Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

509 pm PDT Wed apr 21 2021

Synopsis

Mainly dry and seasonable weather conditions will persist through Friday. Cooling will occur on Saturday as onshore flow and cloud cover increases region-wide ahead of an approaching weather system. Widespread rainfall is likely on Sunday into early Monday as a cold front drops southward across the region. Warming and drying trend is then expected beyond Monday.

Discussion

As of 01:57 PM PDT Wednesday, A deep marine layer continues to impact the region this afternoon with low clouds holding along much of the coastline, through the Golden Gate and around the Monterey Bay region. This will hold temperatures in the 50s where cloud cover persist to the 70s across the interior. Expecting typical ebb and flow of low clouds each day through the remainder of the week with only minor day-to-day fluctuations in temperatures.

Onshore flow will then increase on Saturday ahead of an approaching weather system which will result in a region-wide cooling trend and increased cloud cover. Afternoon temperatures on Saturday will range from the upper 50s at the coast to mid 60s inland (only a few locations of the interior Central Coast may approach the lower 70s). However, dry weather will likely persist.

A cold front will then approach from the northwest late Saturday night into Sunday with increased probability of precipitation. The deterministic and ensembles are in good agreement with rainfall becoming more widespread after sunrise on Sunday as deeper moisture advects inland along the southward moving frontal boundary. Steady light to occasionally moderate rain will spread from north to south through the day Sunday with the potential for post frontal rain showers. Thunderstorm potential currently appears to be low, yet will continue to monitor in the coming days as we get a better handle on how much instability will be associated with this passing system.

From the previous forecast discussion: A new drought monitor will be released later this week and the trends will be for more adverse drought impacts across the region and state. This storm will have little overall impact on the long term drought. However, confidence is increasing that most of the Bay Area will see soaking rains Sunday into early Monday. At the very least this will put a pause into the onset of fire season as fuel moisture readings are trending towards historically dry values. So from an impacts stand point this will bring beneficial rains and if the current timing holds up the roadway impacts would be mostly confined to Sunday traffic. For weekend planning, Saturday will be the better day for outdoor plans. Snow levels will be dropping in the Sierra so trans-Sierra passes will be impacted by snow for return travel later Sunday into Monday. All locations will notice cooler daytime highs Sunday into Monday.

Any lingering showers Monday should end early though official forecast will hold onto some low precip chances into Monday evening. Drying and modest warming trend Tuesday into Wednesday of next week as high pressure builds and the main storm track shifts north of the Bay Area.

Aviation

As of 05:10 PM PDT Wednesday. .For the 00Z TAFs. MVFR at the coastal terminals as onshore WNW flow (10-20kts) allows for mid-afternoon marine stratus intrusion with cigs between 1000-1500ft. Stratus have also quickly exited from the San Bruno Gap and begun to track past the Golden Gate and towards the East Bay and North Bay terminals. Have adjusted TAFs to accommodate for the early intrusion across the Bay Area terminals, with the NARRE-TL members capturing the WNW flow and MVFR cigs gradually moving into the STS/APC/SFO/OAK/SJC terminals between 3-9Z, with the interior terminals beginning to approach MVFR closer to 9Z. Widespread MVFR with occasional IFR conditions will continue across the Monterey Bay terminals through the entire TAF period. In terms of winds, will see an uptick in winds after 17Z at most of the terminals as onshore NNW flow picks up along the coast ahead of an approaching high pressure system, further impeding the chance for any breaks at the Monterey Bay terminals tomorrow afternoon.

Vicinity of KSFO, Marine stratus have remained steadily to the north of SFO as they exit the gap and travel past the Golden Gate. Given the westerly component of winds this afternoon, widespread MVFR intrusion southwards into SFO is not expected until after sunset but may see occasional MVFR as winds begin to lessen. OAK and SJC are also expected to go MVFR given the rapid intrusion of stratus into the Bay Shoreline overnight, so have adjusted timing of its arrival on latest TAFs to account for current obs. MVFR conditions at SFO and OAK expected to last through 17Z followed by a quick afternoon break in the clouds and an uptick in winds to 15-20kts through the end of the TAF period.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, MVFR conditions will dominate the terminals through the entire TAF period as onshore flow carries into the evening and is reignited by an approaching high pressure tomorrow afternoon. As such, primary concerns will be the occasional IFR cigs that will develop overnight along the coast and into portions of the Northern Salinas Valley, along with the chance for HZ and BR. Winds will remain light and variable overnight but will pick up to 10-15kts tomorrow afternoon and through the end of the TAF period.

Marine

F KSFO, Marine stratus have remained steadily to the north of SFO as they exit the gap and travel past the Golden Gate. Given the westerly component of winds this afternoon, widespread MVFR intrusion southwards into SFO is not expected until after sunset but may see occasional MVFR as winds begin to lessen. OAK and SJC are also expected to go MVFR given the rapid intrusion of stratus into the Bay Shoreline overnight, so have adjusted timing of its arrival on latest TAFs to account for current obs. MVFR conditions at SFO and OAK expected to last through 17Z followed by a quick afternoon break in the clouds and an uptick in winds to 15-20kts through the end of the TAF period.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, MVFR conditions will dominate the terminals through the entire TAF period as onshore flow carries into the evening and is reignited by an approaching high pressure tomorrow afternoon. As such, primary concerns will be the occasional IFR cigs that will develop overnight along the coast and into portions of the Northern Salinas Valley, along with the chance for HZ and BR. Winds will remain light and variable overnight but will pick up to 10-15kts tomorrow afternoon and through the end of the TAF period.

as of 04:42 PM PDT Wednesday, Buoy observations and satellite show moderately gusty winds over the outer waters this afternoon, with gale force gusts persisting over the northern waters. These stronger winds are creating hazardous seas for smaller vessels in the most of the outer waters and the coastal waters north of Point Reyes. A moderate locally generated northwest swell will persist through the end of the week. A building southerly swell will move through the waters this weekend. Chance for rain this coming weekend.

Watches, Warnings, Advisories

Tonight, glw, pt arena to pigeon pt 10-60 nm until 3 am sca, pt arena to pt reyes 0-10 nm until 9 pm sca, SF Bay sca, pigeon pt to pt piedras blancas 10-60 nm until 3 am

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