textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Mild daytime temperatures through Thursday.

- Patchy valley fog around sunrise each morning.

- Significant weather change coming Thursday night into Friday.

Mild day-time temperatures will continue across the Northern Rockies through Thursday as high pressure remains in control. Stable conditions under the ridge are promoting the development of daily valley inversions and trapped cold air, with patchy dense fog. Afternoon temperatures will run around 10 degrees above normal for this time of year.

A significant change remains on track late Thursday into Friday, as a low pressure system and cold front tracks across the region. Colder temperatures and widespread precipitation, focused along and south of the Interstate 90 corridor and along the divide in northwest Montana are the main story. Precipitation amounts have trended higher over the last 48 hours, as model guidance has come into stronger consensus for a mid-level low to track along the divide in southwest Montana by Friday morning. Widespread precipitation will wrap around the west-northwest area of this low pointing towards the highest precipitation amounts (0.75-1.50 inches) along the divide across mountain ranges in southwest Montana. As discussed previously, most of the precipitation will fall as rain ahead of the cold front, limiting the potential for winter weather impacts. Snow levels will fall to around 5,000 feet Friday and lower over the weekend as shower activity begins to decrease. Therefore, forecast snow amounts will be generally under an inch across the region and above 5,000 feet. The exception is across higher peaks along the divide, where several inches of snow may fall above 7,000 feet.

There remains considerable uncertainty in the extended forecast as the majority of long-range forecast models have different opinions on the overall pattern early next week. Twenty percent of the models show another ridge of high pressure returning to the region, with the other models suggesting a southwest flow or a cold Canadian trough. Stay tuned!

AVIATION

High pressure will continue to bring stable conditions. Valley inversions and cold air pools will set the stage for patchy areas of valley fog, particularly near large bodies of water and river systems. The primary period to watch will be from 21/1200-1500Z, including KGPI and KMSO. The probability of dense fog impacts at these terminals is generally less than 10 percent, but can not be ruled out.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.