Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
846 am PST Thursday Dec 18 2014
Synopsis...a few light showers will linger into the morning...
otherwise the area will see a brief lull in precipitation today.
The next system to bring rain back to the area will move in late
this evening through Friday. After some residual showers on
Saturday...a ridge of high pressure will build over the West Coast
resulting in a drying trend through early next week.
Discussion...as of 8:45 am PST Thursday...radar continues to show
a few echos this morning and across the County Warning Area several locations have
picked up light amounts of rainfall over the past few hours. The
shower activity will remain spotty through the day although we
could see a few locations getting close to a tenth. Rain will be on
the increase tonight into Friday as another front approaches our
area and then moves northwest to southeast through the region. Although this
will be a far cry from the larger systems we have had
recently...it could still bring a third to two thirds of an inch
to urban locations by Friday evening with more than an inch over
the North Bay hills.
Biggest concern right now will be Friday morning into the early
afternoon hours associated with the front going across. Could see a
brief period of moderate rainfall at that time with a few Hydro
issues not out of the question. Winds look to be light for lower
elevation spots with speeds of 15 to 25 miles per hour above 1000 feet.
Previous discussion...residual light showers are still occurring
with weak radar echoes moving from west to east across portions of
the district with the stronger echoes now well east of our area.
These are expected to diminish this morning with a break in the
rainy weather expected this afternoon as high pressure briefly
builds over the area.
This break will be short-lived...however...as the next system is due to
arrive this evening with light warm frontal rains spreading into
the North Bay after sunset. The main frontal band is not expected to
move through until Friday morning with brief heavier rain possible
over the North Bay as it moves through...with lighter rain south.
Residual showers are expected to continue into Saturday morning.
Rainfall amounts of up to an inch in the North Bay hills are
expected with a half inch or less expected elsewhere.
High pressure is then prognosticated to build along the coast beginning
on Saturday. Some overrunning moisture could bring a few showers to
the North Bay late Saturday...otherwise a drying trend is
indicated for the district through at least early next week. There
are differences amongst the medium range model solutions with the
GFS offering a dry solution through the week...while both the European model (ecmwf)
and Gem bring in an inside slider type trough affecting northern
California late Wednesday into Thursday with cooler temperatures
and possible precipitation. It should be noted...however...that both
the 6-10 day and 8-14 day outlooks from the climate prediction
center keep California under an above normal temperature and below
normal precipitation pattern through December 23-27 and 25-31
Aviation...as of 3:35 am PST Thursday...a few lingering showers
are still impacting terminals...but those showers will taper off
early this morning. Otherwise...MVFR ceilings will be the story this
morning with southerly flow. Ceilings will lift this afternoon ahead
of the next system. Warm frontal rain will overspread the area
tonight...first North Bay then spreading south. Wet runways see likely
Friday morning for ksjc northward.
Vicinity of ksfo...mainly MVFR ceilings through this afternoon. Brief
period of VFR ceilings this afternoon...but MVFR conditions return overnight
with lowering ceilings and -ra by 08z. Winds southeast and light to moderate.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings with a lingering vcsh.
Ceilings lift this afternoon...but re-develop tonight as the next
system comes in.
Marine...as of 3:35 am PST Thursday...large northwest swell to
impact the coastal water starting Friday afternoon. Latest model
guidance puts a forerunner wave of near 20 second as early at 18z
Friday for northern waters. The period will diminish...but the
swell will increase 16 to 19 feet with a period of 18 seconds.
Therefore...hazardous seas will be possible along the coastal
waters. The large swell will also impact the beaches...especially
north of Point Reyes. A high surf advisory has been issued for surf
15-18 feet with a few breakers over 20 feet. Largest surf north of
Public forecast: Bell/Sims
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