textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- A ridge building over the western U.S. will cause above average temperatures this week, reaching more than 20 degrees above average in some places late this week.

- An atmospheric river will continue flowing around the periphery of the ridge into next weekend causing light to moderate precipitation in northwest Montana into the weekend.

The ridge building over the western U.S. will cause cause above average temperatures this week. The warmest temperatures will be along Interstate-90 and south on Thursday and Friday, when Butte is forecast to be more than 20 degrees above average. North of Interstate-90 will also be above average, but less so due to the impacts of an atmospheric river.

Usually when we discuss atmospheric rivers, the main impact is heavy precipitation, but that is not the case this time. While this event does technically meet the criteria of an atmospheric river based on the amount of water vapor in the atmosphere and how fast it is moving, other factors are preventing it from producing heavy rain. The biggest factor is that it will be embedded in anticyclonic flow, which helps to suppress lift and rain production. Still, it will be able to produce light to moderate rain at times, especially in the mountains. It will also produce consistent cloud cover over northwest Montana, preventing it from warming up as much as other parts of the state. The most notable aspect of this atmospheric river is it's duration. It will last all week, and current forecasts have it dying out early Saturday morning. The main concern for impacts will be warm temperatures in the mountains combined with frequent precipitation causing worsening avalanche conditions. For those planning to go into the back country, please monitor the avalanche forecast centers for the latest conditions this week.

AVIATION

With all of the recent precipitation and low elevation snow melt under a building ridge, fog is a concern at terminals KGPI and KMSO. Current forecasts do not have any fog in them though. Cloud cover and strong winds at the ridge top level causing some weak mixing are keeping the fog from forming. As these factors are expected to continue through the morning, we have not put any fog in the forecast, but we are aware that with all the boundary layer moisture, we are on a knifes edge, and if the boundary layer decouples and winds die down before sun rise, fog could form quickly this morning.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.