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October 1, 2020 – Above normal temperatures and very dry conditions will be seen through Friday. Areas of wildfire smoke will impact interior NorCal for the next couple of days. Continued dry weather and a slight cooling trend is expected for the weekend, although temperatures remain 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 Fri before there is some weakening of the ridge over the weekend. 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 14 to 22 degrees above normal today and widespread areas of moderate heat risk for this time of year. Given little movement is forecast with the high pressure cell over NV today, similar to yesterday, this will maintain the Ely and Nly pressure gradients and the Nly flow for another day. We do expect a gradual weakening of the Ely and Nly pressure gradients later in the day due to pressure falls from daytime heating.

Solano Co area likely to encounter another day triple digit temps and heat risk from the adiabatic warming effects from the Vaca Mtns. Record highs for Fairfield 104 set in 1952, and 104 at Vacaville Nut Tree Airport set in 2001, and 101 at Travis AFB in 1970 are possible. In fact, tying the record max at KSUU has a 70% chance of occurring according to our experimental NBM viewer. tying the record high potential is much lower in our other main climo areas with just 26% at KSCK (100 in 2001) and 17% at KRDD (104 in 2001). The wafting/drifting of wild fire smoke also adds to the uncertainty of establishing record temps.

Shifting of the high pressure cell to the west and over the Socal area on Fri will lessen the impact of Nly winds. Temps will not be quite as hot as today and this trend will continue on Sat as high pressure gradually weakens over the region. On Sun, onshore floor will provide some much needed relief with max temps cooling into the 80s in the Valley and mostly 60s and 70s in the high Sierra. This is up to 6 degrees above normal in the lower elevations and about 8 to 12 degrees above normal in the mountains. In addition, humidity recovery will remain poor to moderate over the higher terrain due to a strong subsidence inversion until the weekend.

Now that the calendar has turned to Oct 1, we begin a new water year today. Happy new water year, hopefully it will be (b/w)etter than the last.

Extended Discussion (Monday through Thursday)

GEFS and EC ensemble in general agreement weakening the persistent West Coast ridge and shifting it east by the middle of next week. Pattern will remain dry for NorCal with temperatures still around 10 degrees above average early in the week, returning to only slightly above average by Thursday.