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October 25, 2020 – Strong north to east winds will develop across northern California on today and tonight. These winds combined with very low relative humidity values and dry fuels will lead to extremely dangerous fire weather conditions. Red Flag Warning from 8 AM this morning to 1 PM Tuesday and Wind Advisory from 8 PM this evening to 11 AM Monday.

Discussion

A powerful and dangerous offshore wind event will materialize across northern and central California over the next 12 to 24 hours. This event is forecast to be the strongest event of the year so far for the region. Consequently, the potential fire weather impacts will be extreme. The Storm Prediction Center has a large portion of the region under an “extremely critical” risk, the highest possible categorical assignment for fire weather. Fuels are dry and will only further dry, winds and gusts will be very strong, and relative humidity values will be desert-dry. The meteorological synoptic setup is uncomfortably similar to recent past events in northern California such as October 27-28, 2019 (Kincade Fire rapid growth), November 8, 2018 (Camp Fire rapid growth), and October 8-9, 2017 (2017 Wine Country Fires rapid growth).

For today, positively-tilted upper level trough axis will dive south from the Pacific Northwest towards the Great Basin. Behind this trough, surface high pressure will develop to our north/east along with plenty of cold, dense air. This sets the stage for the north-to-south and east-to-west offshore pressure gradients that will impact the region.

North winds will first increase in the northern Sacramento Valley around mid-morning Sunday. Elsewhere at that time, winds will still be rather light. In fact at sunrise, there may be some elevated stratus with lingering onshore flow in/around the Delta. Northerly winds will increase from north to south along the western half of the Sacramento Valley through the day, peaking in the late afternoon and evening hours between 20 to 30 mph sustained and gust of 40 to 50 mph. Relative humidity values will crash to the single digits to middle teens for most locations in the Valley, foothills, and mountains.

The east-west pressure gradients will significantly strengthen late day and overnight into Monday. This is when the winds in the southern Cascades, northern Sierra, and foothills along the western slopes will dramatically increase and become dangerously gusty. Northeast to east winds of 20 to 35 mph with gusts of 40 to 60 mph will be likely during this period of time. Isolated gusts in favored canyons, gaps, and ridgetops can conceivably exceed 70 mph. Additionally, can’t rule out the potential for some developing mountain waves along the lee west slopes of the Sierra as the cold, dense air descends and accelerates from the crest. Various cross section analyses from the North Bay to the northern Sierra in the latest 3-km NAM support this possibility, which could push localized wind gusts to even more extreme levels. Relative humidity recoveries overnight Sunday and into Monday morning will effectively be non-existent with many locations in northern California holding between 10 to 25 percent.

While winds are forecast to gradually subside through the day on Monday, relative humidity values will drop to remarkably low values — less than 5 percent for some locations — with dew points at or below 0 deg F. Therefore, extremely critical fire weather conditions will continue through Monday and at least into Tuesday morning.

As far as legacy products — Red Flag Warning goes into effect at 8 AM PDT Sunday for the west side of the Sacramento Valley, northern San Joaquin valley, southern Cascades, and the northern Sierra. Red Flag Warning then expands through Tuolumne County at 8 PM PDT Sunday. Additionally, a Wind Advisory goes into effect locations that will experience the strongest winds: at 8 AM PDT Sunday for the west side of the Sacramento Valley and at 8 PM PDT Sunday for the northern Sierra to Yosemite.

To recap…all necessary ingredients will be in place to produce extremely critical fire weather conditions: 1) strong winds, 2) low relative humidity values, and 3) dry wildland fuels. Any new ignitions will be present in an environment that will promote rapid, explosive, and dangerous spread of fire. Have a plan in place if you need to evacuate will little/no notice. While Fire Season 2020 has already been historic from the countless lightning- based ignitions in August, remember that peak offshore wind season is right now for much of the Golden State. Be safe and vigilant during this extremely high-impact and particularly dangerous fire weather event.

Extended discussion (Thursday through Sunday)

High pressure continues to build over the west coast mid-week allowing for mostly clear skies and lighter winds on Wednesday. Highs look to be a few degrees above normal with this ridge, with a few Valley locations possibly reaching into the low 80s on Thursday.

Deterministic models have diverged at this point for next weekend with the GFS continuing high pressure across NorCal and the ECMWF bringing a system across the PacNW with some precipitation chances in far northern California.

Ensemble guidance suggests the former is more likely and have continued a dry forecast through the extended period. With this said, very dry conditions continue from mid week into the weekend with low daytime humidity expected for most of the area and practicing fire safety remains critical.