Fire weather outlook, August 18 through 22

Lack of widespread critical fire weather has kept fire starts down across much of the western US. Expect fire weather to become more critical today in OR/CA and spread eastward into ID/WY through late-week.

A cut-off upper level low off the coast of north CA will provide enough moisture and instability for scattered dry thunderstorms over high elevations. Very dry surface conditions combined with hot temperatures will make fuels very receptive to dry lightning. Tues, the low will drift southward along the coast, but lose ample moisture to provide widespread storms, however a few isolated storms are possible in central/north CA through Tues evening.

A more potent upper level trough will enter dig into the Pacific Northwest Tues, and begin to eject over the Northern Rockies on Wed. In advance of this trough, surface pressure falls are expected across central OR/WY creating breezy westerly winds Tues and Wed. Combined with warm temperatures and dry conditions, fire suppression will become difficult. Higher moisture across extreme northern WA/ID/MT will develop scattered rain and wet thunderstorms over higher elevations bringing some relief mid-week, with a higher heavy rain threat for eastern MT late week.

Surface low pressure will begin to develop across the central Plains in response to the eastward movement of these upper level features. With high pressure over the Rockies and the western US, a strong pressure gradient will set up over WY and southern ID, and northern CO late week. This gradient will develop breezy conditions across these areas leading to critical fire weather. In addition, increased upper level moisture will provide ample conditions for scattered dry thunderstorms across the same areas, especially over higher terrain. Cooler conditions will develop across the northern Rockies late weekend. Meanwhile, warm and dry conditions will continue for CA and western/central OR/WA with isolated locations of gusty winds through the weekend.

Weather Highlights:

CA: Critical fire weather Monday due to dry lightning, warm temperatures and dry conditions. Threat will lessen and drift southward on Tues/Wed. Continued hot and dry through the period.

OR/WA: Scattered dry thunderstorms across higher elevations of southern OR Monday. Increasing winds Tues/Wed with critical fire weather conditions expected Wed. A few wet thunderstorms over higher terrain of extreme nrthern WA.

Southern ID/WY/northern CO: Very breezy conditions developing with warm temperatures and dry conditions continuing into the weekend. Scattered dry thunderstorms possible through the period over higher terrain. Critical fire weather expected Wed through Fri.
MT: Isolated dry thunderstorms possible Wed through Fri for southern MT, with wet thunderstorm chances across northern MT. Otherwise warm and dry through Thurs with heavy rain possible Fri across eastern MT.