Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS66 KMTR 111149
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
349 AM PST Sun Dec 11 2016

.SYNOPSIS...Dry weather is forecast through Monday along with
partly cloudy skies and seasonably cool temperatures. Rain is
forecast to return to the northern part of the region by Tuesday
and to all areas by Thursday. Local heavy rain is possible,
especially on Thursday, along with locally strong and gusty
winds.

&&

.DISCUSSION...As of 3:20 AM PST Sunday...The frontal boundary
that brought soaking rains to much of our region yesterday has
moved to the south of our area and has dissipated. Drier
northwesterly flow has developed across our region, resulting in
partial clearing. Clearing in the North Bay has allowed for
sufficient low level cooling for fog to form in the valleys, with
visibilities dropping as low as 1/4 mile at both the Sonoma and
Napa County Airports overnight. Expect patchy dense fog to
persist until mid morning, mainly in the North Bay Valleys.

Dry and quiet weather is forecast through Monday. The plume of
high precipitable water that extends from near Hawaii to the
California coast has shifted to our south temporarily. But high
clouds associated with this moist flow will stream across our
region over the next two days. 

By Tuesday the moisture plume will lift back to the north and once
again be aligned over our forecast area. At the same time, an
upper trough, currently in the Gulf of Alaska, will be dropping
slowly south offshore. This trough will interact with the
moisture plume and the result will be the formation of a frontal
boundary and renewed precipitation over the Bay Area on Tuesday.
The models have been having a rather difficult time deciding
exactly where the frontal boundary and associated band of rainfall
will be located from Tuesday through Wednesday. But the current
model consensus suggests the rain band will be centered over the
central portion of the San Francisco Bay Area on Tuesday and then
lift northward into the North Bay by Wednesday as the trough
deepens offshore. The boundary is then forecast to shift to the
southeast and intensify Wednesday Night and Thursday as the upper
trough finally moves eastward. Rain rates are expected to mostly
be light to moderate through Wednesday but may become heavy on
Thursday. Thursday looks to be the day of greatest concern for
hydro issues as it will be the third consecutive day of rain in
some areas and also likely the time period of most intense
dynamics and heaviest rainfall. In addition, locally strong and
gusty southerly winds are possible, especially on Wednesday Night
and Thursday. Will issue a hazardous weather outlook to highlight
the upcoming stormy weather this week, drawing attention to
possible hydro and wind issues.

Precipitation is expected to mostly end by Friday as a dry and
cool northwest flow develops behind the departing system. Current
model output indicates mostly dry weather next weekend. Nights
are expected to be cool late in the week and into next weekend
with patchy frost possible.

&&

.AVIATION...As of 3:30 AM PST Sunday...Biggest concern over the
next 6 hours will be patchy fog developing, particularly in
valley areas. VIS could potentially drop to a mile or less with
thickest fog. KSTS is currently reporting VIS at 1/2SM, and KSNS
was reporting fog just a few hours ago. Current infrared satellite
imagery shows high clouds filtering into the region from the
northwest. These high clouds will persist over the next 24 hours.
Overall, VFR conditions will prevail over the area, with light
west/northwest winds this afternoon. Any fog occuring or that
develops will likely lift/dissipate around 17z. Fog may develop
again late tonight or early tomorrow morning, but not confident
enough to include in the tafs.

Vicinity of KSFO...VFR and light winds today.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Possible MVFR/IFR Cigs and Vis through
17z, otherwise VFR and light winds.

&&

.MARINE...as of 03:28 AM PST Sunday...Generally light northwest
winds and seas will continue through early Tuesday morning. The
only exception will be off the coast of Big Sur, where winds will
be moderate to strong due to the presence of a coastal jet this
morning through Monday morning. On Tuesday morning, winds will
switch out of the south and increase ahead of a low pressure
system that will approach the coastal waters by late Tuesday. A resulting
deformation zone will occur with northeast winds over the northern
outer waters and southerly winds over the remainder of the coastal
areas. Light to moderate showers will occur beginning Tuesday
morning, with chances continuing through the forecast period.
Winds are forecast to switch out of the northwest by Thursday
evening.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 AM

&&

$$

PUBLIC FORECAST: Dykema
AVIATION: BAM
MARINE: BAM


Visit us at www.weather.gov/sanfrancisco

National Weather Service Glossary of Abbreviations