Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
404 am PST sun Nov 23 2014
Synopsis...an upper level ridge will produce dry and mild weather
across the region through Thanksgiving day. Daytime temperatures
will warm well above normal by Tuesday. Nights will be
cool...especially in the valleys. A weather pattern change is
expected to occur late in the week...which will result in rain
chances by next weekend.
Discussion...as of 3:35 am PST Sunday...surface high pressure
developing over northern California and northern Nevada has
resulted in north to northeast breezes in the hills early this
morning...and locally to near sea level into the North Bay
valleys as well. This northerly flow is bringing in a drier
airmass which has thus far prevented fog from forming in the North Bay
valleys. Where winds remain light...near the Delta and in the East
Bay valleys...patchy dense fog has developed. May need to consider
a dense fog advisory for the East Bay valley zone if dense fog
becomes more widespread prior to sunrise. Northerly flow aloft is
forecast to increase through the remainder of the night and this
may result in improving visibilities if the winds are strong
enough to mix through the nocturnal inversion.
Besides some patchy dense morning fog...today is shaping up to be
a mostly sunny and mild day with highs mostly in the middle 60s. The
low amplitude upper ridge currently centered offshore along 135w
is forecast to amplify and shift eastward over the next few days.
Subsidence under this ridge will warm and dry the airmass over our
region. 850 mb temperatures are forecast to rise from 7-8 degree c today to
15-16 degree c by midweek. In addition...light offshore flow will
persist through at least Tuesday...helping to maintain a dry
airmass over our area. These developments will result in a warming
trend over the next few days with high temperatures reaching into
the upper 60s to middle 70s range by Tuesday...about 10 degrees
warmer than normal. Daytime temperatures will probably reach their
peak on Tuesday and Wednesday.
The combination of clear skies...light winds...and a dry airmass
will mean cool nights. Patchy frost is likely in the coldest
inland valleys of the North Bay...as well as southern interior
Monterey County...late tonight and early Monday morning. Minimum
temperatures will gradually moderate through the week...but inland
valleys will continue to see temperatures dip into the middle and upper 30s
through at least midweek.
Fog should not be much of an issue after this morning...except
perhaps near the Delta.
The medium range models agree that a pattern change will begin to
develop late in the week. The upper ridge will start to break down
and move to the east by Thursday. However...the ridge will hold on
long enough to maintain dry and mild weather through
Thanksgiving...and possible into Friday as well. Daytime temperatures
will begin to cool by Thursday as the ridge weakens and light
onshore flow develops. Rain chances are forecast to begin in the
northern part of our area by late on Friday as a storm system
approaches California from the west. Rain chances will then develop across
most of the rest of the area Friday night into Saturday. Both the
European model (ecmwf) and GFS then bring a second round of heavier precipitation through
our area during the early part of next week as a deep upper trough
with ample moisture arrives along the California coast. This is still
beyond our seven day forecast period and obviously subject to
change. However...the model agreement is at least encouraging.
Aviation...as of 4:00 am PST Sunday...there has been patchy fog
in the East Bay interior as well as in the Salinas valley. Surface
high building into northern California will bring a light north to
northeast flow to the sfo Bay area. The fog is expected to stay
out of the sfo approach zone this morning as the flow has not
quite established itself. Less fog is expected Monday morning as
the airmass dries out even though the north to northeast flow
becomes established so fog will not be an issue for sfo and Oak.
Vicinity of ksfo...moderate to high confidence VFR forecast for ksfo.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...patchy fog around sns through 15z
with visibilities as low as 1/2 mile.
Marine...as of 4:00 am PST Sunday...high pressure off the
California coast extends into northern California and northern
Nevada. This will bring stronger winds to the southern waters
through Monday night. Winds will become light Tuesday and
Wednesday then switch to southeast Thursday as the high moves into
the Great Basin.
... Pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
Public forecast: dykema
aviation/marine: west pi
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