Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

458 pm PDT Wed Oct 21 2020

Synopsis

Heightened fire weather concerns will continue in the hills through at least Friday morning due to gusty, dry offshore winds combined with persistent low humidity. A stronger and more widespread offshore wind event will get underway tonight for the North Bay Mountains and East Bay Hills and Valleys and continue through Friday morning. Gusty winds are also expected to expand locally into the Santa Cruz Mountains and San Mateo coastline early Thursday morning. Temperatures will remain warmer than seasonal averages today, but then cool to near normal late in the week and into the weekend. Another offshore wind event is possible in the hills from Sunday into Monday.

Discussion

As of 02:00 PM PDT Wednesday, The lull in the offshore winds is well underway in the North Bay this afternoon, with light winds being observed all across northeastern Sonoma County and the mountain passes of the North Bay. Stations west of the 101 are slightly stronger due to some sea breeze influence, but even those winds have begun to decrease as the hot air mass compresses the marine layer over most of the CWA this afternoon. Both the Bodega Bay and Fort Ord Profilers have picked up on the rapid compression, and with the exception of the Central Coast from the Monterey Peninsula southward, are observing clear visibilities all along our coastal waters. Moreover, modest humidity recoveries (at best) were observed across most of the North Bay overnight, with current RH values only in the 20-30 percent range across much of the area and some of the highest peaks only making it into the upper-teens.

589dm 500hPa ridge associated with the Rex Block continues to nose over the Central California Coast. As expected, are already starting to see the finer mid-level pressure gradient over the Mountain West. All in all, the synoptic setup that has been progged over the last few days is a mere hours away from playing out across our CWA. The next burst of offshore winds will begin to affect the northern CWA late tonight, where the Napa Mountains and the high- elevation areas of Sonoma County (primarily those east of the 101) will once again experience moderate winds in the 10-25mph range, with some gusts over 40mph possible (especially in the windiest locations like St. Helena). These offshore winds will rapidly descend over the East Bay Hills and some of the interior valleys (although not immediately along the valley floors). Are expecting strong gusts in the 45-50mph range, if not higher, in locations like Mt. Diablo. As these winds mix down to 1000ft, will also see a rapid drying of the air mass over much of the Northern and Eastern CWA. These offshore winds will eventually make their way down to the Santa Cruz Mountains sometime after midnight on Thursday before being steered towards the San Mateo coast. Essentially, will see RH values in the teens all the way down to portions of the SF Peninsula shoreline. Local WRF has picked up on this drying signature for several days now, so likelihood is high to see winds along the coastal portions of the Red Flag Warning making into the 15-20 mph range, with brief gusts up to 40 mph also possible. Thankfully, these winds will be shorter-lived in the Santa Cruz Mountains and San Mateo Coast, but are still expecting very poor overnight humidity recoveries going into Thursday morning. Another lull will be observed regionwide during the day on Thursday, but short-range guidance has consistently progged RH values in the low teens all across the areas under the Red Flag Warning, with some North Bay ridges expected to come out of tonight's burst only in the single digits.

As the embedded shortwave settles over the northern Great Basin tomorrow afternoon, expecting a cooling trend for the day tomorrow, with max temps falling 5-10 degrees F compared to today's observed max temps, especially across the interior. While these temps are cooler than what has been observed in recent days, they are still running slightly above seasonal average. This cooling trend is progged by mid-range guidance to continue through the next several days. Despite this cooldown, however, a repeat of offshore winds Thursday night and into Friday morning with widespread low RH values. NAM has pushed back on the intensity of these winds over the Santa Cruz Mountains and the San Mateo coast for tomorrow night's burst, but still high confidence that the air mass will remain critically dry not only through Friday morning but over the next several days.

Going into this weekend, ECMWF ENS and GEFS both continue to nestle our CWA in between the dominating upper-level ridge associated with the Eastern PAC Rex Block and the next shortwave to descend from BC and into the Great Basin. ECMWF ENS members are presenting much more robust signatures with regards to the intensity and location of the troughing over the Mountain West, by rapidly digging it and gaining a positive tilt in the process. Even the GEFS members, which trend lighter on the digging and keep the axis of the trough further east from California, still present the same synoptic setup. Despite which members win out, are looking at increasing confidence that the sometime after Saturday night and through early next week we will likely have another offshore wind event and that this one has the potential to be stronger than what we expect from tonight into Friday morning. Timing, precise locations, and variables (e.g. wind speeds, RH values) will become more fine-tuned as we inch closer to the weekend and hi-res model runs continue to come in. For now, will continue to monitor the setup for this weekend.

Looking well into the extended, the precipitation potential remains nearly non-existent for much of the region, with GEFS and ECMWF extended members showing a continuation of dry conditions well into the end of the month. La Nina pattern also looking to carry over into the rest of the season. While no two ENSO events are the same, La Nina events have tended to result in drier conditions all across Southern and parts of Central California. Something to keep in mind as we head into the second half of fall.

Aviation

As of 4:57 PM PDT Wednesday, Stratus is currently confined to the Big Sur coast south of the MRY Bay Area. VFR conditions expected.

Vicinity of KSFO, VFR. Northwest winds 15-18 kt decreasing after 04Z.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR expected. Low chance of stratus developing in Monterey Bay.

Marine

As of 04:47 PM PDT Wednesday, Strong gale force winds with occasional storm force gusts possible over the northern coastal waters through late week. This will result in hazardous steep waves. Mixed seas will persist with a moderate northwest swell and longer period southerly swell.

Watches, Warnings, Advisories

Tonight, red flag warning, caz509-512 red flag warning, caz507-510-511 sca, pt arena to pt reyes 0-10 nm sca, pt reyes to pigeon pt 0-10 nm sca, pigeon pt to pt pinos 0-10 nm sca, pigeon pt to pt piedras blancas 10-60 nm glw, pt arena to pigeon pt 10-60 nm

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