NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 091805

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1005 AM PST Mon Dec 9 2019

.SYNOPSIS...Dry conditions with mild temperatures are forecast
through Tuesday afternoon. Patchy dense fog is likely during the
night and morning hours. A weak weather system may produce light
rain from late Tuesday into early Wednesday. Dry conditions are
then forecast for most of the second half of the work week before
rain chances increase by Friday night or Saturday.

&& of 9:20 AM PST Monday...Fog continues to impact
much of the San Francisco Bay Area this morning. Visibilities at
major airports such as San Francisco and Oakland dropped down to
as low as a 0.25 mile and 0.125 mile, respectively, earlier this
morning. The San Jose Airport managed to avoid the worst of the
fog impacts with visibilities only dropping down to 4 miles there.
Right now the Dense Fog Advisory for the North Bay Valleys, City
of San Francisco, San Francisco Bay Shoreline, San Mateo Coast,
and East Bay Interior Valleys is set to expire at 10 AM PST. While
dense fog is not expected to be widespread after this time, would
not be surprised if a good number of locations still have low
clouds and even reduced visibilities beyond mid-day. No major
changes have been made to the short-term grids. For additional
forecast details, please refer to the previous discussion section.


.PREV DISCUSSION...As of 3:30 AM PST Monday...Primary short-term
weather concern from this morning through Tuesday is fog. Plenty
of low level moisture from recent rains, in combination with long
December nights and a stable airmass, are producing conditions
favorable for fog formation. Dense night and morning fog is a
potential hazard through at least Tuesday. Dense fog has been
reported in the North and East Bay valleys overnight, as well as
along the east shore of San Francisco Bay. Dense fog has thus far
been patchy and not particularly persistent, so will hold off on
issuance of a Dense Fog Advisory for the time being. However, if
dense fog becomes more widespread by daybreak, an advisory will be
issued. Fog will be a factor during this morning`s commute.

In addition to patchy dense fog, areas of low clouds linger this
morning. A low December sun angle and lack of mixing will result
in slow clearing of low clouds and fog today, and it`s possible
some areas will not clear until sometime in the afternoon.

Another round of fog is likely tonight and into Tuesday morning as
a shortwave ridge over California maintains a stable airmass, and
as low level moisture persists. Temperatures over the next few
days will remain mild, with highs ranging mostly from the upper
50s to lower 60s, except locally cooler where fog persists. Low
temperatures will be mostly in the 40s and lower 50s, except upper
30s in the cooler valleys.

Early morning satellite imagery shows a frontal system well
offshore along 140W. The models agree that this system will split
long before it reaches the coast, with the northern branch of the
split moving northeast towards British Columbia and the southern
branch tracking eastward and reaching the California coast by
Tuesday night. The system is forecast to weaken rapidly as it
pushes onshore on Tuesday evening, but the models generally agree
that it will maintain enough moisture and lift to generate
scattered light rain in our area, particularly in northern
coastal areas. Rain could develop by late Tuesday afternoon along
the North Bay coast, but the best chance of rain will be on
Tuesday evening. A few showers may linger through late Tuesday
night or even into early Wednesday. Rainfall totals from late
Tuesday through early Wednesday are forecast to be under a quarter
inch. The ECMWF is a bit more bullish with rain amounts,
forecasting close to a half inch in northwest Sonoma County.

A broad low amplitude upper ridge is forecast to develop off the
southern California coast by midweek and remain there through
Friday. This ridge should maintain dry and mild conditions for
most of our area during the second half of the work week. However,
to the north of the ridge will exist a moist zonal flow with
embedded weak upper level disturbances which may result in light
rainfall at times in the North Bay. Rain chances will increase by
Friday night or Saturday when the ridge to our southwest weakens
and a shortwave trough tracks across northern and central
California. It looks as though this system will move through
quickly and generate only light rainfall amounts early in the
upcoming weekend.

In the longer range, both the GFS and ECMWF ensemble mean
indicate that a progressive pattern will continue into next week,
bringing additional rain chances by Tuesday or Wednesday of next

&& of 9:50 AM PST Monday...For 18z TAFs. Low clouds
and fog appear well entrained in the valleys across the forecast
area this morning. Anticipate a late scattering out of low clouds,
possibly in the early afternoon hours. Some terminals will likely
seeing little to no clearing at all. Have updated TAFs to reflect
this. Otherwise light winds will prevail. Low clouds forecast to
return this evening with patchy fog expected once again.

Vicinity of KSFO...IFR-LIFR conditions due to combination of low
clouds and fog through at least this morning. It is possible
either or both of these could last into the afternoon. VFR
conditions expected by around 20z-22z before low clouds return
overnight. Generally light winds expected through the period at or
under 10 kt.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Patchy IFR/LIFR conditions this morning
due to fog and low clouds over KSNS. MVFR conditions forecast for
KMRY. Low clouds expected to return this evening. Light southeast
winds will turn onshore by this afternoon. Winds will turn
easterly once again this evening.

&& of 8:51 AM PST Monday...High pressure will build in
from the west into today, keeping winds generally light across the
waters. A weak system will then move through the region early
this week bringing a chance of showers Tuesday night into
Wednesday while winds turn southerly. Moderate northwest swell
will persist through much of the week before a larger and longer
period northwest swell arrives late this week.






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NWS MTR Office Area Forecast Discussion