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

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FXUS66 KMTR 240534

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1034 PM PDT Sun Oct 23 2016

...Two rounds of rain slated to impact our region this week...

.SYNOPSIS...A Pacific weather system will approach the northern
California coast tonight and bring rain chances to the region from
Monday through Tuesday with the best chance of rain in the north
bay. Another weather system will likely bring widespread and
heavier rain to the the entire region by late Thursday continuing
into Friday. Unsettled weather will likely continue into next

&& of 8:53 PM PDT Sunday...No updates for this
evening. IR satellite shows lots of mid and high clouds streaming
over the region from the southwest with a deep trough approaching
the northwest California coast. 00z nam brings light rain into
northern Sonoma county by Monday morning with the frontal boundary
only slowly sagging towards the Golden Gate by tomorrow evening.
So right now commute impacts will likely be in the north bay
Monday afternoon/evening. The front will then continue to sag
southward Monday night into Tuesday bringing some light rain to
the peninsula and Santa Cruz mtns. Not expecting much rain south
of Santa Cruz. Boundary will then shift northward on Tuesday as a
warm front in response to digging trough offshore. Dry wx forecast
across the entire district then Tuesday night into Weds.

Will wait for full 00z suite before updating the extended range
but the Thursday storm will be the one with more potential for
moderate to heavy rain and periods of stronger south winds. The
heaviest rain is likely from the Santa Cruz mtns southward to the
Big Sur hills. Will address this more closely with the 00z ecmwf
output and in the coming days. Either way things on track for a
fairly wet October with more potential systems next weekend into


.PREV DISCUSSION...As of 2:00 PM PDT Sunday...Comfortable albeit cool
temperatures across our entire CWA today with a mix of sun and
clouds. Generally spots are running 2 to 4 degrees cooler than
Saturday with temperatures in the 60s to lower 70s. Also worth
noting a southerly surge that is currently bringing clouds and
cooler conditions around Santa Cruz.

Rain will slowly advance into the northwestern portion of our
region late tonight as a frontal boundary associated with a low
west of the PacNW continues to track to the east. Models have
only slightly changed since the past couple of days although in
general timing has slightly slowed. Now looks like rain gets into
the North Bay near sunrise and down to SF Bay starting either
Monday afternoon or evening (could start as drizzle before
changing over to rain). Precipitation will likely not advance
much past Santa Cruz as the front stalls out and dissipates.
Rainfall amounts still look like up to 2" for the North Bay with
generally 1/4"-1/2" around SF Bay and the Santa Cruz Mountains.
Lower amounts can be expected for areas farther to the south.

Rain will mostly come to an end by Tuesday evening with mostly dry
conditions through Wednesday night before a second stronger
system moves back into our region. This is one that the ECMWF has
been showing over the past five days and latest runs have little
change. Synoptically a longwave trough will set up to our west
with an associated surface low that will pull in subtropical
moisture. The low will rapidly deepen and approach the Northern
California coast as a cold front moves through. Although there is
still some disagreement with the details, in general there is good
agreement that widspread rain can be expected Thursday into
Friday. Periods of moderate to heavy rain are possible although at
this time there is a wide range of solutions for where the main
plume of moisture will be focused. In light of the ECMWF
consistency decided to heavily weight the forecast toward its
solution. Rainfall amounts are very tricky due to the uncertainty
of the main focus of moisture, although would expect some of the
coastal hills to get 2 to 4". Southerly winds will also be on the
increase for Thursday and Friday as the gradient tightens. Higher
elevation spots could see gusts well over 30 MPH.

Unsettled weather will likely continue beyond the second system
with a moist unstable flow across our entire region.

&& of 10:31 PM PDT Sunday...Patchy stratus and/or fog along
the coast, inland skies are VFR. Low-moderate confidence cigs forecasts
tonight, winds too will be a bit challenging. Locally gusty winds to
30 knots have been observed this evening at KOAK for instance.

SMX-SFO 3.1 mb and SFO-ACV 6.2 mb either hold or gradually strengthen
tonight and Monday causing the surface winds to favor southeasterly
direction due to cross-isobaric wind flow from high pressure over land
to a developing low pressure offshore. GFS Lamp wind guidance indicates
sustained SE winds up to 20 knots+ by Monday afternoon over much of
the forecast area. Low cigs coverage should mix out early Monday morning.
Mid/high level cloudiness with mid-upper level trough moves eastward
Monday, showers likely confined to North Bay while elsewhere pretty
good chances of downsloping and drying due to gusty (offshore) SE
winds Monday til cold front edges in from the west late in the day.
Surface cold front stalls over northern coastal waters late Monday.

Vicinity of KSFO...VFR. SE winds becoming gusty by Monday. Dry runways
Monday then rain showers arrive late Monday night/Tuesday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Patchy stratus/fog possible tonight, low
confidence on timing and duration due to increasing E-SE wind flow.
Plenty of mixing Monday very likely leading to VFR.

&& of 10:11 PM PDT Sunday...Southerly winds will increase
over the northern waters through tonight and over the southern waters
on Monday as a cold front approaches and then stalls over the northern
outer waters. This front is then forecast to retreat to the north as
a warm front Tuesday. Wet weather conditions return this week mainly
northern coastal waters then to all coastal waters later this week.


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




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