FXUS65 KMSO 090658 AFDMSO Area Forecast Discussion National Weather Service Missoula MT 1258 AM MDT Fri May 9 2025 .DISCUSSION... KEY MESSAGES: - Today into Saturday warmest temperatures so far this year, causing significant increase in river/stream flows - Scattered thunderstorms Saturday and Sunday afternoons, potentially a few strong to severe storms - Snow potentially to 5500 feet across southwest Montana Tuesday into Wednesday. Ridging builds back in today and Saturday with warm and mostly dry conditions across the Northern Rockies. By Saturday afternoon, mid level moisture, instability, and shear combine for scattered high based showers and thunderstorms, mainly for southwest Montana and Lemhi County but may stretch as far north as the I-90 corridor. Watch for strong wind gust potential over 50 mph from any convection as background gradient driven winds will already be gusting at 25-35 mph regardless of convection. Given the shear profile, hail is also a concern. This warm period will accelerate snowpack melt off, with stream and river flows expected to rise significantly in response. Current river/ stream flow modeling does have a couple locations rising close to action stage by May 12-13 but no flooding is expected. Sunday, the ridge breakdown is expected with a signal of increased instability and moisture. Showers and thunderstorms will be present ahead of and along a Pacific cold front. A strong to severe storm or two with hail and damaging winds can not be ruled out in this environment. Also ahead of the front, watch for gusty winds of 30-40 mph across Lemhi County and southwest Montana on Sunday afternoon. There continues to be an indication that the front may be a little slower to move south across Lemhi County and southwest Montana into Monday. The main trough across Washington to northern California begins to exhibit more of a negative tilt. This causes the flow aloft to become more southeasterly, thereby opposing the incoming frontal system and slowing it down. The effects of this could be more rainfall potential across Idaho in the post-frontal regime (anafront), but warmer across southwest Montana. It could also bring another round of storms along the front. Clusters are continuing to show a Yellowstone Low forming Tuesday into Wednesday. Deja Vu? This pattern looks very similar to what occurred just a few days ago. Snow levels could fall to 5500 feet and there could be wintry impacts at high level locations across southwest Montana. While it appears that a majority of the clusters suggest the heavier snowfall amounts will generally be east of southwest Montana, there is one cluster that shows that 44 percent of the ECMWF ensemble members, 20 percent of the GFS ensemble and 10 percent of Canadian ensemble members depict a snowier solution. In addition to the pass level snow, this cluster solution would also allow for widespread rainfall to lower elevation areas along and south of the I-90 corridor. Stay tuned as the details come together! && Aviation: Ridging builds back in today with VFR conditions across the Northern Rockies. High and mid level cloud cover will pass through but no mountain obscurations are anticipated. Breezy afternoon winds with gusts of 15-25 kts are anticipated at most airfields as a result of peak diurnal instability. Winds diminish quickly by sunset with the loss of daytime heating. Changes arrive on Saturday with scattered showers and isolated thunderstorms, mainly across southwest Montana and Lemhi County. Wind gusts of 20-30 kts will be common on Saturday afternoon but thunderstorm outflow will allow for gust potential over 45 kts. && .MSO WATCHES/WARNINGS/ADVISORIES... MT...None. ID...None. && $$