Fire Weather Outlook September 8 through 13

An upper level trough is crossing the western US Monday. This is enhancing the pressure gradient across central WA/OR this afternoon, generating gusty winds in excess of 35mph. Combined with very dry vegetation and relative humidity between 15-20%, red flag warnings have been posted. Weak cold front will cross the region overnight, weakening winds for Tues with slightly cooler conditions and higher RH. A few pockets of lingering gusty winds will persist for northern CA/southern ID/southern OR and the northern Great Basin on Tues. Southern ID will see the strongest winds with an enhanced fire risk. Farther west, very dry vegetation and RHs < 15%, especially for northern NV/north CA will promote fire growth as well, but winds are expected to be weaker. These conditions will still promote continued suppression difficulties of large wildfires in the region.
A disturbance will round the trough as it moves east, passing over the northwest US on Wed. In response, a strong high pressure is expected to develop on the backside of this disturbance across the northern Rockies. This will once again create strong easterly surface flow for much of the west. As winds flow downslope of the Rockies, warm/dry air will situate itself over the western states with above normal temperatures and dry conditions. Some localized breezy conditions are possible Thurs across WA/OR/ID, however they will dwindle Fri as the high moves east with a weakening pressure gradient. Upper level ridging builds back in across the western US this weekend and above average temperatures with very dry conditions are anticipated to continue into next week.
Heavy monsoonal rains (and moisture from ex-hurricane Norbert) will remain possible through Tues in the desert southwest and eventually diminish mid-week. Some very widely scattered dry thunderstorms possible in south/south-central CA. Drying trend will continue into the weekend before another round of monsoonal moisture pushes into NM/NV/UT/CA by early week. This may also be enhanced by another tropical system by mid-week.
Weather Highlights:
CA: Few isolated storms south Mon/Tues, otherwise continued above normal temperatures and very dry conditions. Isolated breezy winds Tues/Thurs for northern CA.
OR/WA: Red Flag Warning Mon. Breezy winds with critically low RH Mon with improving conditions into Tues. Another period of localized breezy conditions Thurs with a warming trend into the weekend. Expect continued very dry conditions, especially for southern OR.
Great Basin/southern ID: Heavy rain possible for the southern Great Basin through Tues. Enhanced fire weather Tues with gusty winds and low RH for northern Great Basin/ID. Dry and warm through the period.


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