September 14, 2020 – Smoke and haze from wildfires will continue to impact air quality and temperatures this week. A weak weather system will bring onshore flow and periods of gusty winds through the week as well as a slight chance of showers in the northern counties and mountains late- week. A Red Flag Warning is in effect today for the southern Cascades and foothills due to gusty winds and low RH.
A trough over the Pacific resides roughly 600 mi west of NorCal and has begun to bring onshore flow and some high clouds over the area. This feature will remain over the EPac the next few days continuing to amplify before beginning to progress over the PacNW Thursday into Friday. The main impacts of this system will be some cooler temperatures and breezy winds through the week.
For today, some breezy winds are expected to develop this afternoon continuing into the evening hours mainly in the vicinity of the Delta and high Sierra. Southwest to west winds in the Delta may be up to 25 mph today, with southwest wind gusts in the Sierra up to 35-40 mph. While the onshore flow will help slightly with humidity values in the Valley, low RH and extremely dry fuels are still present in the mountains. With this said, a Red Flag Warning remains in effect for this afternoon for the southern Cascades and adjacent foothills for 11am through 8pm Monday where minimum daytime relative humidity will remain in the teens and active wildfires could be impacted by the gusty winds.
Smoke will continue to impact the area today and through the week as area wildfires continue to burn. Airnow forecasts air quality to remain between moderate to very unhealthy across NorCal today, although the onshore flow may help improve conditions slightly in the Valley later this afternoon.
For the rest of the week, expect periods of breezy winds each afternoon and evening with average to slightly below average temperatures. As the trough makes its progression towards the coast Thursday, another round of stronger winds may develop over the Sierra which may create additional fire weather concerns.
Precipitation may creep into the Coastal/northern mountains Thursday afternoon and evening. Have kept only the mention of showers at this time, although if instability increases as the low moves in an embedded thunderstorm or two is not out of the question.
Extended Discussion (Friday through Monday)
The evening deterministic model runs were trending a little faster with the upper trof moving across Wrn NOAM at the onset of the EFP than indicated in the 12z Sun 5H Cluster analysis (the latest available at press time). The progged faster model movement has been in error over the last few days as the trof has been held at bay by the strong ridge over the intermountain west, centered near the 4- Corners. With modeling suggesting a closed low will develop along 140W and west of the Pac NW today and Tue before deepening on Wed, little Ewd movement of the trof is expected until later in the week. This will keep Norcal in the warmer/drier SW flow pattern through mid week before a cooling trend of some 8 to 14 deg below normal on Fri. Temps will then rebound over the weekend and into next week as the trof moves away from the region and ridging rebuilds.
Along with the much cooler temps on Fri will come a band of stronger mid level winds as the trof nears the coast and moves inland. Late Thu and into Fri 7h winds are forecast to increase to 45-50 kts over the Nrn mtns including Wrn Plumas Co. But with the winds will come higher humidity and the chance of only some light rain. The 00z 9/14/20 Wx Bell ECMWF ensemble meteogram for RDD shows about one- fifth of the members with some light rain (a couple-few hundredths) on Fri, and only 5% of the GFS members. One thing to monitor is the entrainment of subtropical moisture into the incoming trof from T.S. Karina in the Ern Pac which may spread as far N as the Tahoe area late Thu and Fri (GFS more bullish than the ECMWF for now). Since this is a non-zero PoP, we have introduced a chance of showers over the Mother Lode and W slope Sierra mainly from Tahoe southward on Fri. For now, it appears the instability (thunder chances) will remain well to our S/SE.