074 FXUS66 KMTR 192348 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 448 PM PDT Tue Oct 19 2021 .SYNOPSIS...Unseasonably cool and unsettled weather conditions continue through the upcoming weekend. The first of a series of storms arrives tonight providing beneficial rain to most of the area with a good push of wind. A second similar strength system arrives Thursday into Friday. A more potent storm system looks to arrive late in the upcoming weekend with more widespread rainfall. && .DISCUSSION...as of 02:00 PM PDT Tuesday...The first in a series of storms will impact the Bay Area beginning this evening and linger until storm number two arrive later in the week. KMUX radar has been switched to precip mode as a few showers are already showing up over the northern outer waters. The latest timing is trending a little slower than precip forecasts with the onset of rain for the Bay Area. Rain chances increase early this evening over the North Bay, but even more likely by 8-11 PM. Once the rain does finally move in it will progress southward before stalling over the Bay Area. The heaviest rainfall still appears to be with the leading edge of the precip tonight over the North Bay. Will have to watch this closely as a few models put rainfall rates near critical levels for debris flow (2020 burn areas). Enough dynamics will be possible with the initial fropa that a thunderstorm or two will be possible over the northern waters (less than 20 pct). Last but not least, a potentially bigger impact with the first system are the winds. Latest hi-res models show a robust low level jet taking aim at the coastal waters, coastal North Bay, and San Francisco Peninsula Coast. The mix down potential is pretty high with gusts in the 35-45 mph range. Therefore, a Wind Advisory will be issued for the North Bay Coast/Valleys, San Francisco Peninsula Coast, and Bay Shoreline this evening/overnight. The stalled boundary will waver and slowly drift northward Wednesday into Thursday. Given the lingering boundary and persistent moisture feed off the Pacific scattered showers and periods of rain will continue. Rainfall totals Tonight through Thursday afternoon: N Bay Mts 2.5-4.0", N Bay Valleys, 1-2", SF Bay and Santa Cruz Mts 0.5-1.5", Santa Clara Valley and southward less than 0.25". Far interior Monterey/San Benito may see little to no rainfall. System number two begins to merge with the stalled boundary late Thursday into Friday. System number is more progressive and doesn't stall like the first front. Rain initially develops over the North Bay Thursday night before moving S and E through day Friday. Rain will also end abruptly from N to S behind the front during the day as well. Given the progressive nature of the front rainfall amounts will be a little less. May even see a break in the precip Friday afternoon into Saturday afternoon before system number three arrives. System number three, likely the biggest of the bunch, will move toward the region Saturday night. Rain chances will increase through the day on Sunday, but the bigger push of tropical moisture will occur later Sunday into Monday. Instead of trying to nail down a specific model winner from a deterministic output it's better to leverage ensembles and probabilistic forecasts. Therefore, our messaging hasn't changed that much. The first notable Atmospheric River of the season will impact the region Sunday into Monday. That being said, some of the deterministic output for Sunday night is eye opening with impressive 6 hour rainfall amounts. AR Model output is also off the chart (not sure if that seems real yet). Those higher end bullseyes have been shifting from run to run and is further support by WPC Cluster analysis. Simply put, exact details are hard to nail down this far out, but easier to say solid rain will impact the region. We still have several days to fine tune the details with the AR. Current rainfall amounts with system three: N Bay and all coastal mountains3.0-5.5", elsewhere 1.0-3.0". There will likely be some rain shadowing. Some lingering precip will be possible through the middle of next week. Please follow the forecast as details become more clear over the next few days. && .AVIATION...as of 04:42 PM PDT Tuesday...For the 00z TAFs. A band of rain showers is off the coastline, already being picked up by the KMUX radar, and moving toward the land. The arrival of the front is likely around 6Z for the North Bay, 8Z for SFO, and 12Z for the Monterey Bay. The chance for isolated thunderstorms is possible, but remains low in confidence. The main impacts from this front will be the periods of moderate to heavy rain and the gusty southerly winds accompanied by LLWS. Cigs are still projected to be around MVFR heights, between 2000 and 3000 feet, with occasional reduced vis between 4 to 6 statute miles during the heaviest rain. It can be expected to wet runways during the overnight hours. While the chance for rain with this front remains high, the most difficult part of the this forecast is surface winds. There is high confidence for 30 to 40 knot southerly winds between 1500 and 2500 feet AGL, lowering in the early Wednesday morning hours to around 500 feet by some models. That can create difficult LLWS at Bay Area terminals. However, if the winds mix down to the surface, strong gusts up to 30 knots are possible. Winds are not expected to reduce until after approximately 18Z, but remain out of the south. As Wednesday afternoon approaches, expected MVFR cigs to remain as the front stalls over the Bay Area. The rain showers are forecast to turn to more patchy lingering showers and possible drizzle around the vicnity of most terminals. Winds will remain out of the south around 10 knots and occasional stronger gusts. Short term high res models are projecting another slug of moisture to move through the Bay Area which has the potential to bring another round of light to moderate rainfall late in the TAF period. Vicinity of KSFO...Currently VFR, but increasing mid to high level clouds. Rain approaches along a cold front Tuesday night into Wednesday. Current model projections have the rain arriving around 08Z at the terminal. Expect wet runways overnight and early Wednesday morning. Confidence in thunderstorm development is low. Winds have already turned to the southwest ahead of the front. As the rain arrives, winds will further turn to the south-southeast and increase aloft keeping LLWS in the TAF overnight into early Wednesday morning. These strong winds aloft, between 30 to 40 knots, will likely get as low as 500 feet AGL. Currently projecting winds to mix down to the surface, but there is only moderate confidence. The chance for an airport weather warning for winds in excess of 25 knots is low, but will be monitored. Strongest winds are forecast between 08Z and 12Z. Winds will stay southerly through Wednesday, despite diminishing to around 12 to 16 knots. The front is expected to stall keeping scattered showers around the terminal for much of the day. A secondary chance for showers at the terminal with gustier winds is for late in the TAF period. Cigs will remain MVFR through the end of the TAF. KSFO Bridge Approach...Similar to KSFO. Monterey Bay...Currently VFR as high clouds continue to approach the bay ahead of the cold front. As the front arrives near 09Z, MVFR cig will be featured around the bay. Best chance for rain showers will be for coastal terminals around 12Z. Showers will remain in the vicinity of the terminals through much of Wednesday. Winds will be out of the south and breezy at times. A secondary chance for light showers at coastal terminals is expected at the end of the TAF period. && .MARINE...as of 02:43 PM PDT Tuesday...An approaching cold front will move across the waters between this evening and tomorrow morning. Winds have become southerly and are gradually strengthening across the waters. Latest guidance suggests gale force gusts are likely ahead of the front this evening through the overnight period for the northern waters and into the northern San Francisco Bay. These strongest winds are anticipated just ahead of the front late this evening through the overnight period. Steep to very steep wind waves of 8 to 12 feet are possible with these strongest winds and are hazardous for small craft vessels. Winds will gradually diminish on Wednesday, but remain southerly as yet another wet system approaches. Otherwise, a moderate northwest to west swell will persist in the waters through the week. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...Wind Advisory...CAZ006-505-506-508-509 SCA...Pigeon Pt to Pt Pinos 0-10 nm from 9 PM SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM GLW...Pt Arena to Pt Reyes 0-10 nm until 3 AM GLW...Pt Arena to Pigeon Pt 10-60 nm until 3 AM GLW...SF Bay from 9 PM until 3 AM GLW...Pt Reyes to Pigeon Pt 0-10 nm from 9 PM until 3 AM GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 9 PM until 3 AM SCA...Mry Bay until 3 AM && $$ PUBLIC FORECAST: MM AVIATION: DK MARINE: DRP 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