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Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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523 FXUS65 KTFX 151653 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 1053 AM MDT Mon Jul 15 2024 ...Aviation Section Updated... .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. && .UPDATE... Slight modifications were made to the coverage area for precipitation chances today in response to recent obs and the latest model trends. The main concern for fire weather will be the potential for dry thunderstorms in southwestern Montana. The one inhibitor to watch, though, is the smoke aloft has been moderating thunderstorm development over the past couple days. This trend may continue today with more smoke aloft being advected from wildfires in Idaho, Oregon and California. That being said, there is still a solid 20 to 30% chance of showers and thunderstorms in the area and lightning as well as gusty/erratic winds will be something to watch this afternoon. Haze probabilities were expanded from the base HRRR smoke model based on observations and air quality sensors. For now, the smoke is still expected to remain at higher levels of the atmosphere which will prevent any major deterioration of air quality at the surface outside of locations in the vicinity of local wildfires. Otherwise, no major changes were made to the going forecast. -thor && .AVIATION... 15/18Z TAF Period VFR conditions will prevail throughout the duration of the 1518/1618 TAF period; however, isolated to scattered thunderstorms along the Idaho border may bring gusty and erratic winds to the KWYS terminal, mainly this afternoon. Otherwise, another round of showers and thunderstorms is possible along and north of a KCTB to KHVR line through 06z Tuesday, but confidence in any one shower or storm impacting the KCTB and KHVR terminals was too low to mention in this TAF issuance. Slantwise visibility reductions upon ascent/descent will continue due to smoke/haze across portions of Central and Southwest Montana (i.e. KHLN, KBZN, KEKS, and KWYS). 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 && .PREV DISCUSSION... /ISSUED 540 AM MDT Mon Jul 15 2024/ 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 && .PRELIMINARY POINT TEMPS/POPS... GTF 92 57 90 61 / 0 0 0 0 CTB 90 54 88 57 / 10 20 10 0 HLN 95 58 95 60 / 0 0 0 0 BZN 92 52 92 55 / 0 0 0 0 WYS 80 42 83 41 / 30 20 20 10 DLN 86 50 88 52 / 10 10 0 0 HVR 89 58 89 60 / 10 20 0 0 LWT 85 54 85 56 / 10 0 10 0 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls