Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
454 am PDT Sat Oct 22 2016
Synopsis...temperatures will cool off to closer to normal this
weekend as high pressure currently over the West Coast moves to
the east. An unsettled weather pattern is expected to develop
early next week with highest rain chances on Monday and Tuesday
and then again late in the week.
Discussion...as of 3:00 am PDT Saturday...generally a clear start
to the day today thanks in part to a northerly gradient still
around 4 mb. Only exception of note is close to Monterey where
patchy fog has settled in with visibility values down to 1/4 of a
mile. Models do indicate that clouds could form this morning along
the San Mateo coast and wrap into sf Bay, so for now will level
those in place. Outside of that, generally sunny conditions can
be expected today with highs in the mid 60s to mid 70s in most
locations. Saturday night into Sunday morning will likely be
similar to today.
After that the pattern will undergo changes as a longwave trough
moves toward the West Coast as an associated upper level low takes
aim on the pacnw/British Columbia coastline. As that system nears, some light
rain/drizzle may move across the North Bay by Sunday afternoon.
Rain will then be on the increase as a frontal system moves into
our County Warning Area from the north on Monday. Rain will advance to the south
with the main band forecast to be in the sf Bay region Monday
evening/night. Models continue to show it rapidly
dissipating/stalling out by Tuesday meaning higher rain amounts
will be for locations to the north. By Tuesday 1-2" will likely be
across North Bay mountains with 1/2" to 1" for the rest of the
North Bay. Around sf Bay generally 1/4" to 1/2" is forecast. Less
than 1/4" can be expected for the rest of our County Warning Area with many of
those spots getting under 0.05".
A break in the rain can be expected before a second system moves
into our County Warning Area later in the week. Models continue to have difficulty
with the amount of rain that can be expected plus aerial coverage.
Case in point the GFS which has gone from showing widespread rain,
to focusing it over Monterey County and places to the south, to
now focusing over the North Bay. The European model (ecmwf) continues to be very
steady with its solutions and shows widespread rain for Thursday
into Friday although the main moisture tap with precipitable water values up to
1.30" is positioned more over Southern California. The gefs precip
mean also indicates widspread rainfall, although the values are
generally very light.
For the forecast, decided to go with chance/likely pops for our
entire County Warning Area based on the amount of uncertainty. Rainfall amounts
for the second system remain very hard to forecast with solutions
showing a potential for several inches of rainfall. Unsettled
weather may continue through next weekend.
Anyone with plans next Thursday and Friday should closely monitor
the latest forecasts.
Aviation...as of 5:00 am PDT Saturday...patchy fog has
developed along the coast and the North Bay valleys while VFR
conditions prevail across the sfo Bay area this morning with high
cirrus clouds. The fog is patchy and has not stayed over one
particular area for very long so do not expect any impacts on sfo
and the approach zone.
Vicinity of ksfo...VFR.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...patchy LIFR for vsbys down to 1/2 mile
Marine...as of 5:00 am PDT Saturday...high pressure off the
California coast will weaken as a low pressure system approaches
the Pacific northwest. Northwest winds will decrease today and
switch to southerly Sunday. Southerly winds will continue Monday
through Wednesday as a series of lows move across the Pacific
... Pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm until 9 am
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm until 9 am
Public forecast: Bell
aviation: west pi
marine: west pi
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