textproduct: Great Falls

KEY MESSAGES

- It will be warm and windy today.

- A cold front moving through the area this evening will bring in cooler air for Saturday, with snow possible in the mountains of the Rocky Mountain Front.

- Another Pacific weather system moves through the Northern Rockies and brings more shower activity early next week.

DISCUSSION

- Meteorological Overview:

Ridging aloft will begin to break down late Friday, but not before unseasonably warm and dry conditions persist for another day. A potent negatively tilted Pacific trough and attendant cold front are poised to move through the Northern Rockies late Friday afternoon through early Saturday. H500/H700 south/southwesterly flow increases to the 30 to 50 kt range by Friday afternoon, with some of this reaching the surface amid deep layer mixing. Surface winds will be strongest and most impactful over the high plains along the Rocky Mountain Front during the cold frontal passage Friday evening. While it will be breezy to windy for central/north-central locations to the east, there will be a shorter opportunity for wind gusts exceeding 50 mph and higher dependency on isolated shower and thunderstorm activity transferring the stronger flow aloft to the surface.

There will be little precipitation of consequence with this system; however, H700 temperatures quickly falling to around -5C Friday night will initiate a period of light accumulating snow over the higher terrain (above 6000 ft) of Glacier National Park, including the Logan Pass area by Saturday morning. More seasonable temperatures will filter in behind the front on Saturday, with gusty westerly winds lingering over the plains through the early afternoon hours.

Another Pacific trough looks to move through Sunday into Monday. This one will be weaker with the core of the system's energy staying well south of the forecast area. The primary change to the sensible weather will be increased shower activity for southwestern Montana and continued areawide seasonably cool temperatures. Then a broad trough will bring increased westerly flow for breezy to windy conditions heading towards the mid-week period of next week. - RCG

- Forecast Confidence & Scenarios:

Gusty to Strong Winds Late Friday into Saturday...

The primary concern continues to be strong winds associated with Pacific cold front Friday evening through early Saturday. South to southwesterly winds begin increasing in advance of the front Friday afternoon with gusts mostly in the 25 to 35 mph range. The combination of gusty winds with the unseasonably warm conditions will bring localized fire weather concerns, although lingering ground moisture will preclude widespread elevated fire conditions.

The cold front looks to cross the Continental Divide roughly between 5 and 10 pm MDT. Strong westerly winds will be most widespread and persistent over the high plains along the Rocky Mountain Front due to the cold frontal passage in the evening and then mountain wave activity overnight. There is over a 70% chance for wind gusts exceeding 55 mph and over a 50% chance for sustained winds exceeding 40 mph. Reasonable high end wind gusts (90th percentile) will be around 70 mph, particularly for areas along and west of highway 89 from Chouteau to US highway 2 and then for areas west of I15 north of the Highline. With this in mind, the high wind watch was expanded to include the Southern Rocky Mountain Front Plains zone. Technically high wind criteria is 75 mph for gusts and 50 mph for sustained winds for both Rocky Mountain Front Plains zones, but the high wind watch seems warranted given this is the first high wind event of the fall season, trees being mostly fully leafed, and the amount of lingering summer recreation traffic.

The stronger winds for central and north-central areas east of I15 will be less consistent and more dependent on the dynamics of the front and isolated convection to transfer the stronger flow to the surface. Moreover, the temporal window of opportunity will generally be less than three hours. Given these factors, I have elected to hold off on high wind products for now.

There was some consideration given to extend the high wind watches through Saturday afternoon with diurnal mixing looking to bring another round of gusty winds; however, the upper level flow weakens substantially and ultimately looks to diminishes the occurrence of widespread criteria winds.

Glacier National Park Light Accumulating Snow Saturday Morning...

There will be strong cold air advection behind this front with snow levels crashing to as low as 5000 ft along the Rocky Mountain Front. The 90th percentile of snowfall of 1 to 3 inches seems reasonable for areas of Glacier National Park above 6000 ft given the synoptic setup. Impacts will be most felt for those recreating in the park or for those responsible for maintenance late Friday night and Saturday morning. - RCG

AVIATION

25/06z TAF Period.

Quiet conditions expected the rest of tonight, with just some gradual increase in surface winds. It becomes windy by this afternoon, with strong surface winds likely over the western half of North Central MT this afternoon/evening. A cold front will move from west to east across the CWA this evening, resulting in a few thunderstorms. Brusda

PRELIMINARY POINT TEMPS/POPS

GTF 62 88 49 64 / 0 0 20 0 CTB 50 83 43 57 / 0 10 10 0 HLN 53 85 43 66 / 0 0 10 0 BZN 47 81 42 66 / 0 0 0 0 WYS 32 70 32 59 / 0 0 0 0 DLN 51 79 39 66 / 0 0 0 0 HVR 57 89 48 66 / 0 0 20 0 LWT 51 84 45 67 / 0 0 10 0

TFX WATCHES/WARNINGS/ADVISORIES

High Wind Watch from Friday evening through Saturday morning for Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains.