textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- High-based thunderstorms will develop late this afternoon and evening across north-central Idaho and northwest Montana, bringing a primary threat of erratic, gusty outflow winds. - Widespread rain develops on Wednesday, along with much cooler temperatures, and a return to winter driving conditions over mountain passes by Thursday.

- A highly uncertain but potentially impactful band of heavy, wet snow could disrupt the Thursday and Friday morning commutes across western Montana valleys.

The upper-level ridge that brought us record-challenging warmth will begin to break down this evening. Today will be the warmest day of the week, with valley highs soaring into the upper 70s and mid 80s. The combination of dry fuels, low relative humidity, and increasing southwest breezes will maintain elevated fire weather conditions across southwest Montana through the early evening.

As we head into late afternoon and evening, increasing Pacific moisture and thermodynamic instability will interact with the departing ridge. This will trigger isolated to scattered thunderstorms, initially focusing over north-central Idaho before spreading into northwest Montana. Because the lower levels of our atmosphere remain quite dry, these storms will be high-based with the main threat being localized wind gusts over 40 mph.

A drastic, large-scale pattern shift begins Wednesday morning as a strong Pacific cold front sweeps across the Northern Rockies. Widespread precipitation is expected through Thursday night. The highest liquid precipitation totals, locally exceeding one inch, are expected along the Clearwater Mountains and the Continental Divide. Behind the front, temperatures will plummet 30 to 40 degrees from today's highs.

As this cold air mass settles over the region, snow levels will drop rapidly Wednesday night into Thursday. By Thursday morning, snow levels will fall to between 3,000-4,000 feet. This will bring wet, accumulating snow to higher valleys and mountain passes, including Lookout, Lolo, Homestake, and MacDonald passes. Travelers should prepare for slushy accumulations and winter driving conditions along the Divide.

We are also tracking a narrow, highly uncertain band of heavy precipitation that could impact the Thursday morning commute across Northwest Montana. If this band sets up over the valleys, heavy, wet snow will create periods of low visibility, slushy roads, snapped tree branches, and isolated power outages. Furthermore, a second band of snow could develop over West-Central Montana Thursday night, timed perfectly to disrupt the Friday morning commute.

The cool and showery pattern will persist through the upcoming weekend as a broad upper-level trough stalls over the western United States.

AVIATION

VFR conditions will hold across the airspace through the afternoon, with increasing southwest winds aloft mixing down to terminal sites (KBTM, KSMN). Expect afternoon wind gusts of 20 to 25 knots. By late afternoon (around 22/00Z), increasing instability will trigger high-based thunderstorms, primarily impacting north-central Idaho initially before spreading into northwest Montana. Because of the dry lower atmosphere, these cells will be capable of producing erratic outflow gusts in excess of 35 knots. A strong cold front sweeps from west to east beginning early Wednesday morning. All terminals will experience rapidly lowering ceilings, shifting winds, and widespread MVFR to localized IFR conditions in moderate rain by the afternoon. Extensive mountain obscurations are expected in the high terrain, along with dropping snow levels Wednesday and Wednesday night.

HYDROLOGY

Rapid snowmelt driven by today's temperatures in the 70s and 80s will combine with widespread rainfall Wednesday and Thursday. This will result in notable rises on local rivers and streams. While major river flooding is not expected, water ponding in poorly drained areas is likely. Moderate rainfall over steep terrain will also increase the risk of small rockslides.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory from 6 PM Wednesday to noon MDT Friday for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region...Potomac/Seeley Lake Region.

Winter Weather Advisory from 6 PM Wednesday to 6 PM MDT Thursday for West Glacier Region.

ID...Winter Weather Advisory from 6 PM Wednesday to noon MDT Friday for Eastern Lemhi County...Western Lemhi County.