FXUS65 KTFX 140451 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 951 PM MST Wed Nov 13 2024 Aviation Section Updated. .KEY MESSAGES... - Breezy through much of the upcoming week, especially on the plains. - Light mountain snow at times through Thursday. - Accumulating mountain snow across Southwest Montana Friday into Saturday. && .UPDATE... /Issued 907 PM MST Wed Nov 13 2024/ Main adjustment to the forecast was to decrease winds a bit over the plains for early this evening to better reflect current conditions. However, a shortwave trough is still on track to move northeast over the plains through this evening, which should help translate the gusty southwest down to the surface soon enough. Have also added a slight chance of showers to the plains tonight into Thursday morning, as high-resolution models are hinting toward a few light showers moving over the area with the passage of the shortwave. -Coulston && .DISCUSSION... /Issued 907 PM MST Wed Nov 13 2024/ - Meteorological Overview: Brief upper level ridging across the region is shifting eastward ahead of a broad Pacific trough. A weak wave progressing through the trough late today into Thursday will result in mainly light pass- level snow across the Northern Rocky Mountain Front. Additionally, southerly flow will result in breezy conditions across the region, most notably in north-south oriented valleys across Southwest Montana this afternoon and evening. The broad troughing continues to shift eastward to close out the week, with an additional embedded wave crossing the region Friday afternoon into Saturday morning. This will be discussed further below in the confidence and scenario section, but there is uncertainty with the north/south placement of this wave, which will plays large role on where precipitation will end up falling. After the broad troughing exits late Saturday, zonal flow develops across the Northern Rockies, with another trough quickly entering the Pacific Northwest Sunday morning. The main story with this system will be for breezy to strong winds, with a slightly lesser concern for light snow heading into early next week, mainly across Southwest Montana. Guidance then favors an upper level ridge building in toward the middle of next week, though with surface high pressure looking to develop to our west and relatively low surface pressure over the plains, breezy winds look to continue at times next week. - Forecast Confidence & Scenarios: Winds: The first concern is the remainder of the afternoon and evening across southwest Montana, where a modest pressure gradient will be in place. At this time the chance for a 58 mph gust across Southwest Montana is only greater than 50% across terrain, with chances generally 30% or less across lower elevation locations. The lone exception is around Norris Hill, where the probability is around 40%. Given the limited probabilistic support, we will continue to hold off on High Wind Warnings for the remainder of today and tonight. Areas in north south oriented valleys (Such as the Madison River Valley) will undoubtedly see sustained winds on the order of 25-35 mph, with gusts exceeding 50 mph at times. Isolated instances of gusts in excess of 55 mph will be possible. Another area of low confidence for strong gusts will be along the Rocky Mountain Front out to Cut Bank around daybreak Thursday. Ensemble (GEFS) averaged max gusts tomorrow morning in Cut Bank is around 45 mph, with around 20% of members having a gust in excess of 50 mph. No High WInd Warnings are being considered for this area, but will be an area to watch. The next period of potentially impactful winds is late Saturday night through Sunday night across most areas. Probabilistic guidance gives around a 90% chance for a 58 mph gust in Browning late Saturday night through Sunday, and a 70% chance in Cut Bank over the same timeframe. Confidence in this wind event is higher given ensemble support, but it is too early to warrant a High Wind Watch. Snow: Initial concerns will be for the Northern Rocky Mountain Front through the day tomorrow. Confidence is high that the greatest snow amounts will remain above pass level, hence limiting impacts to commuters. The chance for 3 inches of snow over Marias Pass from 5 PM this evening to 5 PM Thursday is around 40%, and decreases to 0% for East Glacier Park over the same timeframe. Given the limited areal extent of relatively minor impacts, I have opted to hold off from issuing a Winter Weather Advisory for this area at this time. The next round of snow arrives Friday into early Saturday. Deterministic and ensemble guidance have not been consistent with the north/south placement of precipitation over the last couple days and this trend is persisting into today. Confidence is high that there will be accumulating snow in the Gallatins and Madisons, with lesser confidence in snow as you move further north or west. The chance for 3" snow across the aforementioned ranges Friday into Saturday is greater than 60%. Elsewhere in terrain the probability is 50% or less. At lower elevations the location with the best chance for 3" (Around 20%) is the south end of Bozeman. Minor impacts to passes such as Bozeman Pass, Raynolds Pass, and Targhee Pass are most likely, with limited confidence on impacts to other passes. Another round of troughing will be around early next week, but does not look to feature a long duration precipitation event, rather a relatively quick period of snow as the leading edge of the wave moves through. The chance for 6 inches of snow with this system is only greater than 50% in the high terrain of the Northern Rocky Mountain Front and in the Gallatins and Madisons. Ridging mid to late next week: Cluster guidance is nearly unanimous (90% chance) that an upper level ridge will build in late next week, which would favor a return of above average temperatures and a dry period. Conversely there is a small subset of guidance (Around 10%) that does keep upper level troughing around toward the end of next week, leaving the door open that the region remains unsettled. -AM && .AVIATION... 14/06Z TAF Period VFR conditions are expected to continue at the terminals across North Central, Central, and Southwest Montana through at least 15/06Z, but clouds and mountain precipitation may obscure mountain tops on the Continental Divide for much of the period. Otherwise, mid- and high-level cloudiness will decrease after 12Z. Winds will remain the primary concern for the area for much of the period. A passing disturbance will keep mid-level winds strong through at least 12Z, causing mountain wave turbulence, gusty south to southwest surface winds, and widespread low level wind shear. In the wake of the disturbance, winds should decrease somewhat, especially after 15Z, lessening the threat for turbulence and low level wind shear, but surface winds will remain gusty and more westerly through at least 00Z. -Coulston The KWYS TAF will not be issued until airport operations resume next spring. Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 39 53 26 45 / 10 10 0 10 CTB 34 48 23 45 / 10 0 0 0 HLN 33 53 25 45 / 10 10 0 40 BZN 28 51 22 42 / 10 0 0 50 WYS 18 36 11 38 / 30 30 10 70 DLN 25 47 21 38 / 10 10 0 50 HVR 35 56 25 49 / 10 20 0 0 LWT 35 52 26 45 / 10 0 0 30 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls