313 FXUS66 KMTR 191754 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1054 AM 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. 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 09:14 AM PDT Tuesday...A cool start to the day with widespread morning MinTs in the 40s and mid to upper 30s in the colder inland valley locations. Mostly clear skies and cool 850mb temps allowed for a good amount of radiational cooling at the surface. A look at visible satellite imagery shows mid and high level clouds beginning to stream into the area from the northwest, ahead of a frontal boundary approaching the NorCal/PacNW coast. The first of a series of rain events arrives tonight and is expected to provide widespread beneficial rain, with the possible exception of southern interior portions of Monterey and San Benito Counties, which will see minimal amounts. Latest hi-res models show rain arriving with the frontal boundary to coastal Sonoma Co just after sunset, working its way southeast towards SF/Oak 10pm-midnight, and reaching Monterey Bay after midnight. There should be relatively brief steady rainfall with the frontal boundary, along with gusty southerly winds, and scattered post-frontal showers through the day tomorrow, as the front falls apart as it works its way through the Bay Area. While flooding/debris flow concerns with this system are minimal due to limited rainfall rates and duration, it is still a good time to prepare for the upcoming wet season by clearing your gutters and storm drains of debris. This can help prevent ponding and localized flooding. The weekly outlook is for more beneficial rain Thursday into Friday and a more potent AR system arriving Sunday. Will be keeping an eye on model trends as the week progresses, especially for the potent Sunday system. Please see the previous discussion for more details. && .PREV DISCUSSION...as of 03:00 AM PDT Tuesday...Generally quiet weather conditions persist early this morning as a weak short-wave ridge shifts inland over the region. This has resulted in mainly clear sky conditions with only patchy fog and/or low clouds across the valleys of Sonoma County as well as around the western Santa Clara Valley. With weak ridging aloft, look for a slight warm-up across the region this afternoon ahead of an approaching mid/upper level trough. Increased onshore flow both at the surface and aloft will advect a deeper plume of moisture into the Bay Area and Central Coast with PWAT values forecast to range between 1.00"-1.25". This moisture will interact with a frontal boundary forecast to sweep into the North Bay late this evening bring widespread light to moderate rainfall. Precipitation along the frontal boundary will push inland and southward through the night. There appears to be enough instability to produce a slight chance of thunderstorms tonight, mainly out over the Pacific off of the North Bay coastline. Breezy to locally gusty onshore winds are also likely ahead of and in wake of the frontal passage. The frontal boundary looks to weaken/dissipate across the Central Coast during the day Wednesday, yet modest moisture advection looks to persist region-wide through Thursday within more zonal flow. Thus, expecting lingering showers and/or light rain to persist at times through Thursday. Rainfall amounts from late today through Thursday look to range from 2.50"-3.50" across the North Bay Mountains, 1.50"-2.00" across the North Bay Valleys, 1.00"-1.50" in the Santa Cruz Mountains, 0.25"-0.75" for much of the greater San Francisco Bay Area and Santa Lucia Mountains, and generally less than 0.25" elsewhere. This pattern will also support afternoon temperatures remaining below seasonal averages through midweek. Another mid/upper level trough axis is forecast to push inland into the Pacific Northwest Thursday night into Friday which will drive another frontal boundary southward into the region. This system looks to produce rainfall amounts of around 1/2 to 1/3 of the aforementioned totals from earlier in the week. Nonetheless, beneficial rainfall amounts for this early in the season. Unseasonably cool conditions are also likely through late week. Forecast ensembles indicate moist zonal flow will persist over much of the West through the upcoming weekend as well. The EC and GFS ensembles and deterministic models indicate the potential for an even more potent system to sweep through the region late in the weekend. This system looks to tap into an even deeper moisture tap with more widespread