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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 202043

Area Forecast Discussion
National Weather Service San Francisco Bay Area
143 PM PDT Fri Sep 20 2019

.SYNOPSIS...A warming trend will 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. A trough of low
pressure will move through the region late next week resulting in
significantly cooler temperatures.

&& of 1:43 AM PDT Friday...Under sunny skies,
afternoon temperatures have warmed nicely with 70s and 80s across
the board. At 81 degrees, Santa Rosa is 10 degrees warmer than
yesterday at this time. Additional warming is expected on Saturday
as high pressure aloft moves over the state. Max temperatures are
expected to increase another 3-5 degrees with highs ranging from 
the 70s at the coast, 70s and 80s around the Bays, to the 80s to 
lower 90s interior areas.

On Sunday, the weather pattern will begin to change when an upper
level low drops south from the PacNW into eastern California and 
western Nevada. This will bring cooler temps to the area on
Sunday, with increased humidities. On Monday the surface high will
nose into the northern Great Basin creating an offshore flow
pattern across northern California. Current model runs indicate
slightly less of a pressure gradient developing, however, offshore
winds are still indicated across the North Bay Mountains and East
Bay Hills late Monday through Tuesday morning, and again Tuesday
night. High temperatures will warm both Monday and Tuesday, with
Tuesday expected to be the warmest day, with warmest inland max
temps approaching 100 degrees, and near-coastal sites possibly
approaching the lower 80s.

Another significant pattern shift is then forecast to begin  
midweek bringing with it a substantial drop in temperatures. 5-10
degree temperature drop is forecast from Wednesday to Thursday as
the ridge weakens and an upper level trough approaches from the 
PacNW. Additional cooling is then expected Friday and into next 
weekend along with the possibility of precipitation.


.AVIATION...As of 10:51 AM PDT Friday...For 18z TAFs. VFR will 
continue for the terminals through the TAF period. Air mass is dry
with no cloud development at the terminals. Onshore winds will 
increase this afternoon and become locally breezy. For tonight, 
light offshore flow will develop at several of the terminals. This
should continue to advect dry air into the area with VFR 
prevailing through Saturday morning. 

Vicinity of KSFO...VFR. Onshore winds becoming breezy 21-22z with
speeds around 15 kt. Light and variable winds overnight tonight. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Onshore winds this afternoon with
speeds around 10kt. Light east to southeast winds developing


.FIRE of 02:55 AM PDT Friday...A quick look at Energy
Release Component (ERC) and 100 hour fuel values are both well
below normal for the region. The below normal readings are likely
in response to the recent rain and cool weather. A warming and 
drying trend is on track to develop into the weekend. There is
even a brief period of offshore flow Friday night across the North
and East Bay Hills. Winds will be breezy with moderate to
possibly poor humidity recoveries Friday night. Not strong enough
or long enough to warrant any Red Flag warnings. What it may do 
is help chip away at the low ERCs and high 100 hour fuels. The 
inside slider mentioned above will bring a more pronounced 
offshore flow event Monday into Tuesday with poor overnight 
humidity recovery and gusty winds in the hills. One potential 
limiting factor is the actual offshore flow gradient. Models put 
the WMC-SFO gradient at 7-8 mb, decent but not strong. If this 
pattern holds true a Fire Weather Watch may be in order over the 
weekend. Climatologically this is the favored time of year for 
offshore flow events and Red Flag warnings for Wind/RH in the Bay
Area. Taking a look at the bigger picture - far NorCal has had a 
few wetting rain events so fire season may be near the end to the 
north. SoCal - not so much and fire season will keep on going. 
What about the Bay Area? Fuels have been below normal for much of 
the summer and recent cool/wet weather has kept them that way. 
That being said, the recent wet weather wasn't wet enough to end 
fire season. Therefore, we're not out of the woods yet and we 
should remain weather/fire aware through the Fall season.

&& of 8:50 AM PDT Friday...Northwest winds ease today, 
becoming generally light along the coast north of Point Sur and 
locally moderate in the outer waters and along the Big Sur coast. 
For Saturday, generally light winds across the waters except 
locally moderate in the outer waters north of Point Reyes. Winds 
increase somewhat into Sunday. A 9 to 11 second northwest swell 
will be the predominate wave in the waters today, with locally 
hazardous steep fresh swell. A lighter long period southerly swell
will also be mixed in.


     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM




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