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

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FXUS66 KMTR 080630
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1130 PM PDT Fri May 7 2021

.SYNOPSIS...An early season offshore wind event has prompted a
Red Flag Warning to be issued starting 11 pm tonight through 6 am
Monday for the interior North Bay mountains (Eastern Sonoma
Ranges and all of the Napa Ranges) and East Bay hills as well as
the interior valleys. Otherwise expect a sunny and pleasant
weekend with mild weather near the coast and bays and sunny and
warm weather inland. Offshore winds will ease by Monday but high
pressure will keep the pattern sunny, warm and dry through most of
next week.

&&

.DISCUSSION...as of 9:13 PM PDT Friday...GOES-WEST Water Vapor
Imagery this evening shows the rapidly- approaching dry air mass
starting to snake its way into northeastern Sonoma County and
northern Napa County as a potent north-south pressure gradient
continues to increase in response to a strong surface high.
Currently running at a 10mb difference between the higher pressure
over Acadia (ACV) and SFO. This strong pressure gradient has
quickly translated into winds that are starting to pick up across
the highest peaks in the interior North Bay ranges (e.g. 25mph
gusts at Atlas Peak).

Coordinated with our partners in Sacramento earlier in regards to
the timing of the winds as this air mass will quickly spread into
the Napa and Eastern Sonoma County ranges just before midnight.
Winds will then also quickly pick up over the East Bay hills
overnight as the air mass gets redirected from the Central Valley
just after midnight there. The majority of these offshore winds will
initially remain limited to the highest peaks (e.g. Mt. Diablo, Mt.
St. Helena, etc) but will mix down to the foothills overnight. Are
looking at wind gusts between 35-45mph above 1500 ft with gusts
approaching 55mph above 2000 ft and 60mph above 2500ft. Winds right
along the foothills will also gust between 25-35mph. These dry,
offshore winds will peak right before sunrise and level-off during
the day on Saturday but will still remain breezy with gusts
between 15-25mph along the foothills and particularly in the
interior East Bay hills as offshore flow continues to get
transported in from the Central Valley. Moreover, the air mass
will continue to dry through the afternoon and evening, with
relative humidity values dropping into the teens across the North
and East Bays. Nonetheless, the coastal ranges of the North Bay,
along with coastal San Mateo and the Santa Cruz Mountains, will be
spared from the bulk of the dry air mass as onshore flow returns
by Sunday afternoon to the coast following the weakening of the
north-south gradient. Nonetheless, breezy offshore conditions,
along with the low relative humidity values, will make for Red
Flag conditions to continue in the Eastern Sonoma County ranges,
the Napa ranges, and the East Bay hills/interior valleys until
sunrise Monday. The marine air mass simply will not have the
energy required via ebb and flow processes to extend that far
inland tomorrow night and into Monday morning.

Following the weakening of the north-south pressure gradient, winds
will greatly calm regionwide as the axis of the mid-level ridge
comes ashore, promoting onshore flow and a return of the typical ebb
and flow pattern of the marine layer. As a result, will see max
temperatures in Bay Shoreline gradually go from upper-70s/low-80s F
on Monday to more widespread 70s F by the second-half of the
week. The far-interior mountains and hills will remain warm in the
80s and low 90s F but winds will be much calmer given the
widespread ridging influence. As for coast, short-range ensemble
members do hint at the gradual return of marine stratus by early
next week, while temps will remain seasonal in the 60s to low 70s
F.

ECMWF and GFS ENS members show a relatively zonal pattern as we
transition towards a more ridge-dominated pattern through the
middle of next week. Should note that a couple of the members,
along with the ECMWF deterministic, hint at some momentum transfer
from the Gulf of Alaska descending into the PAC NW in the form a
long-wave mid-level trough. The most recent deterministic
solutions hint at the potential for the divergent quadrant of the
trough moving over Northern California by next weekend. Will need
to assess the potential for some moisture to tap into the region
around that timeframe, but given the weak moisture transport
across the PAC, currently not looking favorable. Will continue to
monitor over the next few days.

&&

.AVIATION...As of 11:30 PM PDT Friday...For the 06z TAFs. VFR.
Strong northerly pressure gradients are 9.4 mb ACV-SFO and 2.6 mb
UKI-STS at 11 pm. Onshore gradient is 2.1 mb SFO-SAC. Surface
winds decoupling with low level wind shear developing tonight and
Saturday morning. Low level wind shear redeveloping Saturday night
into Sunday morning.

Vicinity of KSFO...VFR. West to northwest winds 10 to 20 knots
tonight, gusty up over 30 knots Saturday afternoon and evening.
Low level wind shear with north to northeast winds aloft.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Mainly west to northwest winds 10 to
15 knots except gusty to 20 knots Saturday afternoon and early
evening. Low level wind shear tonight and Saturday morning.

&&

.MARINE...as of 08:32 PM PDT Friday...Northwest winds continue to
produce strong wind gusts across the waters. There will be a
brief period on early Saturday where the winds taper off, but then
restrengthen later Saturday and remain elevated through the
weekend. Winds out over the outer waters can expect to see gale
force gusts into Saturday night. Seas will be dominated by these
wind driven waves though a very small northwest swell will also
exist through the coming days.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ507-510-511
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             GLW...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM

&&

$$

PUBLIC FORECAST: Diaz
AVIATION: Canepa
MARINE: DK

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