textproduct: Great Falls

KEY MESSAGES

- Hottest day of the week for most today.

- Showers and thunderstorms in a few areas tomorrow afternoon ahead of a cold front bringing much cooler temperatures, gusty winds, and winter weather into Thursday.

- Unsettled with cool temperatures Friday into the weekend.

DISCUSSION

- Meteorological Overview:

A long fetch of southerly flow aloft ahead of an upper level low just off the Pacific coastline will result in the warmest day of the week for many today. While the day looks to be dry for the vast majority of areas, a few showers cannot be completely ruled out given the ample surface heating and diffluent flow aloft this afternoon and evening. A few elevated showers or thunderstorms cannot be ruled out tonight as better forcing aloft arrives.

As the upper low devolves more into a series of stronger open waves, the mass drifts northeastward toward the Northern Rockies and Northern Great Plains Wednesday. Sufficiently cool air aloft looks to combine with forcing from the troughing and still very warm surface temperatures across much of the region to result in showers and thunderstorms Wednesday afternoon and evening. The overall risk for a strong to severe thunderstorm appears quite conditional and localized at this time, but bears monitoring for changes in trends.

A cold front then moves across the region Wednesday night, bringing gusty northwesterly winds (Mostly across the plains) and much cooler temperatures. Northwesterly surface flow behind this cold front and additional forcing from a trough diving South/Southeast from BC late Wednesday night into Thursday will result in a stretch of winter weather, mostly in the mountains and in northwesterly upslope favored areas at lower elevations like the Lewistown area and areas southeast of Great Falls toward the Little Belts. Although overall snowfall amounts may not accumulate to large totals at lower elevations, the combination of gusty northwest winds and falling snow will result in difficult travel conditions and stress infrastructure late Wednesday night and Thursday.

The aforementioned trough diving south or southeast from BC slows and lingers near the Continental Divide in Southwest Montana Thursday into Thursday night. This will aid in additional snowfall along the Continental Divide and across Southwest Montana over this timeframe.

Confidence in specifics wane after Thursday, but the overall pattern of upper level troughing across the region persists through the weekend. The main takeaway for this timeframe is for temperatures to continue to be on the cooler side, with additional rounds of precipitation at times. -AM

- Forecast Confidence & Scenarios:

Thunderstorms Wednesday:

Latest HREF guidance favors areas near the Canadian border for the greatest instability Wednesday afternoon. The probability for 500+ J/Kg cape in this area is roughly 40%. Forecast soundings from the most aggressive guidance does feature effective shear in excess of 30 kts in the favorable thermodynamic environment. All that to say there is a conditional risk for strong to severe thunderstorms near the Canadian border Wednesday afternoon, though the ingredients have to come together perfectly for it to happen.

Active Weather Wednesday Evening into Thursday:

Temperatures are not rapid to respond to the cold front at lower elevations Wednesday evening (At least with respect to becoming cold enough to result in snow), which looks to delay the onset of snow at lower elevations til late night. Nevertheless, the gusty winds and snow that does develop at lower elevations (The best overlap for this looks to be in northwesterly upslope areas over the plains) looks to result in localized impacts to travel and infrastructure through the day Thursday. The trend in amounts with this system has generally been down slightly over the past day or so at lower elevations, largely associated with better forcing trending eastward into eastern Montana. Mountain areas still look to see impactful snow, with the Little Belts seeing the greatest probability for 9 inches of snow (Greater than 80% at this time).

As for the gusty winds that develop behind the front, forecast soundings are not overly concerning for high wind criteria. Localized areas southeast of terrain in Fergus and Blaine counties will have the greatest probability to see strong winds/gusts. Given this threat appears localized and not high probability at this time, no High Wind Watches have been issued. -AM

AVIATION

21/06Z TAF Period

VFR conditions prevail through the TAF period. Westerly flow aloft backs to southerly Tuesday afternoon with some increase in mid to high level cloud-cover. Generally light surface winds persist through Tuesday morning with breezy south winds developing at southwest MT terminals Tuesday afternoon. Hoenisch

PRELIMINARY POINT TEMPS/POPS

GTF 80 48 72 30 / 0 10 60 100 CTB 77 43 65 27 / 0 10 60 90 HLN 81 47 66 32 / 0 0 80 100 BZN 78 42 64 32 / 0 0 50 100 WYS 67 34 48 26 / 0 20 80 100 DLN 74 43 59 33 / 0 0 70 90 HVR 82 48 78 30 / 0 10 30 80 LWT 79 50 72 29 / 10 10 50 90

TFX WATCHES/WARNINGS/ADVISORIES

Winter Storm Watch from Wednesday evening through Thursday afternoon for Big Belt, Bridger and Castle Mountains-Gallatin and Madison County Mountains and Centennial Mountains-Little Belt and Highwood Mountains.

Winter Storm Watch from late Wednesday night through Thursday afternoon for Fergus County below 4500ft-Judith Basin County and Judith Gap-Snowy and Judith Mountains.