Scientific Forecaster Discussion
fxus66 kmtr 150606
Area forecast discussion
National Weather Service San Francisco Bay area
1006 PM PST Fri Dec 14 2018
Synopsis...a weak frontal system will remain over the San
Francisco Bay area through Saturday, producing scattered light
showers, mainly over the North Bay. A stronger and wetter system
is forecast to impact the entire region Sunday and Sunday night,
with widespread moderate rain. Brief heavy rain is possible Sunday
afternoon and evening. In addition, gusty south winds are likely
Sunday and Sunday evening, mainly near the coast and in the hills.
Showers will taper off by late Sunday night and end by midday
Monday. A very large wave train arrives late Sunday into Monday.
Dry and mild weather is forecast from Monday afternoon through
much of next week.
Discussion...as of 9:15 PM PST Friday...today's weak frontal
system produced nearly a half inch of rain in portions of the
North Bay, but rainfall amounts were generally less than a tenth
of an inch for locations south of the Golden Gate. The front that
produced today's light rainfall has stalled over the San Francisco
Bay area this evening, and is forecast to drift slowly northward
tonight and Saturday. Scattered light showers are forecast to
continue along the frontal boundary, from about San Mateo County
northward tonight and Saturday morning and then primarily over the
North Bay on Saturday afternoon and evening. For areas from Santa
Cruz and Santa Clara counties southward, the first half of the
weekend is expected to be mostly dry with mild temperatures.
Current satellite imagery shows a robust storm system dominating
the northeastern Pacific. The models have shown good agreement and
consistency regarding this system, and so forecast confidence is
relatively high that moderate to locally heavy rain will fall
across most, if not all, of our forecast area Sunday and Sunday
night. Widespread light rain is forecast to begin spreading into
the North Bay by late Saturday night and then increase in
intensity Sunday morning. It's likely that most precipitation will
remain confined to the North Bay through Sunday morning. Then, on
Sunday afternoon and evening, a vigorous upper trough will
approach the coast and push the front to the south and east.
The upper trough will also likely enhance the surface front and
so Sunday afternoon and evening is when forecast intensity is
expected to peak, with at least a brief period of heavy rain
possible as the front sweeps through. The Storm Prediction Center
indicates a general threat of thunderstorms across our area late
Sunday as colder air aloft arrives with the upper trough.
Thunderstorm chances are not currently in our forecast, but we
may need to consider adding them. It should be noted that the 00z
NAM shows a trend towards decreased instability from previous
model runs, which would suggest a lower potential for
Post-frontal shower activity is forecast to continue through much
of Sunday night, but then taper off quickly from north to south on
Monday morning and end by midday as a shortwave ridge quickly
build across the region.
Rainfall totals with the Sunday/Sunday night system are forecast
to range from 1 to 3 inches in the North Bay and from 0.5-1.5
inches across the rest of the forecast area, except locally up to
2 inches in the Santa Cruz and Santa Lucia mountains.
Gusty southerly winds will develop ahead of the incoming system on
Sunday and continue quite strong and gusty through Sunday evening.
Strongest winds will be near the coast and in the hills where
local gusts of up to 40 mph are expected. Winds are not expected
to be strong enough to warrant wind advisories.
Longer range models agree that an upper ridge will maintain dry
and mild weather conditions across our region from Monday
afternoon through at least Thursday of next week.
From previous discussion...in addition to the rain and wind, the
incoming storm system will be responsible for bringing the
largest waves so far this season to the coast from late Sunday
through Monday. A high surf warning has been issued and is in
effect from 9am Sunday through 9pm Monday. The earlier start time
accounts for the arrival of the very long forerunner waves which
present a significantly increased risk of rip currents and sneaker
waves. Please review the beaches section for more information
about the upcoming high surf event.
Aviation....as of 10:00 PM PST Friday...VFR conditions prevail
at all terminals except ksts, which is presently directly under
the residual cloud band and scattered light showers associated
with the stalled weak frontal boundary. Little change is expected
through the night and into Saturday morning, except at ksfo and
koak where ceilings and/or visibilities look likely to dip into
the MVFR range beginning after midnight. Winds will continue
generally light and southeasterly.
Vicinity of ksfo...visibilities and then ceilings considered
likely to reduce into the MVFR range during the overnight and
morning hours. Light south-southeasterly winds. Moderate
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR conditions expected to continue to prevail
through the night and on Saturday. Light southeasterly winds,
increasing a bit during the morning hours in the northern Salinas
valley. Moderate confidence.
Beaches...as of 03:09 PM PST Friday...a potent storm system
passing south of Aleutian Islands will move nearly due east over
the coming days. This eastward trajectory will create a dynamic
fetch zone where the strongest winds of the storm system will
continuously increase the energy within a swell train on the
southern flank of the storm, resulting in a very large, long
period west-northwest wave train aimed at the California coast. Very long
period forerunners will arrive through the day Sunday and will
bring a significantly increased risk of rip currents and sneakers
waves to the coast. The largest waves are then forecast to arrive
Sunday night through Monday morning, with peak swells of 17 to 21
ft at 19 to 21 seconds currently expected. Large breaking waves of
25 to 40 ft will be possible at west and northwest facing
locations, with breakers up to or exceeding 50 ft at favored break
points along the coast. As a result, a high surf warning has been
issued from 9 am Sunday through 9 PM Monday along the entire
Marine...as of 08:51 PM PST Friday...a moderate west to northwest
swell will build tonight and result in hazardous conditions through
Saturday. Generally light winds will prevail tonight as a weak
frontal boundary remains stalled over the region. South to
southeasterly winds will then increase Saturday into Sunday ahead
of the next and stronger approaching Pacific storm system. An even
larger and more powerful long period west to northwest swell will
move in Sunday afternoon through Monday.
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt 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...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: dykema/drp
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