California heat could threaten long-standing temperature records. How long will warm weather last?

San Francisco Chronicle​

2 days ago

Hot weather will grip California for one more day, threatening long-standing temperature records before a significant cool-down Sunday.
A high-pressure system will continue to keep skies generally clear across Northern and Central California on Saturday, trapping hot air beneath it. The air mass in the lower atmosphere is expected to warm by a couple of degrees Saturday, which would typically translate to similar warming at ground level, but a complex wind pattern could throw off forecasts.
The area of high pressure will be flanked by two areas of low pressure, one spinning off the coast of Southern California and another approaching the Oregon coast. Together, these two low-pressure systems will complicate wind and temperature patterns across the Golden State on Saturday.
In the Bay Area, a few degrees of cooling is forecast nearly everywhere as marine-chilled winds increase. A heat advisory will be in effect from 11 a.m. to 8 p.m. Saturday for the interior East Bay valleys and the North Bay interior mountains. On Friday, the heat advisory covered a larger swath of the Bay Area, but relatively cooler winds should keep the South Bay in the 80s to low 90s Saturday, short of heat advisory criteria. Triple-digit temperatures should also be avoided in the East Bay valleys Saturday as the sea breeze hits in the midafternoon, but highs will still be well above average, in the mid-90s.
San Francisco's microclimates will again be on full display. Ocean Beach may struggle to hit 60 degrees while neighborhoods on the east side of the city should surpass 70 degrees. Oakland will probably wind up a degree or two shy of 80 degrees.
In the Central Valley, Saturday will probably be even hotter than Friday in many locations. Highs are forecast to range from 101 to 108 degrees. The National Weather Service forecasts highs of 105 degrees in Merced (Merced County) and Hanford (Kings County), which would tie daily records in each city. A few other cities could be just short of their hottest May day in recorded history.
The delta breeze may keep temperatures in west Sacramento around 100 degrees, but areas just north and east of the city, such as Roseville, will be close to 105 degrees. If the delta breeze holds off longer than expected, Sacramento Executive Airport could challenge a monthly temperature record. Red Bluff and Chico could reach 107 degrees Saturday.
Sunday will mark an abrupt end to the hot spell. The high-pressure system is predicted to break down rapidly with temperatures dropping to the 60s to low 70s near the bay shoreline and 70s to low 80s in Wine Country and the interior East Bay. Even the Sacramento Valley will drop to the 80s to low 90s, only 5 to 7 degrees above average.
Sierra, Southern California thunderstorms
The low-pressure system spinning off the Southern California coast is expected to absorb remnant moisture from Tropical Storm Alvin this weekend, which will raise the chance of thunderstorms.
The Storm Prediction Center forecasts thunderstorms across the Sierra Nevada and the Mojave Desert on Saturday. Thunderstorms will likely become more widespread Sunday, as the threat expands to the Antelope Valley and Transverse Ranges. A mix of wet and dry lightning strikes are possible, along with erratic wind gusts.
It's possible rare May rain could make an appearance in Palm Springs and the desert areas if thunderstorms pop up. Palm Springs averages just 0.05 inches in May, but Sunday's storms could add up to a tenth of an inch or more.