September 13, 2020 – Smoke and haze from wildfires will continue to impact air quality and temperatures. Weak weather system will bring onshore flow and periods of gusty winds beginning today through early next week as well as a slight chance of showers in the northern counties by mid- week. A Fire Weather Watch is in effect Monday for the southern Cascades and foothills due to gusty winds and low RH.
Discussion
Main concerns with the near term forecast continue to be smoke, breezy winds, dry humidity values, and possible precipitation in the far northern counties by mid-week.
Smoky skies cleared a bit in the Valley yesterday afternoon, with hires guidance suggesting this trend may continue today. Onshore flow develops this morning as upper level troughing begins to replace ridging over NorCal today. HRRRx guidance suggests only light smoke in the southern Sacramento Valley this morning, with conditions slightly improving in the northern Valley this evening. Expect haze and at least light smoke to continue across the area as wildfires continue across the state. Airnow forecasts for today still indicate areas of moderate to unhealthy air quality. Still expecting similar afternoon highs to those of yesterday, with the Valley in the upper 80s and low 90s, and mountain temperatures generally in the 70s.
Breezy winds are expected to develop this afternoon and evening continuing into early next week as this trough continues to amplify and move east. South to southwest winds in the Valley may gust up to 20-25 mph each afternoon with the strongest winds in the northern Sac Valley and the Delta. In the high Sierra, expect some west to southwest gusts up to 35 mph, with the stronger winds seen Monday afternoon. Although the onshore flow should slowly increase humidity, these winds will bring some local fire weather concerns as fuels continue to be dry. A Fire Weather Watch continues for the southern Cascades and adjacent foothills Monday morning through 8pm Monday evening where minimum daytime relative humidity will remain in the teens and active wildfires could be impacted by the gusty winds.
Stuck with NBM PoPs for mid-week chances for precipitation with only small chances in the Coastal mountains by mid-week. Relatively “better” chances seen later in the week as the trough finally moves onshore. Temperatures through mid-week remain near or slightly below normal with continued localized gusty south to west winds in the afternoons and evenings.
Extended Discussion (Thursday through Sunday)
GEFS and EC ensemble have settled into a pretty consistent forecast indicating the closed low off the coast will approach NorCal on Thursday before becoming an open wave on Friday as it moves across the PacNW. The system may bring a few showers to the northwest corner of the forecast area on Thursday, with a little better chance possible across the northern mountains and far north end of the Sacramento Valley on Friday. Locally breezy conditions along with a little cooler temperatures (and higher RH’s) are expected.
Ridge rebuilds for warmer and drier conditions next weekend. It appears a relatively brief period of north/east winds will be possible, but that could change depending on whether the late week system digs further into the Rockies as it moves east of NorCal.