Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
Issued by NWS San Francisco Bay Area, CA
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575 FXUS66 KMTR 220013 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 413 PM PST Thu Nov 21 2024 ...New AVIATION, MARINE... .SYNOPSIS... Issued at 222 PM PST Thu Nov 21 2024 Atmospheric River continues to remain stationary across the North Bay with increasing flood risks developing. Prolonged rainfall is additionally resulting in an increased risk of landslides, downed trees, and downed powerlines across the North Bay. More widespread moderate rain will spread south on Friday into the rest of the Bay Area and Central Coast. && .SHORT TERM... (This evening through Friday) Issued at 222 PM PST Thu Nov 21 2024 Key Messages: -Aerial Flood Warning now in effect for central Sonoma County -Numerous Flood Advisories cover the North Bay -Flood Watch in effect for the North Bay through Saturday morning -Wind Advisory in effect from 10 PM tonight through 10 PM tomorrow -High Surf Advisory in effect through Friday morning A stationary rain band producing moderate to at times heavy showers continues to train over the North Bay with the highest rain totals focused in Sonoma County. Most areas in Sonoma and Napa counties have seen between 5" to 10" of rain in the last 48 hours. Locally higher amounts between 10" to 15" have been observed across the higher elevations of the coastal and interior North Bay Mountains. Southwards in Marin County, rain totals are slightly lower with most sites seeing between 3" to 6" over the last two days. Rain totals drop off significantly outside of the North Bay with most sites in the northern San Francisco Peninsula and East Bay seeing less than an inch so far. Reports of flooding and downed trees have started to increase across the North Bay. As such an Aerial Flood Warning is now in effect for portions of central Sonoma County, including urban Santa Rosa, with creeks and streams starting to rise in the vicinity of the city. Heading into Friday, we can expect more widespread moderate showers to spread into the rest of the Bay Area and Central Coast late tonight through tomorrow. This will bring an additional 2" to 5" across the North Bay with locally higher totals possible in the elevated terrain. The majority of the Bay Area and coastal mountain ranges will see between 1.5" to 3" of rain while portions of the South Bay and Central Coast will see up to an inch. A slight (less than 10%) chance of thunderstorms will continue through Friday evening. The main things to focus on heading into Friday are 1) increased flood risk across the North Bay, 2) flood risk across the rest of the Bay Area and Central Coast as rain spreads southward, and 3) elevated wind threat tonight into tomorrow. Showers have not let up over the North Bay in over a day and a half. As such two things are happening 1) streams and creeks in Sonoma County are starting to rise with many already reaching the action to minor flood stages. Among the most notable, official forecast points have the Russian River at Geyserville peaking at minor flood stage, Russian River at Guerneville peaking at the action stage, and the Mark West Creek near Mirabel Heights peaking at the moderate flood stage tomorrow evening. Streams and creeks throughout the North Bay are expected to continue rising as moderate to at times heavy rain continues to train over the North Bay today and tomorrow. This brings us to our second point, 2) soil saturation. Prior to this event soils were fairly dry across the North Bay, but, after the last two days, soils are becoming fairly saturated. As the soil becomes more saturated, it is able to absorb less water and results in increased surface runoff, resulting in more widespread flooding. As such, a Flood Advisory remains in effect across the entire North Bay through early Saturday morning and a more targeted Aerial Flood Warning is now in effect for urban Santa Rosa. Elsewhere, flooding potential will increase slightly across the rest of the Bay Area and Central Coast as moderate showers spread southward Thursday night into Friday. Soils remain dry in these regions which help to slightly decrease flooding concern for this region. However, nuisance flooding is still a possibility especially if moderate to heavy showers train over a particular location for an extended period of time. Strong, gusty southerly winds return tonight with a Wind Advisory in effect from 10 PM tonight to 10 PM tomorrow night along the coast and across areas of elevated terrain. HRRR model guidance supports a narrow frontal rainband developing overnight Thursday into Friday and pushing southwards through the evening. Strong winds are expected to develop ahead of and along this rain band but will quickly ease after frontal passage occurs. Widespread wind gusts up to 50 mph are possible but locally higher 60+ mph gusts may develop above 2,500 feet. This will contribute to an increased risk of downed trees and power outages, particularly across the North Bay, as soils become saturated. Anyone who is able to should avoid being out during the worst of the storm (early Friday morning