FXUS65 KTFX 192350 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 540 PM MDT Mon Aug 19 2024 Aviation Section Updated. .SYNOPSIS... Showers and thunderstorms will continue across North- central, Central, and Southwestern Montana through Wednesday. Isolated thunderstorms will produce strong to severe wind gusts and large hail. An upper-level trough begins to move over the area on Friday which will bring strong wind gusts on Friday and Saturday and cooler temperatures for the weekend. && .DISCUSSION... This afternoon through Tuesday... This afternoon an upper-level ridge remains over North-central, Central, and Southwestern Montana. However an upper-level shortwave moves through the upper-level flow and will bring showers and thunderstorms to the area this afternoon. These showers and thunderstorms will continue through about midnight tonight. Isolated thunderstorms will produce strong to severe wind gusts and/or large hail. All thunderstorms will produce frequent lightning and brief heavy rainfall. The Storm Prediction Center (SPC) has issued a slight risk (15% chance within 25 miles of a point) for severe winds from Toole County to Western Hill County down to western Jefferson to eastern Granite County. The rest of North-central, Central, and Southwestern Montana has a marginal risk (5% chance within 25 miles of a point) for severe winds and a marginal risk for hail for northern Beaverhead to northern Gallatin County up to Cascade County and west. This afternoon through tonight there is a 15 - 40% chance for a tenth of an inch of rain or greater. On Tuesday an upper-level ridge remains over North-central, Central, and Southwestern Montana. However an upper-level shortwave moves through the upper-level flow and will bring showers and thunderstorms to the area Tuesday afternoon and evening. Isolated thunderstorms will produce strong to severe wind gusts. On Tuesday the SPC has issued a marginal risk (5% chance within 25 miles of a point) for severe winds and large hail from Bozeman to Butte north and west. On Tuesday there is a 5 - 20% chance for a tenth of an inch of rain or greater across the area. On Tuesday across Southern Beaverhead, Southern Gallatin, and Southern Madison there will be elevated fire weather conditions due to low RH and wind gusts greater than 25 mph. Wednesday through Friday... On Wednesday the upper-level ridge remains over North-central, Central, and Southwestern Montana. In the afternoon through the early evening the ridge begins to shift east and break down. This will cool down temperatures and bring showers and thunderstorms to North-central, Central, and Southwestern Montana on Wednesday. On Wednesday the SPC has issued a marginal risk (5% chance within 25 miles of a point) for severe thunderstorms west of a Havre to Great Falls to Bozeman Pass line. On Wednesday there is a 15 - 45% chance for greater than a tenth of an inch of rain in North-central Montana. On Wednesday across Southern Beaverhead, Southern Gallatin, and Southern Madison there will be elevated fire weather conditions due to low RH and wind gusts greater than 25 mph. On Thursday an upper-level ridge builds back in over North-central, Central, and Southwestern Montana. Thursday afternoon/early evening an upper-level shortwave moves through the upper-level flow. On Thursday it will be pleasant and dry. On Friday the upper-level ridge begins to break down and an upper-level trough begins to move into the area. This will bring strong wind gusts primarily to north-south oriented valleys of Southwestern Montana on Friday. This will continue to be monitored for details. Due to warm air advection from the upper-level trough temperatures will warm up on Friday. On Friday the upper- level trough will bring isolated showers to North- central, Central, and Southwestern Montana today. Saturday through next Monday... On Saturday three of the four clusters (86% of ensemble members) show an upper-level trough moving over North-central, Central, and Southwestern Montana. The other cluster (14% of ensemble members) keeps the upper-level ridge over the area. For the upper-level trough scenario this would transition North-central, Central, and Southwestern Montana to a wetter and cooler pattern with breezy winds on Saturday. The upper-level ridge scenario would keep status quo weather for the area. On Sunday the clusters show an upper-level trough of varying strengths over North- central, Central, and Southwestern Montana. This would transition the area to a wetter and cooler pattern with windy conditions as the trough moves directly over the area. On next Monday clusters, ensembles, and deterministic models begin to diverge. Two clusters (64% of ensemble members) show a weak upper-level trough over North- central, Central, and Southwestern Montana which would keep the cooler/wetter weather in place over the area. One cluster (25% of ensemble members) has zonal flow over North-central, Central, and Southwestern Montana which would keep it cool but dry across the area. The other cluster (11% of ensemble members) has an upper-level ridge over North-central, Central, and Southwestern Montana which would warm up temperatures and bring unsettled weather back to the area. This will be mentored for more details. -IG && .AVIATION... 20/00Z TAF Period Numerous showers and thunderstorms are spread across Central Montana. Observations indicate cloud bases around or above 10kft. Heavy rain, hail up to 1 inch diameter, and gusty winds to 45kt are possible impacts at terminals including KHLN/KGTF/KLWT. Storms are moving north, and may impact KCTB/KHVR with primary gusty wind hazard through 04Z. Skies clear quickly behind the thunderstorms, with quiet, VFR conditions prevailing overnight through early Tuesday afternoon. Widely scattered showers and isolated thunderstorms are expected again Tuesday evening, but there is not enough confidence to add these impacts to the TAF at this time. Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PRELIMINARY POINT TEMPS/POPS... GTF 57 89 55 82 / 50 20 30 20 CTB 55 83 52 76 / 50 20 40 30 HLN 55 92 55 83 / 40 20 10 30 BZN 51 90 50 84 / 30 30 10 20 WYS 43 80 42 77 / 10 10 10 10 DLN 49 84 48 78 / 20 10 0 10 HVR 61 92 58 85 / 50 10 70 20 LWT 55 90 52 81 / 40 20 30 20 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls