397 FXUS66 KMTR 190329 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 829 PM PDT Mon Oct 18 2021 .SYNOPSIS...Cooler conditions persist this week with rain chances returning Tuesday. Additional rain chances will arrive Thursday evening and again next weekend as a more powerful storm system arrives. && .NEAR TERM [Through TDY MORN]..18/830 PM. GOES-WEST Mid-level Water Vapor Imagery is capturing the elongated moisture tongue that is being advected along the PVA zone to the east of the elongated 523dm 500mb trough centered at ~140W. This tongue is currently blocked off from the California Coast by a weak mid-level ridge in the wake of yesterday's disturbance. While some remnant moisture manifested in the form of faint, isolated showers and high-base CUs in the morning, have remained relatively today across the CWA. The axis of this mid-level ridge will gradually move ashore over the next 18-24 hours, allowing for higher PWAT values to stream into the Coastline by tomorrow afternoon. For now, are on track for a clear and quiet night across the CWA with patchy clouds along portions of the immediate coast and some radiational fog in the interior North Bay valleys by dawn. The lower RH values tonight relative to last night will also help the environment cool off much faster, and so have progged min temps for tomorrow morning in the 40s F for much of the CWA, along the mid to upper 30s at some of our elevated ranges > 2000 ft. .SHORT TERM (TDY-THU)...18/830 PM. The moisture tongue that is currently upstream from the Northern and Central California coastline will continue to travel up and over the mid-level ridge Tuesday afternoon, and so will begin to observe greater concentrations of high-base clouds across the North Bay and eventually the rest of the Bay Area and portions of the Central Coast by early evening. The latest short-range deterministic solutions continue to favor the eventual progression of the moisture tongue into the Bay Area by late Tuesday evening as the ridge exits the region. The showers associated with the tongue will travel towards the S and E along the cold front, starting in the coastal ranges of Sonoma County and streaming south of the Golden Gate and into the rest of the Bay Area overnight before eventually streaming into the Central Coast before daybreak on Wednesday morning. If this setup sounds familiar it is because the synoptic-scale environment will hold a resemblance to the one last night, down to the axial tilt of the trough. Only this time around expect greater PWAT values exceeding one inch. The key difference is that the parade of storm systems is now well underway. Following the FROPA, will continue to observe lingering showers across the region Wednesday and into Thursday, albeit with lower rain rates than what is progged for the FROPA. Needless to say, rainfall totals have been nudged upward from previous forecast. Rainfall amounts Tuesday afternoon through Thursday: up to 2.5-3.5" N Bay Mountains, N Bay Valleys 1.5-2.0", SF, E Bay and S Bay 0.25-0.75", elsewhere less than 0.25". .LONG TERM (THU NIGHT-MON)...18/830 PM. The amount of momentum transfer occuring between the Western and Eastern PAC will continue to increase over the next few days. Both the GFS and EC deterministic runs pick up on this ocean-wide shift in recent trends, where the usual mid-ocean ridge that sits north of the Hawaiian Islands has lost some of its intensity. In its wake, have observed greater 200mb streaming of flow from the Sea of Japan over to the Central PAC. In doing so, that wave energy has greatly amplified the depth of some of the troughs that have developed over the Gulf of Alaska. This is where the Parade of Storms comes into play: as more energy is able to transfer across the PAC, will also see deeper troughs over our neck of the woods. Moreover, these deeper troughs will be able to host greater zones of PVA, along with the advection of moisture from the Central PAC over to the California Coast. The latest multi-model ensembles have picked up on the shift over the past few days and continue to trend towards waves of greater PWAT concentrations latching on the coast (1.3-1.6 inches) Thursday into Friday, along with another potential round during the weekend. Latest IVT values on the EC and GFS out to this period remain well over 250, with values by the weekend and into early next week approaching 700+ kg m-1 s-1 at times along the coast. All of this is to say that the early indications for this time-frame are that a late weekend system will have the most available moisture from an AR perspective. Will be monitoring for any changes in ensemble and deterministic trends with regards to this moisture tongue, but current consensus is that this event has the potential to be the largest float out of the parade of storms that we expect to move over the region this week. In terms of potential totals, for the Bay Area: N Bay 5-8", SF, E and S Bay 1-3", Santa Cruz Mts 3-5", and areas southward 0.5-1.5". Be advised that these values are subject to change owing to the large uncertainties between now and the weekend. As the previous forecaster had described it, this setup could stick a fork in fire weather concerns for most of the forecast area, save for the Central Coast. && .AVIATION...as of 05:00 PM PDT Monday...For the 00z TAFs. Mostly VFR this evening. Chance for FG/BR over KSTS/KMRY tonight, with better confidence in KSTS seeing reduced visibilities and IFR/LIFR ceilings. Light, locally driven winds tonight. VFR skies for Tuesday, winds will start to shift S/SE after 21Z as the next system approaches the area. Vicinity of KSFO...VFR will prevail through the period. Winds will be light tonight. S/SE winds will increase to 10kts after 21Z, as our next system approaches the area. KSFO Bridge Approach...Similar to KSFO. Monterey Bay...VFR conditions could become interrupted by patchy BR/FG forming around 10Z. Low confidence, so it is left out of the TAF. Better confidence in VFR prevailing through the period. Winds will be light, with SW/S winds forming towards the end of the period. && .MARINE...as of 08:39 AM PDT Monday...Moderate post-frontal northwest winds prevail on Monday. The next front with additional rain chances approaches the waters on Tuesday afternoon, causing the winds to shift to the south, strengthening Tuesday night into Wednesday morning when gale force gusts will be possible over the northern outer waters. These winds will generate steep waves resulting in hazardous seas conditions, particularly for smaller vessels. Southerly winds will ease Wednesday and then persist through much of the forecast period. Seas remain out of the northwest with a weaker southerly swell. The primary swell will shift to more westerly by mid week. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...NONE. && $$ PUBLIC FORECAST: Diaz AVIATION: Dhuyvetter MARINE: DK 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