Missoula, Montana

• Valley rain and mountain snow expected through Monday morning, focused along and south of I-90. Northwest Montana stays mostly dry.

-  High pressure bringing stable conditions and valley fog next
   week.

-  Potential return to unsettled pattern with chances for mountain
   snow by next weekend (Nov. 7-8).

What has changed with today`s system? Not a whole lot. So far the rainfall amounts have ranged between 0.01 and 0.10" south of Hamilton. A fairly unexpected shortwave brought rain showers amounting up to 0.24" from McCall north and west to Lewiston, Idaho. Areas from Lemhi County east and north will receive the highest precipitation amounts ranging from 0.25 to 0.40" through Monday morning. Some snow accumulation is expected at the higher elevation mountain passes through tonight.

With skies partially clearing from northwest Montana southwards as the system shifts off to the east tonight, fog and low cloud development are possible. With not much mixing expected to occur on Monday, the boundary layer moisture is expected to remain high that by nightfall, and more fog or low clouds will be possible. Rinse and repeat this several times through Wednesday. If clouds end up hanging around some locations, high temperatures could end up only getting into the mid to upper 40s rather than the 50s.

A quick-moving shortwave trough will bring a chance for showers on Thursday. Light snowfall accumulation is possible from the Idaho Panhandle to the northern Clearwater Mountains and across northwest Montana down to 5000 feet. There is a 25 to 35 percent chance for rain showers from central Idaho east and north to the Glacier Region.

Yet another trough passage may occur on Friday. Our clustering tool shows that at least 50% of the ensemble members have some sort of wave passage. After this, the clusters depict a very large amplitude ridge encompassing the region. This would support above average temperatures. As for precipitation, around 51% of the clusters show a flatter looking ridge which would support more clouds and possibly moisture moving into the Idaho Panhandle and northwest Montana by Sunday. The NBM, which has well over 100 weather model members from around the world, kind of supports this idea and has a 20 to 30% probability of precipitation of 0.01" or more occurring by the 8th.

The pattern has an El Nino look to it with deep troughing over the northeast Pacific. This acts to advect milder air into the western U.S. The caveats for the Northern Rockies: We are getting into the time of year when the effects of the sun become weaker and weaker. This may translate to the increase of low clouds and fog that may not dissipate very fast. While Pacific trough passages can give us sporadic precipitation, cloud cover can prevent us from getting to full potential for temperatures.

Low clouds have been persistent from GPI to MSO this morning. Rain has already developed in Lemhi County north to the Bitterroot Valley and at KBTM as of 0400MST. Expect further degradation of ceilings and visibility as rain continues, especially from Interstate 90 and points south and east. Mountain obscuration will be a problem from the Bitterroot Mountains eastwards. As the main moisture plume moves away from the region tonight, upper clouds will dissipate, but low clouds may not. Further fog and low cloud issues are expected through Monday morning.

MT

None.

ID

None.