Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

407 am PST Mon Jan 18 2021

Synopsis

Warm, dry weather will continue through Monday, along with gusty offshore winds in the hills of the San Francisco Bay Area. A strong, and much more widespread, offshore wind event is forecast from this evening through Tuesday morning. During this time, damaging winds are possible in the hills and locally in the valleys and near the coast. Winds will gradually diminish on Tuesday afternoon. A cooling trend is forecast from Wednesday through Friday and slight rain chances develop by the end of the week.

Discussion

As of 02:45 AM PST Monday, A dry mid/upper level short-wave trough is currently dropping southward across the region with surface high pressure developing to the north and northeast. While there is an increase in high level clouds, surface conditions are beginning to dry out as offshore flow develops and strengthens in response to a tightening pressure gradient. Wind gusts of 40 to 60 mph are being reported in the higher elevations of the Bay Area with Mount Diablo seeing gusts in excess of 70 mph. Winds are forecast to increase region-wide today and become more widespread tonight into Tuesday resulting in strong, damaging winds in the region's higher elevations. While the the stronger winds look to be mostly confined to the higher elevations through the day today, stronger wind gusts may occasionally mix down to the lower elevations. This threat greatly increases late tonight into Tuesday morning as wind gusts increase to 60 to 80 mph in the peaks and ridges potentially mix down locally to the valleys and near the coast. All this said, a Wind Advisory is in effect for much of the Bay Area today and then expands to the Central Coast tonight. A High Wind Warning will go into effect this afternoon through Tuesday morning for the region's higher elevations. For additional details, please see NPWMTR for the latest wind products.

While conditions aloft begin to cool slightly in response to the mid/upper level trough, dry offshore flow will allow for unseasonably warm temperatures again this afternoon. Additionally, moderate to high fire weather concerns are expected over portions of our region through Tuesday in response to the strong offshore winds and low relative humidity. While precipitation totals so far this winter season remain well below normal, there has been enough rainfall across the San Francisco Bay Area, particularly in the North Bay, to moisten fuels and limit the threat critical fire weather concerns. That said, fuels remain drier over the Central Coast, enough so to warrant a Fire Weather Watch for elevations above 1,000 feet across Monterey and San Benito Counties. To allow for the coordination with our fire partners later this morning, will hold off on upgrading this watch to a Red Flag Warning at this time. For additional details, please see the Fire Weather Discussion below.

Winds will begin to diminish region-wide during the day on Tuesday, yet remain locally strong and gusty into Tuesday night across the North and East Bay. Temperatures will also cool both during the daytime and overnight through the remainder of the week as a mid/upper level ridging noses into the Pacific Northwest and a cut- off low spins off of the southern California coast. Medium to longer range ensembles indicate the Pacific ridge will retrograde to the west late this week and into the upcoming weekend. This will allow for a series of mid/upper level trough to drop southward along the West Coast providing cooler and potentially unsettled weather conditions. Confidence in the details remain low at this time and will be fine tuned in the coming days, especially once short-term wind and fire weather concerns pass.

Safety remainders from a previous discussion, The offshore wind event continues to look strong and now is the time to act. In case of power outages: make sure cell phones/devices are fully charged, gather up flashlights and extra batteries, and have a supply of non- perishable food/water. If there are widespread power outages, remember that it may take crews longer to get the power back on so make sure you have what you need in case you have to go a day or two without power. Other things to remember: secure loose outdoor objects, trim dead tree branches on your property, be cautious when driving as occasionally strong gusts may suddenly hit your vehicle, making it difficult to control.

Aviation

As of 4:05 AM PST Monday, For the 12Z TAFs. Widespread VFR as potent dry trough shifts SSE across Bay Area and Central Coast early Monday morning. A bank of broken to scattered clouds AOA 18000-20000 feet will accompany this trough with very dry, very wind N/NNE winds on the lee of the trough. Low level wind shear of 30-40kt can be expected overnight at Bay Area terminals, with these winds mixing down to the surface later today, bringing rather gusty surface winds. Offshore winds expand and become stronger, even at the surface, late today through tonight and into Tuesday. Low level wind shear of 35-50kt can be expected at most terminals Monday night into early Tuesday, with wind gusts at these terminals reaching similar ranges Tuesday morning, including at KSFO. Strongest winds will be around 1500-2000 feet AGL this morning, with them lowering to 1000-1500 feet tonight, even as winds may become weak at ground level - thus the elevated low level wind shear concerns through the current TAF package. Otherwise -- rather strong winds of 65-85mph over mountain peaks/ridges and upper lee slopes through Tuesday.

Vicinity of KSFO, VFR with sct high clouds and light winds. Increasing NNE winds aloft overnight will generate LLWS concerns from through the morning and possibly into early afternoon. Surface winds may become gusty tomorrow afternoon with even stronger offshore winds expected tomorrow night. Surface wind gusts up to or in excess of 50mph are possible early Tuesday morning.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR with few high clouds and calm winds. VFR should prevail through the TAF period. Breezy northerly winds expected in the afternoon, becoming gusty near the end of the TAF period. Some localized low level wind shear concerns are possible at KMRY/KWVI late Monday night into early Tuesday morning as the offshore wind event strengthens and expands southward to encompass portions of the Central Coast.

Marine

As of 02:45 AM PST Monday, Breezy north winds will prevail over the waters through Monday. The strongest winds are expected over the northern outer waters where occasional low end gale- force gusts will be possible. Locally breezy conditions will also be present along the Big Sur coast south of Point Sur. These winds will persist into tomorrow before a burst of offshore winds over land spills over into the waters starting late this evening through tonight. This will bring gale force conditions across the entire coastal waters and through portions of the San Francisco Bay into Tuesday resulting in widespread hazardous conditions. A large, very long period northwest swell will impact the waters through late tomorrow bringing additional hazards. The next long period northwest swell arrives mid-week.

Beaches

As of 09:45 PM PST Sunday, A large, very long period northwest swell has arrived at the coast creating hazardous conditions through Monday afternoon. Large swell heights will lead to breaking waves of 18 to 22 feet, locally higher at favored breakpoints, along west to northwest facing beaches. As a result, a High Surf Advisory is in effect through Monday afternoon for west and northwest facing beaches while a Beach Hazard Statement has been issued for the northern Monterey Bay including Santa Cruz. With unseasonably warm temperatures this holiday weekend, it will likely attract more people to the coast. Individuals should be reminded that our coastline and the cold Pacific waters remain dangerous as these larger than normal waves impact the coast. Extreme caution is advised when visiting area beaches. Never turn your back to the ocean.

Watches, Warnings, Advisories

Today, wind advisory, caz518-528-530 high surf advisory, caz006-505-509-530 high wind warning, caz507-511-512-517 wind advisory, caz507-511-512 wind advisory, caz006-505-506-508>510-513-529 sca, pt arena to pt reyes 0-10 nm sca, pigeon pt to pt pinos 0-10 nm sca, pt pinos to pt piedras blancas 0-10 nm sca, pigeon pt to pt piedras blancas 10-60 nm sca, pt arena to pigeon pt 10-60 nm sca, SF Bay until 3 pm sca, pt reyes to pigeon pt 0-10 nm until 3 pm glw, SF Bay from 3 pm glw, pt reyes to pigeon pt 0-10 nm from 3 pm sca, mry bay from 9 am

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