Area Forecast Discussion
Issued by NWS Missoula, MT
Issued by NWS Missoula, MT
711 FXUS65 KMSO 020855 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 255 AM MDT Sun Jun 2 2024 ...HEAVY RAIN TONIGHT INTO MONDAY LEADING TO INCREASED RISK OF SMALL STREAM FLOODING, ROCK AND DEBRIS FLOWS, ACROSS FLOOD PRONE AREAS... .DISCUSSION...An atmospheric river remains on track to impact the Northern Rockies later this afternoon and evening through Monday night. Pacific moisture and precipitation will begin moving into the region this afternoon as a trough approaches the WA/OR coast. Widespread moderate to locally heavy rain will develop across central Idaho and along the ID/MT border this evening into Monday morning as the core of the atmospheric river moves inland. Ensemble model systems continue to indicate deep Pacific moisture, with integrated water vapor values reaching 400-500 kg/ms, which typically occurs once every 10 years during early June. Mountain areas will see impressive precipitation totals given strong orographic forcing associated with a strengthening pressure gradient and upper level jet aloft. The latest precipitation forecast generally has 1.00-2.50 inches across mountain ranges in central Idaho and northwest Montana, with localized amounts of 3.00+ inches. The higher end totals continue to focus across the Clearwater Mountains, where the HREF has >70% probability for 2.00 inches or more. Primary impacts include an increased risk of small stream flooding and rock and/or debris flows in steep terrain and flood prone areas. Flood Watches remain in effect for the Clearwater Mountains and Orofino/Grangeville region to address this concern. The risk for main stem river flooding is reduced given antecedent river flows running below normal. The Clearwater River at Stites continues to be an exception, with probabilities for flows to reach minor flood stage at 20%. Forecast winds have increased across central Idaho and northwest Montana Monday afternoon and evening. Pressure rises associated with Pacific cold front passage Monday afternoon/evening have trended higher, with the risk for gusts of 40-50 mph increasing. Furthermore, high resolution models suggest convective showers and thunderstorms along and behind the front, which will produce localized gusts of 40 mph or greater. The EPS EFI suggests climatologically extreme winds for early June as Camas Prairie of Idaho, the Flathead and Mission Valleys, the mountain ranges in northwest Montana. NBM wind gust probabilities for gusts over 40 mph reach 60-80% across aforementioned valleys, with higher probabilities in the terrain. Wind advisories may be needed if this trend continues. Beyond the atmospheric river, a ridge builds over the region by mid-week through next weekend. Temperatures will warm to above average by Wednesday and into the upper 80s and low 90s by Thursday and Friday. /Lukinbeal && .AVIATION...Widespread rain, associated with an atmospheric river will reach the ID/MT border between 02/2200-2400Z, before pushing eastward across the divide by 03/0300Z. Widespread terrain obscurations are expected as ceilings lower below 5,000 ft AGL. Turbulence, associated with mountain wave activity, will increase after 03/300Z. /Lukinbeal && .MSO WATCHES/WARNINGS/ADVISORIES... MT...ID...Flood Watch from this evening through Monday evening for Northern Clearwater Mountains...Orofino/Grangeville Region...Southern Clearwater Mountains. && $$