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)

                            
000
FXUS66 KMTR 192338
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
438 PM PDT Thu Sep 19 2019

.SYNOPSIS...Seasonably cool conditions will persist today under
mostly sunny skies. 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 02:00 PM PDT Thursday...Water vapor imagery 
indicates an upper trough is centered over western Nevada and is 
moving eastward. The southern portion of the trough extends over 
our region, with cool temperatures aloft and northwesterly flow a 
result of this feature. Dry air and subsidence on the back side of
this trough has resulted in mostly sunny skies with a little bit 
of cumulus development over the terrain and no real marine layer 
to speak of. Temperatures today have been mild, with upper 60s to 
mid 70s observed across the area. Highs will end up being a few 
degrees below seasonal averages, which combined with generally 
light winds and mostly sunny skies is making for a very pleasant 
afternoon. 

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 4:40 PM PDT Thursday...A few afternoon cumulus
clouds over the area. Drier air moves into the low levels tonight
so VFR conditions expected through Friday. 

Vicinity of KSFO...A few cumulus clouds at 3000-4000 ft through
04Z then clear skies expected. West winds 15 to 20 kt with gusts to
25 kt through 04Z. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions are forecast to prevail
through Friday. Could see some localized stratus try to develop 
overnight along the MRY Bay shoreline but confidence of occurrence
is rather low.

&&

.MARINE...as of 04:32 PM PDT Thursday...Moderate and locally gusty
northwest winds winds will continue into the evening hours over
the waters, with the strongest winds  in the outer waters north of
Point Reyes and locally south of  Point Sur along the Big Sur
coast. Winds will then gradually  subside overnight. An 11 second
northwest swell will be the  predominate wave in the waters today,
bringing the potential of  hazardous conditions from squared seas
to portion of the waters. A much lighter southerly swell will also
be mixed in.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Mry Bay until 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             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: ST
AVIATION: W Pi
MARINE: Rowe

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