textproduct: Great Falls

KEY MESSAGES

- Freezing rain and drizzle across portions of the Hi-Line this evening into early Wednesday, with areas of fog also developing.

- Another round of freezing rain over portions of the plains late Wednesday evening into Thursday morning, with largely light mountain snow occurring over the same timeframe.

- A Canadian cold front moves south over the plains into some Central Montana valleys Friday night into Saturday, bringing at least briefly colder temperatures and an opportunity for lower elevation snow.

DISCUSSION

- Meteorological Overview:

Southwesterly flow aloft is in place across the region ahead of upstream troughing off the Pacific coast. At the surface a cooler airmass is in place over portions of the plains, with more mild temperatures further south/west. Weak forcing moving through the southwesterly flow aloft late this afternoon and evening will result in light precipitation developing over the plains, largely across the Hi-Line and adjacent areas. The cooler air remaining at the surface combined with warmer air aloft will result in a period of freezing rain this evening before precipitation lifts north of the border tonight. Lingering easterly upslope flow near the surface will result in either freezing drizzle or fog tonight across similar areas near the Hi-Line. Given the combination of freezing rain and the potential for freezing drizzle has resulted in a Winter Weather Advisory being issued for these areas. Slick roads are forecast to develop, which result in difficult travel conditions. Low clouds and or fog linger into the day Wednesday over the plains, resulting in a cooler day.

Further southwest across Southwest Montana valleys, the remainder of the day today and into most of tomorrow remains dry. The main impact will be from increasing southerly winds through the day Wednesday as the pressure gradient increases. North south oriented valleys will see the most potent winds, with the Madison River Valley favored for the strongest winds. A High Wind Warning has been issued for this area.

As the aforementioned Pacific troughing nudges eastward toward the Western US coastline, another wave ejects toward the Northern Rockies Wednesday evening into Christmas day. This wave looks more potent and will result in a frontogenetic band of precipitation developing, oriented from southwest to northeast. Mountain snow will fall at times, with high peaks near the Idaho border most favored for accumulating snow Christmas day. The greater impact looks to be from freezing rain however, which develops over portions of the plains Wednesday night into Christmas morning. Icy roads are forecast to develop where this freezing rain ultimately develops, which is most favored in the Golden Triangle and points eastward. There remains uncertainty with respect to surface temperatures on the southern portions of the plains, which makes it difficult to pin down the exact extent of freezing rain at this time horizon. Given this is the case, no Winter Weather Advisories are being issued at this time for this timeframe. Should confidence in location of freezing rain increase, Winter Weather Advisories would become necessary.

Additional weaker waves move across the Northern Rockies on Friday from the Pacific troughing, which will result in light, mainly mountain snow at times into Friday night. Thereafter a more potent wave diving southeastward from BC looks to propel a Canadian cold front across the plains and into some Central and Southwest MT valleys Friday night into Saturday. Pacific moisture overrunning this colder airmass will result in some lower elevation snow Friday night into early Saturday.

The region looks largely dry thereafter into early next week, with the greatest uncertainty associated with just how quickly the colder surface airmass over the plains moves out. Areas in and near the Milk River valley will need an extra day to see the cold air move out compared to most other areas over the plains. -AM

- Forecast Confidence & Scenarios:

Freezing rain and drizzle tonight:

High resolution guidance as well as some other guidance is wringing out just enough moisture across portions of the Hi-Line this evening to result in a glaze of freezing rain. The precipitation moves north of the border after midnight, though lingering low clouds will result in either fog or some freezing drizzle. Given the risk for freezing drizzle extends into at least the early morning Wednesday, the Winter Weather advisory extends through this timeframe. Should the drizzle not materialize, fog would be favored, and would result in visibility reduction heading into the day Wednesday.

Freezing Rain Wednesday night into Christmas:

The aforementioned frontogenetic band of precipitation looks to develop Wednesday evening into Wednesday night, shifting northeastward across the plains into Christmas. There remains some uncertainty in the exact track of this band, with additional uncertainty related exactly how far south the colder surface air mass is able to make it by the time precipitation starts. Should the colder air struggle to make inroads further south, the risk for freezing rain would be in a rather narrow corridor. Conversely, should the colder air make it all the way to Great Falls and Lewistown, a more widespread risk for freezing rain would develop Wednesday night into Christmas Day. These trends will be monitored closely over the next day or so.

Canadian cold front this weekend:

A push of much colder looks to arrive late Friday night into Saturday. The probability for a high of 10 F or less across the Hi- Line Saturday is greater than 50%. Although snow will accompany this push of colder air, the probability for snow in excess of an inch at lower elevations on the plains is at or below 20% Friday night into Saturday. -AM

AVIATION

23/18Z TAF Period

Main concern will be the potential for freezing rain later this afternoon and into this evening. Light precipitation currently falling west of the divide will move northeast into Northeast MT by evening. It is likely that surface temperatures will remain below freezing in the Havre area, thus the higher chances for freezing rain. After the freezing rain exits, a light freezing fog will develop over portions of North Central MT as well. Another round of mixed precipitation is looking likely over much of North Central and Southwest MT Wed afternoon/evening. Mountains/passes will be obscured at times across the period. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 20 30 20 45 / 20 0 50 30 CTB 16 19 11 38 / 20 0 10 10 HLN 30 50 36 49 / 20 0 70 30 BZN 31 53 40 49 / 10 10 70 50 WYS 29 41 36 39 / 10 60 100 90 DLN 27 50 38 45 / 10 20 80 30 HVR 17 28 20 31 / 30 0 30 30 LWT 37 55 33 46 / 10 0 40 30

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory until 9 AM MST Wednesday for Eastern Toole and Liberty-Hill County-Northern Blaine County.

High Wind Warning from 6 AM to 8 PM MST Wednesday for Madison River Valley.