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

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405 FXUS66 KMTR 200536 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 1036 PM PDT Thu Sep 19 2019 .SYNOPSIS...A warming trend will begin on Friday and continue through Saturday. A low pressure system moving southward to our east early next week may create the potential for continued warming temperatures, dry offshore flow and increased fire weather concerns. && .DISCUSSION...as of 10:03 PM PDT Thursday...After a generally cooler start this morning compared to Wednesday morning the mid September sun helped recover temperatures to the mid 60s coastside through the 70s bayside to lower 80s inland valleys today; average temperatures varied up to a few degrees cooler inland locations to a couple degrees warmer coastal locations compared to the daily normals. The upper level trough is continuing to move slowly eastward over the Intermountain West. Clear skies tonight continue with exception of a few patches of coastal low clouds developing by early Friday morning. Sunny to mostly sunny skies return Friday with highs reaching the mid to upper 60s on the coastline to the 80s inland valleys. Large scale downward vertical motion under increasing 500 mb height ridging will result in steady lower level warming beginning Friday thus we should see the return of a marine temperature inversion on the profilers by Friday afternoon or evening. Patchy coastal stratus may return either late Friday night or early Saturday morning. Away from the coast skies will continue clear with more or less steady warming over the weekend and early next week with daytime highs in the 90s much more common. With the passage of an upper level trough to our east early next week offshore winds increase fire weather and heat risks. For late next week, today`s 12z medium range models showed much greater agreement on a change to unseasonably cooler weather. The 00z GFS still leans in this direction with 500 mb height falls to the mid 550s decameters over the Bay Area late next week, close but not quite as low as the earlier 18z run. For reference, 550 decameters is the minimum for all September Oakland upper soundings for 1948-2014 period of record. For additional details please see previous forecast discussion. && .PREV DISCUSSION...as of 02:00 PM PDT Thursday...An upper ridge centered over the eastern Pacific will begin to build over the West Coast on Friday and continue to strengthen through Saturday. Temperatures aloft will warm, with 850 mb temps reaching approximately 15C on Friday and 17C on Saturday. The warming aloft will translate to warming at the surface, especially away from the coast. Inland valley locations look to warm well into the 80s on Friday, with mid 80s to lower 90s common on Saturday. Onshore winds will keep temperatures from rising too much at the coast, but an increased offshore component on Saturday will help temperatures reach the 70s to lower 80s. North to northeast winds will be breezy at times in the North Bay Hills on Saturday morning, but not windy or dry enough for major fire weather concerns. On Sunday, the ridge will begin to flatten as another trough approaches the Pacific Northwest. In response high temperatures should cool a few degrees. The trough will then take a northwest to southeast trajectory and is likely to follow an "inside slider" type path, with the core of the trough moving just east of the Sierra crest. The past several runs of the GFS deterministic have kept the trough over Utah and Colorado, but this appears to be an outlier compared to GFS and ECMWF ensemble means. As the trough moves into the Desert Southwest, offshore flow will develop as surface high pressure strengthens over the Intermountain West. This will likely result in a significant warming and drying trend for our area Monday through Wednesday. The exact track and strength of the trough early next week will determine the strength of the offshore flow and amount of drying/warming we see. This pattern will need to continue to be closely monitored as it could lead to increased fire weather concerns. Warm and dry conditions look to persist through the middle of next week. The long range ensembles are starting to hint at a trough dropping down the West Coast next weekend, which would result in cooler conditions for the region. && .AVIATION...As of 10:35 PM PDT Thursday...Cool northerly flow aloft bringing drier air into the low levels tonight so VFR conditions expected through Friday. Vicinity of KSFO...VFR. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...VFR. && .MARINE...as of 10:29 PM PDT Thursday...High pressure building off the California coast will bring moderate to locally gusty northwest winds through tonight. Winds will decrease slightly over the weekend. An 11 second northwest swell will be the predominate wave in the waters tonight. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM && $$ PUBLIC FORECAST: Canepa/ST AVIATION: W Pi MARINE: W Pi 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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