Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service San Francisco Bay area
1020 PM PDT sun Apr 20 2014
Synopsis...dry weather will continue through Monday...but cooler
weather is expected as onshore flow strengthens. A weak storm
system will impact the area Monday night into Tuesday...bringing a
chance of light rain and cooler temperatures. Dry weather and
near normal temperatures return Wednesday before rain chances
return at the end of week.
Discussion...as of 9:00 PM PDT Sunday...temperatures moved
sharply higher today in response to strong subsidence under an
upper ridge and reduced onshore flow. Afternoon highs were
mostly in the 70s and 80s. Only areas along the immediate coast
remained in the 60s. Highs today were as much as 15 degrees warmer
than yesterday. Today will be the warmest day of the week.
Skies remain mostly clear this evening with virtually no coastal
stratus. Onshore pressure gradients are beginning to increase and
The Fort Ord profiler shows that a marine inversion has recently
developed at about 1000 feet. So...look for areas of coastal low
clouds and fog to develop overnight and move locally inland late.
In addition...high clouds will increase as a Pacific weather
system approaches. The models continue to forecast only scattered
light rainfall from this incoming system...mainly on Monday night
when the upper trough axis approaches the coast. Based on latest
NAM...Euro...and Canadian models...shower activity may make it as
far south as the Big Sur coast on Monday night. Therefore...a
forecast update earlier this evening extended rain chances farther
to the south on Monday night. Indications are that rain chances
will quickly diminish on Tuesday morning as the upper trough axis
moves inland. Rain totals with this system still look to be around
a tenth of an inch...at most...in the North Bay with just a few
hundredths or less elsewhere.
Cooling will begin on Monday as onshore flow strengthens in
advance of the incoming system...but southern interior sections of
our area will remain relatively warm still. More significant and
widespread cooling will occur by Tuesday as a much cooler airmass
sweeps across our area. In addition...Tuesday will be a blustery
day...especially near the coast...as brisk northwest winds
develop. Gradual warming is forecast for Wednesday and Thursday as
a flat upper ridge develops over central and Southern California.
The medium range models have definitely been trending faster with
the system at the end of the week. The GFS...ECMWF...and Gem
models all spread rain across most of our district on Thursday
night and Friday. But rainfall intensity varies...with the GFS being
the wettest and the European model (ecmwf) the driest. There is also disagreement
regarding a second system that comes in over the weekend. The
European model (ecmwf) sweeps light amounts of rain across most of our area on
Saturday night...while the GFS and Canadian models confine warm
advection rain mainly to the North Bay on Sunday before building
in an upper ridge. Therefore...considerable uncertainty exists
late in the extended forecast period...although there does appear
to be a good chance that most of our area will get wet with that
first system on Friday.
Aviation...as of 10:15 PM PDT Sunday...clear skies over the area
this evening. Inversion layer will allow patchy low clouds to form
along the coast late tonight which may impact the mry Bay.
Vicinity of ksfo...VFR.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...IFR ceilings after 12z.
Marine...as of 4:30 PM PDT Sunday...large northwest swell has arrived
with buoys reporting waves as high as 15 to 16 feet with 15-16
second interval. Waves will slowly decrease tonight with periods
around 13 seconds. A high surf advisory is in effect until 5 am PDT
... Surf advisory...entire coast from Sonoma County
south to Monterey County until 5 am
Small Craft Advisory...rough bar advisory for sf bar until 5 am
Public forecast: dykema
aviation/marine: west pi
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