textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Forecast challenges with valley fog will continue.

- Ensembles continue to be challenged with the change to colder weather and snow next weekend.

Valley fog continues to be a forecast challenge for the region. The overall surface pressure pattern will change little in the next 24-hours, suggesting a status quo and continued fog and stratus in the valleys. However, tonight a shortwave moves across the region from north to south, which will generate some weak lift, which may help lift some of the fog into a low stratus. The shortwave is also bringing quite a dry air mass aloft with it, which will allow more efficient terrain cooling overnight, which will enhance the valley fog potential. Overall, we expect the fog will continue through Wednesday morning. Subtle changes in the flow aloft will cause it to pile up in different places in the valley for valleys that don't get totally filled, but overall, the fog will remain.

Ensembles continue to be challenged by the potential for a cool down and some heavy snow next weekend. The challenge lies with the positions of the upper level ridge and the core of the cold air at the surface. Thus far, most ensembles have kept the core of the cold air far to the east along the Montana-North Dakota border. A colder model solution would have the core further west which would push more of the cold air over the Continental Divide. If the upper level ridge were to move further westward, that would support pulling more of that cold air to the west. But recent runs have trended further east with the ridge and core of cold air. So it will get much colder next weekend, but won't be an Arctic intrusion of really, really cold air; that's all going to the east.

Ensembles have also trended towards much less snow next weekend. Compared to 24 hours ago, most ensemble systems have really reduced the high end (90th percentile) snow forecasts for the northern Rockies region. The high end forecast for the weekend in places like Missoula and Kalispell has dropped from 4 to 6 inches down to 2 to 4 inches. The forecast is still calling for plenty of snow in the mountains, so that hasn't changed, it's just that the more extreme forecasts are becoming less likely. Ensemble clusters based on QPF (the liquid equivalent of an precipitation) are split into two main scenarios. The most likely scenario with 61% of ensembles supporting it has less than 0.5 inches of liquid precipitation across the northern Rockies, and that would mainly be in the mountains. About 35% of ensembles are still supporting the wetter forecasts with 0.75 to 1.5 inches of liquid precipitation in the mountains. So, in the last 24 hours the ensembles have veered to the drier solutions.

In any case, we're still expected a cool down and snow this weekend. Don't let the trends fool you, there will still be winter weather impacts, especially in the mountains. So if you plan on being out and about next weekend, be prepared for winter driving. It's just that right now forecasts are trending more towards a winter weather advisory level of impact instead of a winter storm warning level of impacts.

AVIATION

Valley fog and stratus are expected to continue through Wednesday morning across the region. A shortwave moving over the region tonight may help the stratus lift some, but it is also bringing a lot of dry air aloft with it. The dry air will enhance the radiational cooling, strengthening the cold pools and ultimately making the fog and stratus worse. Terminals KGPI and KSMN are the most affected by the fog and stratus. Terminal KBTM is too elevated to be affected by the cold pools, and light down valley flow and drier air has kept the fog out of KHRF so far. Terminal KMSO remains on the boundary of the fog daily, but with the cold pools getting stronger, expect the fog to become more frequent and persistent at KMSO over the next couple of days.

MSO WATCHES/WARNINGS/ADVISORIES

MT...ID...Air Stagnation Advisory until 4 PM PST Tuesday for Northern Clearwater Mountains...Orofino/Grangeville Region... Southern Clearwater Mountains.