Issued Tuesday July 29, 2025 8:35 P.M. MDT

Tonight…Fair skies. Lows in the mid 40s to mid 50s except upper 60s to near 70 in breezy areas and in thermal belts on slopes and hillsides with near 60 around 5000 feet. Light wind except local east wind 10 to 20 mph through favored canyons, along portions of Lake Koocanusa and across ridge tops.

Wednesday…Mostly sunny in the morning then partly cloudy and hot with isolated afternoon showers and thunderstorms, mainly along the Canadian border, across eastern mountains and south of Happys Inn to Thompson Falls. Some thunderstorms will contain heavy rain, frequent lightning, gusty winds and small hail. Highs in the upper 80s to upper 90s with mid 70s around 5000 feet. Light wind except local east wind 5 to 15 mph through favored canyons, along portions of Lake Koocanusa and across ridge tops in the morning, decreasing in the afternoon. Gusty winds possible near thunderstorms.

Wednesday night…Partly cloudy with isolated showers and thunderstorms along the Canadian border, east of Highway 93 and south of Thompson Falls in the evening. Warm with lows in the mid 50s to mid 60s except lower 70s in breezy areas, thermal belts on slopes and hillsides with lower 60s around 5000 feet. Light wind except local east winds 5 to 15 mph with gusts to 20 mph through favored canyons, along portions of Lake Koocanusa and across ridge tops.

Thursday…Partly cloudy and hot with isolated afternoon showers and thunderstorms especially across Sanders County. Highs in the upper 80s to upper 90s with upper 70s around 5000 feet. Light wind except local east wind 5 to 15 mph through favored canyons, along portions of Lake Koocanusa and across ridge tops in the morning decreasing in the afternoon. Gusty winds possible near thunderstorms.

EXTENDED OUTLOOK (FRIDAY THROUGH TUESDAY):

Friday and Saturday…Cooler with a chance of showers and thunderstorms. Locally heavy rainfall possible. Lows in the 50s all elevations. Highs in the mid 70s to mid 80s with mid 60s around 5000 feet..

Sunday through Tuesday…A slight chance of showers and thunderstorms along the Canadian border and across the eastern mountains otherwise dry. Locally breezy in the afternoon. Lows in the lower 40s to lower 50s except mid 50s in thermal belts on slopes and hillsides with lower 50s around 5000 feet. Highs in the mid 70s to mid 80s with lower 60s around 5000 feet.

In-Depth Weather Discussion for Northwest Montana/North Idaho/Northern Rockies

ISSUED 07/27/2025 9:45 P.M. MDT/8:45 P.M. PDT

SHORT TERM (SUNDAY NIGHT THROUGH TUESDAY):

Shortwave that was not picked up very well by models passed through the region on Sunday bringing some surprise showers and thunderstorms across much of North Idaho and the northern portions of Lincoln County in Northwest Montana. Unfortunately at least one fire was sparked by these passing thunderstorms as a dry strike occurred outside of the heavier rain cores of these high based cells. The HI-RES NAM/WRF model which often does well in depicting thunderstorms did not pick up on the shortwave that passed through the region on Sunday however the 00Z run has picked up on another weak shortwave disturbance that passes through during the morning hours on Monday and generates a few fast moving, high based cells passing across North Idaho. After what happened on Sunday have gone ahead and included mostly silent 10% pops in the forecast but left out wording for showers and thunderstorms for Northwest Montana but included it in North Idaho. The timing of these weak features in the developing upper level pattern is problematic at best and cannot completely rule out at least isolated showers and thunderstorms over the next few days as a general weakness remains along the California coast with a weak and diffuse yet diffluent south to southwesterly upper level steering flow remaining across the region. Any thunderstorm clusters that develop associated with the upper level weakness off the California coastline could spin up a vort max or weak shortwave that would lift north into our region and when it encounters some upper level jet dynamics can kick off some fast moving high based thunderstorms as was the case on Sunday. Upper level ridging will continue to slowly build along the spine of the Continental Divide through Tuesday causing a deepening Gulf of Alaska adjustment wave or upper level low to amplify some and dig further south. As this transition occurs our upper level steering flow will back to a more southerly pattern which will help steer weak shortwave rotating around the Intermountain western ridge to move north into our area. This will have potential large implications on our weather pattern this week. Read below for details.

LONG TERM (TUESDAY NIGHT THROUGH SUNDAY):

The airmass across our region will become increasingly more hot and unstable as the continental ridge remains anchored along or just east of the Continental Divide with its axis extending up into southern Canada while the Gulf of Alaska adjustment wave continues to also amplify offshore. The flow between these two opposing features will be almost due southerly which will also become increasingly diffluent across our region helping to generate vertical motion through the atmosphere and lift which, in turn will result in increasing chances for showers and thunderstorms. The atmosphere will NOT be “advecting" monsoon moisture” into the region as some forecasters are suggesting but rather as the airmass remains rather stagnant and daily thunderstorms increase in coverage especially across Oregon and Central Idaho the airmass will continue to modify over time and become increasingly moist and unstable. Despite increasing surface dewpoints and higher relative humidity values much of the thunderstorms that develop across our region from Wednesday through Friday will be of the elevated, forced convective type as surface based convection looks to be largely capped by the presence of the upper level ridge. With our convection largely being elevated and dynamically forced much of the activity is likely to occur during the evening and overnight into early morning hours before waning during the later morning into the afternoon hours. By later Friday and into Saturday the weak upper level low that has been spinning off the California coast will finally move northward with colder air aloft and will aid in finally allow convection to become surface based in addition to elevated. Showers and thunderstorms will become increasingly heavy rain producers as the airmass modifies with cloud bases lowering and convection starting earlier in the day. The Gulf of Alaska adjustment wave will finally begin to push east into British Columbia by Sunday which will shift our upper level steering flow to a more southwesterly and westerly flow which will shunt any additional shortwaves rotating around the Continental ridge to also shift further east. This will result in a drying trend with near seasonal to slightly below seasonal high temperatures by Sunday with convection likely becoming focused more along the Canadian border and eastern mountains of Montana. Interestingly enough we are now at our climatologically hottest time of the year for high temperatures which range from 85-93 degrees, hottest in Northwest Montana valleys of Troy, Libby, Thompson Falls. As for low temperatures those readings begin to decrease from a climatological scale on July 28th so the nights on average will now begin to cool as the daylight continues to decrease each day allowing for more effective radiational cooling. We are now loosing almost 3 minutes of daylight each day now. Autumn will be here before we know it.