surface from day time vertical mixing. The combination of the warm, breezy, and dry conditons will bring elevated to critical fire weather conditions to the region. Temperatures are also expected to peak today, with numerous lower elevation locations reaching into the 80s.

Saturday, an elongated trough coming in from Canada will bring a multi-day weather system through Montana. A cold front passing from north to south Saturday morning bring the first rush of cooler air to the region along with mountain snow and lower elevation rain/snow. Lee troughing with northerly cold air advection continues behind the front, and transitions rain to snow at lower elevations by Saturday evening. Meanwhile the front progression slows down heading into Southwest MT, which delays the switch over the snow later than North-Central MT. Strong convergence and frontogenesis also occurs along this front as it head into North-Central/Central MT, which can develop moderate to heavy snow banding. Northerly flow in North-Central MT will allow for enhanced snowfall for northerly upslope areas of the Little Belts/Central Continental Divide. Temperatures don't look to warm up much heading into Sunday. Increasing moisture from the remnant southwest tropical storm system will continue to feed moisture into this storm system Sunday into Monday before exiting later in the day Monday.

Behind this system, another quick moving trough looks to swing through Wednesday into Thursday, which will keep the cooler temperatures and chances of precipitation. However, details with this will be heavily dependent on what happens this weekend/Monday.

- Forecast Confidence & Scenarios:

Even though we are closer in time to this next weather system, there remains some uncertainty in the progression of this system. This is due to the remnant of the tropical system Rachel lifting north from the Southwest U.S. and into the Rockies. The first uncertainty is the cold front timing Saturday. Some recent trends slow down the front progression later into the afternoon Saturday across North-Central MT which will bring a slower transition to rain to snow. Some solutions have this earlier cold front timing Saturday morning, which bring a quicker transition to rain to snow as temperatures will struggle to heat up with diurnal daytime heating. Another factor is if any moderate to heavy snow banding can dynamically cool the atmosphere quicker, leading to a quicker rain to snow transition.

The second uncertainty is where the better convergence and frontogenesis enhanced snow banding sets up. Models were trending towards Central/North-Central MT, but some recent runs have that cold front and surface low stalling by the MT and ID border. As a result, there also could be moderate to heavy snowfall in Southwest MT Sunday into Monday. For Central/North-Central MT, any moderate to heavier snow banding can crash surface/road temperatures, leading to quicker accumulations. Probabilities for 6" of snow along the Montana Highway 200 corridor and northerly upslope areas of Cascade and Judith Basin Counties run 50% chances. However, the concerns of these snow bands developing gave the nudges to expand the Winter Storm Watches into those areas and the Meagher County Valley. That front stalling out near the MT/ID border combined with the warm, moist tropical airmass overriding the boundary may also bring concerns for a wintry mix along the state border.

The main impacts from this weather system will be wet snow weighing down power lines and trees, leading to power outages and damaged trees. Temperatures dropping into the 20s Sunday morning will allow for wet pavements to freeze and become slippery. Colder temperatures will also impact any outdoor recreation in the mountains/back country and travel over mountain passes will be impacted by heavy, wet snow. -Wilson

AVIATION

09/06Z TAF Period.

Primary concern this TAF period will be for the development of gusty south to west winds Friday morning, lasting through the early evening. A Canadian cold front will begin to move southward across the region late evening Friday, bringing a wind shift to a northerly direction. -AM

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 78 52 81 43 / 0 0 0 0 CTB 73 51 72 33 / 0 0 0 10 HLN 76 47 82 43 / 0 0 0 0 BZN 75 43 82 43 / 0 0 0 30 WYS 72 31 70 36 / 0 0 0 0 DLN 78 41 81 45 / 0 0 0 20 HVR 74 47 84 36 / 0 0 0 10 LWT 75 51 83 42 / 0 0 0 20

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 Cascade County below 5000ft-Elkhorn and Boulder Mountains-Helena Valley-Judith Basin County and Judith Gap- Little Belt and Highwood Mountains-Meagher County Valleys.

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.

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.