rainfall likely. While confidence is high for this to occur, the details remain difficult to pin-point at this time as to exactly where and when the heaviest rainfall will occur. When all said and done, 7-day rainfall totals could range from 5-8" over the North Bay, 3-5" in the Santa Cruz Mountains, 1-3" across the San Francisco Bay Area and Santa Lucia Mountains, and 0.50"- 1.50" elsewhere. Stay tuned for the latest forecast information as we continue to monitor this series of storm systems and their impacts on the region. && .AVIATION...as of 10:55 AM PDT Tuesday...For the 18z TAFs. Satellite imagery depicts approaching storm system and increasing high clouds with predominately VFR conditions this morning, aside from some lingering VLIFR/LIFR fog/stratus in the Sonoma area near KSTS. Winds are predominately light and southerly, except locally breezy through favored SE oriented valleys, such as near KSNS in the Salinas Valley. Cold frontal boundary expected to arrive and transition through the area late tonight into early Wednesday, and with that, bring periods of moderate to heavy rain, gusty southerly winds, LLWS, potential thunderstorms, MVFR cigs & visbys, and wet run ways. Overall, confidence is moderate at best in terms of timing of the frontal passage as well as precise wind directions and speeds. Model to model confidence is fairly low in terms of wind speed and direction, but sufficient to say predominately S-SE winds will prevail, peaking at around 10-20kt, with gusts up to 20-30kt possible. No significant wind shift to the W or NW is expected in the wake of the front as additional frontal boundaries will pass over the coming days. Predominately southerly winds persist beyond tonights initial cold front. Finally, a S-SSE low level wind jet of 30-45 knots has been mottled by most near term models, descending as low as 500-1000 feet overnight and strongest at around 1500-2000 feet. This will bring a threat of LLWS to most terminals overnight. Models are also showing some instability with the frontal passage, mainly over the waters and over the North Bay coast. Therefore, there is a remote chance of a thunderstorm or two. Vicinity of KSFO...Light winds this morning before S/SE winds increase to 10+ kts after 21Z, as our next system approaches the area. Models and guidance show stronger southerly wind gusts possible this evening and overnight (~20-30 kt). Rain looks to arrive ~08z along with lowering cigs and visbys. A S-SSE low level jet at 500-2000 feet AGL will result in LLWS overnight into early Wednesday morning. No significant wind shift towards the W or NW in the wake of the front. Instead, winds remain southerly due to additional storm systems arriving later in the week. KSFO Bridge Approach...Similar to KSFO. Monterey Bay... VFR today with high clouds beginning to move in to Monterey Bay this evening ahead of the incoming frontal system. Moderate southerly winds at SNS currently that will veer around to be out of the NW this afternoon. Winds in MRY to be more southwesterly as the afternoon progresses. MVFR CIGs with lower clouds coming in. VCSH entering the Monterey Bay late tonight ~09z. Light rain forecasted for MRY terminal ~12-13z. Will continue to monitor and updated TAFs with the timing of the potential rain. && .MARINE...as of 10:41 AM PDT Tuesday...Winds have transitioned and become southerly ahead of an approaching storm system this morning. These southerly winds are forecast to strengthen through the day, peaking overnight and into tomorrow morning. Gusty conditions with periods of moderate to heavy rain are expected to accompany the front as it moves across the waters this evening into tomorrow morning. Peak wind gusts in the northern outer waters will approach and exceed gale force for several hours, thus a gale force warning is in effect. In addition, these gusty winds will generate steep waves resulting in hazardous seas, particularly for smaller vessels. Winds will gradually diminish on Wednesday, but remain southerly as yet another wet system approaches. Mixed seas persist with a moderate northwest swell and a weak embedded southerly swell. The main northwest swell will shift more westerly by mid week. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM GLW...Pt Arena to Pigeon Pt 10-60 nm from 3 PM SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM SCA...Pt Arena to Pt Reyes 0-10 nm from 3 PM && $$ PUBLIC FORECAST: Lorber/RGass AVIATION: DRP/SMM 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