textproduct: Great Falls

KEY MESSAGES

- Strong winds develop this morning with a long duration of potentially damaging winds expected through this afternoon.

- Snow affects areas along the continental divide and over some mountain passes today with brief but intense showers and isolated snow squalls possible with the passage of a cold front this morning. - An active weather pattern continues through the rest of the week with another period of strong winds and mountain snow accompanying a fast moving weather system Thursday night and Friday.

DISCUSSION

- Meteorological Overview:

A vigorous shortwave trough moving onto the SW BC/Pac NW coast early this morning will rapidly translate east today with surface low pressure emerging and intensifying in southern AB this afternoon. Very strong mid-level flow (70-80kts at 700MB and locally higher across N-central MT) spreads east beneath the shortwave and across the region late this morning through this afternoon. Potent dynamic forcing associated with the shortwave and enhanced atmospheric mixing beneath cold advection will promote the downward transport of these winds to the surface today aided by the anomalously deep surface low that develops in southern AB and tracks southeast across NE MT by this evening. All of these factors support the advertised unusually strong, widespread and long duration wind event expected to develop today.

On the precipitation side of this event, a fairly robust surge of Pacific moisture is spreading rain and snow into western MT early this morning with precipitation expected to intensify along northern portions of the continental divide ahead of and for a period following the frontal passage. Snow levels initially around 6000 ft there will fall rapidly with the frontal passage with a period of moderate to heavy snow likely at Marias Pass this morning. Further south and east, a period of rain quickly changing to snow will impact some central and southwest MT pass locations with the rapidly changing conditions being more impactful than total snowfall amounts there. Across the valleys and plains, scattered to numerous bands of convective showers will accompany and follow the cold front this morning. Isolated lightning cannot be ruled out but the main impact with these showers is the potential for a quick transition from rain to intense snow or graupel along with the onset of strong winds to much of central and southwest MT this morning.

As the upper level and surface low track across NE MT tonight, winds slowly diminish with colder air and snow showers wrapping around the low across portions of north-central MT during the overnight period. While strong winds slowly subside overnight, the combination of continued gusty northwest winds and snow showers could bring some additional winter weather impacts to eastern portions of North- central MT. A short break between weather systems is expected Thursday before another energetic/fast moving disturbance tracks east along/north of the Canadian border Thursday night and Friday. Another period of strong winds is likely, but looks to be of shorter duration and magnitude than todays event. Winter weather impacts are also possible, mainly across the mountains but some light rain/snow is also possible on Friday as a band of frontal precipitation sags south through central and southwest MT. Hoenisch

- Forecast Confidence & Scenarios:

Short range/hi-resolution models and ensembles continue to give high confidence in the unusually strong and long duration of todays wind event with a high (80-90%) risk of gusts exceeding 60-65mph and moderate to high (60-80%) risk of gusts 65-75 mph across most of the forecast area. Higher gusts are possible where terrain enhancement or brief convective elements occur with the passage of strongest winds aloft early this afternoon.

What makes this wind event stand out is the widespread nature of the expected strong winds, likely to impact not just the plains but also the typically somewhat protected valleys of central and southwest MT, including the Helena and Bozeman areas. Multiple hours of winds of this magnitude will increase the likelihood for damage to equipment or infrastructure that can handle our typically shorter duration wind events.

Satellite imagery this morning indicates darkening indicative of strong downward motion and potential mixing of stratospheric air trailing the upper level wave now moving into the west coast. A concern here is the potential for sharp drying and decrease in humidity as this feature shifts across the region this afternoon. This would increase the risk of blowing dust and locally reduced visibility as well as bring at least a short period of increased grassland fire risk despite recent moisture. Hoenisch

AVIATION

17/12Z TAF Period

The main concern during this TAF Period is widespread strong winds. The strong winds will begin to die down around 18/04Z. Wednesday morning a cold front will push through Southwestern Montana and bring a brief heavy burst of rain and/or snow to the KHLN, KEKS, and KBZN terminals. This will briefly reduce visibility to at least MVFR-levels. After this front pushes through there will be widespread wind gusts in excess of 45 kts across the KEKS, KBZN, and KHLN terminals with gusts in excess of 55 kts across the KCTB, KHVR, KLWT, and KGTF terminals. Additionally, associated with the frontal passage in Southwestern Montana there is a low chance for an isolated lightning strike. The strong wind gusts and winds will lead to blowing dust with a low chance of reducing visibility to MVFR-levels. There will be widespread mountain wave turbulence, low-level turbulence, and low-level wind shear during the duration of this TAF Period. The KHVR terminal will have MVFR- level ceilings at the very end of this TAF Period.-IG

PRELIMINARY POINT TEMPS/POPS

GTF 51 26 47 37 / 60 0 10 30 CTB 44 21 40 28 / 40 10 0 30 HLN 48 26 40 33 / 90 10 30 70 BZN 51 23 41 32 / 90 10 30 50 WYS 40 11 30 23 / 100 10 90 90 DLN 48 23 39 33 / 80 0 40 70 HVR 46 8 34 23 / 50 30 10 40 LWT 48 18 42 31 / 60 10 10 20

TFX WATCHES/WARNINGS/ADVISORIES

Winter Storm Warning until 11 PM MST this evening for East Glacier Park Region-Southern Rocky Mountain Front.

High Wind Warning until midnight MST tonight for Bears Paw Mountains and Southern Blaine-Beaverhead and Western Madison below 6000ft-Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Cascade County below 5000ft-East Glacier Park Region- Eastern Glacier, Western Toole, and Central Pondera-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Elkhorn and Boulder Mountains-Fergus County below 4500ft-Gallatin Valley- Gallatin and Madison County Mountains and Centennial Mountains- Gates of the Mountains-Helena Valley-Hill County-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Madison River Valley-Meagher County Valleys-Missouri Headwaters-Northern Blaine County-Northern High Plains-Northwest Beaverhead County- Ruby Mountains and Southern Beaverhead Mountains-Snowy and Judith Mountains-Southern High Plains-Southern Rocky Mountain Front-Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County.

Winter Weather Advisory until 11 PM MST this evening for Gallatin and Madison County Mountains and Centennial Mountains- Little Belt and Highwood Mountains-Northwest Beaverhead County- Ruby Mountains and Southern Beaverhead Mountains.