FXUS65 KTFX 171449 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 749 AM MST Tue Dec 17 2024 .KEY MESSAGES... - Warm front lifts through the area on Wednesday, bringing some brief snow before high winds develop in the late morning and afternoon - Winds will be strongest along the Rocky Mountain Front, but winds over 60 mph will be possible across much of North Central and Southwestern Montana - Generally warmer and tranquil, if breezy at times, weather expected through the rest of this week as ridging builds in, though active weather may return next week && .UPDATE... Early morning update: Satellite and surface observations are showing fog developing this morning in areas where skies are clearing in wake of light snow late last night and early this morning. Fog in most areas is patchy in nature, but areas on the plains adjacent to the Rocky Mountain Front, in Cascade County, and in river valleys of Gallatin County are seeing the fog be a bit more widespread in nature. Hence, a Dense Fog Advisory was issued for these areas through noon. In addition to reduction in visibility, elevated surfaces such as bridges may become slick where fog is most dense given below freezing temperatures. Elsewhere, where fog is more patchy in nature, a Special Weather Statement has been issued to describe potential impacts. -AM && .DISCUSSION... /Issued 540 AM MST Tue Dec 17 2024/ - Meteorological Overview: Upper level ridging will begin to build into the area today in the wake of a weak trough that moved through overnight. As this ridge builds in, expect a fairly quiet weather day across North Central and Southwestern Montana, though weak high pressure sliding in across the plains will keep the plains on the cool side even as the southwestern valleys begin to warm up under the ridge’s influence. The ridge that builds in today will not last long, as a strong, vigorous shortwave trough roars into the area, along with it’s associated jet streak. While some light snowfall (and isolated freezing drizzle) will be possible as the system’s warm front surges through the area, the bigger story will be the strong winds associated with this system (more on that in the Confidence & Scenarios section). After this shortwave, upper level ridging will take a firm hold across the Western CONUS, which will result in warming conditions and generally quiet weather, though it will be a bit breezy at times. Looking into next week, there is fairly good agreement in the models suggesting a slightly active, but still warm, weather pattern for at least early next week heading into Christmas. - Forecast Confidence & Scenarios: For today, confidence is high in a mostly quiet day, aside from some early morning fog and isolated light snow shower before high pressure slides in from the north. Wednesday will be the highest impact period of this forecast as high winds impact most of the area. Confidence continues to steadily increase in a seasonably notable high wind event, with many areas of North Central and Southwestern Montana now having a 50-70%+ chance of wind gusts over 55 mph, and areas along the Rocky Mountain Front, Judith Gap, and some of the ridges of Southwestern Montana having over a 50% chance of wind gusts over 65 mph, I have elected to expand the existing High Wind Watch to much of Southwestern Montana, along with some expansion further north across the Plains. Supporting this decision are two things: earlier timing and, in turn, atmospheric profiles more favorable to the transport of the strongest winds to the surface thanks to daytime thermal mixing still being in play. Recent atmospheric profiles suggest that most low elevation areas will have about approximately a 3-4 hour window for wind gusts over 60 mph, namely between noon and 4, when thermal mixing will begin to weaken as the sun sets. This will also coincide with the arrival of cooling aloft, which will make the air aloft even more ready to mix to the surface via mechanical mixing of the air. As confidence increases, I would expect that most areas see the High Wind Watches upgraded to High Wind Warnings, though we are just a bit outside the time window for the upgrades. Regardless of what product is out at the end of the day today, be prepared for a very windy day across North Central and Southwestern Montana, and ensure that any loose Christmas/Holiday decorations are secured. Beyond Wednesday, with upper level ridging becoming entrenched across the Western US, expect temperatures to gradually warm, with little to no precipitation expected. This ridge will hold through Saturday, though there are some indications that our weather could become a bit more active early next week. Ludwig && .AVIATION... 17/12Z TAF Period A weak disturbance will exit the area this morning with low VFR to MVFR ceilings remaining across much of the region through Wednesday morning. By Wednesday afternoon, a strong disturbance will arrive bringing a chance for snow and strong winds with low- level wind shear, mountain wave turbulence, and mountain obscuration possible through at least Wednesday evening. North-central Montana (KCTB, KHVR): Periods of IFR due to snow and fog will continue at KHVR through 17/18Z. After that, MVFR ceilings will persist through the remainder of the TAF period with another chance for snow after 18/10Z. Low clouds will persist at KCTB through much of the period with a chance for snow after 18/06Z. Central Montana (KGTF, KHLN, KLWT): Periods of fog and low clouds are expected at KGTF and KLWT through the first 6 hours of the period with temporary reductions into MVFR during this time. By 17/20Z, low clouds are expected to lift giving way to mid and high level clouds through the remainder of the period. With the arrival of the disturbance, there is a chance for snow at both locations after 18/06Z. Otherwise, VFR conditions and generally light winds will persist through the entire period at KHLN. Southwest Montana (KBZN, KEKS): Periods of low-VFR to high-MVFR will persist through the first 6 hours with mountain obscuration possible during this time. After that, VFR conditions are expected to persist through the remainder of the period. Winds will pick up at KEKS after 18/00Z with gusty winds arriving by 18/05Z. Winds will start to pick up at KBZN after 18/12Z with gusty winds arriving after the end of the 12Z TAF period into Wednesday afternoon. -thor Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 23 16 52 21 / 20 40 20 10 CTB 17 10 45 14 / 30 50 40 10 HLN 37 26 52 29 / 20 30 40 10 BZN 36 21 49 22 / 10 20 30 10 WYS 30 16 32 13 / 50 60 60 20 DLN 37 24 46 20 / 20 10 20 0 HVR 19 5 42 6 / 30 50 60 30 LWT 27 14 48 18 / 30 30 30 20 && .TFX WATCHES/WARNINGS/ADVISORIES... Dense Fog Advisory until noon MST today for Cascade County below 5000ft-Gallatin Valley-Northern High Plains-Southern High Plains. High Wind Watch from late tonight through late Wednesday night for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains- Southern Rocky Mountain Front. High Wind Watch from Wednesday morning through late Wednesday night for Bears Paw Mountains and Southern Blaine-Eastern Pondera and Eastern Teton-Northern Blaine County-Western and Central Chouteau County. High Wind Watch from Wednesday morning through late Wednesday night for Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Cascade County below 5000ft-Elkhorn and Boulder Mountains- Fergus County below 4500ft-Gallatin Valley-Gallatin and Madison County Mountains and Centennial Mountains-Gates of the Mountains- Helena Valley-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Madison River Valley-Meagher County Valleys- Missouri Headwaters-Snowy and Judith Mountains-Upper Blackfoot and MacDonald Pass. Winter Weather Advisory until 9 AM MST this morning for Gallatin and Madison County Mountains and Centennial Mountains. && $$ http://www.weather.gov/greatfalls