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September 15, 2020 – Smoke and haze will continue to impact air quality and temperatures. A weak weather system will bring onshore flow and periods of gusty winds through the Delta and higher terrain this week. Stronger winds, cooler air, and a slight chance of showers and thunderstorms in the far northern counties and mountains possible late Thursday into Friday. Dry weather returns for the weekend.

Discussion

Low pressure remains off the Pacific Northwest coast which is expected to deepen and eventually move across the NW CONUS Thursday and Friday. Onshore flow continues and periods of breezy winds can be expected this afternoon and evening, and again tomorrow. These winds may help improve smoky conditions in the Valley this afternoon, although portions of the foothills and mountains near wildfires will continue to see the smoky skies. Increasing humidity this week will help alleviate fire weather concerns some.

Near average temperatures today and tomorrow will begin to cool Thursday into Friday as the aforementioned weather system makes its way across the area. Along with the cooler temperatures (with temperatures roughly 6-12 degrees below average on Friday), some gusty winds over the Sierra/Southern Cascades are expected Thursday into Friday as the pressure gradient increases with the frontal passage.

Precipitation chances with this system remain limited mainly to the northern portion of the CWA. Best chances look to be in the Coastal mountains and Shasta County beginning late Thursday continuing through Friday. Thursday evening may see enough instability that an isolated thunderstorm will be possible. Dry weather and decreasing winds begin Friday evening into the weekend with the exit of this system.

Extended Discussion (Saturday through Tuesday)

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If weren’t for the unknown of the amount of influence from lingering wildfire smoke in the area, the forecast synoptic pattern would present a benign wx pattern over Norcal during the EFP. The upper level trof will have moved east of the CWA on Sat with NWly flow over Norcal in its wake.

The air mass will warm with max temps recovering to near to slightly below normal on Sat and 5 to 10 degrees warmer than Fri. The flow backs a little with W to SW flow from very weak troffing along the coast later in the weekend and into early next week. The warmest day is expected to be Sun with max temps just a few deg above normal, before settling to near to slightly below normal Mon/Tue due to a little increase in onshore flow from the weak troffing.

Generally “blended” the Ensemble member Max T guidance which varied slightly with generally the ECMWF and GFS a little warmer than the NBM. Comfortable mins are expected as the days shorten/nights lengthen as the Equinox nears.