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September 29, 2020 – Above normal temperatures and very dry conditions will continue through much of the week. Areas of wildfire smoke will drift over interior NorCal for the next couple of days.

Discussion

A strong and highly amplified 2+ height anomaly ridge with a 5 to 10 year return interval will remain stationary over the W coastal states into Old Mexico through the short-term forecast period. This will result in well above normal temps with max temps some 10 to 18 degrees above normal and widespread areas of moderate heat risk for this time of year. In fact, the Solano Co area may encounter high heat risk Wed/Thu as winds turn down valley and adiabatic warming effects from the Vaca Mtns result in the potential for triple digit temps there, and in the northern Sacramento Valley in the afternoons.

The strong ridge and periods of downslope winds will result in a dry air mass, but fortunately the easterly winds will weakening today with relatively a light wind regime through the rest of the forecast period. Humidity recovery will remain poor to moderate over the higher terrain due to a strong subsidence inversion.

Experimental smoke forecast indicates the areal coverage of smoke from area wildfires will spread over most of the CWA during the next couple of days. Not only will this suppress max temps in the areas of dense smoke, it will also negatively impact air quality generally over the coastal range and into the Valley over the next day or two. As the easterly gradients weaken, upslope flow may transport some smoke over the W Slope Sierra at times as well.

Extended Discussion (Saturday through Tuesday)

Upper ridging over the area Saturday, weakens and shifts into the Great Basin Sunday as upper low approaches California then lingers along the coast early next week. This will result in dry weather with a minor cooling trend. High temperatures remain slightly above normal, but could locally be impacted by smoke from wild fires.