textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Warming and drying trend with the weather pattern during weekend that lasts through early next week.

- Showers and isolated thunderstorms possible Tuesday evening, with gusty outflow winds possible.

- Widespread rain and much cooler temperatures arrive Wednesday through Friday.

A robust ridge of high pressure will amplify over the Northern Rockies this weekend, initiating a significant warming trend through early next week. Regional temperatures will peak on Tuesday, with valley highs reaching the 70s and 80s approximately 20 degrees above seasonal norms. Record breaking daily highs are probable for several locations. By Tuesday afternoon, increasing instability and mid-level moisture will support the development of high-based thunderstorms, primarily across north- central Idaho. The primary hazards with these cells will be frequent lightning and gusty outflow winds.

A cold front will cross the region Wednesday ahead of an approaching broad upper-level trough, ushering in widespread precipitation and breezy conditions. While the precise track of the subsequent closed low remains uncertain for late next week, high confidence exists for a transition to much cooler and more unsettled weather. Moderate to heavy rainfall is expected Wednesday through Thursday, with the highest probabilities for >1.00" of liquid accumulation focused over the Clearwater Mountains and the Continental Divide. Ensemble clusters highlight a 30% secondary scenario where a deep moisture plume wraps into the circulation, potentially yielding 2.00" to 3.00" of liquid along the Divide.

Impacts will include rises on area waterways, though flooding is currently not anticipated. Forecast users should remain alert for ponding in poor drainage areas and localized rock or debris fall on steep terrain. Additionally, snow potential increases by Thursday for the higher valleys and mountain passes of southwest Montana, including the Butte area and Homestake/MacDonald passes. The degree of winter impacts remains highly sensitive to the eventual track of the closed low.

AVIATION

VFR conditions will prevail throughout the period as a strengthening ridge of high pressure maintains clear skies and light, terrain-driven winds. Some patchy fog is possible near river bottoms Sunday morning, but impacts to airfields are not anticipated.

MSO WATCHES/WARNINGS/ADVISORIES

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