Scientific Forecaster Discussion
fxus66 kmtr 102156
Area forecast discussion
National Weather Service San Francisco Bay area
156 PM PST Tue Dec 10 2019
Synopsis...a weak weather system will produce light rain from
late today into early Wednesday, mainly in the San Francisco Bay
area and North Bay. A moist zonal flow will keep a chance of
light rain in the forecast for much of the week, with the best
chances across the North Bay. A system will bring another chance
of rain and cooler temperatures on Saturday with dry weather
returning Sunday and Monday.
Discussion...as of 01:55 PM PST Tuesday...a weak storm system is
approaching the California coast this afternoon and will bring
light rain to the area tonight mainly north of Monterey Bay.
Temperatures this afternoon across the southern portion of the
forecast area have been mild in the upper 50s to middle 60s, where
skies have been sunny since early this morning. Low stratus and
now high clouds have kept temperatures cooler around sf Bay and
the North Bay with temperatures in the middle to upper 50s. Clouds
will continue to increase this afternoon ahead of the system.
Kmux radar has begun to detect a band of rainfall over the
coastal waters. Light rain will move into the North Bay this
afternoon and spread south to the Golden Gate and greater Bay area
this evening. The trough associated with the rain is weakening
and will move through quickly. A brief period of steady light rain
will become more showery overnight tonight and taper off by
Wednesday morning. Rainfall totals will be light, with 0.25 to 0.5
inches across the North Bay (locally higher in the mountains),
0.1 to 0.25 across the Bay area, and only a few hundredths of an
inch south of the Santa Cruz Mountains.
A moist zonal flow will develop Wednesday through the end of the
week. A subtle disturbance in the flow and warm advection will
bring a chance of rain mainly north of the Golden Gate on
Wednesday night and Thursday. Moist onshore flow will continue
into Friday with the best chance of light rain continuing across
the North Bay. Rainfall through the end of the week will be light
with minimal impacts. A slightly warmer airmass will also arrive
with highs on Thursday in the lower to middle 60s. The moist flow
will also provide for mild overnight lows, mainly in the upper 40s
to lower 50s. The moist zonal flow will bring mostly cloudy skies
through the week.
A shortwave trough will then move through northern California on
Saturday. Currently, most of the precipitation with this system is
forecast to remain to our north. The trough will bring a cooler
airmass to the region on Saturday with highs dropping back into
the 50s. A cool, drier northwesterly flow will develop on Sunday
followed by upper level ridging early next week. A progressive
pattern still looks to persist into next week, but the Euro
ensemble has trended towards less amplified troughing.
Aviation...as of 10:00 am PST Tuesday...for 18z tafs.
Challenging aviation forecast as high clouds are masking the near-
surface stratus decks on satellite imagery from time-to-time.
Latest taf package in the short-term is based on present satellite
trends... terminals are ranging anywhere from VFR to IFR
conditions. Confidence is low to moderate for sites that are
reporting mfvr/IFR ceilings. A weak system will approach the Bay
area late afternoon/overnight and bring generally light rain to
terminals. Winds are forecast to remain light.
Vicinity of ksfo...difficult forecast for sfo as high clouds have
periodically blocked satellite view of the stratus. Latest
satellite trends show stratus developing and surging north from
the terminal toward the city. Confidence is low in the short-term
as to how this stratus deck will behave. With the low December
sun angle, it may take several more hours for the terminal to
transition to scattered. Light easterly winds will likely prevail
into the afternoon hours, becoming light out of the southeast
overnight. Chance of light rain enters the picture after sunset as
a weak system approaches the region.
Sfo bridge approach...southern half of the San Francisco Bay is
largely clear according to the most recent visible satellite
trends. Stratus is located just to the north of the San Mateo bridge
and extends north all the way to the terminal. Expect gradual clearing
over the next several hours, but the low December sun angle could
slow down the stratus mix-out. Otherwise, similar to the terminal.
Monterey Bay terminals...VFR conditions are expected through much of the
day. Southeast winds around 10-15 kt will continue through midday at
sns with lighter winds at mry. Possible return of the low clouds
late afternoon or evening.
Marine...as of 08:10 am PST Tuesday...a weak cold front will
turn winds southerly today and bring a chance of showers this
evening and into Wednesday. Light to moderate southerly winds will
then continue through much of the week before shifting back out
of the northwest towards the weekend. Moderate northwest swell
will also persist before a longer period northwest swell arrives
on Thursday. Wave heights will gradually build heading into the
coming weekend creating hazardous seas conditions, particularly
for smaller vessels.
... Arena to Pigeon pt 10-60 nm until 3 am
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 9 PM until 3 am
Public forecast: St
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