Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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000 FXUS66 KMTR 230048 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 548 PM PDT Sun Oct 22 2017 .SYNOPSIS...After a seasonably mild afternoon across the region today, very warm to hot temperatures are likely on Monday and Tuesday afternoons. Along with the heat, dry conditions will persist through the first half of the week as a result of weak offshore winds. A gradual cooling trend is then forecast during the second half of the week, yet temperatures will remain above seasonal averages. && .DISCUSSION...as of 01:47 PM PDT Sunday...High pressure continues to build in over the region from the south and will strengthen through midweek. As the air mass aloft warms, surface temperatures will also increase with afternoon highs reaching 15 to 20 deg F above seasonal averages on Monday and Tuesday. This combined with weak offshore flow will result in temperatures reaching into the 80s at the coast with lower/middle 90s inland. With this event, the warmest temperatures are forecast to occur on Tuesday across the Central Coast where the warmest and driest air mass will be in place. Cannot rule out a few of the warmest inland areas such as Pinnacles National Park reaching into the upper 90s. Overall, winds are forecast to remain light for most urban locations but may become breezy and locally gusty at times in the peaks and ridges beginning tonight before gradually tapering off Monday night into Tuesday. These conditions will raise fire weather concerns to some extent, but rainfall late last week is providing some temporary relief, at least initially. By the time conditions dry out late Monday and Tuesday, wind speeds are forecast to be weaker. Heat risks will also increase into the moderate category both Monday and Tuesday afternoon, mainly from the Santa Cruz Mountains southward through the Central Coast. However, longer nights and modest overnight cooling under clear skies will limit the overall heat risks region-wide through this heat event. Thus, folks are urged to remain vigilant and use extra caution during the peak heating of the day through early this week. The mid/upper level ridge is then forecast to weaken and shift back offshore by midweek in response to an upper level trough digging southward across the Great Basin and Central Plains. This will allow for a cooling trend from Wednesday through at least Friday, yet temperatures are forecast to remain above seasonal averages. In addition, medium to longer range models maintain dry weather conditions and near to slightly above average temperatures heading through the end of October. && .AVIATION...
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As of 5:48 PM PDT Sunday...High confidence VFR due to offshore flow tonight through Monday night. Increasing air mass stability under an amplifying thermal and height ridge will result in surface winds tending to decouple from increasingly gusty NE winds aloft in the Bay Area hills and mountains tonight into Monday morning; low level wind shear is possible due to 925 mb NE winds reaching 25-35 knots based on NAM/WRF output. Warming due to large scale air mass compression is showing up on the profiler data from Bodega Bay to Fort Ord with shallow inversions based at sea level to 500 feet. Vicinity of KSFO...VFR. W-NW winds gusting to near 20 knots til 04z this evening then winds shift and become light NE tonight. Low level wind shear possible 06z-17z. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...VFR. W-NW winds gradually becoming light E-SE this evening, winds becoming increasingly gusty from the SE in the Salinas Valley late tonight through Monday morning.
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&& .FIRE WEATHER...As of 3:20 AM PDT Sunday...Gradual warming and drying will continue across the district today. Offshore flow is then forecast to develop tonight, resulting in much warmer and drier conditions by Monday and Tuesday. Locally moderate and gusty north to northeast winds are forecast in the hills late tonight and Monday morning. The latest WRF has scaled back on the magnitude of wind speeds in the hills tonight and Monday morning. Even so, local gusty winds are likely over the higher ridges and peaks of the North Bay, particularly across eastern Napa County where gusts up to 40 mph are possible. In addition, nighttime humidity recoveries will be poor in the hills tonight. Were it not for the recent wetting rains on Thursday night, these developments may have resulted in critical fire weather conditions. However, finer fuels are expected to retain enough moisture in the short term to mitigate fire weather concerns to some extent. Will continue to draw attention to local gusty winds and drying conditions with headlines in the Fire Weather Planning Forecast. Fuels are expected to dry significantly by late Monday. Fortunately, winds in the hills on Monday night are not expected to be as strong as tonight, with only some isolated patches of gusty winds expected at the higher elevations of the North Bay. Will continue to monitor fuel moisture over the next few days to determine if critical fire weather conditions may occur. Winds are expected to subside in all areas by Tuesday afternoon, but very warm and dry conditions will continue through midweek. Slight cooling is expected during the second half of the week as high pressure over California weakens. && .MARINE...
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as of 4:42 PM PDT Sunday...Increasing northwest winds later today and overnight. Moderate northwest swells will gradually subside through the evening. A different moderate to long period northwest wave train will arrive late tonight, peaking in energy later Monday.
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&& .MTR WATCHES/WARNINGS/ADVISORIES...
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.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
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&& $$ PUBLIC FORECAST: RGass AVIATION: Canepa MARINE: DRP FIRE WEATHER: Dykema 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

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