Scientific Forecaster Discussion
fxus66 kmtr 200545
Area forecast discussion
National Weather Service San Francisco Bay area
945 PM PST Fri Jan 19 2018
Synopsis...isolated showers will continue for the remainder of
the evening, mainly in southern portions of our region. Otherwise
clearing skies and cool temperatures are expected overnight. Dry
and cool weather is forecast for Saturday. The next system is
expected to spread rain across primarily the northern portion of
the forecast area from late Sunday through Sunday night. Rain
could be heavy at times in the North Bay. The weather pattern
will remain active into next week, with more rain likely by
Discussion...as of 9:00 PM PST Friday...kmux radar currently
shows scattered showers continuing across our southern coastal
waters as well as isolated showers over central Monterey County.
Otherwise dry weather prevails along with clearing skies. This is
consistent with short-term model data which generally indicate dry
conditions for the remainder of tonight. A forecast update was
completed earlier in the evening to reduce rain chances overnight.
The airmass that has moved into our area behind last night's cold
front is the coldest air our region has seen since the first of
the year. This cold airmass combined with clearing skies will
mean chilly overnight lows, particularly in the interior valleys
where temperatures are expected to drop to near freezing by
Saturday is expected to be a dry and cool day under mostly sunny
The next system, currently seen on infrared satellite imagery in the
Gulf of Alaska, is expected to bring renewed rainfall to our
forecast area by late in the weekend. The models agree that a
frontal boundary will drop south into northern California on
Sunday, with rainfall projected to develop in the North Bay on
Sunday afternoon. The frontal boundary is then expected to sag
slowly south of the Golden Gate by late Sunday night and then
dissipate as it reaches the Monterey Bay area by Monday afternoon.
Both the 00z NAM and 00z GFS indicate strong warm advection
across the North Bay coupled with a slow moving frontal boundary
from late Sunday afternoon through much of Sunday night. Rainfall
in the North Bay during this time period could be heavy at times.
Both the 00z NAM and 00z GFS forecast as much as 2.5 inches of
rain across northwest Sonoma County by late Sunday night with up
to 1.5 inches across the North Bay burn scars. This raises at
least slight concerns about the potential for debris flows/flash
flooding over and near the North Bay burn scars. If models
continue to trend wetter, we will need to consider issuing a
Flash Flood Watch for the North Bay burn scar areas for late
Sunday and Sunday night.
Precipitation totals south of the Golden Gate with this upcoming
system are projected to be much less compared to the North Bay,
with forecast amounts of less than a half inch. Rain should end in
most areas by Monday afternoon. The evening forecast update
included a reduction in rain chances for the southern portion of
our area Sunday night and Monday.
In addition to potential heavy rain in the North Bay, this system
may generate locally strong and gusty south winds late Sunday
afternoon and evening, particularly in coastal areas from about
Half Moon Bay northward where gusts of up to 40 mph are possible.
Another storm system is projected to impact our area by the
middle of next week. Rain may develop in the North Bay as early as
Tuesday as a warm front moves through, but the bulk of the
precipitation is expected sometime between Wednesday and Wednesday
night as a strong cold front moves through. Unlike the Sunday
night system, the midweek system is forecast to dig south through
central California and bring widespread rain to our entire
Aviation...as of 9:45 PM PST Friday for 06z Saturday tafs. VFR
conditions are generally expected to prevail at most terminals
through the period. There remains some uncertainty with morning
fog potential for ksts as we now have a cooler, drier airmass in
place across the region. We'll continue to monitor and amend if
Vicinity of ksfo...VFR conditions are expected to prevail.
West winds around 5 to 10 kt overnight will increase Saturday
afternoon to around 15 kt.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions are generally expected to
prevail at kmry and ksns, though, can't rule out the possibility
of some low clouds around kmry.
Marine...as of 8:01 PM PST Friday...swells will continue to
gradually decrease throughout the coastal waters overnight.
Despite the decreasing swells, increasing winds will maintain the
hazardous seas. Scattered shower activity will continue to
... Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar
Small Craft Advisory...mry Bay
Public forecast: dykema
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