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NWS Forecast Area Discussion - San Francisco Area

134
FXUS66 KMTR 230616
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1016 PM PST Sun Jan 22 2017

.SYNOPSIS..Scattered showers along with a possible thunderstorm or
two can be expected through Monday night. Drier weather returns
starting on Tuesday.

&&

.DISCUSSION...as of 8:25 PM PST Sunday...Several bands of
moderate to locally heavy rain went through our CWA during the
afternoon and early evening hours, however in general the rend has
been for spottier coverage with lighter amounts over the past few
hours. Radar does show an area of mostly light precipitation over
our coastal waters that is moving to the coast. This will bring
another shot of rainfall to the South Bay down to the SLO county
line for the remainder of the evening. Rainfall will mostly be
under a tenth of an inch in urban spots with locally over 1/3rd
for coastal ranges.

Post-frontal instability will continue to combine with moisture in
the atmosphere to keep scattered showers going through all day
Monday and likely through Monday night. SPC continues to also have
us in the slight risk for convection with small hail a definite
possibility. Since any of those cells could bring brief heavy
rain, the flash flood watch will continue into tomorrow.

All signs point to much drier conditions starting on Tuesday and
likely going through at least the weekend. Temperatures will also
warm back to normal/slightly above normal values after Thursday.

.PREVIOUS DISCUSSION...Satellite imagery shows a 512MB upper low lingering
approximately 250 miles offshore of the Pacific Northwest early
this afternoon. The surface cold front associated with this upper
level system moved into the forecast area overnight, bringing
moderate to heavy orographically enhanced rains and strong
southerly winds, before exiting into Southern California earlier
this afternoon. The Big Sur region reported the strongest wind
gust, at 79 MPH, early this morning as a strong low level jet
moved through the area, while strong winds of 60 MPH or greater
were also observed over other coastal waters and ridges. The
heaviest rain also occurred overnight, especially over the Big Sur
and Santa Cruz mountains, where locally up to 2-3.5" fell with the
main cold frontal rain band. Urban areas picked up lesser amounts
ranging from .25 to 1.25". This upper low is expected to be the
last significant Pacific storm system to bring strong winds and
heavy rain to the San Francisco and Monterey Bay areas through the
remainder of the month of January.

Also evident on satellite imagery is a deep field of cumulus
development in area stretching across the northern half of
California in the wake of the surface cold front to approximately
750 miles offshore. This unstable air mass will continue to funnel
into our warning area through the remainder of today and Monday,
allowing for ongoing convective initiation through the coming day
and a half. The convective parameters associated with this
unstable air mass (0-1km shear 10m/s or greater, 0-6km shear 35m/s
or greater, LCL 1500 feet or below, CAPE 500j/kg or greater, and
some helicity) suggest embedded convection is likely, meaning
that all the thunderstorm- related hazards should be expected.
These include brief, but intense rainfall, copious amounts of
small hail, up to marble sized hail, frequent lightning, and
erratic wind gusts. Also possible, primarily late this
afternoon/early this evening, are brief water spouts, funnel
clouds, or even a weak tornado. Convective precipitation
accumulations through tomorrow could rival the stratiform
precipitation accumulations that came through with the main cold
frontal rain earlier today.

As of midnight tonight (not including precipitation from this
last system), January month to date precipitation for our urban
areas are anywhere between 225-360% of normal. This excessive
rainfall has helped to mitigate the years long drought the region
has experienced but also resulted in supersaturated soils,
spilling of reservoirs, overtopping creeks, flooded roadways, and
countless downed trees and land slides. The flash flood watch
covering our forecast area has been extended until 6PM Monday as
a result of the aforementioned wet antecedent conditions, the
ongoing flood related issues, and the forecasted convective
precipitation accumulations through tomorrow.

After a few hour hiatus following the surface cold frontal
passage, KMUX radar now shows numerous to widespread rain showers
and isolated to scattered thunderstorms across the forecast area.
The strongest storm cells thus far have been offshore and showed
mid level mesocyclone development before weakening as they
approached the coast. The bulk of the convective activity
currently on radar has developed along an air mass boundary which
is expected to shift onshore just after 3pm. Additional flash
flooding and urban and small stream flooding is expected to occur
as these, and further, rain showers and thunderstorms shift
inland. The highest rain rate intensities with these thunderstorms
could also trigger rock and mud slides as well as trigger flash
flood warnings for the Soberanes, Loma, and Chimney burn scars.

The core of the upper low is forecast to begin sliding southward
from the PACNW coast towards the California coast on Monday,
before weakening overnight Monday, and exiting into Southern
California by late morning Tuesday. A short wave ridge will setup
along the Pacific coastline for the remainder of the day Tuesday,
providing a respite from the wet weather into early Wednesday. The
North Bay will experience a glancing blow from a weak upper
disturbance by Wednesday afternoon, bringing only very marginal
rainfall accumulations there.

There is high model consensus that dry weather with seasonal
temperatures will return Thursday as high pressure develops aloft.
This high pressure system will then dominate the local weather
pattern through the upcoming weekend and likely through the
remainder of the month of January. Long term models show the next
Pacific storm system arriving around Feb 1st.

&&

.AVIATION...As of 10:00 PM PST Sunday...Showers continue to
move through the area as cold air cu from an upper low off the
Oregon coast funnels in moisture. Convection has diminished due to
loss of daytime heating but there is still the possibility of a
thunderstorm tonight. The low will move south on Monday passing
through the area Monday night. Weak subsidence ahead of the low
will provide a short break from the showers but will pick up again
Monday night.

Vicinity of KSFO....Showers through 12Z. Shower activity
decreasing Monday but may pick up again Monday night after 03Z
an upper level low approaches.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Showers through 12Z with an isolated
thunderstorm possible. Shower activity decreasing Monday.

&&

.MARINE...as of 09:00 AM PST Sunday...A 972 mb low centered 350
miles west of Astoria will fill in as it moves south. It will
move across the coastal waters as a weak low Monday night.
Scattered showers can be expected through early Tuesday. Large
westerly swell will persist through Tuesday morning keeping
hazardous sea conditions.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Flash Flood Watch...CAZ006-505>513-516>518-528>530
             High Surf Advisory...CAZ006-505-509-529-530
             SCA...Mry Bay
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar

&&

$$

PUBLIC FORECAST: Bell/DRP
AVIATION: W Pi
MARINE: W Pi


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NWS MTR Office Area Forecast Discussion

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