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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 201146

Area Forecast Discussion
National Weather Service San Francisco Bay Area
446 AM PDT Fri Sep 20 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. A trough of low pressure will move through the region
late next week resulting in a big cooldown.

&& of 03:07 AM PDT Friday...Satellite fog product 
reveals clear skies across the entire Bay Area. Automated sensors 
at Bay Area airports also show clear skies with no fog. 
Synoptically speak - an upper level trough is swing through the 
Great Basin with upper level ridge building in from the west. 
Building high pressure, warming 850 mb temps, and ample sunshine 
will leading to warmer temperatures today. Highs will be in the 
upper 60s to mid 70s near the coast, 70s and 80s Bay shoreline and
interior locations. High pressure will continue to build over the
region into the weekend, but the ridge will flatten slightly 
limiting full potential of warm up. None the less, temperatures 
will increase another 3-5 deg F from Friday to Saturday. Highs 
will be in the 70s at the coast, 70s/80s Bay shoreline, and 80s to
lower 90s interior. Hi-res models indicate a slight increase in 
low level moisture with the building ridge aloft early Saturday 
morning. Therefore, a few patchy low clouds will be possible along
the coast. 

A change in the longwave pattern begins to unfold on Sunday as the
ridge weakens and an upper level trough moves in from the NW. The
lower 500 mb and cooler airmass will lead to a slight drop in
temperatures for the northern half of the Bay Area. Monterey/San
Benito counties will hold steady. By Sunday night the trough
develops into an inside slider type of pattern - generally dry and
in the fall months can trigger hot weather and fire weather 
concerns. It definitely appears that this pattern shift will do
all three: no precip, increase fire weather concerns with
offshore flow, and raise temperatures dramatically. For fire 
weather concerns see Fire Weather below. Temperatures will 
increase a tad on Monday for some locations, but the bigger warm 
up will occur on Tuesday and Wednesday. A 597 DAM high parked off 
the West Coast for Tuesday and Wednesday. To put this into 
perspective the strength of the high for late September is roughly
2-3 std deviations above normal. Temperature anomalies during 
this time period are impressive as well - 850mb temps approaching 
22-23C or about 6 std deviations. Latest forecast point soundings 
for KOAK put 850mb temps close to 22C, which compared to 
climatology it is above the 90 percentile. So what does all of 
this mean for the Bay Area? A late season warm up is forecast with
high temperatures well above normal. Highs on Tuesday and 
Wednesday are forecast to be is forecast to build 70s/80s at the 
coast, 80s Bay shoreline and 90s to lower 100s interior. Thermal 
belts will be in full effect at night as well thanks to warm 
airmass and offshore flow. The hills will remain in the 60s and 
70s at night. However, valley locations (more populated areas) 
will see some minor relief at night. Said relief will limit heat 
risk potential, but latest guidance does put interior locations 
in the moderate risk for heat concerns.

Another pattern shift is then forecast on Thursday with a pretty
substantial drop in temperatures. 5-10 degree temperature drop is
forecast from Wednesday to Thursday as the ridge weakens and a 
trough approaches from the NW. The strength of the low varies from
one model to the next, but all show an decent upper low. Further 
cooling is then expected Friday and into next weekend. Interesting
to note that the pattern aloft goes from anomalously above to 
below in a matter of a few days. If the Euro is to be believed 
not only will it be colder, but possibly wet. 

Stay tuned for this roller coaster ride of weather the next 7-10


.AVIATION...As of 4:50 AM PDT Thursday...Widespread VFR with light
AM winds. Air mass remains slightly too dry for cloud formation
again this morning. Onshore winds increase in the afternoon to 
become locally breeze. Similar setup for tonight into tomorrow
morning but will need to watch advection of mid to upper level
moisture aloft for possibility of tighter dew point spreads/cloud
formation for Friday night into Saturday morning.

Vicinity of KSFO...VFR. Breezy PM winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR.


.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 04:38 AM PDT Friday...Northwest 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.


     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM




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