Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

507 am PDT Sat Sep 22 2018

Synopsis

Cooling is expected region-wide through the weekend as onshore flow increases. A renewed warming trend is then likely during the first half of next week as high pressure builds near the West Coast and light offshore flow develops. Dry and gusty northerly winds may develop locally in the hills Sunday night into Monday morning, which will lead to increased fire weather concerns.

Discussion

As of 3:35 AM PDT Saturday, Early morning satellite imagery shows widespread low clouds and fog along the coast, over San Francisco Bay, in the valleys of Sonoma and Marin Counties, and up the length of the Salinas Valley. According to Fort Ord profiler data, the marine layer depth is at about 1200 feet.

The cooling trend that was initiated by yesterday's southerly surge is forecast to continue through the weekend. Surface pressure gradients are more strongly onshore this morning compared to yesterday and so we should see marine air surge farther inland today. In addition, the marine layer will likely deepen later today and tonight as an upper trough moves in from the northwest. And, models indicate stronger onshore flow after the trough passes on Sunday. Thus, we can expect more widespread low clouds tonight and Sunday morning along with continued cooling on Sunday, especially inland. Most of our area will see temperatures cool a bit below seasonal averages by tomorrow.

Sunday looks to be the coolest day of the week ahead as a warming trend is forecast to get underway on Monday. By late Sunday, after the upper trough has moved off to our east, the models agree that an upper ridge will build off the coast of Oregon and far northern California, while a surface ridge develops to our north and east by Sunday night. These developments will set the stage for dry north to northeast winds in the hills Sunday night into Monday morning, something that could result in critical fire weather conditions, particularly in the North and East Bay Hills where winds are projected to be strongest. However, recent models have trended lighter with these offshore winds and current WRF model output indicates lighter winds compared to the last offshore wind event on Wednesday night. On the other hand, it doesn't take a whole lot of wind to create Red Flag conditions if relative humidity values are low and fuels are very dry. For now will continue to highlight fire weather concerns with a Hazardous Weather Outlook. Will wait to see what the wind forecast looks like with the 12Z models before determining if a Fire Weather Watch is warranted.

The offshore flow on Sunday night and Monday will kick start a warming trend that is expected to last through midweek. High temperatures are forecast to quickly climb into the 90s across inland areas by Tuesday. Coastal areas may climb into the 80s, or even lower 90s, if offshore flow is strong enough. But given latest model forecast wind trends, coastal highs in the 70s and lower 80s are more likely. The warmest days next week are expected to be Tuesday and Wednesday. By midweek the upper ridge is forecast to begin drifting to the northwest, and a cooling trend will likely get underway on Thursday. At this point the models diverge somewhat. The ECMWF forecasts an upper low to undercut the ridge and drift slowly towards the California coast during the second half of next week while the GFS keeps the low well offshore. The ECMWF solution would result in greater cooling late in the week, with even the potential for showers next weekend.

Aviation

As of 5:07 AM PDT Saturday, We're in the soup. Model output did very well capturing Friday's quickly evolving southerly surge; stratus and fog are well entrenched this morning with onshore flow. Presently meso-scale model boundary layer humidity forecasts are under-forecast. VFR well inland otherwise clearing of stratus and fog will likely be a slow process today. The 12z tafs lean more heavily toward VLIFR-IFR, gradual improvement by late morning and afternoon then an early evening return to stratus.

Vicinity of KSFO, IFR stratus ceiling has filled in over the San Francisco Bay overnight. Ceiling forecast to lift to low end MVFR by 19z, westerly wind increasing to near 15 knots by 21z. Bkn cig tempo sct 21z-24z otherwise early stratus return in the evening. IFR cig likely returning by 05z. Slow clearing process again Sunday morning.

SFO Bridge Approach, IFR ceiling at the San Mateo Bridge, slow clearing by later morning and afternoon. Early stratus return tonight.

Monterey Bay Terminals, VLIFR-IFR this morning, partial improvement possible by noon with cigs lifting to MVFR, possibly an all day stratus coverage, evening IFR cigs return by 03z.

Marine

As of 3:04 AM PDT Saturday, A surface trough will remain nearly stationary along the California coast while strong high pressure over the eastern Pacific drifts eastward becoming nearly stationary off the Pacific Northwest coast early next week. By mid to late next week the high pressure system will drift northward into the Gulf of Alaska merging with a stronger high pressure system over Alaska. Increasing northwest winds will return to the California coastal waters while onshore winds become gusty over the bays each afternoon and evening over the weekend.

Watches, Warnings, Advisories

Today, sca, pt arena to pigeon pt 10-60 nm from 2 pm sca, pt reyes to pigeon pt 0-10 nm from 5 pm sca, pigeon pt to pt piedras blancas 10-60 nm from 5 pm sca, pt arena to pt reyes 0-10 nm from 9 am sca, SF Bay from 2 pm

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