September 27, 2020 – Strong north and east winds have developed across much of the area this morning and we will see these continue at least into the early part of the afternoon before they let up some. Winds will pick up once again over the Sierra and Foothills tonight and will be similar to a bit stronger than what they were last night and this morning. Winds will begin to diminish during the morning Monday but will remain breezy into the afternoon. Low daytime humidity and poor to moderate overnight humidity recoveries will accompany these winds.
This will bring critical to locally extreme fire weather conditions into Monday afternoon. High pressure building into the region will continue to bring warming temperatures and we will see them push into the 90s to near 100 for the Valley and Foothills today and for much of this week. The changing wind direction and heat will likely bring the return of smoke impacts to much of the region, you can monitor air quality at fire.airnow.gov.
A prolonged critical fire weather is unfolding overnight across northern California. Northerly winds are already at 10 to 20 mph with some gusts above 30 mph in portions of the Sacramento Valley, combined with extremely poor relative humidity values for nocturnal standards. Winds will continue to increase through sunrise with relative humidity values holding steady (or even dropping). GOES-West satellite is providing high-resolution imagery with a 1-minute temporal refresh rate over much of the Golden State through at least 11 AM PDT Sunday to aid in fire weather support and services.
California is presently positioned between an upper level ridge axis oriented north-south a few hundred miles offshore, as well as a trough that extends from the Northern Rockies southwest towards the Great Basin. This results in northwest flow aloft, a pattern setup that is generally concerning this time of year as we begin to enter offshore wind season. The north-south and east-west surface pressure gradients are continuing to strengthen as a surface high builds in Oregon and Nevada. As of 12 AM PDT, the gradients were +13.2 mb from Medford to Sacramento and +7.0 mb from Winnemucca, Nevada to San Francisco. The stronger the pressure gradients, the better potential for stronger winds.
For the Sacramento Valley…the strongest winds are forecast to impact the western half of the valley from now through mid- morning, then marginally ease during the day on Sunday. That said, winds will remain strong and gusty with relative humidity values bottoming-out during the afternoon hours continued critical fire weather conditions. A renewed period of strong winds will redevelop overnight Sunday and continue into Monday. Peak winds during this event for the Sacramento Valley will be north to northeast at 20 to 30 mph sustained with gusts to 45 mph, locally 50 mph. Minimum relative humidity values will fall to the low teens and possibly single digits on Sunday and Monday afternoons.
For the Southern Cascades, Northern Sierra, and foothills… strongest winds will impact the southwest-northeast oriented canyons/gaps (e.g., Jarbo Gap) through mid-morning before gradually easing during the day on Sunday. A renewed burst of possibly stronger winds will develop overnight Sunday and into Monday. Peak winds will be northeast to east at 20 to 30 mph with gusts to 45 mph, locally 55+ mph. Relative humidity values will fall to the 10-20 percent range on Sunday and Monday afternoons with exceptionally poor overnight recoveries.
The Red Flag Warning is in effect through 9 pm PDT Monday during this prolonged period of extreme fire weather conditions. Additionally, a wind advisory is in effect for locations expected to be impacted by the strongest winds: 2 am PDT Sunday to 8 pm PDT Sunday for the western Sacramento Valley, and 2 am PDT Sunday to 2 pm PDT Monday for the northern Sierra Nevada foothills.
Shifting from fire weather to heat…the upper level ridge offshore will move onshore over Oregon on Monday. We’ll see widespread 90s throughout the Sacramento and northern San Joaquin Valleys through much of this next week, equating to about 10 to 15 degrees above late September climatology. Some forecast models go even hotter than the official forecast (which leans on the National Blend of Models), suggesting low 100s for a good portion of the Sacramento Valley. Heat risk will generally be low to moderate, mainly impacting sensitive groups and/or those outdoors for prolonged periods.
Extended Discussion (Thursday through Sunday)
Above normal temperatures are forecast into the first week of October. For those keeping tally, Downtown Sacramento has recorded 105 days this year at/above 90 deg F, ranking second all-time. The record is 110 days, which occurred in 1984. If the present forecast remains on track, 2020 will overtake 1984 by the end of next week.
Dry conditions are forecast to prevail through the extended period.