Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

519 am PDT Sat Oct 21 2017

Synopsis

A warming trend will begin over the weekend and continue into early next week. Much warmer and drier conditions are expected to develop by Monday and Tuesday. Slight cooling is likely during the second half of the week, but temperatures will remain well above seasonal averages.

Discussion

As of 3:30 AM PDT Saturday, Quiet night across our region today with cool temperatures (mostly in the 40s to the lower 50s) that are running 5 to 10 degrees cooler than this time yesterday. Drier air is also in place with dew points in the 40s. Only real question is if fog will form before sunrise. Best bet is in the North Bay -- Santa Rosa is currently at 100% RH -- so will leave it in the forecast for that region. Today should be generally sunny with slightly warmer temperatures as a ridge of high pressure off the coast builds toward our region. Highs will be in the mid 60s to the lower 70s.

Temperatures will continue to trend warmer into next week as the ridge expands and builds into the southwestern Conus. This has been a change in the synoptic pattern that has been advertised for over a week by the models. Latest guidance has 500 MB heights at or above 595 DM for Monday and Tuesday. At the same time 925 MB temperatures will be in the 25-30C range which would put it close to some of the highest values ever for the entire month of October. With winds forecast to be offshore, very warm temperatures will make its way across our entire region -- including local beaches. Highs will be in the 80s to mid 90s. Records will likely fall. Overnight lows will remain cool providing relief from the daytime heat.

After that we see a large divergence with solutions. The GFS has a system dropping into the Great Plains with the ridge/Rex block focused more to the northwest of our area. On the other hand the ECMWF shows that on Wednesday the ridge will flatten while a system dives southward from British Columbia into the interior Northwest. This system will continue into Eastern Nevada/Four Corners region late on Thursday. If this solution verifies it will lead to gusty northerly winds particularly for the North Bay Mountains and East Bay Hills. ECMWF already has 925 MB speeds forecast to exceed 30 KT. Although this is still a few days out, we will be keeping a very close eye on it to see where the system will end up. If the GFS is correct it will be a non-event.

No indication of rain through Halloween.

Aviation

As of 4:30 AM PDT Saturday, Satellite image is partially obscured by high clouds but indications are stratus is nil or patchy at the coast. VFR conditions today but there could see some vsbys down to 3-5 miles through 16Z in the North Bay as the ground may still be saturated from the recent rain.

Vicinity of KSFO, VFR. West winds 13-15 kt after 21Z.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR.

Marine

As of 3:30 AM PDT Saturday, High pressure off the central California coast will weaken tonight and Sunday as a frontal system moves into the Pacific Northwest. The high will strengthen and build northward Sunday night and Monday after the front passes. A long period northwest swell will continue to taper down through the day.

Watches, Warnings, Advisories

Today, sca, rough bar advisory for sf bar until 3 pm sca, pt pinos to pt piedras blancas 0-10 nm

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