textproduct: Great Falls

KEY MESSAGES

- Generally milder and drier conditions prevail for much of the workweek, but a weak front will bring slightly cooler temperatures on Wednesday.

- Patchy fog development will increase over the next several nights over the plains and southwest valleys.

- Breezy to windy conditions develop over the plains of Central and North Central Montana to end the work week and through the weekend, especially along the Rocky Mountain Front.

DISCUSSION

- Meteorological Overview:

Ridging aloft will shift east of the Northern Rockies to over the High Plains through Thursday as a shortwave trough over the Gulf of Alaska (this afternoon) slides east and over Western Canada and the Pacific Northwest (Thursday morning).

The aforementioned shortwave trough will bring increasing westerly winds and spotty mountain precipitation on Thursday as it moves over the Northern Rockies from the afternoon through evening hours. This trough passage will then usher in an extended period of strong westerly flow aloft that will persist into early next week despite dominant ridging building for the first half of the weekend. The trend of the strongest winds aloft staying north of the Canadian border on Friday continues to gain traction, but a tightening surface pressure gradient and mountain wave activity still looks to enhance stronger winds along the Rocky Mountain Front Friday into early Saturday.

Ensembles are favoring the ridge flattening and sending a fast moving trough into the forecast area Sunday into Monday. This will bring another round of winds, strongest along the Rocky Mountain Front, and some more light precipitation along the northern divide. Temperatures look to remain above average this weekend into early next week, with only a slight cool down following any shortwave passages. - RCG/Moldan

- Forecast Confidence & Scenarios:

High Winds from Friday through Sunday

Overall confidence in high winds materializing along and west of the Interstate 15 corridor to the Continental Divide across portions of Central and North Central Montana remains low. ECMWF EFIs continue to remain on the low end and below 0.6, which considering that November is typically when high wind events become more prevalent indicates a borderline event at best. In addition to the ECMWF EFIs the NAEFS with respect to cross barrier flow at H700 indicates wind speeds that are generally only up to 1 standard deviation above normal. This again would suggest the threat for a marginal high wind event.

While the pressure gradient will be strong at times, approaching 0.2mb/km across western portions of Glacier County, the that fact BUFKIT soundings depict veering winds with height to out of the northwest within the critical layer raises concerns about how much momentum will actually be transferred to the surface, especially from Friday through Saturday outside of mountain wave activity. For now we will continue to hold off on issuing a High Wind Watch given the lack of support for high winds outside of the more bullish GEFS members.

The table below lists the NBM5.2 probabilities for wind gusts to exceed 55, 65, and 75 mph at select locations across the plains of Central and North Central Montana over the aforementioned timeframe.

Fridays Wind Gusts Probabilities LOCATION > 55 mph | > 65 mph | > 75 mph East Glacier 90% | 70% | 20% Browning 95% | 75% | 30% Cut Bank 50% | 25% | 5% Havre <5% | 0% | 0% Choteau 5% | 0% | 0% Great Falls 10% | <5% | 0% Lewistown 0% | 0% | 0%

Saturday Wind Gusts Probabilities LOCATION > 55 mph | > 65 mph | > 75 mph East Glacier 55% | 25% | 10% Browning 60% | 35% | 15% Cut Bank 35% | 20% | 10% Havre 5% | 0% | 0% Choteau 5% | <5% | 0% Great Falls 5% | 0% | 0% Lewistown 0% | 0% | 0%

Sundays Wind Gusts Probabilities LOCATION > 55 mph | > 65 mph | > 75 mph East Glacier 75% | 50% | 10% Browning 80% | 65% | 30% Cut Bank 55% | 30% | 10% Havre 15% | 5% | 0% Choteau 5% | <5% | 0% Great Falls 15% | <5% | 0% Lewistown 0% | 0% | 0%

- Moldan

AVIATION

05/00Z TAF Period

The forecast region and most terminals will have prevailing VFR conditions with light winds this evening and through tonight with the exception of KWYS and KHVR. LIFR with temporary VLIFR due to freezing fog and low stratus is expected to develop tonight at and in the areas of KWYS and KHVR with some scattered low status at KEKS until late morning. Gustier winds up to 25KT out of the southwest will develop across the central and north central areas tomorrow by early afternoon and will continue to increase beyond this TAF period. Primary hazards will be visibility restrictions from freezing fog and mist, mountain obscurations, and potential ice accretion in freezing fog. - Akins

PRELIMINARY POINT TEMPS/POPS

GTF 31 54 35 54 / 0 0 0 0 CTB 29 49 33 51 / 0 0 0 0 HLN 28 49 29 51 / 0 0 0 0 BZN 27 47 28 47 / 0 0 10 0 WYS 20 42 21 39 / 0 0 10 0 DLN 26 44 25 46 / 0 0 10 0 HVR 27 51 32 53 / 0 0 0 0 LWT 31 55 31 53 / 0 0 0 0

TFX WATCHES/WARNINGS/ADVISORIES

None.