textproduct: Missoula

DISCUSSION

A very unusual synoptic pattern to discuss this afternoon. The MRMS radar loop shows a massive area of precipitation wrapped around a low in southern Alberta, Canada. This area of heavier rainfall, possibly a "seclusion", is making its way south and west towards northwest Montana. This system will bring in 200% of normal of atmospheric moisture and set the stage for heavy mountain rainfall in the Glacier Region south and west to the Mission Mountains.

Due to the strong pressure/wind gradient of 18 millibars between the 987 millibar surface low pressure in southeast Alberta and 1005 millibars at Glacier Park International, the winds have been sustained over 20 mph at Flathead Lake. Because of this, there is a Flathead Lake Wind advisory in effect through late tonight.

Winter storm warnings continue to be in effect for the Bitterroot Mountains and southwest Montana locations through Monday morning. Recent model trends continue to show snowfall in these area. The intensities may increase once the slug of moisture from Canada arrives late tonight.

By Tuesday, winds will relax and we will transition into a weak ridge of high pressure with slightly unstable air. Given the widespread precipitation, fog and low clouds could cause travel issues across the region Tuesday morning. The lingering moisture will also support scattered showers later in the day.

A moderating trend is expected later this week with possible thunderstorms each day. With the added boundary layer moisture from all the rain, this could enhanced the instability each day.

AVIATION

A large upper low over Montana will bring in heavier rainfall from northwest Montana by this evening 29/0000Z/6PM MDT and spread southwards with time. There will be plenty of low clouds around, potentially IFR (Instrument Flight Rules) at times throughout the night tonight into Monday. Moderate to heavy rain intensities will be possible and with the added moisture, reduced visibility from rain and/or fog could be an issue. GPI has the greatest chance for heavy rain rates this evening. MSO could experience occasional heavy rain rates later tonight. Other terminals will experience steady rain at times into Monday. Gusty south to west winds to 25 knots are possible across western Montana this afternoon and evening due to the tight 10+ millibar gradient. The elevated moisture levels continue on Monday there could be moderate showers and a few thunderstorms capable of very heavy rainfall across the region. The higher probability for any thunderstorms will be north of I-90 to northwest Montana.

HYDROLOGY

Anomalously high moisture content (200% of normal) will fuel heavy precipitation through Monday across western Montana, especially northwest Montana tonight through Monday. Snow levels rising toward 8000 feet will support efficient, heavy rainfall. When combined with south- to-westerly winds picking up extra moisture off Flathead Lake, rain intensities will increase. High-resolution weather models show possible rain rates over 0.30 inches per hour in the Lewis, Northern Swan, and Mission Ranges tonight into Monday morning. Fortunately, existing snowpack is minimal, so this will translate purely to increased stream runoff. Areas prone to rockfalls or mudslidesincluding upper elevations of the Going-to-the-Sun Roadcould be vulnerable depending on exactly where the heaviest rain sets up. Campers and RVs parked close to streams or the Hungry Horse Reservoir should consider repositioning to higher ground today. Flash flood watches were hoisted for both Flathead and Lake Counties to address the heavy rain potential.

By Monday afternoon, more moderate to heavy showers could impact the Lincoln, Sanders, and Lake County areas. High resolution models depict possible 0.50 to 1.00 per hour rain-rates primarily in the foothills and mountains in these areas. This has been a recent change and we are contemplating issuing further watches if run-to-run consistency continues.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Warning until 9 AM MDT Monday for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region.

Flash Flood Watch through late tonight from the Glacier Park Region to the foothills and Mission Mountains.

Lake Wind Advisory until 3 AM MDT Monday for Flathead/Mission Valleys.

ID...Winter Storm Warning until 9 AM MDT Monday for Eastern Lemhi County...Western Lemhi County.