through Friday evening) should do so. If you have any outdoors decorations, now is the time to secure them. If you have to be out on Friday, make sure to leave extra time to get to your destination and avoid driving through flood waters. Waters overtopping roadways may be deeper and swifter than they appear and can result in a dangerous situation developing. Flooding can be particularly hard to see in the dark so use extra caution when traveling at night. Remember - turn around, don`t drown. && .LONG TERM... (Friday night through next Wednesday) Issued at 222 PM PST Thu Nov 21 2024 Friday night through Saturday, light showers will continue across the region in the wake of cold frontal passage. Totals from these showers will be relatively minimal (amounting to less than a few tenths of an inch) with a 10% chance of thunderstorms persisting through the day on Saturday. However, the storm door remains open as we see low pressure lingering off the coast of Washington/Oregon and increased moisture transport into Central California through midweek. While there is still some uncertainty as to how long rainy conditions will persist for, it is looking increasingly likely that rain will persist through midweek. As such, an additional 1" to 2" of rain is expected across the North Bay, Santa Cruz and San Mateo Mountains, and Santa Lucia Mountains from Sunday to Wednesday and up to an inch of rain elsewhere. Some potential light at the end of the tunnel, models have trended drier beginning Wednesday afternoon into Thursday but, given uncertainty in the forecast almost a week out, this may change as we get closer. && .AVIATION... (00Z TAFS) Issued at 413 PM PST Thu Nov 21 2024 Stratus continues to funnel through our region creating a mix of IFR, MVFR, and VFR conditions. North Bay continues to have lower visibility and ceilings as the showers remain pretty heavy. As the heavier rain bands move over some of the terminals in SF Bay, East Bay, and South Bay ceilings will lower to MVFR with a chance of IFR conditions starting early Friday morning going into the afternoon. Higher confidence that KHAF will be socked in with IFR conditions with some moments of MVFR through the TAF period. Southerly winds remain relatively breezy to strong with gusty winds near SF Bay terminals. The gusty winds have eased a bit over the North Bay, but will make a return late tonight for KSTS and early tomorrow morning for KAPC. Otherwise, most sites will continue to see gusty southerly winds through the TAF period. Vicinity of SFO...MVFR as a heavier rain band have moved over the terminal. Some models call for a break in MVFR ceilings near 08-09Z but had low confidence to include at this moment. Higher confidence that MVFR/IFR conditions will prevail after 14/15Z along with occasional heavier rain bands from 15Z to the end of the TAF period. Southerly winds have eased a bit with gusty winds diminishing. There is a chance for moments gusty winds between now until 14Z but low confidence to keep gusty winds prevailing through 14-15Z. Higher confidence that winds start to rebuild after 15-16Z to breezy and strong. Will monitor winds closely for any updated changes. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR through most of the TAF period. There is a chance for MVFR ceilings to develop tonight but confidence is moderate. VCSH were added to the TAF closer to the end of the TAF period, but confidence is low, as showers may start after 00Z. Southerly winds will build to relatively breezy, but moderate confidence of gusty winds forming by the afternoon hours. && .MARINE... (Tonight through next Wednesday) Issued at 413 PM PST Thu Nov 21 2024 Widespread gale force winds will continue the hazardous marine conditions. Winds will range between 25 knots to 40 knots. Seas up to and just above 15 feet are anticipated with long period swell continuing to roll in. Widespread rain showers will result in gusty and erratic outflows and subsequent rough waters. Boating conditions will improve this weekend, but still remain quite hazardous, particularly to smaller vessels and crafts. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...High Surf Advisory until 6 AM PST Friday for CAZ006-505-509-530. Wind Advisory from 10 PM this evening to 10 PM PST Friday for CAZ006-502>505-509-512-514-515-517-518-530. Flood Watch through late Friday night for CAZ502>506. Beach Hazards Statement through late tonight for CAZ529. PZ...Small Craft Advisory until 3 AM PST Saturday for Mry Bay-SF Bay N of Bay Bridge. Gale Warning until 9 AM PST Friday for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM. Gale Warning until 9 PM PST Friday for Pigeon Pt to Pt Pinos 10- 60 NM-Pt Reyes to Pigeon Pt 0-10 nm. Small Craft Advisory until 3 AM PST Friday for Pigeon Pt to Pt Pinos 0-10 nm. Gale Warning from 3 AM Friday to 3 AM PST Saturday for Pigeon Pt to Pt Pinos 0-10 nm. Small Craft Advisory until 9 AM PST Friday for Pt Pinos to Pt Piedras Blancas 0-10 nm. Gale Warning from 9 AM to 9 PM PST Friday for Pt Pinos to Pt Piedras Blancas 0-10 nm. && $$ SHORT TERM...Kennedy LONG TERM....Kennedy AVIATION...SO MARINE...Murdock 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