FXUS65 KTFX 171657 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1057 AM MDT Mon Jun 17 2024 .SYNOPSIS... Well below normal temperatures are expected across the Northern Rockies through the middle of the work week as an anomalously cold June storm moves overhead. This storm will also bring beneficial rain and mountain snow to Southwest through North Central Montana. && .UPDATE... Main change to the forecast was to ensure that any mention of rain today was of a more steady rain vs the shower wording a few areas had, given that most of the rainfall today will be a stratiform rain. I also re-ran the hourly temperatures to bring them in line with current trends. Ludwig && .AVIATION... 17/12Z TAF Period Widespread rain/snow snow showers continue to spread across terminals this morning and will last through the TAF period. Precipitation will transition VFR ceilings to MVFR ceilings this morning (afternoon for KBZN/KEKS). During the evening, ceilings will drop to IFR ceilings (except KGTF dropping to IFR morning). KLWT has a 70% of dropping to LIFR ceilings around midnight Tuesday. There's a 10-30% for thunderstorms this afternoon, which can drop visibilities to MVFR/IFR categories. Mountain obscuration will be persistent through the TAF period from low ceilings and mountain snow showers. -Wilson Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 347 AM MDT Mon Jun 17 2024/ Today through Wednesday...Positive tilted longwave trough over the Western CONUS and Canada, with an attendant closed low over the Pacific Northwest, will slowly lift northeast and over the High Plains through Tuesday afternoon. This track combined with the mid- level (H700) low tracking from over Northern Idaho (this morning) to Central Montana (this afternoon/evening) and eventually Northeast Montana/Western NoDak (Tuesday morning/afternoon) will place much of Southwest through North Central Montana beneath favorable dynamics for widespread lower elevation rain and mountain snow. This precipitation will be very beneficial for the entire CWA, with most ASOS climate sites running a June precipitation deficit of between 0.75" and 1.50". Anomalously, by 2-3 standard deviations, cold air associated with this disturbance combined with strong dynamics have made for a tricky snow level forecast, with the operational NBM showing snow levels ranging from 6000-8000 feet throughout the event. Recent June snowstorms (i.e. June 7-8th, 2020 and June 13-14, 2022) with equally anomalous cold air and strong dynamics have brought impressive snowfall to areas along the Continental Divide, with actual snow levels being much lower than the operational NBM guidance. Given the higher snow level bias of the operational NBM during these types of events, decided to lean into and below the NBM10th percentile for this event, which brought snow levels down as far as 4500-6500 feet across Southwest through North Central Montana. Winter Weather Advisories remain in effect for most mountainous zones; however, a few zones (i.e. Southern Rocky Mountain Front, Little Belt and Highwood, and Gallatin and Madison County Mountains and Centennial Mountains) across Central and Southwest Montana may require an upgrade to a Winter Storm Warning should snowfall amounts trend any higher. Held off on upgrading these areas given that half of the snow will be falling during the daylight hours today, with a high June sun angle working against snow accumulations. None-the-less, cold and raw conditions will befall anyone out in the backcountry, with the risk for hypothermia being a significant risk for those who are unprepared. Additionally, the heavy, wet nature of the snow will likely lead to tree damage at higher elevations and the potential for power outages. On the liquid side of the storm, NBM probabilities for greater than 0.5" range from 20-40% across the Hi- Line and far Southwest Montana to 80%+ across Central Montana, with the probability for 1" of QPF being 30-60% north of the US Hwy 12 corridor and southeast of a Havre to Choteau line. High temperatures over the next several days will be well below normal, with today being the coldest as highs struggle to rise out of the 40s to low 50s. Overnight temperatures will fall into the 20s to 30s tonight and Tuesday night, with wind chill values falling to as low as the teens in the mountains over this same timeframe. Thursday through Friday...ensemble clusters favor southwesterly flow aloft over the Northern Rockies as shortwave troughing moves from over the Eastern Pacific/Western Seaboard and towards the Northern and Central Rockies. This southwesterly flow regime will help to maintain scattered to numerous showers and storms, particularly over Central and North Central Montana, each day. Additionally, temperatures will moderate to near normal. Saturday through next Monday...quasi southwest to zonal flow is favored within the multi-model ensemble mean during the period, which is expected to translate to well above normal temperatures and isolated showers/storms each day across the Northern Rockies. - Moldan && .PRELIMINARY POINT TEMPS/POPS... GTF 45 37 53 36 / 100 90 90 30 CTB 48 35 51 32 / 90 70 80 20 HLN 49 40 58 40 / 100 90 90 40 BZN 52 33 54 32 / 100 100 90 30 WYS 50 27 49 24 / 90 90 80 20 DLN 51 31 55 33 / 90 90 70 20 HVR 52 39 58 38 / 100 80 70 30 LWT 48 34 51 33 / 100 100 90 30 && .TFX WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory until 6 PM MDT Tuesday for Big Belt, Bridger and Castle Mountains-East Glacier Park Region-Elkhorn and Boulder Mountains-Gallatin and Madison County Mountains and Centennial Mountains-Little Belt and Highwood Mountains- Northwest Beaverhead County-Snowy and Judith Mountains-Southern Rocky Mountain Front. && $$ http://www.weather.gov/greatfalls