Fire weather outlook, September 3 through 8

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Yet another upper level trough providing some active fire weather conditions today across northern CA/southern OR and most of WY/western SD. With this trough, a lack of mid-level moisture is preventing any thunderstorm activity across the northwestern US. However, very dry conditions (aggravated by drought in CA/OR) combined with windy conditions are developing critical fire weather conditions in both these regions. A surface cold front associated with the trough will slide southeast across the northern Rockies Wed evening and into Thurs. This front will increase humidity across WY/SD lessening fire danger and shifting winds to the north for Thurs.

The same cold front will push south/west across OR/CA overnight Wed. This front will continue mixing overnight with breezy winds and poor RH recovery-continuing elevated fire danger through Wed night. Through late week and into the early weekend, easterly winds will provide additional downsloping off the Rockies and continue increased fire risk with poor overnight RH recovery through Sun. With an off-shore trough developing late-week and weak ridging over the Rockies, temperatures will increase to 90F+ across much of CA/OR/WA. Very dry conditions and RHs at/below 20% will expand eastward/northward across the region and into the central Rockies through the weekend. Breezy easterly winds will continue through Fri for much of OR/northern CA/NV and gradually diminish overnight Fri. As the off-shore upper level trough approaches the West Coast Sun/Mon, westerly winds will increase for much of the northern CA/OR/WA/ID, which may develop widespread critical fire weather conditions. No precipitation is expected for northern CA/NV and WA/OR/ID through the period.

The other major weather highlight will be soon-to-be Hurricane Norbert near the California Peninsula. Current forecast model trends depict moisture streaming northward into the southwestern US by late weekend. Developing monsoonal conditions across AZ/NM/UT/CO late week will provide chances of scattered thunderstorms. However, much more widespread rains/storms could develop for portions of southern CA/NV and much of AZ/UT/CO with the influence of Norbert. Great uncertainty still exists on the exact track of this system, though confidence is increasing on widespread heavy rains late weekend/early next week.

WPC 5-day OPF
Quantity of precipitation forecast for September 3 through 8, 2014. Rains continue to avoid much of the Pacific Northwest and western US, with some much needed relief for southwest US. Image from the Weather Prediction Center.

Weather Highlights:

WY/western SD/northern CO/NE Panhandle: Critical fire weather Wed with breezy westerly winds, RH’s <20%, and warm conditions. Cold front crossing region tonight increasing RH and winds increasing and turning northerly post-front. Becoming very dry/warm Sun/Mon with increasing fire weather threat.

OR/northern CA/northern NV: Critical fire weather Wed through Thurs with breezy northerly wind, turning easterly Thurs AM. Very warm/dry with poor overnight RH recoveries expected through Sat with continued enhanced fire weather conditions. Additional critical fire weather conditions possible Mon/Tues with breezy westerly winds.

WA/ID/MT: Increasing fire weather threat as temperatures warm into the weekend with breezy westerly winds possible late weekend. Continued dry conditions with no precipitation expected (with exception of northern ID/MT Wed/Thurs).

AZ/UT/NM/southern CO: Monsoonal flow returning Thurs/Fri/Sat with scattered thunderstorms, rain becoming more numerous over the weekend, especially across AZ/UT with flooding possible.

Southern CA/southern NV: Very dry and warm through late week. Increasing showers/storms Sun/Mon with flooding possible.


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