Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 191123
Area Forecast Discussion
National Weather Service San Francisco Bay Area
323 AM PST Thu Jan 19 2017
.SYNOPSIS...Scattered showers can be expected today before the next
front arrives tonight into Friday with another round of wind and
rain. Lingering showers Saturday before the final front brings
more wind and rain Sunday before tapering to showers Sunday night
into Monday and possibly Tuesday.
.DISCUSSION...as of 3:20 AM PST Thursday...Widespread rainfall
yesterday afternoon and evening along with gusty winds both
diminished during the overnight hours. Rainfall totals ranged from
less than half and inch for southern portions of our CWA all of
the way to greater than 6 inches near Venado in Sonoma County. In
general the forecast did fairly well for both rain and wind.
KMUX radar continues to indicated scattered showers especially for
the southern third of our CWA and that will be the main story for
all spots as we will be between systems. Generally less than
1/10" will fall from sunrise to sunset although favored coastal
ranges could see locally over 1/4". The "break" in the weather
will be short as round two moves into the picture starting this
evening. Models bring the leading edge of precip first to the
North Bay then down through our entire region during the overnight
hours. Rain will likely start out light but quickly become
moderate or possibly heavy as the main front approaches. At the
same time, instability will increase leading to the slight chance
of some embedded thunderstorms (which would help to increase rain
rates). Rain will then switch back over to post- frontal showers
Based on the timing and amount of moisture, this second system is
not expected to produce as much rainfall as the one we had
yesterday. However, still am expecting 1/2" to 1" for most urban
spots with 1 to 2.5" for most coastal ranges. Even with the
smaller rain amounts, local rivers and streams will be quick to
respond due to multiple previous systems over the past week.
Southerly winds are also not forecast to be as strong as they were
yesterday. Generally speeds of 15 to 25 mph with local gusts to 40
mph are expected.
Showers the remainder of Friday will decrease Friday night into
Saturday before the third system moves to our region late Saturday
into Sunday. Rain will likely be moderate to heavy at times with
totals forecast to be close to the first round of rain yesterday.
In addition, IVT guidance suggests a longer term event compared to
the one that will go through tonight with the highest values from
Santa Cruz County southward. Southerly winds are also expected to
be close to the numbers we saw yesterday. Based on current
guidance, wind advisories and either a flood advisory or flash
flood watch appears likely as the event gets closer.
Rain will again switch back to showers for the start of next work
week with precipitation possibly into Tuesday. ECMWF had
indicated an impressive slug of moisture going through on Monday,
however it has backed away from that solution. Now it and the GFS
depict mostly scattered precipitation with drier weather by late
Longer range guidance from CPC going into February favors drier
than normal conditions.
.AVIATION...As of 10:06 PM PST Wednesday...The cold front that
brought widespread rain to the region this evening will move out
of the area overnight leaving in its wake a few lingering showers
and lighter winds. Clouds will scatter out behind the front with
mainly VFR cigs expected into the morning hours. As we transition
to the next system, that is expected to arrive late Thursday night
into Friday morning, scattered showers can not be ruled out.
Therefore I have maintained VCSH to -SHRA in the TAFs for
Vicinity of KSFO....VFR cigs will prevail through tonight.
Vicinity showers will persist through Thursday. Light south winds
around 10 kt through tonight. Winds will strengthen slightly and
veer to the southwest Thursday afternoon. Winds will switch back
to the south ahead of the next system with Thursday night.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR cigs will prevail through tonight.
Vicinity showers will persist through Thursday. Southeast winds
of 10 to 20 kt with gusts to around 28 kt expected to ease to
around 10 kt after 09z-10z tonight. Winds will temporarily veer
to the southwest Thursday afternoon. Winds will switch back to the
south ahead of the next system with Thursday night.
.MARINE...as of 02:41 AM PST Thursday...Winds will increase
tonight and friday as a frontal system moves through the area.
winds will increase more saturday night into sunday as another
system approaches. very large swells arrive friday night with
swell heights building to 25 to 28 feet friday night and saturday.
.Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Rough Bar Advisory for SF Bar
PUBLIC FORECAST: Bell
MARINE: W Pi
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