Area Forecast Discussion

DISCUSSION

KEY MESSAGES:

- Low confidence in frost this morning due to persistent cloud cover, though brief and patchy valley fog remains possible where clearing occurs.

- A cold front drops north-to-south Friday bringing showers and isolated thunderstorms from northwest Montana to the I-90 corridor.

- Strong warming early next week will push temperatures into the 80s for western Montana and well into the 90s for the lower river valleys of north-central Idaho.

Zonal flow will maintain generally dry and seasonable conditions today. While a Frost Advisory remains in effect for Lemhi County, persistent overnight overcast skies have limited radiational cooling, making widespread frost unlikely. Instead, patchy valley fog could be possible in localized areas that manage to have skies briefly clear out around sunrise.

An upper-level trough diving south out of British Columbia and Alberta will push a cold front through the region on Friday. This feature will introduce a tight window of frontal instability moving north-to-south. Showers and isolated thunderstorms will initiate near the Canadian border by mid-to-late morning, progressing southward toward the I-90 corridor by the late afternoon and evening. While overall moisture is limited and duration of shower activity will be short, brief convective downpours could produce rainfall rates as much as a third of an inch per hour. Valley totals will range from a few hundredths up to 0.20 inches with decreasing amounts heading southward, and 0.20 to 0.50 inches looking possible along the Continental Divide. Activity drops off sharply south of I-90. Cool northerly flow on Saturday will maintain breezy conditions (gusts to 25 mph) and a few isolated mountain showers.

An elongated west-east ridge establishes across the western US by early next week. Although this keeps the Northern Rockies under westerly flow aloft positioned on the eastern side of the ridge axis, strong subsidence will clear skies and initiate a robust warming trend. Temperatures will peak around Tuesday, reaching the upper 70s and 80s for western Montana valleys, and 90s to near 100 in the lower river valleys of north-central Idaho.

AVIATION

The main short-term aviation concern is the low-end potential for patchy valley fog through 11/1500z, dependent entirely on how quickly and where the overcast deck breaks up. Otherwise, VFR conditions will dominate today under drying zonal flow. Breezy west-northwest winds will develop after 11/1800z with afternoon gusts of 15-20 knots across most terminals and decoupling after sunset. Note: KGPI is experiencing communication issues this morning likely resulting in an AMD NOT SKED for the 12z TAF issuance.

MSO WATCHES / WARNINGS / ADVISORIES

MT...None. ID...Frost Advisory until 9 AM MDT this morning for Eastern Lemhi County.