October 24, 2020 – Strong north to east winds will develop across northern California on Sunday and Sunday night. These winds combined with very low relative humidity values and dry fuels will lead to extremely critical fire weather conditions.
A powerful offshore wind event is forecast to develop in the next 24 to 48 hours, impacting many throughout northern and central California. All the necessary ingredients will be there to produce extremely critical fire weather conditions: 1) low relative humidity values, 2) strong winds, and 3) dry fuels. Forecast models suggest that this may be the strongest offshore wind event of the year. More details below.
Upper level trough axis will drop from British Columbia and into the Pacific Northwest on Saturday. This trough is progged to drop south into the Great Basin as an “inside slider.” At the surface, high pressure will develop along with a cold airmass in the trough’s wake. Surface pressure gradients will then strengthen initially in the north-south component during the day on Sunday, and then in the east-west component late Sunday and overnight into Monday.
High resolution models are now beginning to better capture the finer details of this event. The latest 3-km NAM ramps up the 925 mb winds (about 2500 ft MSL) over the northern Sac Valley on Sunday morning, and then winds will subsequently increase over the southern Cascades, northern Sierra, and lower foothills later on Sunday and Sunday night. The 3 km NAM has these winds at 925 mb peaking over 60 kt. The downslope/katabatic effects of the winds at the surface will result in an astonishingly dry airmass with little/no relative humidity recoveries on Sunday night, and widespread single digit humidity values on Monday afternoon — less than 5 percent in some places! Dew points will bottom out around 0 deg F, possibly lower. The already dry fuels will become even drier.
For the Sac Valley, strongest winds will focus along the west side with gusts of 40 to 50 mph likely during the day on Sunday. Winds don`t appear to be as strong on Monday, but extremely critical fire weather conditions will persist with the rock- bottom relative humidity values.
For the southern Cascades, northern Sierra, and foothills, offshore winds will begin to develop around mid-day Sunday, and accelerate during the the evening and overnight hours into Monday. These powerful winds will gust anywhere from 50 to 70 mph — conceivably higher in wind-prone canyons, gaps, or in the vicinity of mountain waves. Single digit relative humidity values will be widespread in the foothills/mountains on Monday afternoon as winds slowly subside.
Red Flag Warning is in effect beginning Sunday morning for the Sac Valley and southern Cascades. The Red Flag Warning for the northern Sierra begins Sunday evening. During Red Flag conditions, any new and/or existing fires have the potential to spread rapidly and behave erratically. Information for other locations in northern California can be found in discussions/fire weather forecasts from NWS San Francisco Bay Area, NWS Eureka, NWS Reno NV, and NWS Medford OR.
To recap, an extremely critical and dangerous fire weather event is expected to unfold across northern and central California over the next 24 to 48 hours. The strongest winds in the local forecast area impact 1) the southern Cascades and northern Sierra and adjacent foothills, and 2) the western side of the Sacramento Valley. The entire region can expect extremely low relative humidity values. Looking back on past powerful offshore wind events, there are meteorological similarities to the October 27-28, 2019 event as well as November 8, 2018.
Extended discussion (Wednesday through Saturday)
Upper ridging builds into Northern California bringing lighter winds and above average temperatures for the latter half of the week. Extremely dry conditions will still remain through Saturday with daytime humidity values in the teens most afternoons and moderate to poor overnight recoveries in the foothills. Continuing to practice fire safety remains essential with the prolonged dry conditions. The warmest day of the week looks to be Thursday with highs back into the low 80s in many Valley locations. Highs look to be a few degrees cooler Friday and Saturday as a trough moves across the PacNW. Precipitation chances with this system look to stay north of California, with the main impact being some additional high clouds across the area.