FXUS66 KMTR 230455 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 855 PM PST Fri Feb 22 2019 .SYNOPSIS...Dry and seasonably cool conditions will prevail across most of the region through the weekend. The exception may be the North Bay where there is a chance of light rain late in the weekend. Heavier and more widespread precipitation is possible from Monday through the middle of next week, particularly across the North Bay, as a frontal boundary with significant moisture sags slowly south across northern California. Expect warmer temperatures next week, particularly at night. Another potentially wet system may bring more rain to the region late next week and into the following weekend. && .DISCUSSION...As of 8:55 PM PST Friday...It's another cool evening across our region under mostly clear skies. The boundary layer has moistened up a bit over the past 24 hours with surface dewpoints running anywhere from 5 to 15 degrees above what they were at the same time last evening. Thus, expect overnight lows to moderate slightly tonight and generally be a few degrees warmer than last night. Even so, patchy frost is likely once again in the interior valleys late tonight and early Saturday. The cold upper trough that brought snow to southern California and southern Arizona the past couple of days is moving quickly off to the east this evening. Another cold trough over British Columbia is beginning to drop south. However, the models agree that this system's southern progression will halt over the Pacific Northwest this weekend as energy and moisture over the eastern Pacific undercuts the ridge out along 140W, resulting in zonal flow across California by late in the weekend. In addition, a moisture plume is forecast to undercut the ridge and arrive over northern California by late in the weekend. This pattern evolution is now expected to unfold more slowly than previously thought and models are now keeping all precipitation north of our area through the bulk of the weekend, with rain not expected to develop in the North Bay until late Sunday afternoon at the earliest and possibly not until Monday. Thus, nearly all of our area can expect a dry weekend, along with a gradual warming trend as a more mild airmass arrives from the west. The most difficult part of our forecast is in regard to the frontal boundary and moisture plume that will be in place across northern California from Sunday night through early Wednesday. Where exactly that boundary/plume will be focused, and for how long, will have major implications for the timing and amount of precipitation during the first half of next week. For the past few model runs the GFS has been focusing the moisture plume over the SF Bay Area, and especially the North Bay. The 00Z GFS continues this trend as it forecasts moderate to heavy rain over the North Bay, and occasionally slightly south of the Golden Gate, from Monday afternoon through Tuesday evening. The 12Z ECMWF, by contrast, brings the boundary south into the Bay Area much later (not until Tuesday night) and generates much less rainfall compared to the GFS. For example, the 00Z GFS forecasts more than 4 inches of rain in Santa Rosa through Tuesday night, while the 12Z ECMWF accumulates less than 0.75" in Santa Rosa. Also, the 12Z GEFS ensemble members forecast a wide range of precipitation values over the Bay Area with individual ensemble members forecasting rainfall amounts at SFO ranging from less than a half inch all the way up to more than 4 inches. The upshot is that there remains a great deal of forecast uncertainty regarding rainfall amounts early next week. The wettest model solutions would likely result in at least minor flooding by late Tuesday, especially across the North Bay. Also, strong and gusty winds are possible early next week, especially on Tuesday. Those living in the Bay Area, and especially in the North Bay, should prepare for the wettest potential outcome and be prepared to take action if flood watches or warnings are ultimately issued. The airmass to the south of the moisture plume next week will be very mild, and much warmer than the airmass currently over our area. Thus, expect daytime temperatures to warm into the lower to mid 60s by Tuesday of next week and overnight lows to be in the low to mid 50s. Another system that will be tapping into subtropical moisture is expected to impact our area by late next week or into the following weekend. That system also has the potential to produce heavy rain in our area, but probably for a shorter period of time compared to the system early in the week. && .AVIATION...As of 9:00 PM PST Friday...VFR with a dry air mass, only minor and a mainly delayed up trend in surface relative humidity arriving along the immediate coastline and bays per recent metar observations. Meso-model output is over-forecasting surface and boundary layer relative humidity; little temperature differences between land and sea surface temperatures today limited onshore pressure gradients, gradients are somewhat weaker than forecast, likely limiting and delaying the inland extent of ocean based humidity this afternoon. Clear tonight then cirrus clouds advance over the cwa Saturday, clear skies will allow decent nocturnal radiative cooling tonight with cold air drainage winds developing bringing drier air back to the immediate coastline and bays, setting up a light offshore wind flow. Onshore wind returns Saturday by mid to late afternoon. Vicinity of KSFO...VFR. West wind near 10 knots becoming light NE overnight into Saturday morning. Light NE wind Saturday becoming onshore again in the mid to late afternoon. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR. W-NW winds 5 to 10 knots gradually becoming E-SE tonight. E-SE cold air drainage winds returning to onshore winds by mid to late Saturday afternoon. && .MARINE...As of 7:53 PM PST Friday...Breezy northwest winds will diminish tonight. Moderate northwest swell will continue into next week. Southerly winds will develop later this weekend and into early next weekend. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 PM && $$ PUBLIC FORECAST: Dykema 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