textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Widespread Winter Travel Impacts (MonTue): Snow is expected to cover roads, creating slick conditions.

- Active Pattern Continues (Late Week): A mix of moisture and cold ground temperatures could bring additional rounds of slick travel.

Low clouds have developed in the last 24 hours across the region. This is possibly due to the release of moisture from lakes and rivers within the modified Arctic air mass in place. Forecast soundings depict this continuing through Monday morning. In the Flathead Valley, extending northwards, the soundings show the cloud layer thickening enough that there could be concern that fine snow crystals or even freezing drizzle could start to fall. This is very tough to predict, but just wanted to throw that out there that there is a low probability but impactful if it occurs.

Confidence continues to grow that a widespread snow event will develop on Monday and last through Tuesday. Across northwest Montana, the snow could develop as early as Monday morning, but more likely in the afternoon. A key impact to this will be the fact that any melting snow during the afternoon could freeze-up after nightfall Monday evening. You add relatively dry snow on top of that, it could become very slick. With that in mind, we wanted to get the message out early enough for northwest Montana, so we issued advisories. Snow amounts could vary widely during this event, as seen in the weather models. Much of the variance is likely due to resolution differences in being able to accurately describe the Arctic airmass in place (a known bias). There are indications that Flathead Lake could help to enhanced snowfall from Bigfork to points north and east (i.e. Many Lakes Region, foothills of the northern Swan Range). Other regions may need advisories issued in successive shifts. The Missoula and Bitterroot Valleys will be a challenging forecast as total precipitation amounts range from several hundredths up to a couple of tenths. This could easily bring a dusting to several inches of snow. The GFS model seems to be handling the cold air in place better than some others, and shows snow developing in Missoula by mid-afternoon Monday, but then shows some drying Monday evening before another round of snow develops Monday night.

This pattern with northerly flow can favor the lower elevations of Idaho, particularly the Camas Prairie. As the trough passes southwards on Tuesday, snow may develop and linger for several hours. This could also be a black ice scenario that needs to be watched.

After snow showers wane by Wednesday morning, there will be a brief lull until the next system starts to arrive by later Thursday! Temperatures will be near seasonal levels, although some locations may have persistent clouds that could keep temperatures cooler.

The firehose of moisture gets pointed in our direction by Friday and Saturday. A deep trough of low pressure situated near Hawaii is responsible for a long fetch of subtropical moisture reaching the northern latitudes! The details on precipitation type and how much are a little bit more fuzzy. Both the ECMWF and GFS ensembles show a high probability for the colder airmass hanging on over much of our region, except across southern Idaho County and Lemhi County later Thursday into Friday. Then they both show overrunning precipitation during the day with some type of erosion of the cold air in place. It could be quite the sloppy day around here with freezing rain, snow, rain, wind, and moderating temperatures. They both show a little bit of cooling by Saturday which could equate to lower snow levels. Also to throw into the mix is the fact that this northwest flow pattern can be a considerable snow- making pattern for both valleys and mountains. Also northwest flow can leave the Glacier Region and/or northwest Montana vulnerable to Arctic air intrusions.

AVIATION

Low clouds will be stuck in the valleys through Monday. There is a low probability for light freezing drizzle or flurries across northwest Montana emanating out of the stratus. Widespread light to moderate snowfall will develop by Monday afternoon across northwest Montana, then a second surge of moisture impacting much of the region by Monday night into Tuesday. Mountain obscurations, low ceilings and visibility will be possible Monday into Tuesday area-wide.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 1 PM Monday to 11 AM MST Tuesday for Kootenai/Cabinet Region.

Winter Weather Advisory from 2 PM Monday to 5 PM MST Tuesday for Flathead/Mission Valleys...Lower Clark Fork Region... Potomac/Seeley Lake Region...West Glacier Region.

ID...None.