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Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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270 FXUS65 KTFX 181502 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 902 AM MDT Sun Aug 18 2024 .SYNOPSIS... A southwesterly flow aloft will bring a chance for scattered showers and thunderstorms to Southwest and North Central Montana through Monday, mainly during the afternoon and evening hours. Some of these storms could become strong, with gusty winds and hail the main hazards. Temperatures will remain 5 to 10 degrees above normal through Monday, but they will cool back closer to normal for the mid to late week period, with a continued chance for showers and thunderstorms each day. && .Update... On-going forecast is performing well this morning and no update is needed. High temperatures will warm into the mid-80s to mid-90s this afternoon across Southwest through North Central Montana, with isolated to scattered showers and thunderstorms developing during the mid- to late afternoon across Northeast Idaho and Southwest/West Montana. These showers and thunderstorms will then lift northeast across the remainder of Central and North Central Montana through the midnight hour; however, an isolated shower or storm may linger along the Hi-Line and mountains along the Idaho border even beyond this hour. A few of the thunderstorms this afternoon and evening could become severe, with the primary severe weather threat being damaging winds (less than a 15% chance of occurring). - Moldan && .AVIATION... 18/12Z TAF Period Aside from a few lighter end showers or virga along the Northern Continental Divide this morning, mostly dry conditions are expected until this afternoon. Some southwest valleys south of I90 may still see some patchy fog through 18/15Z, but probabilities for visibility falling less than 6sm are running less than 20% for most locations including KWYS. A shortwave embedded in the southwesterly flow aloft will initiate widely scattered showers and thunderstorms over the higher terrain of West/Southwest MT around 18/20Z before spreading north and eastward into the valleys/plains through the late evening hours. Some of the stronger cells may produce strong, gusty winds, brief downpours, and cloud to ground lightning. - RCG Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 524 AM MDT Sun Aug 18 2024/ Today through Monday... This will likely be the most active period of the next week or so. A southwesterly flow aloft will remain over the Northern Rockies to begin this week, between a deep low pressure trough along the Pacific Northwest coast and a high pressure ridge over the Great Plains. This pattern will keep temperatures 5 to 10 degrees above normal across North Central and Southwest Montana, which will help create mainly afternoon and evening instability and bring a chance of scattered showers and thunderstorms. A nearly stationary north-south oriented frontal boundary will also provide a good focusing mechanism for thunderstorm development. Dry sub-cloud layers with afternoon relative humidity in the 20 to 30 percent range at lower elevations will make strong and erratic wind gusts the main hazard, prompting the Storm Prediction Center (SPC) to put much of the forecast area under a Marginal Risk for severe thunderstorms each day (5+ percent chance of 58+ mph gusts). The tap into Pacific and monsoonal moisture in the flow will also keep precipitable water values between 0.75 and 1.00 inches, which is slightly above normal, maintaining the potential for isolated brief heavy showers. The differences between today and Monday will be the strengths of passing disturbances overhead and their associated wind shear. Today`s relatively weak disturbance will keep bulk shear values in the 20 to 30 kt range, while a stronger disturbance on Monday will bring 30 to 40 kt of shear. As a result, the potential for large hail (1+ inch diameter) is higher on Monday, as included in the SPC Marginal Risk (5+ percent probability). Tuesday... A shortwave trough will eject from the main Pacific trough and move over the forecast area. This will help shift the frontal boundary east more so over Central and Eastern Montana, cooling temperatures off a few degrees, and putting the focus for additional shower and thunderstorm activity there. However, the passage of the shortwave will also increase the resulting west- southwesterly flow aloft, which will translate down to the surface in the wake of the frontal boundary. While strong winds are not expected, it will be a somewhat breezy day; NBM guidance indicates a 30 to 50 percent probability of at least 15 mph sustained winds and 30 mph gusts Tuesday afternoon on the Rocky Mountain Front and over Southwest Montana south of Interstate 90. Wednesday through next Sunday... Ensemble model cluster guidance is slowly coming into better agreement with at least the early portion of this period. Most (85 percent) of the clusters start to bring the low pressure trough onshore into the Western United States through Friday, while 15 percent of them keep it along the coast. As a result, the NBM cools temperatures back closer to normal, while keeping a low chance for additional showers and thunderstorms. Over the weekend, there is slightly less agreement, but again the majority (70 percent) of the clusters keep at least a weak low pressure trough over Montana, prompting the NBM to cool temperatures even more with a slightly increased chance for showers and thunderstorms. -Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 92 58 92 57 / 20 30 30 30 CTB 86 55 87 55 / 10 20 20 30 HLN 93 59 90 56 / 30 30 50 30 BZN 91 55 88 51 / 20 20 40 20 WYS 79 45 77 43 / 20 30 60 20 DLN 85 53 84 48 / 30 20 40 10 HVR 95 63 93 61 / 0 30 10 40 LWT 91 58 90 55 / 20 20 40 30 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls