textproduct: Great Falls

KEY MESSAGES

- Light snow/rain showers continue today across Central and Southwest Montana through Wednesday morning.

- A spring storm system will bring heavy mountain snow and widespread rain/snow to lower elevations late Wednesday through Friday.

- Drier and milder weather move in for the weekend, with low end chances for mountain precipitation.

UPDATE

No updates to the evening forecast. Isolated thunderstorms have diminished over Southwest MT. Scattered rain/snow showers are also decreasing, and will continue to decrease overnight. Main concern is the stronger storm system expected to arrive late Wed into Thu. No changes to current winter storm watch. Brusda

DISCUSSION

- Meteorological Overview:

Westerly flow aloft today will continue to stream in Pacific moisture into the region. The light rain/snow band in Southwest MT earlier this morning has lifted to Central MT. Continued embedded vorticity maximums in this flow aloft will continue light rain/snow across Central MT through Wednesday morning and develop scattered rain/snow showers across Southwest MT through Wednesday morning. This afternoon, weak CAPE by the Idaho and MT border will bring an isolated chance for a thunderstorm, though overall confidence is low.

Heading into Wednesday, The main closed low trough pushes onshore across the Pacific Northwest. Chances for rain/snow showers increases throughout the day Wednesday, with better chances for widespread precipitation arriving with the surface low Thursday morning. Strong moisture advection with this spring storm system will allow for heavy snow to develop in the mountains. Higher snow levels Thursday morning will allow for a rain/snow mix at lower elevations initially. A cold front passage Thursday will help decrease snow levels for accumulating snow to develop at lower elevations Thursday evening through Friday morning. Weak instability ahead of/along the cold front in Southwest MT will bring the concern for moderate to heavy convective snow bands Thursday.

After this system clears out during the day Friday, an upper-level ridge builds in for the weekend. As a result, milder and drier conditions are expected during the weekend and into next week. However, there may be a few weak passing systems that try to develop light precipitation over the mountains.

- Forecast Confidence & Scenarios:

Snow This Afternoon Through Wednesday:

Synoptic forcing isn't too strong with this snow band lifting north into North-Central MT. Warm pavement temperatures will limit snow accumulations through the afternoon. After sunset, road temperatures cool down enough for light snow accumulations. However, light moisture amounts will only accumulate 1-3" of snow along mountain passes. Therefore, any Winter Weather Advisory was held off.

Snow Wednesday Evening Through Friday:

There is good confidence in widespread, wetting precipitation with this system. There remains some uncertainty in how much moisture come out of this system. Snow levels Wednesday afternoon rises to 6,000ft to 7,500ft, falling to only 6,000ft in SW MT, and 4,000 to 5,000ft in Northern MT. This will limit snow accumulations to mainly the mountain passes overnight Wednesday. The cold front passage during the day Thursday will help transition lower elevations to snow, with the best chances for accumulating snow Thursday evening through Friday morning when road temperatures drop to freezing.

There have been some variability still with model run to model run for total moisture/snow amounts. Snowfall probabilities did decrease slightly in the pass 24 hours. Though there still is a 40-80% chance for 9" of snow along the mountain passes in Central and Southwest MT. There is a 30-60% chance for 6" of snow along the MT Highway 200 corridor between Belt and Standford. However, north to northwest flow wrapping snow around the north side of the low Thursday and Friday, gives enough confidence for upslope snow to enhance snow totals in Cascade, Judith Basin and Fergus Counties. Therefore, the current watches remain on track. The snow will be mostly wet in nature, with turning a bit more powdery in the mountains Friday morning. This brings a concern for wet snow to weigh down trees and power lines, causing power outages. Also, the cold and raw conditions pose a risk to newborn livestock.

Southwest MT with this cold front Thursday complicates the forecast a bit. Weak instability ahead of the front poses a small risk to an isolated thunderstorm along the frontal passage in the valleys. Strong cooling behind the front can cause some frontogenesis along the front. There is a possibility for a quicker switch from rain to snow along the frontal passage, and the instability and frontogenesis can result in some moderate to heavy convective snow banding. If road temperatures crashes and accumulate snow sooner as a result, than a Winter Storm watch/warning may be needed for the Gallatin Valley. -Wilson

AVIATION

01/00Z TAF Period

Initial concern will be for a mix of rain and snow showers through the evening, primarily across Southwest Montana. After a reprieve the remainder of the overnight and Wednesday morning, another round of showers looks to develop Wednesday afternoon across similar areas. Mountain obscuration is forecast for Southwest Montana and adjacent areas this evening.

Patchy fog cannot be completely ruled out Wednesday morning in areas that manage to clear out tonight, though that probability is too low to warrant any mention in TAFs at this time. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 26 56 35 43 / 10 20 70 100 CTB 19 51 30 38 / 0 10 50 100 HLN 30 56 34 44 / 30 60 90 100 BZN 28 55 33 48 / 70 70 80 100 WYS 31 48 30 40 / 90 80 100 100 DLN 33 57 35 44 / 50 70 70 100 HVR 21 52 29 42 / 0 0 30 90 LWT 21 50 29 46 / 30 20 50 100

TFX WATCHES/WARNINGS/ADVISORIES

Winter Storm Watch from Thursday morning through late Thursday night for Elkhorn and Boulder Mountains-Northwest Beaverhead County-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Storm Watch from Thursday afternoon through Friday morning for Big Belt, Bridger and Castle Mountains-Cascade County below 5000ft-Fergus County below 4500ft-Gallatin and Madison County Mountains and Centennial Mountains-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Snowy and Judith Mountains.