FXUS65 KTFX 031747 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1050 AM MST Sun Nov 3 2024 .SYNOPSIS... An active pattern will continue across the Northern Rockies to begin the work week, with strong winds and accumulating snow expected. && .UPDATE... Minor adjustments were made to the winds and precipitation chances to reflect minor updates in the models. Otherwise the going forecast remains on track. The one main deviation with this update was adding blowing snow probabilities to the northern Rocky Mountains for Monday morning based on the expected wind speeds/gusts coinciding with expected snowfall amounts. The blowing snow threat along the higher elevations, including the mountain passes, is starting to stand out as a much larger threat than previously expected. Further investigation into the finer details will need to be made and currently we are waiting on updated model guidance later this morning to confirm the potential but it is something that will be monitored closely heading into the afternoon forecast package. -thor && .AVIATION... 03/18Z TAF Period Expect VFR conditions to be the rule over the next 24 hours as a brief stretch of quiet weather develops for North Central and Southwestern Montana. Main concern at present will be a few lingering SHSN at KWYS along with some low VFR cigs, though these will dissipate over the next few hours. Elsewhere, expect winds to increase overnight tonight and tomorrow as the next system approaches. Ludwig Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 442 AM MST Sun Nov 3 2024/ Key Points : - Overall quiet conditions are expected over the next 24 hours; however, isolated rain/snow showers are possible over the plains with a higher coverage of snow showers in the mountains - A period of strong, and in the case of the Rocky Mountain Front and immediate eastern plains, high winds develop Monday morning/afternoon and persist through Tuesday night - Significant snow accumulations are possible along the Continental Divide north of Montana Hwy 200 corridor, in addition to the island ranges of Central and North Central Montana from Monday afternoon/night through Tuesday night - The first, accumulating snowfall of the season for most lower elevations of Central and North Central Montana is growing more likely from Tuesday morning through Tuesday night Changes to Forecast : - High Wind Watch for the Rocky Mountain Front and adjacent foothills and plains, and Eastern Glacier, Western Toole, and Central Pondera Counties has been upgraded to a High Wind Warning from 8AM MST Monday through 8AM MST Tuesday - Winter Storm Watch has been issued for the Snowy, Judith, and Bears Paw Mountains, and Southern Blaine, Judith Basin, and lower elevations of Fergus County from late Monday night through late Tuesday night Today through tonight...Heights will be rising throughout the day as longwave troughing slides over and east of the Northern Rockies, with transient ridging over the Eastern Pacific (this morning) quickly sliding east and towards the region by Monday morning. This will place Southwest through North Central Montana beneath cold and unstable northwesterly flow aloft through much of the daylight hours. This cold and unstable flow aloft combined with surface heating is expected to help support isolated rain/snow showers over the plains and valleys, with better coverage of snow showers across the Continental Divide and mountains of Southwest Montain. While most locations will receive little to precipitation from these showers, a few of the stronger showers over aforementioned higher terrain could produce a quick 1-2" of snow. Given how isolated these higher amounts will be through the day, no winter weather highlights are planned at this time. Monday through Tuesday...will be the most impactful period of the next week, with multiple hazards to address. The first hazard will be potential for high winds as transient ridging over the Northern Rockies on Monday morning is quickly flattened by a fast and compact shortwave, which will be digging southeast from the Pacific Northwest/British Columbia (Monday morning) and over the Northern Rockies and High Plains through Tuesday. Mid- to upper level winds ahead of and behind this digging shortwave are not overly anomalous (generally 1 to 2 standard deviations above normal); however, favorable thermal profiles from late Monday morning through early Monday evening will help to support mountain wave activity. As the shortwave digs south and east over the Northern Rockies Monday night, strong cold air advection and mechanical mixing along the first of two cold fronts (i.e. Pacific cold front late Monday evening/night and re-enforcing cold front Tuesday morning) will continue to support the potential for strong mid-level winds translating to surface. At this time NBM probabilities continue to favor (i.e. a 70% or greater chance) wind gusts in excess of 55 mph along the Rocky Mountain Front and immediate eastern plains out to the I-15 corridor, which is why the High Wind Watch has been upgraded to a High Wind Warning for these areas. It should be noted that NBM probabilities and EFI values have been increasing further to the east over some plains locations of Central and North Central Montana during the Monday night through Tuesday afternoon timeframe, most notably in downsloping areas of the island ranges, with this trend needing to be monitored for the potential need to expand the current High Wind Warning further east. The second hazard of concern will be the potential for significant snowfall and/or impactful accumulations from Monday afternoon through Tuesday night. Orographic snow will begin to fall along the Continental Divide by late(early) Monday morning(afternoon), with a (convective) band of snow then spreading east and over Southwest through North Central Montana Monday night as the Pacific cold front slides east. Additional snow, potentially convective in nature, will then sag south from Alberta and across the plains of North Central and Central Montana through Tuesday morning as a re-enforcing cold front crashes south. Strong, northerly flow in wake of this re- enforcing cold front will then bring ideal upslope conditions for the island ranges of Central and North Central Montana through the remainder of Tuesday, in addition to the US Hwy 89 and Montana Hwy 200 corridor between Armington Jct and Lewistown. While NBM probabilities have been slow to pickup on the potential for significant (i.e. greater than 6" across lower elevations or 9" in the mountains) snowfall across the the Continental Divide, island ranges, and lower elevations of Central Montana between Great Falls and Lewistown; latest EFI values have risen to between 0.7 to 0.95 with respect to snow across these areas. These increasing EFI values and ensemble support within the ECMWF, Canadian, and GEFS has increased confidence that impactful snow is possible across all of the island ranges and portions of Central Montana plains, which has necessitated the expansion of the Winter Storm Watch. Additional Winter Weather Advisories are likely to be needed to buffering the current Winter Storm Watch, but have held off for now. Wednesday through next Sunday...ensemble clusters largely favor broad, upper level ridging building in over the Western CONUS through the period behind the departing shortwave trough; with the aforementioned shortwave trough digging south into the Four Corners and becoming cut off. This will generally lead to moderating temperatures and overall dry conditions; however, and isolated mountain shower can't be ruled out Wednesday morning and then over the upcoming weekend. - Moldan && .PRELIMINARY POINT TEMPS/POPS... GTF 49 26 49 37 / 10 10 20 20 CTB 46 25 45 33 / 20 10 20 40 HLN 46 27 49 34 / 20 10 10 40 BZN 41 20 45 27 / 30 10 10 60 WYS 32 5 30 16 / 70 20 30 90 DLN 40 18 43 26 / 20 10 10 50 HVR 48 23 50 31 / 10 10 10 50 LWT 45 23 47 30 / 10 10 10 40 && .TFX WATCHES/WARNINGS/ADVISORIES... High Wind Warning from 8 AM Monday to 8 AM MST Tuesday for East Glacier Park Region-Eastern Glacier, Western Toole, and Central Pondera-Northern High Plains-Southern High Plains-Southern Rocky Mountain Front. Winter Storm Watch from Monday afternoon through Tuesday evening for East Glacier Park Region-Southern Rocky Mountain Front. Winter Storm Watch from late Monday night through late Tuesday night for Bears Paw Mountains and Southern Blaine-Fergus County below 4500ft-Judith Basin County and Judith Gap-Little Belt and Highwood Mountains-Snowy and Judith Mountains. && $$ http://www.weather.gov/greatfalls