FXUS66 KMTR 171833 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1033 AM PST Tue Dec 17 2024 ...New AVIATION, MARINE... .SYNOPSIS... Issued at 407 AM PST Tue Dec 17 2024 Dense Fog overnight will make for a difficult commute at times this morning, please drive slowly and give yourself extra time to reach your destination through the mid-morning hours. After the fog lifts this morning, we'll see warmer temperatures under mostly sunny skies and light winds across our entire area. An unsettled pattern develops later this week, and will persist through the weekend into next week. && .UPDATE... Issued at 844 AM PST Tue Dec 17 2024 Widespread stratus and fog across the valleys has been slow to mix out this morning. As a result, the Dense Fog Advisory currently in effect for most of the Bay Area, the Monterey Bay region, and the northern Salinas Valley has been extended through 11 AM, which aligns the end time with the Dense Fog Advisory that our neighbors in Sacramento have issued. Will continue to monitor observations in case the fog does mix out earlier than anticipated. Otherwise, the key forecast concern will be the active weather pattern that's set to begin this upcoming weekend, potentially beyond the Christmas period. There will be a fuller discussion of this possibility with the next forecast update later today. DialH && .SHORT TERM... (Today and tonight) Issued at 407 AM PST Tue Dec 17 2024 As mentioned earlier in the evening update, dense fog has developed across much of our area and will continue for 2-3hrs past sunrise this morning. A Dense Fog Advisory remains in effect through 9AM PST for much of area. Low amplitude, progressive ridging today and tomorrow will result in a pleasant increase in daytime highs of about 5 degrees from Mondays high temperatures. Boundary layer flow will mostly maintain an offshore component under the upper level ridging through the middle of the week. Wednesdays highs from the East Bay south through the remainder of the interior Central Coast will jump about another 5 degrees to around 70 under mostly sunny skies and light winds. && .LONG TERM... (Wednesday through Monday) Issued at 407 AM PST Tue Dec 17 2024 The offshore boundary level flow will persist through Thursday evening with high temperatures in the 60s to mid-70s across interior valleys. Thursday into Friday a shift in the pattern will result in increased southerly flow up to 30kts at H85 Friday morning into the afternoon, as H50 heights begin to drop out ahead of a broad deepening trough to our west. The first of two waves of precipitation will arrive late Friday night, brushing the the North Bay with light rain, expected to be under a tenth of an inch. The second wave associated with the trough axis moving onshore across NorCal and southern Oregon will bring more impactful rainfall, with the North Bay now highlighted by the WPC in the Day 5 Excessive Rain Outlook for moderate to heavy rain possible Saturday afternoon through late Saturday night. The active longwave synoptic pattern looks to continue through and beyond the extended forecast. Get out enjoy the pleasant weather the next few days. && .AVIATION... (18Z TAFS) Issued at 1033 AM PST Tue Dec 17 2024 LIFR-IFR conditions are slowing improving this morning, overall conditions vary from LIFR to VFR, otherwise cirrus clouds are moving eastward through a medium 500 mb ridge over the coastal waters. A stable air mass will prevail during the 18z TAFs, along with light offshore winds except locally variable and onshore during the mid to late afternoon, then light offshore tonight and Wednesday morning. Lower to mid level thermal ridging and 500 mb height ridging will continue to steadily develop over the forecast area today, tonight and Wednesday. Recent soaking rain over much of the forecast area, long December night-time radiative cooling and light winds support redevelopment of fog tonight and Wednesday morning, there's potential for dense fog redevelopment as well. Thinking is most recent HREF and WRF for example are under-forecasting fog in our forecast area including with respect to the TAF sites; for 18z have included lowering visibilities to IFR with evening onset of fog for KSTS, KAPC, KOAK and KSFO. Dense fog may also develop at KSJC, not yet indicated in 18z TAF rather advertising IFR at least for the moment. NAM statistical guidance is showing low surface visibilities tonight and Wednesday (again Wednesday night and Thursday morning) for the aforementioned airport locations. Vicinity of SFO...IFR ceiling holding steady per recent surface observations and satellite imagery, lifting to MVFR 19Z, allowing to 23Z to mix out to VFR; an east wind with weak OAK-SFO pressure gradient is currently noted in the surface observations keeping IFR feed going from SF Bay. It's a near persistence forecast for tonight and Wednesday morning which means potential for fog redevelopment. Seeing low to very low surface visibility (category 1, less than 1/2 mile) in the NAM statistical guidance as early as 06z this evening, and extending to 15Z Wednesday before slow clearing takes place. Heads up for Wed night and Thu morning, similar with fog and low visibility (persistence). Mainly light offshore wind. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...LIFR-IFR due low ceilings and fog slowly lifting, otherwise it's VFR. VFR for the afternoon and early evening, then areas LIFR-IFR redeveloping tonight and Wednesday morning. Mainly light offshore wind. && .MARINE... (Today through Sunday) Issued at 947 AM PST Tue Dec 17 2024 High pressure returns today with dry weather and mainly light offshore winds through Thursday night. Long period westerly swell will arrive midweek causing an increase in shoaling on harbor entrances and elevated sneaker wave risk along the coastline. Another low pressure system is set to arrive Friday into the weekend which will result in elevated wave heights and create hazardous conditions for small crafts. && .BEACHES... Issued at 437 AM PST Tue Dec 17 2024 A combination of moderate period northwesterly swell and long period westerly swell will contribute to elevated potential for sneaker wave development early Wednesday morning through late Thursday morning. The combination of northwesterly and westerly swell will lead to elevated concerns across all coastal areas, including normally sheltered areas such as the northern Monterey Bay. Sneaker waves are likely to break between 10 to 13 feet and may pose a risk to any beachgoers who are caught unaware. Visitors to the coast midweek are advised to take extra care and never turn their backs to the ocean as sneaker waves may sweep unaware beachgoers into the ocean. A second higher confidence period of elevated sneaker wave risk is likely to develop next weekend as another system approaches the West Coast. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Dense Fog Advisory until 11 AM PST this morning for CAZ006- 502>506-508>510-512-513-528>530. PZ...Small Craft Advisory until 9 PM PST this evening for Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM. Small Craft Advisory until 3 AM PST Wednesday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...JM LONG TERM....JM AVIATION...Canepa MARINE...Canepa 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