Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 151030

Area Forecast Discussion
National Weather Service San Francisco Bay Area
330 AM PDT Sun Sep 15 2019

.SYNOPSIS...The cooling trend that began in coastal areas on
Saturday will spread inland today and Monday as a weather system 
moves in from the northwest. There is also a chance of rain on 
Monday, mainly across the northern half of the area. A second 
weather system may bring renewed rain chances around the middle of
next week, and also keep temperatures on the cool side of normal 
through much of the week. 


.DISCUSSION...As of 3:30 AM PDT Sunday...Temperatures have cooled
back to normal across area airports, with current readings in the
50s to lower 60s. The marine layer has bee steadily ramping up
over the past 24 hours, and now is over 1000 feet. More notable is
the tightening of the onshore surface pressure gradient over the
past 24 hours, with 3.0 mb from SFO to SAC, as compared to 0.8 mb
yesterday at this time. These factors are in response to an
approaching upper trough, and as a result, inland areas today 
will enjoy much cooler temperatures as was already observed near
the coast yesterday. Highs today are forecast to range from the
mid 60s to lower 70s at the coast, to the 70s and 80s inland.
Warmest inland locations should only reach the lower 90s. 

Meanwhile, the approaching upper level trough is robust enough to
bring rainfall to northern California on Monday, possibly as far 
south as the Monterey Bay. There are still differences in model
solutions regarding rainfall amounts as well as how far south it 
will occur. Models have been trending drier each day as the event
approaches, however, the 06Z NAM is now more bullish, with San
Francisco getting over a quarter of an inch by mid day Monday. The
00Z GFS and ECMWF both have around a tenth of an inch for that

Medium range models indicate another trough bringing rain chances
to the northern portion of the area on Wednesday. Have added
chance or slight chances of rain to mainly the north Bay for this
time frame. This trough will keep temps cool. In the longer 
range, models indicate rising heights for slightly warmer temps 
again by next weekend. 


.AVIATION...As of 11:02 PM PDT Saturday...Coastal stratus and fog
are redeveloping under nocturnal cooling and as northwest winds 
keep steady cool air advection going in the lower levels. VFR 
except IFR ceilings on the coast steadily moving inland overnight 
as cooling continues through early week steadily lifting and 
weakening marine inversions especially Sunday night into Monday. 
Additional cloud layers develop later Sunday and Monday with the 
arrival and passage of a cold front Monday morning, showers 
accompanying the cold front and sharply amplified upper trough.

Vicinity of KSFO...IFR ceiling has developed, though with lower
level cool air advection ceilings will probably tend to lift to
MVFR early Sunday morning. Low confidence on timing of stratus mix
out Sunday. Cloud layers then arrive Sunday evening into Monday
morning with showers on the doorstep Monday morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR ceilings developing overnight. Low 
confidence on timing of ceilings lifting to VFR Sunday, best chances
late Sunday morning into the afternoon. MVFR ceilings likely returning
early Sunday evening.

&& of 2:51 AM PDT Sunday...Light to moderate northwest 
winds will persist over the coastal waters today. Expect locally 
breezy afternoon and evening winds along the inner coastal waters 
south of Pigeon Point and over the bays. Winds will ease early 
this week as high pressure over the eastern Pacific weakens. A 
cold front will move through the waters on Monday bringing a 
chance of showers. Mixed seas will continue with moderate period 
northwest swell and a slightly longer period southerly swell.


     .Tday...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...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 11 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...SF Bay from 2 PM
             SCA...Mry Bay from 2 PM




Visit us at

Follow us on Facebook, Twitter, and YouTube at: