textproduct: Great Falls

KEY MESSAGES

- One last mild day, with afternoon showers and thunderstorms.

- Cold front arrives this evening and tonight, bringing a sharp transition to colder/wetter conditions through Thursday, including impactful snowfall in the mountains and at some lower elevation locations.

- Additional snow lingers near the Continental Divide and across Southwest Montana into Friday before slowly subsiding.

- Remaining unsettled through the weekend into early next week.

DISCUSSION

- Meteorological Overview:

The first of several embedded waves within broad troughing across the west moves overhead today across the region. Initial impact is for gusty south to southwest winds in many areas developing this morning. Precipitation largely near the Continental Divide this morning slowly spread eastward in the form of showers and thunderstorms in the afternoon and evening. A few of the most robust thunderstorms will pose a gusty wind threat, but the confidence in timing and location of any of these thunderstorms is low at this time.

A cold front then begins to move in late day and into the evening, bringing sharply colder temperatures to the entire region, along with gusty northwest winds over the plains. A more widespread precipitation develops along and behind the front, impacting mountain areas and much of the plains. Snow levels fall quickly behind this front, though rain looks to persist behind the front for a couple hours at lower elevations before the sufficiently cold air arrives to transition precipitation type to snow. Impactful snow is forecast to develop in many mountain areas this evening and persist into Thursday. Areas at lower elevations of greatest concern for impacts will be in the Armington Junction area and adjacent areas near the Little Belts, as well as near Lewistown as the northwest winds combine with snow. There is some uncertainty with just how impactful this snow and wind will be for these areas, which is largely dependent on how quickly snow levels fall in these areas. More about this concern will be discussed in the uncertainty section.

Lingering gusty northwest winds near and east of US-87 during the day Thursday will slowly subside toward Thursday evening and night. All the while, another wave within the broad upper troughing will keep snow in the forecast along the Continental Divide and across Southwest Montana Thursday night into Friday as precipitation further east wanes Thursday evening. Additional impacts from the snow are most favored in the terrain in these areas, but will reach the valley floors at times into Friday.

Although the bulk of the upper troughing moves east/northeast this weekend into early next week, an unsettled northerly or northwesterly flow aloft develops. Temperatures largely look to remain on the cooler side, with at least low-end opportunities for precipitation most days through the weekend. -AM

- Forecast Confidence & Scenarios:

Thunderstorms today:

While latest high-resolution ensembles do not favor any one area over another by too much in terms of instability this afternoon, any stronger thunderstorms that do develop will be capable of producing gusty and erratic winds. If there was one area that was slightly favored over the rest for development of greatest instability it looks to be across the Hi-Line in the Shelby area where there is approximately a 70% probability for 250+ J/Kg CAPE today.

Potential for Blizzard Conditions in/near the Little Belts Late Tonight and early Thursday:

The main source of uncertainty stems from how long of an overlap there will be between the strongest winds and heaviest snowfall in these areas tonight. High resolution guidance favors the strongest winds in between Armington Junction and Geyser this evening before slowly subsiding through the overnight. Meanwhile this same set of guidance keeps precipitation type as rain in this area through midnight or so. A scenario that features temperatures that are quicker to cool or that has the stronger winds lingering longer would result in at least a few hours of blizzard conditions in this area tonight.

South of Armington Junction on US-89 on the way to Kings Hill Pass precipitation looks to flip over to snow quicker than on 87. That said, the winds do not appear quite as strong. All that to say there is still quite a bit of uncertainty in the higher-end impact scenario at this time. Regardless of whether blizzard conditions materialize in these areas, impactful snow is still forecast, which will warrant some sort of winter highlight (Whether it be a Winter Weather Advisory, Winter Storm Warning, or Blizzard Warning is to be determined).

There also appears to be a conditional risk for localized blizzard conditions near Lewistown late tonight and Thursday morning, but uncertainty in overall snow amounts are keeping concerns lower for now.

Potential for strong northwest winds tonight and Thursday:

Latest high resolution guidance favors areas across the plains downwind of terrain as the most likely to see gust of 60 mph materialize tonight into early Thursday. Areas with a greater than 50% probability to observe a 60 mph gust are in eastern Fergus county and across a southern and eastern Blaine county. -AM

AVIATION

22/06Z TAF Period

Numerous impacts to aviation are forecast over the next day or so. Initial concern will be for gusty southerly to southwesterly winds across many areas developing mid-morning. Mountain precipitation will likely be ongoing by mid-morning near the Continental Divide, spreading eastward in the form of showers and a few thunderstorms in the afternoon before becoming more stratiform in nature toward the evening. A cold front bringing gusty northwest winds, mainly over the plains, moves through in the evening Wednesday, bringing along with it a mix of rain and snow. Mountain obscuration will become increasingly common over the next day or so. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 70 30 41 24 / 80 100 80 50 CTB 67 26 40 23 / 80 90 40 30 HLN 64 32 40 25 / 80 90 100 70 BZN 61 31 43 23 / 70 90 90 70 WYS 46 25 38 18 / 90 90 100 80 DLN 56 32 45 24 / 80 90 80 70 HVR 77 30 42 27 / 40 90 50 20 LWT 70 27 36 22 / 40 90 70 30

TFX WATCHES/WARNINGS/ADVISORIES

Winter Weather Advisory from 6 PM this evening to 6 PM MDT Thursday for Big Belt, Bridger and Castle Mountains-East Glacier Park Region-Northern High Plains.

High Wind Watch from late tonight through Thursday afternoon for Bears Paw Mountains and Southern Blaine-Eastern Toole and Liberty-Fergus County below 4500ft-Hill County-Northern Blaine County-Western and Central Chouteau County.

Winter Weather Advisory from midnight tonight to 6 PM MDT Thursday for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Snowy and Judith Mountains.

Winter Weather Advisory from 6 PM this evening to noon MDT Friday for Elkhorn and Boulder Mountains-Gates of the Mountains- Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

Winter Storm Watch from this evening through Thursday afternoon for Little Belt and Highwood Mountains.

Winter Storm Warning from 6 PM this evening to 6 PM MDT Thursday for Gallatin and Madison County Mountains and Centennial Mountains.

Winter Storm Watch from late tonight through Thursday afternoon for Judith Basin County and Judith Gap.

High Wind Warning until 6 PM MDT this evening for Madison River Valley.

Winter Storm Watch from Thursday evening through Friday morning for Gallatin Valley.