Scientific Forecaster Discussion
fxus66 kmtr 101800
Area forecast discussion
National Weather Service San Francisco Bay area
1000 am PST Tue Dec 10 2019
Synopsis...dry conditions with mild temperatures are forecast
through most of today. A weak weather system will produce light
rain from late today into early Wednesday, mainly in the San
Francisco Bay area. 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 on Saturday with dry weather returning Sunday and Monday.
Discussion...as of 09:00 am PST Tuesday...stratus persists this
morning across the Bay area as the boundary layer remains moist
after the weekend's rainfall. High clouds last night likely
prevented the stratus from lowering to fog. Today, stratus should
thin and dissipate as the heating of the day gets underway. A
brief period of sunshine should develop in many places late this
morning or early this afternoon before a system spreads high
clouds over the area. Light rain should begin late this afternoon
across the North Bay and spread south to the greater Bay area
this evening. Models weaken the trough as it approaches the
California coast with further weakening as it moves south and
east. As such, only light rainfall is expected across the area
with very little rainfall south of the Santa Cruz Mountains.
Scattered showers will persist overnight and end by early
Wednesday morning. The North Bay looks to receive the most rain
with amounts locally up to a half inch, and around a tenth of an
inch in the greater Bay area and East Bay. Winds will also remain
relatively light, so overall impacts will be minimal.
Highs today will range from the mid 50s to lower 60s. A slight
warming trend will develop on Wednesday and Thursday, with most
locations seeing highs in the 60s. Overnight lows will remain mild
with most sites in the upper 40s to low 50s. A moist zonal flow
will develop behind today's system, with light rain again possible
on Thursday and Friday. The North Bay once again looks to see the
highest chance of rain but some models bring showers into the Bay
area. The current forecast remains on track, and no updates this
morning. For additional details, see the previous discussion.
Previous discussion...as of 3:10 am PST Tuesday...a band of thick high
clouds has spread over our forecast area overnight, reducing
outgoing longwave radiation. This is not only helping to keep
overnight temperatures somewhat elevated, but also inhibiting fog
formation. Surface relative humidity values are anywhere from 5 to
15 percent drier than 24 hours ago and fog is currently absent in
most areas. Present conditions are much different than early
yesterday morning when areas of dense fog had formed. High clouds
are expected to persist for the remainder of the night and so any
fog that does develop by daybreak is likely to remain patchy.
Less low level moisture this morning will mean earlier clearing of
patchy low clouds today. Also, high clouds are expected to
diminish by this afternoon. More sun today should mean slightly
warmer temperatures than yesterday.
Early morning satellite imagery shows a system approaching from
the west out near 130w. Although this system is relatively weak,
it has tapped into a relatively moist airmass from the southwest
with precipitable water values as high as 1.4 inches. So, although
models indicate weakening as the system moves inland over
California, there should be sufficient moisture to produce at
least light rain across much of the sf Bay area from late this
afternoon through tonight. A few showers may linger into
Wednesday morning. Rain totals through Wednesday morning are
forecast to be less than a quarter inch, except locally up to a
third of an inch in the coastal hills of Sonoma County. Winds are
not expected to be a factor with this system, with wind gusts of
only about 20 mph forecast along the coast overnight.
Once the incoming system passes tonight, the models agree that a
moist zonal flow will develop across the eastern Pacific and into
the Pacific northwest and far northern California. Weak impulses
moving through this moist zonal flow may generate light rain as
far south as the North Bay from time to time from late Wednesday
through Friday night. A shortwave trough moving in from the
northwest late in the week may then generate more widespread
rainfall across our area on Saturday. However, the models have
been backing off on precipitation potential on Saturday, and where
rain does fall, amounts are expected to be light. Rainfall totals
for the entire week, from late today through Saturday, are
forecast to be a quarter of an inch of less from San Francisco
southward...and from 0.25 to 1" in the North Bay. Onshore flow
through the week will maintain seasonable daytime temperatures and
mild nighttime temps.
Dry weather is expected to return to the entire forecast area on
Sunday and Monday. Longer range models maintain a progressive
pattern into next week, with at least two systems potentially
impacting our area. The systems next week, while not currently
expected to be overly strong or wet, at least appear likely to
generate more widespread and significant rainfall than what is
forecast this week.
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.
Public forecast: St/dykema
Visit US at www.Weather.Gov/sanfrancisco