Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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000 FXUS66 KMTR 191136 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 436 AM PDT Mon Apr 19 2021 .SYNOPSIS...A cooling trend begins today with breezier onshore winds and a deepening marine layer. More widespread cooling is forecast for Tuesday as temperatures return to seasonal averages. Dry and seasonable conditions then persist through Friday. The weekend will bring cooler temperatures and increasing chances for rain late Saturday and Sunday. && .DISCUSSION...as of 03:35 AM PDT Monday...The marine layer has deepened to around 1000 ft, as the upper ridge begins to weaken its grip over our area. Stratus has developed along the coast from Sonoma through the Central Coast and will advance farther inland as the morning progresses. Temperatures are running a few degrees warmer than 24 hours ago at most sites, as a result of the much above normal temperatures experienced yesterday. Today will be another above normal day away from the coast, as a weak ridge remains over our area and 850 mb temps peak. Areas closer to the ocean and bays will cool by a couple to perhaps up to 12 degrees, as stronger onshore flow returns. This will translate to 50s and 60s along the immediate coast, upper 60s to low 70s along the SF Bay shoreline, and low to mid 80s in the inland valleys. Winds may also become pretty gusty in the late afternoon and evening as an upper low forms off the Oregon coast and moves over northern CA. Tuesday, the area will experience more widespread cooling under the influence of the upper low, with the most significant cooling occuring inland. This basically means a return to near-normal temperatures region-wide. Unfortunately, models keep all precip associated with the low to the north and east of our area. Wednesday through Friday do not bring any changes to our area in terms of sensible weather. As the low moves east, we become situated between a trough over the inter-mountain west and a ridge several hundred miles off the coast. Temperatures will be near normal, and a marine layer will provide continued night and morning low clouds. Saturday, an upper low approaches the PacNW, bringing cooling to the coast. Both the ECMWF and GFS bring precip to northern CA Saturday afternoon, but leave our area dry until Sunday morning. There is some disagreement on the path and timing of the low at this point. The ECMWF develops a deep trough off the northern CA coast, which pivots over our area late Sunday, bringing wetting rain to much of the state. The GFS keeps the closed low off the northern coast until Monday afternoon with wetting rain preceding it Sunday afternoon and evening. Ensemble members are also coming together for light to moderate precip in the late Sunday to early Tuesday timeframe. Our forecast reflects this increasing confidence, with chance pops and light qpf across the region Sunday and Monday. Guardedly optimistic at this point, as we`re still six days out. && .AVIATION...As of 04:36 AM PDT Monday...For the 12Z TAFs. Low clouds have filled in along the coast as profilers show the marine layer has deepened to to about 1200 ft AGL. Stratus has also pushed through the Golden Gate Gap and made its way around to Oakland. Clouds expected to continue to fill into the Bay and towards SFO this morning. Current observations record cig heights between 300 and 800 ft AGL with conditions ranging from VFR to LIFR. Expect low clouds to persist through mid to late morning (~15z-18z) with later clearing times around the Monterey Bay. Slight chance of fog around KSTS early this morning. VFR conditions this afternoon before low clouds return this evening. W/NW winds at or below 10 kt this morning increasing this afternoon and evening. Models and guidance continue to suggest gusts between 20-25 kt possible, generally, through early tomorrow morning with stronger gust potential for SFO. Vicinity of KSFO...Satellite shows low clouds slowly expanding south and westward towards the terminal. Intermittent IFR conditions anticipated until ~16z before VFR conditions prevail. Low clouds forecast to return tonight. W/NW winds throughout to increase this afternoon to 20-25+ knots lasting through mid to late tomorrow morning. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...The marine layer has deepened to about 1200 ft AGL as stratus has filled into the Bay and expanded down the Salinas Valley. IFR/LIFR conditions currently, later clearing times expected today compared to yesterday (~17z-19z). Still a slight chance of fog early this morning. VFR conditions expected this afternoon before low clouds return this evening and tonight. Light winds this morning increasing this afternoon to around 15 kt. Breezy conditions may linger into the evening. && .MARINE...as of 02:02 AM PDT Monday...Northwest winds will increase through the day as a disturbance moves southward from the Pacific Northwest into northern California. Gale force gusts are expected to develop over portions of the coastal waters starting with the Big Sur Coast by late this afternoon before winds increase over the northern waters this evening and persist into Tuesday. These winds will produce steep wind waves resulting in hazardous seas conditions, particularly for smaller vessels. Moderate northwest swell through the period along with a lighter longer period southerly swell. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...SCA...Mry Bay from 2 PM SCA...Pigeon Pt to Pt Pinos 0-10 nm from 2 PM SCA...Pt Arena to Pt Reyes 0-10 nm SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 PM GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM SCA...Pt Reyes to Pigeon Pt 0-10 nm SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm && $$ PUBLIC FORECAST: Lorber AVIATION: AS MARINE: AS Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea

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