Moderate Heat Risk through Friday, continued smoke and haze

September 30, 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. A slight cooling trend is expected for the weekend, although temperatures remain slightly above normal.

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. The “Omega Block” is locked in place over Wrn NOAM by 2 long wave trofs, one over the Ern third of NOAM, and the other near 150W. 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 Nrn Sac Vly in the afternoons, provided dense smoke at least thins partially there.

Pressure rises today over the Pac NW will increase the Nly gradients sufficiently for periods of downslope winds, the strongest over the Srn Sac Vly and Nrn SJV today and into the evening. Then lighter Nly/NWly winds Thu and Fri. This will result in an unseasonably hot/dry subsiding air mass today and Thu over Norcal as supported by the NAEFS geopotential heights and return intervals for today and Thu, then weaker signals for Fri. In addition, humidity recovery will remain poor to moderate over the higher terrain due to a strong subsidence inversion until the weekend.

Experimental smoke forecast has not updated since yesterday so our smoke forecast is based on the moderate northerly flow developing today over NorCal. We expect to see the smoke layer spread southward today and Thursday over a good portion of our CWA. Not only will this suppress max temps in the areas of dense smoke, it will also negatively impact air quality over many areas over the next day or two. The dense smoke can limit the max temps some 10 deg or so and makes the temp forecast more problematic. Weakening of the ridge on Sat will allow for onshore flow and cooler temps for interior NorCal.

Extended Discussion (Sunday through Wednesday)

Upper ridging shifts east into the Great Basin Sunday as meso low moves through SoCal Monday into Wednesday. Dry weather expected with high temperatures remaining around 3 to 7 degrees above normal.