textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Cold pools remain dominant across the region causing low clouds and stratus.

- Some easterly flow tonight will help erode some of the cloud cover, but ultimately it will reinforce the cold pools.

- The upper level ridge progresses east slowly into next week further reinforcing cold pools and stable conditions.

- There is about a 20 percent chance of a freezing rain pattern mid next week.

Satellite imagery shows cold pools in place over much of the region with the stratus clouds persisting even in Hamilton most of today. With some high cloud cover streaming over and weak winds in the lower atmosphere, we expect those cold pools to remain locked into place through this evening.

Tonight a short wave moving south across the region will help some of the very cold air east of the Continental Divide move into the region. As this is a down-slope flow for most locations, it will help erode some of the stratus clouds in the valleys of western Montana. There isn't enough upper level support for a full strength arctic blast into the region though, so the cold air will just really seep through the passes along the divide. This is very cold air, and ultimately it will settle in the valleys and reinforce the cold pools already there.

Then the ridge slowly starts to build back into place over the weekend. There will be a few more shortwaves that move along the boundary of the ridge that may affect the valley cold pools, but it won't be enough to wipe them out. By early next week the region will be fully under the influence of the ridge. So fog and clouds will continue to blanket much of the region.

Mid to late next week, about 20% of models continue to progress the ridge axis farther east. If this happens, it will position us under some moderately moist flow into the ridge. With the established cold pools in the valleys, this is a recipe for light snow, or more likely some freezing rain. At this point though, there is only a 1 in 5 or 20% of that outcome occurring next week.

AVIATION

Stratus will continue across the region through most of the day today. Some relief is expected tonight as a shortwave moving north-to-south across the region supports some light easterly flow across the Continental Divide. That is generally a down-slope flow direct, and could help erode some of the stratus in the valleys. It will also help to deepen the cold pools in the valley, which will ultimately result in slightly higher cloud bases in the stratus.

MSO WATCHES/WARNINGS/ADVISORIES

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