Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS66 KMTR 191816

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1016 AM PST Sun Feb 19 2017

.SYNOPSIS...High impact event set to unfold during the next 24 to
36 hours as a potent Pacific storm system and atmospheric river
brings widespread rainfall and strong winds to the region. Periods
of heavy rainfall are likely from late this evening through Monday
evening, potentially resulting in flooding across portions of the
region. In addition, southerly winds will increase late today and
be locally strong and gusty through Monday evening. Widespread
rainfall will then taper off late Monday night into Tuesday.
Cooler and somewhat drier weather conditions are expected late in
upcoming week.

&& of 10:00 AM PST Sunday... The next wet and windy
storm system is nearly at our doorstep, with KMUX radar already
showing scattered rain showers moving inland from offshore. These
precursor rain showers are associated with the initial push of a
1-1.5" TPW plume originating from the tropics and extending to the
California coastline. Precipitation accumulations from these early
rain showers have already brought over half of an inch of rain to
some portions of the coastal North Bay, including at the Venado
and Oak Ridge observation stations. Additional weak to moderate
rain showers are expected to continue through midday and into the
early afternoon hours. Meanwhile, south-southeast winds will
gradually ramp up in strength through the same time period,
especially along the coast and over higher peaks and ridges.

By late this afternoon into early this evening, the deeper
moisture surge will begin to advect inland and bring heavy
rainfall. Models continue to disagree on where this initial surge
of moisture will arrive, however, there has been a noticeable model
trend to nudge the main precipitable water plume northward from
Big Sur towards the Santa Cruz/San Mateo coastal mountains over
the last 24 hours. As such, have modified the QPF forecast grids
to reflect this, with lesser amounts for Big Sur and other
portions of Monterey/San Benito county as the most noticeable
change. The remainder of the overnight package appears on track at
this time, so see previous discussion for details on the current

.PREVIOUS of 03:29 AM PST Sunday...Light
precipitation being reported this morning mainly across the North
Bay and down through the San Mateo coast inland into portions of
the East Bay. This precipitation is associated with increased
moisture advection across the region along with a weak mid/upper
level system approaching from the west. Rainfall will generally
remain light through the early morning and begin to increase in
coverage and intensity later in the day as warm air advection
spreads across the region. By late this afternoon and evening, an
atmospheric river (AR) with PWAT values forecast to exceed 1.35"
will take aim at the Central Coast and further increased rainfall
rates, especially along the coastal ranges as onshore surface
winds also increase. Breezy to locally gusty winds greater than 45
MPH will be possible, especially along the coast and in the higher
elevations. By late tonight, the heaviest rainfall will likely
become more narrow in focus along a frontal boundary that is
forecast to lift northward somewhere between the Santa Cruz
Mountains and the greater North Bay region. Rainfall will likely
further intensify and the threat for flash flooding, flooding of
area creeks, streams and low lying, poorly drained areas will
significantly increase. With that said, the forecast models
continue to struggle on exactly if/when the AR will stall and
where this might occur. Regardless, considerable rainfall will be
possible from late tonight into Monday across portions of the
region. With super saturated soils across just about the entire
region, downed trees and isolated power outages along with rock/
mud slides, shallow landslides and debris flows across recent burn
scar areas remain of great concern.

The heavier band of precipitation is then forecast to slowly sag
southward from Monday morning through the afternoon and evening
hours. However, some of the forecast models, such as the NAM show
the boundary stalling over the North Bay and not progressing
southward until late Monday into Monday night. This event will
have to be closely monitored and folks in flood prone areas are
urged to remain vigilant, stay up to date on the latest forecast
information and take necessary precautions to protect life and
property. Rainfall amounts of 4 to 7 inches will be possible along
the coastal ranges with the potential for some locations to see
upwards of 10 inches. Meanwhile, North Bay valley locations,
coastal areas in and around the San Francisco Bay Area and areas
around Santa Cruz have the potential to receive 2 to 5 inches of
rainfall through Monday night. Further south and inland, rainfall
totals will range from 1 to 2 inches with lesser amounts possible
in inland valley locations from the Santa Clara Valley southward
through the Salinas Valley due to rain shadowing effects. Given
the antecedent conditions from a very wet winter so far, a Wind
Advisory and Flood Watch remain in effect from this afternoon
through early Tuesday morning.

By late Monday into Tuesday, the forecast models indicate the deeper
plume of moisture will finally shift southward as an upper level
trough approaches the Pacific Northwest. This should result in the
more organized rainfall to shift south of the region as well with
lingering showers likely from Tuesday into Wednesday as the parent
mid/upper level trough shifts inland across northern California.
While any additional rainfall after the AR event will only
exacerbate any ongoing flooding, widespread heavy rainfall is not
currently forecast from Tuesday into Wednesday.

Dry weather conditions are then expected from late Wednesday into
early Friday as a short-wave ridge builds across the region in wake
of the exiting trough. Forecast model solutions diverge from late in
the week into next weekend, however the GFS and ECMWF now show the
potential for additional unsettled weather conditions late in the
forecast period, mainly Friday night into Saturday as a cold,
mid/upper level low drops southward down the West Coast. Stay tuned
as forecast confidence remains low beyond early next week.


.AVIATION...As of 10:16 AM PST Sunday...Areas MVFR cigs and
visibilities, and mainly light SE winds today along with
occasional light rain. Weather conditions deteriorate quickly by
evening as a warm front approaches from the SW causing winds to
pick up strongly by this evening. Cold front to move through
tomorrow for lowering cigs and occasional heavier rain.

Vicinity of KSFO...VFR/MVFR, light rain today becoming moderate
to heavy rain by late afternoon or early evening, also strong and
gusty SE winds likely by 03z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR/MVFR, light rain developing today becoming
moderate to heavy by this evening with increasing SE winds. Low level
wind shear (llws) developing during the evening.

&& of 09:18 AM PST Sunday...An approaching storm system
will bring gusty southerly winds to the coastal waters through
monday with gale force winds developing near the coast. Marginally
large swell monday over the southern coastal waters will result in
hazardous seas.


     .Tday...Flood Watch...Entire San Francisco/Monterey Bay region
             Wind Advisory...Entire San Francisco/Monterey Bay region
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Rough Bar Advisory for SF Bar




Visit us at

Follow us on Facebook, twitter, and youtube at: is the U.S. government's official web portal to all federal, state and local government web resources and services.