textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Multiple wrap-around precipitation bands start late tonight across southwest Montana, then another one Sunday night into Monday. Multiple inches of precipitation possible area-wide.

- Periods of heavy snow will develop late tonight across southwest Montana generally above 6500 feet. Risks include: Hypothermia, rough travel in high country roads, and possible tree-fall

- Second wrap-around band Sunday night: Will draw in 200% normal moisture from east of the Divide and snow levels will be above 8000 feet across northwest Montana. Moderate to heavy rainfall rates are possible in the mountains/foothills and could lead to rises in streams, and rockfall from Glacier National Park region to the Mission Valley. Campers near streams/reservoirs should consider moving to higher ground in these areas.

- Fog and low clouds are possible Tuesday morning which impact aviation and travel through the region.

- We remain in cyclonic/unstable flow through next week which could support daily showers/thunderstorms, and keep highs near to below average for the start of July.

There have been lulls in the precipitation today which has allowed warming to occur with temperatures rising into the 60s in some locations. Instability has been meager with a few intra-cloud lightning flashes in a few of the cells. Think that there could be a few showers that develop this afternoon across western Montana that could produce moderate rainfall rates. There's a low probability (< 10%), for lightning occurrence in the terrain, except across southern Lemhi County to Butte (20-30% chance).

Travel over high mountain passes across southwest Montana will become dangerous for travel later tonight due to snow. Hypothermia will increase tonight due to the snow, cold temperatures and breezy winds. A more robust upper low will move into southwest Montana by early Sunday helping to intensify precipitation rates and cause snow levels to fall to around 6500 feet or lower. Upstream soundings indicate rapidly cooling temperatures aloft, which complicates our snow level forecast, but it means snow could realistically fall to 6000 feet by Sunday morning. Both passes Homestake on I-90 and MacDonald Pass on Highway 200 (6300 feet) could see a switchover to a mixture of rain and snow, or could experience a few periods of heavy wet snow as precipitation rates increase with the mesoscale band. The focus of the heavier mesoscale bands may lie somewhere east/west from between I-90 and Butte/Lost Trail Pass.

The pressure/wind gradient will tighten tonight so westerly winds in the high country will be breezy with gusts 25 to 35 mph possible during the snowfall. This will definitely increase the hypothermia threat. There are winter storm warnings in effect to address the snow and cold threats in the backcountry.

The secondary low mentioned above will become quasi-stationary over central/eastern Montana later Sunday and pull in a massive amount of moisture, elevating runoff risk.(see Hydrology section).

By Tuesday, the wind gradient relaxes a little bit and we may be under weak ridging or weakly unstable conditions aloft. With all the precipitation that has fallen, fog or low clouds could be an issue across the region. Also with the higher dew points (increased moisture levels), that could bode well for scattered shower activity later in the day.

After Tuesday, the ensemble model clusters depict us being under cyclonic flow as a broad ridge of high pressure over the eastern Pacific keeps northern stream energy from over the Gulf of Alaska pointed southeastwards towards the Pacific Northwest. This means that we could be experiencing daily showers and thunderstorms through the rest of the week. Highs may get back to average for this time of year. A sneak peak into the 4th of July (Saturday) forecast, 45% of the clusters depict an unstable pattern that would be conducive for shower/storm activity, while 55% of the clusters point to an exiting disturbance which could pave the way for a drier evening for fireworks. Both individual runs of the GFS and ECMWF show the departing system (going along with the 55%).

AVIATION

A closed low will stall across south-central Idaho this weekend, with snow levels falling as cold air moves overhead aloft. Model guidance suggests a widespread shield of precipitation forming over areas along and south of the I-90 Corridor this evening into Sunday morning. Widespread rainshowers will periodically impact visibility and bring lower ceilings for all aviation sites in western Montana and north central Idaho. KBTM, KSMN, and KHRF will be under an area of more instability, where isolated thunderstorms have the ability to form between 27/2200Z and 28/0300Z this evening. Brief heavy rainfall and small hail, along with lightning will be the main threats. As precipitation intensifies overnight across southwest Montana and Lemhi County, ceilings and visibility will likely lower to between MVFR and IFR levels, mostly between 28/0600z and 28/1500z. In additional breezy northwest winds gusting up to 25 kts will increase through this afternoon and evening. As valleys stabilize this evening, mechanical turbulence is possible at ridge top levels.

HYDROLOGY

As mentioned in the key points, anomalously high moisture content (200% of normal) will be associated with wrap- around moisture developing later Sunday and continuing into Monday across northwest Montana. Rising snow levels towards 8000 feet will support more warm process rainfall, and when combined with south to westerly winds picking up moisture off Flathead Lake, rain intensities could increase. High resolution WRF runs depict possible rain- rates over 0.30"+ per hour in the Lewis, Northern Swan and Mission Ranges Sunday night into Monday morning. Thankfully there isn't much in the way of existing snowcover, so this will purely be increased runoff in streams in these locations. Areas prone to rockfall or mudslides could be vulnerable depending on locations that get the higher rates. The upper elevation locations of the Going-To-The-Sun road could be affected by this pattern. Campers and RVs that are parked very close to streams or Hungry Horse Reservoir may consider repositioning to higher ground Sunday night.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Warning from 6 PM this evening to 9 AM MDT Monday for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region.

ID...Winter Storm Warning from 6 PM this evening to 9 AM MDT Monday for Eastern Lemhi County...Western Lemhi County.