Area Forecast Discussion
Issued by NWS Missoula, MT
Issued by NWS Missoula, MT
037 FXUS65 KMSO 161906 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 106 PM MDT Mon Sep 16 2024 .DISCUSSION...GOES Satellite imagery this afternoon shows a closed low spinning over the Great Basin, with southerly flow sending mid and upper level moisture northward, as evident by widespread cloud cover. Moisture and instability will increase across north-central Idaho and southwest Montana this afternoon, with high resolution models suggesting convective initiation ramping up between 200-500pm MDT across area mountain ranges. As mentioned in previous discussions, locally heavy rain is a concern this afternoon given atmospheric moisture values of 150-200% of normal and weak storm motions of 10-20 mph. Idaho County in particular will be monitored closely as the latest run of the HREF has a 20-40 percent chance of 1-hour rainfall rates exceeding 0.50". A closed low remains on track to bring widespread precipitation across the Northern Rockies Tuesday into Wednesday. The general trend over the last few days has been for the low to track moreso from southeast Idaho through Yellowstone National Park into north-central Montana. Shower activity, with embedded thunderstorms, will become widespread Tuesday afternoon and evening as the low tracks over Yellowstone. As the low moves into north-central Montana late Tuesday night into Wednesday morning, modeling systems are in strong agreement for a band of moderate- heavy rainfall to set up along the north-northwest side of the low. Forecast uncertainty on where this band sets up is higher than normal, with models showing the band anywhere from the US-93 corridor eastward into central portions of the state. This uncertainty can be visualized by comparing the low end (10th percentile), most likely, and high end (90th percentile) rain forecast amounts projected by the NBM during a 24-hr period ending 6pm Wednesday. On the low end, the NBM has 0.05-0.10" across western MT, with mostly like rain amounts of 0.25-0.50" and 1.00" along the divide. High end rain totals are in the neighborhood of 1.00-2.00 inches, with 2.5 inches along the divide in Glacier Park. These closed lows are notoriously difficult to predict. The higher end rain solution would increase the risk for runoff in flood prone areas across Glacier National Park, especially on Going-to-the-Sun Road. Precipitation will largely come to an end for Thursday into the early part of the weekend, with temperatures largely rebounding to seasonal normals in the upper 60s to mid 70s. Looking ahead to next weekend, ensembles previously showed a 10-15 percent chance of a high impact, high elevation snow event across northwest Montana. Recent model runs have backed off on this, with probabilities for a foot of snow above 7,000 ft in Glacier National Park dropping from 10-15% to 5% or less. /Lukinbeal && .AVIATION...Shower and thunderstorms will increase in coverage through 17/0000Z, with storms developing across central Idaho and slowly drifting northwestward into west-central Montana through 17/0600Z. With atmospheric moisture running well above normal, storms will be capable of producing locally heavy rain, with visibility reductions under 5SM under cores. A closed low will bring widespread showers Tuesday into Wednesday. /Lukinbeal && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. && $$