textproduct: Great Falls

KEY MESSAGES

- There will be one more day of cool temperatures, scattered to numerous snow showers, and gusty northwesterly winds before much warmer and drier conditions return this weekend into early next week.

- The next Pacific trough approaches the Northern Rockies on Wednesday and will bring breezy to windy conditions and scattered showers and thunderstorms.

- Cooler temperatures, some mountain snow, and lower elevation shower activity move in for next Thursday and Friday.

DISCUSSION

- Meteorological Overview:

Cold northwesterly flow aloft will bring another day of below average temperatures and gusty northwesterly breezes. Shortwave passages and H700/H500 temperatures running around -15C/-30C respectively will also maintain scattered snow shower activity, with the most numerous snow shower activity or even general light snows occurring over central and southwestern MT this afternoon thanks to subtle warm air advection aloft and diurnal heating.

Ridging builds in this weekend through early next week for drying and seasonably breezy conditions. Temperatures quickly warm into the 50s and 60 on Saturday before 60s and 70s, and even higher readings, become more common Sunday through Wednesday.

Southwesterly flow aloft begins to develop as early as Tuesday night in response to a split flow trough moving onto the Pacific coast. As this system moves eastward, moisture and forcing from associated shortwave energy will become sufficient for widely scattered shower and thunderstorm activity as early as Tuesday evening, though Wednesday looks to be most favorable day in terms of CAPE up to 300 to 800 J/kg combined with better forcing from the approaching trough. The primary hazard looks to be localized strong to severe winds and occasional lightning given warm/dry surfaces and inverted V forecast soundings.

The evolution of this trough during the second half of the workweek is a bit complex. It appears that the greatest forcing will be either south or north of our forecast area; however, around half of the ensembles support the southern closed low close being close enough to produce more widespread precipitation over central and southwest MT on Thursday and Friday. - RCG

- Forecast Confidence & Scenarios:

Periods of light snow and snow showers today...

There continues to be around a 50 to 70% chance for an inch snowfall or more for areas near and over the central and southwest mountain ranges. The timing still looks to be in the late morning and afternoon hours, so this should decrease the overall accumulation effectiveness with diurnal heating coming into play. Impacts look to be localized, but may include sharp visibility reductions, gusty winds, and some minor slushy roadway accumulations with the heaviest showers.

Mid- week breezy to windy conditions and widely scattered showers and thunderstorms...

Ensembles continue to highlight Wednesday for the next chance for widely scattered showers and thunderstorms. The timing of the Pacific trough and its attendant cold front will have an impact on the phasing of instability and synoptic forcing and will ultimately determine storm coverage/intensity. A Canadian cold front will also have an impact, especially if it moves prior to peak diurnal instability. An earlier passage may push the most widespread activity to the eastern portions of the forecast area. Overall, localized strong to severe wind gusts look to be the primary impact, but hail and brief downpours may also come into play dependent on instability and moisture.

Mountain snow and lower elevation rains Thursday into Friday...

There is no defined area of low pressure that ensure the forcing required to produce widespread precipitation next Thursday into Friday. But some of the newer deterministic models are introducing various methods that result in similar widespread precipitation scenarios, mostly for central and southwestern MT. Currently, NBM probabilities for 1 inch or more of liquid equivalent precipitation are running around 20 to 40%, again highest over the higher terrain of central and southwest MT. With ensembles favoring warmer temperatures aloft, any accumulating snow looks to be generally confined to mountain areas. - RCG

AVIATION

17/06Z TAF Period

Initial concern will be for an area of light snow working from northwest to southeast across the region through the overnight. This activity largely diminishes by sunrise, but lingering low clouds continue into the day. By late morning and early afternoon, numerous snow showers are forecast to develop within a breezy northwest flow in most areas, with KLWT and KHVR the least favored for snow showers. This snow shower activity diminishes in the evening. -AM

PRELIMINARY POINT TEMPS/POPS

GTF 42 26 59 30 / 50 20 0 0 CTB 40 22 57 26 / 20 20 0 0 HLN 43 25 59 32 / 60 20 0 0 BZN 39 18 57 26 / 70 20 0 0 WYS 31 9 46 16 / 50 30 0 0 DLN 39 20 56 27 / 20 10 0 0 HVR 44 25 61 29 / 10 10 0 0 LWT 39 23 56 30 / 20 20 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.