Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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391 FXUS65 KTFX 150209 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 809 PM MDT Fri Jun 14 2024 .SYNOPSIS... After a few warm days, cooler temperatures will return to the Treasure State over the weekend and into next week, along with some gusty winds Saturday afternoon and evening. Monday and Tuesday will bring widespread precipitation, with rain at lower elevations and snow in some of the higher terrain above 6000 feet. && .UPDATE... Southwesterly flow aloft will allow for ongoing showers and isolated thunderstorms to persist and shift off to the northeast through the overnight. Additional showers and perhaps a few thunderstorms will develop from SW to NE, primarily across SW and Central MT, as the next system aloft continues its approach later tonight. Overall, no significant changes with the forecast this evening. -AM && .AVIATION... 15/00Z TAF Period VFR conditions are mostly expected to continue through at least 16/00Z across North Central (KCTB, KHVR), Central (KLWT, KGTF, KHLN), and Southwest (KBZN, KEKS, KWYS) Montana, unless otherwise noted. A low pressure trough just off the Pacific Northwest coast will keep the area under a southwesterly flow aloft, which will bring some mountain wave turbulence. Initially, a weak disturbance in the flow will bring scattered showers and thunderstorms to Southwest Montana through around 02Z, with the primary threats being erratic wind gusts and lots of lightning, but brief periods of mountain-obscuring MVFR conditions are possible in heavier rain. Another disturbance will then bring a chance for scattered showers to portions of Southwest and Central Montana between 09Z and 15Z. As the trough approaches Montana after 12Z, the flow aloft, along with the mountain wave turbulence, will increase. Also, as the airmass becomes unstable once again, scattered showers and thunderstorms will move east across the area after 17Z. An associated frontal boundary will also spread gusty westerly winds across the area, with portions of the Rocky Mountain Front receiving gusts in excess of 50 kt at times. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 550 PM MDT Fri Jun 14 2024/ Key Points: -A few thunderstorms are expected across far SW MT this afternoon, with a few more thunderstorms across North Central and Southwestern Montana on Saturday -Strong wind gusts to 60 mph are expected Saturday evening and early Sunday morning along the Rocky Mountain Front to Cut Bank -Widespread rainfall still expected early next week, though widespread heavy rainfall is looking less likely -Mountain snowfall is also expected with this system, mainly above 6000 feet -Unsettled weather with showers and thunderstorms late next week as temperatures gradually rebound Short term (through Sunday)... Upper level ridging centered over Eastern Montana this afternoon is resulting in southwesterly flow aloft across the western portion of the Treasure State. This southwesterly flow aloft is promoting warm temperatures area wide, with a few thunderstorms also developing along the Montana/Idaho border. While these storms should exit the area by sunset, we will have to watch for a few instances of gusty winds and small hail with these storms as they remain south of a Bozeman to Dillon line. The upper level ridge that was mentioned above will push further east tomorrow as an upper level trough begins to work its way into the area. This trough will lead to increasing clouds through the morning, with a cold front beginning to push through the area during the afternoon, which will be accompanied by a few showers and even a few thunderstorms. Temperatures will fall sharply behind this front, going from the low 70s to mid 50s within an hour or two of the fronts passage. Gusty winds will be responsible for the spreading of these cooler temperatures, and winds should remain gusty across North Central Montana through the evening. Winds may linger along the Rocky Mountain Front and the adjacent plains, where a few wind gusts to 50-60 mph remain possible. Looking at the state of the atmosphere tomorrow evening, there will be the potential for a wind gust or two to around 60 mph in and around Cut Bank, though this will be highly dependent on the exact timing of the arrival of a mountain wave and if it arrives before the lowest 500-1000 feet of the atmosphere decouple from the days mixed layer, these wind speeds will be attainable. With that said, the NBM does support the idea of a 60 mph wind gust, though a lot of the higher resolution guidance suggests that the higher winds will remain along the immediate Rocky Mountain Front. Given the way these types of events play out, I think it is more likely than not that these winds do remain primarily along the Front, but I have elected to leave the High Wind Warning for now to see if the next 12-24 hours bring some more certainty one way or the other. Sunday will start out breezy across the area, though winds should gradually weaken through the day as this first low pressure system moves further east. Extended (Monday through next Friday)... The first half of next week will be cool and wet as a shortwave trough works its way through the Northwestern CONUS. Rain and mountain snow will begin to work in from the Southwest on Monday as a surface low pressure system begins to develop over Southern WY and Northeastern UT in response to the arrival of this upper level trough. For the most part, most areas will see at least .2 inches (generally the 25th percentile) of liquid precipitation between Monday and Tuesday night, with many areas seeing at least a half inch at the 50th percentile. Where things get interesting is at the high end of the distribution at any particular location, with many areas seeing the potential for 1.5-2 of rain at the 90th percentile. Why the large range? From what I have been able to gather through a few of the operational forecast models (such as the GFS), it appears that there will be some mesoscale features embedded within this system, which will be capable of producing a sustained period of moderate to heavy rainfall for the areas that wind up seeing these features. Given that most areas will not see these features, I have begun the process of nudging precipitation down across the area, as the inherited forecast was generally above the 75th percentile (or higher) with the more recent runs of the NBM. Even though a lot of areas will likely see less precipitation than the current forecast suggests, I still want to be cognizant of the potential for higher amounts of precipitation where the small scale features develop. Beyond midweek, zonal flow aloft will allow our temperatures to gradually increase through the second half of the week, while also allowing us to see the development of afternoon showers and thunderstorms each afternoon. Ludwig && .PRELIMINARY POINT TEMPS/POPS... GTF 51 72 42 64 / 20 60 0 10 CTB 46 64 41 58 / 0 40 0 30 HLN 54 71 42 67 / 20 50 0 0 BZN 50 74 36 65 / 40 50 0 0 WYS 43 68 35 60 / 20 10 0 0 DLN 50 74 34 65 / 20 10 0 0 HVR 51 72 44 62 / 10 60 20 20 LWT 49 75 37 63 / 40 90 10 0 && .TFX WATCHES/WARNINGS/ADVISORIES... High Wind Watch from Saturday afternoon through late Saturday night for Eastern Glacier, Western Toole, and Central Pondera- Northern High Plains. Winter Storm Watch from Monday morning through Tuesday evening for East Glacier Park Region-Southern Rocky Mountain Front. Winter Storm Watch from Monday evening through Tuesday evening for Big Belt, Bridger and Castle Mountains-Elkhorn and Boulder Mountains-Gallatin and Madison County Mountains and Centennial Mountains-Little Belt and Highwood Mountains-Northwest Beaverhead County. && $$ http://www.weather.gov/greatfalls