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

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FXUS66 KMTR 250441

Area Forecast Discussion
National Weather Service San Francisco Bay Area
941 PM PDT Sun Mar 24 2019

.SYNOPSIS...A Pacific storm system will spread moderate rain
across the North Bay late tonight and Monday morning, with light
rain then expected to develop south across much of the rest of the
area on Monday afternoon and evening. Periods of rain are then
likely to continue from Monday night through Thursday, with the
most widespread rainfall expected late Tuesday night into
Wednesday. Dry weather is forecast to return late in the week and
into next weekend.


.DISCUSSION...As of 9:00 PM PDT Sunday...Evening IR satellite
imagery shows a large upper low centered about 800 miles off the
coast of the Pacific Northwest. To the south and east of this low
center is the initial frontal boundary that is currently
approaching the northern California coast. Models forecast light
warm advection rainfall to develop out ahead of the frontal
boundary and into the North Bay by around midnight tonight, and
the KMUX radar is already showing weak returns near northwest
Sonoma County. Latest HRRR and the 00Z NAM indicate that rain
rates will increase in the North Bay late tonight as the frontal
boundary approaches our coast. Moderate to locally heavy rainfall
can be expected in the North Bay from late tonight through the
Monday morning commute. The latest HRRR also forecasts moderate
rainfall rates developing as far south and east as the San
Francisco Peninsula and western Alameda County by midday tomorrow.
The frontal boundary is then forecast to weaken significantly as
it pushes south into the south Bay and Monterey Bay Area on Monday
afternoon and evening. Rainfall totals in the North Bay through
Monday evening are forecast to range from 0.5-1.5 inches with as
much as 2 inches possible in northwest Sonoma County. Rain totals
through late Monday for the remainder of our area are expected to
be a quarter inch or less except up to a half inch along the SF
Peninsula, and locally in the Santa Cruz Mountains and East Bay

The weakened frontal boundary is forecast to stall along a line
from about San Jose to Monterey late Monday night and persist
there through Tuesday. During this time frame, scattered showers
will be most likely near the stalled frontal boundary, but could
occur most anywhere in our area. In any case, rainfall amounts
from late Monday night through Tuesday evening are expected to be

The upper low will finally begin to approach the far northern
California coast late Tuesday night. A vigorous shortwave trough
rotating around the low is expected to bring another round of
widespread rainfall to our area from late Tuesday night through
Wednesday morning. Scattered showers will continue into Wednesday
afternoon and evening. In addition, the airmass will become
increasing unstable late Tuesday night and Wednesday morning as
the upper low approaches and we could see isolated thunderstorms
on Wednesday. The upper low will be very slow to move to the
northeast and out of our area, so scattered showers may continue
through Wednesday night and into Thursday, primarily across the
northern portion of our area. Additional rainfall from Tuesday
through Thursday is forecast to range from 0.50-1.50" in most
areas, with as little as 0.25" in our far southern valleys and as
much as 2 inches in northwest Sonoma County. Forecast rainfall
from late tonight through Thursday will mostly be beneficial,
although some minor nuisance flooding is possible in the North
Bay, particularly on Monday morning.

Gusty winds can be expected at times through midweek. South winds
may gust as high as 35 mph along the coast of the SF Bay Area
from late tonight through early Monday afternoon. More widespread
gusty winds are then likely from late Tuesday through Wednesday
when southwest winds may gust up to 40 mph at the coast and in
the hills.

Southerly flow ahead of this system will result in mild
temperatures through early Wednesday, with cooling then expected
late Wednesday into Thursday. A forecast update earlier this
evening included increasing temperatures, particularly at night,
from Monday night through Tuesday night.

Longer range models indicate dry conditions will return by Friday
and continue into the weekend, although rain may develop in the
North Bay by Sunday afternoon.


.AVIATION...As of 9:41 PM PDT Sunday...VFR extending through the
evening with southerly wind flow, drying lower levels; the WRF
model`s boundary layer and surface humidities initialized well
except RHs are over-forecast along the Monterey county coast. High
pressure moving to the Central Coast is setting up a SMX-SFO
pressure gradient, currently 2.0 mb, and the SFO-ACV gradient is
3.1 mb. An eastern Pacific trough will bring lowering ceilings,
mainly to the Bay Area, late tonight and Monday. MVFR ceilings
with light to moderate rain preceding an incoming cold front, post
frontal wind shifts over to SW-W direction Monday. IFR ceilings
and visibilities in moderate to heavy rain over the North Bay
mainly Monday morning. Light rain with a slow moving and weakening
cold front eventually arrives along the north Central Coast Monday
evening, front may stall in vicinity of Central Coast Monday night
and Tuesday.

Vicinity of KSFO...VFR, wind SE shifting to westerly by 23z Monday.
Tempo light rain 11z-15z Monday with gradually lowering ceilings
becoming MVFR in rain by 15z. Showers with frontal passage diminish
through Monday night.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR, wind E-SE with SE wind increasing
in the Salinas Valley Monday morning and afternoon prior to cold
frontal passage late in the afternoon or evening. Light rain
developing 21z-22z and lasting into the evening as slow moving,
weakening cold front arrives, possibly stalling in the vicinity of
the Central Coast Monday night and Tuesday.


.MARINE...As of 8:44 PM PDT Sunday...Winds southerly and rain
developing through Monday morning ahead of approaching low
pressure system and cold front. Gusty winds can be expected ahead
of the front. Showers linger Monday night and Tuesday. Rain
develops again during the mid-week then dry weather returns late


     .Tngt...SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm




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