FXUS65 KMSO 021947 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 147 PM 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...The main change from previous forecasts has been to issue wind advisories for much of north-central Idaho and western Montana for Monday afternoon. As can be typical with atmospheric river events, gusty winds generally to 45 mph are possible in these areas. The recent HREF ensemble depicted multiple solutions of wind gusts greater than 50 mph in the Camas Prairie and Teakean areas in Idaho, and also in the vicinity of Ashley Lake, west of Kalispell. It will be hazardous for small-craft on Flathead Lake as waves to 2 feet are expected. Opted to not issue a wind advisory for the Missoula/Bitterroot valleys as probability thresholds for gusts greater than 40 mph were not very high. A few high resolution models depict a small section of the western Bitterroot Valley experiencing mix-down winds over 40 mph late Monday afternoon. Successive shifts will continue to monitor if further advisories may be needed. The rain part of this system continues to be similar as previous updates. There remains a flood watch out for Clearwater and Idaho counties as there continues to be an elevated risk for small stream flooding and water issues with clogged drains and ditches. The Clearwater Mountains of Idaho County continue to have up to 30 percent probability for greater than 2 inches of total rainfall through Monday night. The Cabinet Mountains in northwest Montana have around a 85 percent chance for greater than 1 inch of rainfall. Mountain showers continue with breezy winds on Tuesday. The higher probability for gusts greater than 30 mph includes Flathead Lake, along the Divide and portions of southwest Montana including Deer Lodge, Anaconda and the Butte area. A warm-up begins Wednesday as a broad upper level ridge builds over the western U.S. in response to a digging upper level low over the northeastern Pacific Ocean. There is a 90% probability for the lower elevations of north-central Idaho and Lemhi County to exceed 80 degrees on Wednesday. The remainder of the region has a 75 to 90 percent chance of getting into the 80s by Thursday, except around Flathead Lake where water temperatures are around 50 degrees. While the lower elevations of Idaho have a high chance for getting into the 90s by Friday and Saturday, probabilities in western Montana are lower, ranging from 28 percent in Kalispell and 50 percent in Missoula. The reason-being, there have been discrepancies in how the upper ridge evolves next weekend. ECMWF ensembles have depicted the ridge axis further west...while it has less amount of thunderstorm activity, this could open the door for a somewhat cooler airmass from Canada to keep Montana from getting to the 90s. Both the Canadian and GFS ensembles have the ridge centered more over us with an increased chance for thunderstorms. && .AVIATION...An atmospheric river is poised to move through the region later this evening through Monday. General showers are possible this afternoon before steady rain arrives into north- central Idaho by 1400 PDT and into northwest Montana by 1700 MDT. The highest probability for periods of heavy rain, and lower ceilings/visibility will be over north-central Idaho between 0100 and 1100 PDT Monday, including Grangeville. There is a 30 percent chance for periods of moderate rainfall rates at GPI, MSO, and HRF around daybreak Monday through early afternoon. There is a moderate icing potential above 9,000 feet MSL tonight. Mountain wave activity will ramp up over the Divide by Glacier National Park tonight. A cold front is expected to arrive in north-central Idaho by 11 am PDT Monday, and in western Montana by 2 pm MDT. Gusty winds to 35 knots are possible with the frontal passage, and gusts to 45 knots are possible with intense showers along and behind the front. Showers and winds decrease by Monday night. Mountain wave activity with mid-level westerly winds around 50 kts are expected over the Cabinets, Bitterroot, Mission, Rattlesnake mountains, and over the Divide Monday afternoon after the cold front moves through. && .MSO WATCHES/WARNINGS/ADVISORIES... MT... Wind Advisory from 1 PM to 8 PM MDT Monday for Kootenai/Cabinet Region. Wind Advisory from 1 PM to 9 PM MDT Monday for Potomac/Seeley Lake Region...West Glacier Region. Wind Advisory from noon to 9 PM MDT Monday for Flathead/Mission Valleys...Lower Clark Fork Region. Wind Advisory from 2 PM to 9 PM MDT Monday for Bitterroot/Sapphire Mountains. Wind Advisory from 3 PM to 9 PM MDT Monday for Butte/Blackfoot Region. ID...Flood Watch from 11 PM PDT this evening through Monday evening for Northern Clearwater Mountains...Orofino/Grangeville Region...Southern Clearwater Mountains. Wind Advisory from 11 AM to 4 PM PDT Monday for Northern Clearwater Mountains...Orofino/Grangeville Region. && $$