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

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FXUS66 KMTR 191245

Area Forecast Discussion
National Weather Service San Francisco Bay Area
445 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 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 4:39 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, however, residual cold pooling of air from the surface to
near surface may result in strong low level wind shear (llws) if/where
surface winds remain de-coupled.

The leading edge of warmer lighter air from lower latitudes is moving
over the top of chillier denser air associated with yesterday`s gusty
W winds and surface pressure rises in the Bay Area. The collision
of these air masses is resulting in increasing stratiform cloud
development and light rain over the Bay Area. As mentioned, areas
MVFR cigs along with mainly light SE winds today, however, cigs
and vsbys lower later today and expect a period of strong and
gusty SE winds to sweep quickly eastward across the forecast area
by early-mid evening as a warm front quickly approaches from the
SW. Heavy rain tonight and Monday with a very slowly crawling cold
front moving in from the west, winds will be nearly parallel to
this front thus it`ll be slow moving forward.

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) indicated in 12z tafs.

&& of 03:29 AM PST Sunday...A very moist sub-tropical
flow reaches northern California today into Tuesday. Occasional
light rain this morning over the coastal waters and bays becomes
moderate to heavy rain by late afternoon and evening. Gusty
southerly winds will precede a very slow moving cold front late
tonight and Monday. Showers later Tuesday into Wednesday with
another chance of rain Friday into Saturday. Marginally large
swell Monday over the southern coastal waters.


 .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 from 7 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm from 7 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 7 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 7 AM
             SCA...Rough Bar Advisory for SF Bar
             SCA...SF Bay from 9 AM




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