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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 182344

Area Forecast Discussion
National Weather Service San Francisco Bay Area
444 PM PDT Thu Apr 18 2019

.SYNOPSIS...Warmest temperatures of the year today will give way
to a cooling trend through the weekend. Occasional sprinkles are
possible Saturday morning as a weak system limps through the
region followed by a slow warming trend into the middle of next

&& of 02:33 PM PDT Thursday...Bottom line, the 
weather remains clear, dry, and warm for the remainder of today. A
slight cooling trend on Friday into the weekend as a weakening 
system approaches the Bay Area. Another warm up next week with 
possible rain a little over a week from now.

Temps as of 2 pm are generally in the 70s and 80s with South Bay
being one of the warmest areas currently around Campbell and
Saratoga. Even here at the office we are currently at 75 degrees.
This warming is due in part to an upper level ridge over the area
which started the day with light offshore flow. This helped to get
the warm temps started early and now the onshore flow remain weak,
allowing for the solar insolation to continue. The onshore flow
will remain weak through the afternoon and into the early evening
which will keep most areas warm into the evening hours. One item
to watch tonight will be the temperature drop. The forecast
remains the same, but there is an outside chance that we could see
slightly warmer temps overnight if the onshore flow does not kick
in later. Don't be surprised if overnight low remain in the upper
50s over even low 60s in some areas. This won't last long though
as an upper trough approaches the region, pushing the ridge to the
east. This will turn the flow onshore and bring a slight cooling 
to the area for Friday. While 70s will be the norm for Friday, the
80s will be tough to reach for a lot of areas. This upper trough
will also begin to stream some high clouds over the area ahead of
a surface front expected Friday night into Saturday. This frontal
boundary is forecast to move into North Bay Saturday morning and
move toward the SE through the day. Some models continue to hint
as some light rain with frontal passage, but this meteorologist is
still not confident enough to claim any measurable rain in the
forecast. There have been way too many cases we see these late
spring fronts slide into the area only to fall apart on the
doorstep. Instead of going with any measurable qpf, I have opted 
to introduce occasional sprinkles to the forecast. This may 
manifest itself as brief very light rain too. This introduction 
could be a stepping stone toward introducing higher pops and some 
qpf. If the models continue to show widespread light rain, we 
could end up with a few hundredths or even a tenth of an inch in 
favored locations. Again, confidence is not there yet.

Behind this front, look for a decent cooling with Saturday daytime
high struggling to get out of the 60s and overnight lows dipping
into the 40s. However, another ridge is forecast to build in from
the west bringing another bout of warming and drying conditions.
It looks like this ridge axis should be square over the area by
midweek, which means more warm temps and clear skies for midweek.
This second bout of spring warmth could kick it up a notch, if
ridge strength is believable. Again, like today, this will be
short lived as another front is slated for the weekend. Models are
showing some fairly widespread precip for the weekend after this,
but it's too far out to claim with any certainty.

Short of the long, temps are warming, but last gasps of spring are
not done yet.

&& of 4:45 PM PDT Thursday...Skies are clear today as
high pressure moves over the area. The marine layer is very 
shallow and this is verified by the report of 1/2 mile vsby at
Farallon Island. Low clouds will stay out of SFO Bay tonight but
question remains about MRY and SNS. Cannot ignore the fog which is
spreading into Carmel and the tip of the Monterey Peninsula. Kept
the possibility of vsbys down to 1 mile or less at MRY and SNS 
between 11Z and 15Z. The North Bay Valleys could also see some
light fog. 

Vicinity of KSFO...VFR through Friday. West to northwest winds to
15 kt through 04Z. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Possible vsbys reduced to 1 mile or less
between 11Z and 15Z. 

&& of 04:35 PM PDT Thursday...Light to moderate
northwest winds will continue over the coastal waters with high
pressure overhead. Northwest winds will increase on Friday and
continue building over the weekend as low pressure  moves inland
and high pressure strengthens offshore.






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