FXUS65 KTFX 171803 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1103 AM MST Tue Dec 17 2024 ...Aviation Section Updated... .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... Mid-morning update (1000 AM): Satellite trends indicate fog is becoming less widespread across regions where the Dense Fog Advisory was in effect. Hence, the Dense Fog Advisory was canceled early. The main update this morning was to add a Winter Storm Warning for the Northern Rocky Mountain Front tonight into Wednesday and to add High Wind Warnings for most areas for Wednesday. An impressive mid level shortwave will eject eastward out of a slightly more broad trough this evening, heading toward the Northern Rockies. Most moisture looks to be limited to the northern portion of this wave, hence impactful snow looks to be largely reserved for the Northern Rocky Mountain Front. This wave will also have potent mid level flow, which will be translated to the surface, initially along the Rocky Mountain Front and adjacent plains late tonight, spreading eastward and southward through the day Wednesday. Blowing and drifting snow will be a concern in areas immediately adjacent to the Northern Rocky Mountain Front, but confidence is not high that it will extend much out onto the plains given surface temperatures are forecast to rise well into the 30s if not into the 40s at the onset of strong winds. Trends will need to be closely monitored for additional impacts further east on the plains during the day Wednesday. Probabilistic guidance gives a greater than 50% chance (Mostly greater than 70%) chance for 80 mph gusts along the plains immediately adjacent to the Rocky Mountain Front Wednesday. In other areas north of I-90 the chance for 58 mph gusts is mostly greater than 50%. Hence, High Wind Watches were upgraded to a warning, with various start and end times, largely starting earlier further west, and later further east. High Wind watches were added/remain in effect for most areas south of I-90 (Madison River Valley was upgraded to a warning). More minor changes were made to the forecast today to reduce highs, especially across the plains where low clouds look to linger through the day. No other changes were made at this time. -AM Early-morning update (745 AM): 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 749 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... 17/18Z TAF Period Patchy, but locally dense, fog is expected to persist over/near the KGTF and KHLN terminals through 19z this afternoon before dissipating thereafter. IFR to MVFR CIGS over the plains of Central and North Central Montana (i.e. KCTB, KHVR, KGTF, and KLWT terminals) will persist through at least 21z this afternoon, with the latest TAF issuance potentially being too optimistic with improving conditions thereafter. Latest thinking is that the low level flow will turn back to the south between 21-00z over the plains, which will then help to scour out the low level moisture and subsequent stratus deck here. Focus then shifts to mountain wave turbulence and low level wind shear concerns beyond 06z this evening as cross barrier flow increase significantly, with surface wind gusts in excess of 40ts expected at all terminal sites on Wednesday. - Moldan 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... Winter Storm Warning from 9 PM this evening to midnight MST Wednesday night for East Glacier Park Region. High Wind Warning from 2 AM Wednesday to midnight MST 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 Warning from 5 AM Wednesday to midnight MST Wednesday night for Cascade County below 5000ft-Eastern Pondera and Eastern Teton-Eastern Toole and Liberty-Gates of the Mountains- Judith Basin County and Judith Gap-Madison River Valley-Upper Blackfoot and MacDonald Pass-Western and Central Chouteau County. High Wind Watch from Wednesday morning through Wednesday evening for Beaverhead and Western Madison below 6000ft-Gallatin and Madison County Mountains and Centennial Mountains-Northwest Beaverhead County-Ruby Mountains and Southern Beaverhead Mountains. High Wind Warning from 11 AM Wednesday to 5 AM MST Thursday for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Hill County-Northern Blaine County-Snowy and Judith Mountains. High Wind Warning from 11 AM Wednesday to midnight MST Wednesday night for Big Belt, Bridger and Castle Mountains-Canyon Ferry Area-Elkhorn and Boulder Mountains-Gallatin Valley-Helena Valley- Little Belt and Highwood Mountains-Meagher County Valleys- Missouri Headwaters. && $$ http://www.weather.gov/greatfalls