Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

943 am PST Fri Dec 15 2017

Synopsis

Dry weather will continue into next week. A mainly dry cold front will sweep south over the area late Friday night and early Saturday morning which then increases N-NE winds along the ridges and peaks Saturday.

Discussion

As of 7:40 AM PST Friday, The ridge of high pressure that has been overhead for an extended period will finally flatten and progress to the east today as a system approaches from the northwest. Pressure gradients both at the surface and aloft will increase into tonight causing stronger winds to develop. Little change in thinking from the previous few packages -- dry and gusty conditions for higher elevation locations leading to fire weather concerns especially Saturday and Saturday night. Not expecting to make any substantial changes to the package.

.PREVIOUS DISCUSSION, Change is literally on the horizon, more directly the Northern horizon. After a period of persistent high pressure it's finally weakening as an upper level trough and weak surface front approach the region. The offshore gradient has weakened as well. Last night the SFO-WMC gradient Sat at 13 mb and now it's down to 8 mb. Winds are much weaker as a result. One interesting item of note is the continued dry airmass over the region, especially from 925mb-550mb. Santa Rita Peak and Chews Ridge are currently 7% and 13% respectively. Once again, a different story in the valleys with higher humidity, cooler temperatures and even patchy fog for the N and E Bay Valleys.

For today, another day of above normal temperatures as a cold front approaches from the north. Max temps will be close to yesterday's values and in the mid 60s to mid 70s with a few interior locations near 80 for Monterey/San Benito Cty. Latest model guidance brings the cold front into the North Bay 00Z and then slowly southward tonight. As mentioned on previous discussions, associated moisture will remain north of the Bay Area. More importantly, the offshore gradient ramps back up with very gusty winds. Local WRF model continues to show winds of 15-30 mph with gusts exceeding 50 mph tonight through early Sunday. Winds will be strongest above 1000 feet, especially N and E Bay Hills. A wind advisory will remain in effect from 10 PM tonight through 10 AM Sunday. The offshore flow will also result in low RH. The combination of winds and low RH will result in very critical fire weather conditions. A Red Flag Warning is now in effect. See fire section below for more details.

Post frontal conditions will also bring slightly cooler temperatures Saturday and Sunday, but still several degrees above normal for mid December. The offshore flow pressure gradient relaxes on Sunday resulting in weakening winds.

500mb heights and 850mb temps rebound slightly on Monday/Tuesday with warmer weather. Models still continue to struggle with the much anticipated mid-week system. Run to run reliability of the models has been much less than desire. 00Z run continues to show a drier solution with better precip chances NE of the Bay Area and more over far NorCal and the Sierra. Confidence stills remains low and will not include any mention of rain. If we don't get any precip the middle of next week it's not looking good. The GFS and EC build a highly amplified pattern with high pressure over most of the West Coast. Interesting factoid, the EC is forecasting 500mb heights of near 588 dam around Dec 22/23. That would rival all time record values for soundings at KOAK.

Aviation

As of 9:45 AM PST Friday, For 18z Tafs. Light winds this morning will gradually give way to breezy, then gusty northerly winds through the TAF duration. These northerly winds will arrive as the result of a dry cold front sweeping in from north to south. The strongest winds are anticipated late tonight into early Sunday, with sustained winds as high as 15 to 25 kt and gusts to 35 kt at select TAF locations (namely KSFO). Winds aloft over mountain ridges and through gaps will also be stronger than normal. Otherwise, VFR, with some lingering smoke and high clouds ahead of the arriving cold front.

Moderate to high confidence.

Vicinity of KSFO, VFR. Light and variable winds today with a few high clouds. Westerly winds increase tonight along a cold front, becoming gusty after sunset. Gusts may approach 35 kts late in the TAF period. An airport weather warning for strong wind gusts may need to be issued once confidence improves.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR. Light winds along the coast this morning, with breezy up valley winds in the Salinas valley. Winds will turn onshore this afternoon and increase late tonight, moreso at KMRY than KSNS.

Marine

As of 09:36 AM PST Friday, A dry cold front will sweep over the coastal waters from north to south through the next day. This will bring strong gusty north winds to the waters, particularly overnight tonight and into Saturday morning, with gusty north winds persisting through Sunday. Gale force winds are likely across all outer water zones beginning late tomorrow night through at least Sunday morning. The strong and gusty winds will create very steep wind waves and fresh swell, creating very rough and hazardous seas through the majority of the weekend.

Watches, Warnings, Advisories

Today, sca, pt arena to pigeon pt 10-60 nm from 12 pm sca, pt arena to pt reyes 0-10 nm from 3 pm

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