407 FXUS66 KMTR 150552 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1052 PM PDT Sun Mar 14 2021 .SYNOPSIS...A cold front will continue to spread rain south through the region this evening. Scattered showers are expected behind the front late tonight and Monday, along with low snow levels and possible thunderstorms. Brief gusty southerly winds will precede the cold front. Winds will switch to the west and northwest behind the front later tonight and increase, becoming locally strong and gusty from late tonight through Monday morning, especially near the coast. Cold temperatures and possible frost is forecast for Monday night and early Tuesday. Dry weather appears likely on Tuesday and Wednesday. Rain chances redevelop on Thursday and showers may continue into next weekend. && .DISCUSSION...As of 8:55 PM PDT Sunday...A cold front began to spread rain into the North Bay early this afternoon and by this evening rain had developed as far south as Monterey bay. Although radar indicated the presence of an NCFR along the boundary off the far northern California coast early this afternoon, by the time the front reached our area there were no signs of an NCFR and only light to moderate rain rates have occurred. In any case, six hours of light to moderate rainfall in the North Bay has resulted in rain totals of a half inch to an inch in the coastal hills of the North Bay, with 1.12" reported at Venado in northwest Sonoma County thus far. Rain totals elsewhere have generally been less than a half inch. Based on the latest KMUX radar data, the frontal boundary, along with most widespread shower activity, is currently moving through the South and East Bay. Short-term model data indicate the front will reach the Monterey Bay Area within the next hour, and then the far southern reaches of our forecast area by midnight. Models also indicate that shower activity will taper off behind the frontal boundary overnight. However, at least widely scattered showers will continue behind the front through Monday as the cold upper trough, currently centered just off the CA/OR border, tracks from NNW to SSE across California. Snow levels will drop rapidly behind the cold front tonight as a the cold core of the upper trough sweeps in from the northwest. Snow levels are expected to drop as low as 1500 feet in the North Bay by late tonight and to 2500 feet in Monterey County, then drop as low as 1000 feet on Monday morning. However, the bulk of the precipitation is expected to fall before the coldest air arrives late tonight and Monday. So, while some showers may briefly drop snow as low as 1000 feet on Monday morning, accumulating snow is only likely on the higher peaks and ridges in the Bay Area. Higher locations such as Mount Hamilton may pick up a few inches of snow. The NAM has been forecasting the most persistent shower activity on Monday to occur across northern Monterey County. Thus, some of the higher elevations of the Santa Lucias may accumulate up to 6 inches of snow, or more, by Monday afternoon. Lapse rates will steepen overnight and into Monday as the cold core of this system drops to the southeast. However, thunderstorms chances seem to be dwindling given that the lightning detection network has not yet picked up any lightning strikes offshore behind the front. Also, the Storm Prediction Center, which had been including thunderstorm potential for much of northern and central CA in its convective outlook for Monday, has now dropped that potential. In any case, given the cold airmass aloft, feel that slight thunderstorm chances are justified in our forecast tomorrow. Even if no thunderstorms develop, the more intense showers late tonight and Monday will likely produce small hail. Southerly winds picked up late this afternoon and evening as the front rolled in, and winds are currently gusting as high as 40 mph in the North Bay Mountains and Santa Cruz Mountains. Winds will switch to the west and northwest behind the front later tonight and are expected to increase, especially along the coast where gusts up to 50 mph are forecast to develop by late tonight. A Wind Advisory for the North Bay Coast, San Francisco, San Francisco Peninsula Coast, and San Francisco Bay Shoreline goes into effect at 10 pm this evening and continues through 11 AM Monday. Based on the lastest local WRF, strongest winds will occur between about midnight and 9 am, although breezy conditions will persist into the afternoon hours. A few showers may linger into early Monday evening across the southern portion of our area, but skies should clear overnight and cold conditions are expected by late Monday night if winds subside quickly enough. A Frost Advisory may be needed for some of our interior valleys late Monday night into Tuesday morning, most likely the North Bay Valleys, but possibly also for the interior valleys of Monterey County. Dry conditions, along with a warming trend, are forecast for Tuesday and Wednesday as a shortwave ridge briefly builds over California. However, overnight temps are forecast to remain cool on Tuesday night. In the longer range, models agree reasonably well in developing widespread precipitation later in the week as a system moves through Thursday and Thursday night. Although an upper level trough is forecast to persist on into next weekend, the GFS and ECMWF (deterministic and most ensemble members) indicate low chances of significant rainfall beyond Thursday night. && .AVIATION...as of 10:52 PM PST Sunday...For the 06Z TAFs. A cold front continues to move over the cwa, moving south of Monterey and San Benito counties tonight. Conditions along and out ahead of the front include rain lowering ceilings and visibilities to MVFR and IFR and gusty winds. Southerly winds and low level wind shear developing ahead of the front then post frontal northwest winds strengthening tonight and Monday morning. 850 mb temperatures lowering to -5C to -6C late tonight and Monday, unseasonably cold for mid March, exceeding previous 850 mb minimum temps by at least 1 degree Celsius for all Oakland upper air soundings for the period mid March through mid April since 1948. On Monday VFR to MVFR with blustery northwest winds especially Monday morning, also isolated to widely scattered showers. Vicinity of KSFO...Showers in the post cold frontal environment tonight with gusty winds. Post frontal winds ramp upwards to 20 to 30 knots, possibly higher gusts as strong cold air advection on northwest winds aloft also ramp upwards; none issued yet, but will be close to airport weather warning criteria for winds tonight and early Monday morning. Blustery still on Monday with an isolated shower or two. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...Cold front is sweeping through with showers, gusty winds and cigs/vsbys lowering to MVFR, possibly briefly IFR. Gusty post frontal winds tonight and Monday with isolated to scattered showers. && .MARINE...As of 8:45 PM PDT Sunday...A strong cold front is quickly sweeping across the coastal waters and bays tonight. Gusty southerly winds, infrequently gale force, will accompany moderate to heavy rain reducing visibility, and a rapid wind shift to northwest direction will occur tonight. Northwest winds will strengthen rapidly to gale force late this evening into Monday and generate very steep seas of 15 to 20 feet at roughly 10 to 12 seconds. The strongest wind gusts are expected to be along the coast north of Point Reyes and up to 55 knots between from roughly midnight to noon Monday. These very steep seas and gusty winds can capsize small vessels and bring other hazardous conditions across the seas. Winds and seas ease midweek. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...Wind Advisory...CAZ006-505-508-509 GLW...Pt Reyes to Pigeon Pt 0-10 nm GLW...Pigeon Pt to Pt Pinos 0-10 nm GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm GLW...Pt Arena to Pigeon Pt 10-60 nm SRW...Pt Arena to Pt Reyes 0-10 nm SCA...SF Bay until 3 AM SCA...Mry Bay until 3 AM GLW...SF Bay from 3 AM GLW...Mry Bay from 3 AM && $$ PUBLIC FORECAST: Dykema AVIATION/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