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Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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731 FXUS65 KTFX 151142 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 540 AM MDT Mon Jul 15 2024 UPDATED AVIATION SECTION .SYNOPSIS... Above normal temperatures with generally dry conditions will continue through this week across Southwest, Central, and North Central Montana, as a high pressure ridge gradually builds into the region. However, a few thunderstorms cannot be ruled out each day, mainly over the mountains of Southwest and Central Montana, but also along the Hi-Line today. && .DISCUSSION... Today through Tuesday... A lingering northwesterly flow aloft will keep temperatures 5 to 10 degrees above normal for the next couple of days across North Central, Central, and Southwest Montana. A disturbance in the flow will keep the threat for mainly afternoon and evening thunderstorms along the Hi-Line and over the mountains of Southwest Montana. Relatively weak instability, lift, and upper level dynamics will likely hinder any strong thunderstorm development, but the warm and dry air at the surface will make erratic wind gusts from any storms the main concern. However, with precipitable water values around 1 inch over the plains (a bit above normal), storms on the Hi-Line may also produce a brief heavy shower. The weak afternoon instability may be enough to mix some of the breezy and gusty west to northwest winds down to the surface today. However, model probabilities of gusts exceeding 30 mph remain below 40 percent. The upper level high pressure ridge over the Western United States will gradually build into the Rocky Mountain region tonight through Tuesday, which will help decrease winds aloft and limit the risk for mixing down of gusty winds during the day on Tuesday, as well as the risk for thunderstorm development. Wednesday through next Monday... Forecast model ensembles continue to predict a very warm and dry period into early next week. They indicate that the high pressure ridge will remain fairly strong for this time of year as it lingers over the Four Corners region, extending north into Montana. This shift of the ridge axis to just east of the region sets up the potential that at least some monsoonal moisture could be drawn into the area. As of now, guidance keeps the chance for thunderstorms at less than 40 percent and focused over the mountains of Southwest and Central Montana, but we`ll monitor the situation for potentially higher chances. Temperature-wise, current NBM guidance keeps high temperatures well into the 90s through this period at lower elevations (10 to 15 degrees above normal), with some ensemble guidance hinting at some locations potentially exceeding 100 degrees over the weekend into early next week. This could threaten daily record highs for some locations. Also, despite the relatively dry air in place, this kind of a prolonged hot period may limit how much overnight cooling will occur. This may require excessive heat highlights if models start to forecast warmer temperatures, but highlight-worthy temperatures are not being forecast at this time. -Coulston && .AVIATION... 15/12Z TAF Period VFR conditions are expected to continue through at least 16/06Z across North Central (KCTB, KHVR), Central (KLWT, KGTF, KHLN), and Southwest (KBZN, KEKS, KWYS) Montana. A west-northwest flow aloft will keep scattered to broken mid- and high-level cloudiness over the area. An exiting disturbance will take the lingering showers and thunderstorms north and east of KHVR east out of the area through 15Z. However, the band of cloudiness associated with it still linger over the Hi-Line (including KCTB and KHVR) through much of the period. Otherwise, another disturbance in the flow will bring a chance for isolated showers and thunderstorms to the Hi-Line and Southwest Montana between 18Z and 03Z as the airmass becomes weakly unstable, but the terminal with the best chance to see a thunderstorm or two will be KWYS, so it`s been addressed with a PROB30 group. Breezy west to northwest winds will also become gusty at times between 18Z and 03Z. Warmer than normal temperatures will likely cause density altitude issues, and smoke from wildfires will cause reduced slantwise visibility from KHLN south through Southwest Montana. -Coulston Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .FIRE WEATHER... High temperatures today and Tuesday will remain between 5 and 10 degrees above normal with afternoon humidity in the 20 to 30 percent range over the plains and in the 10 to 20 percent range in the valleys. Gusty west to northwest winds are also forecast to mix down to the surface today, but model guidance keeps the chance for gusts of 30 mph or higher below 40 percent. As high pressure builds into the area on Tuesday, the threat for additional gusty winds will decrease. The forecast trend continues to show that the high pressure ridge will gradually move east and settle over the Rockies through at least this weekend, which should warm temperatures to between 10 and 15 degrees above normal through the period, with some guidance hinting at slightly higher temperatures (highs possibly exceeding 100 degrees this weekend). This is forecast to keep afternoon humidity at lower elevations in the 10 to 20 percent range with a daily threat of at least isolated thunderstorms over the mountains of Southwest and Central Montana from monsoonal moisture. This will likely result in at least an increase in fire weather concerns. -Coulston && .PRELIMINARY POINT TEMPS/POPS... GTF 90 57 90 61 / 0 0 0 0 CTB 90 55 88 57 / 10 10 10 0 HLN 93 58 95 60 / 0 0 0 0 BZN 91 53 92 55 / 0 0 0 0 WYS 80 42 82 42 / 30 20 20 10 DLN 84 50 88 52 / 10 10 0 0 HVR 89 59 88 60 / 10 10 0 0 LWT 84 54 85 56 / 10 0 10 0 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls