Technical NOAA Weather Discussion

FXUS66 KMTR 261223

Area Forecast Discussion
National Weather Service San Francisco Bay Area
423 AM PST Thu Nov 26 2020

.SYNOPSIS...Breezy offshore winds will bring sunny and mild
weather for Thanksgiving Day. Temperatures will quickly cool
tonight as a drier airmass settles over the region and lighter
offshore winds continue. High pressure builds Friday through the
weekend with sunny and mild days followed by clear and cool
nights. Longer range forecast into next week looks to remain dry.

&& of 3:27 AM PST Thursday...Skies are clear this
morning with offshore wind pattern in place which is keeping the
region fog free. It'll be a sunny start this Thanksgiving morning
but with some noted northeast winds. The northerly gradient is a
robust 7 mb with an increasing SFO to Winnemucca of around 3-4
mb. So far were seeing northeast wind gusts to around 30-35 mph
across the North Bay hills with gusts to 56 mph for Mt Saint
Helena and 52 mph for Mt Diablo. Humidity values remain moist at
this time but the persistent offshore winds will drive much drier
air over the region today that will allow humidity values to
lower to around 25-25%. As the morning inversion breaks around
16-17z some of the winds in the hills may mix down into lower
elevations. The benefit of the offshore flow will be adiabatic
downslope warming for the North Bay valleys and regions around SF
Bay as well as the immediate coastline. With sunny skies temps
should warm into the mid and upper 60s this afternoon.

The dry airmass will allow temperatures to cool quickly after
sunset this evening as dry offshore winds continue across the
region but at lighter wind speeds in the hills tonight. Overnight
lows into the 30s for the valleys by sunrise Friday morning.

High pressure builds Friday through the weekend with sunny and
mild days followed by clear and cool nights. Some lower 70s will
be likely across the Central Coast as light offshore winds
continue, the airmass aloft warms and dries out. The persistent
upper ridge will eventually allow the airmass to dry further with
humidity values into the teens this weekend for the Central Coast.
Though fire danger has been mitigated by some November rains the
threat does still exist, especially for those locations that
received little or no rain so far this fall. In terms of sensible
weather through the long holiday weekend, it looks ideal for
outdoor activities.

The next shortwave of note is now progged to pass to our north on
Monday with ensemble PoP guidance around 10-15% for the North Bay
and dry elsewhere. General consensus keeps a dry ridge for much of
next week. Models keep wanting to show a weak system trying to
break down the ridge by later next week with some stronger
westerlies trying to undercut the ridge by the second week of
December. Ensemble guidance from both the gfs and ecmwf suggest
the PNA teleconnection will trend negatives towards the middle of


.AVIATION...As of 4:18 AM PST Thursday...VFR with offshore flow.
Winds aloft will be gusty north to northeast this morning. These
winds are mixing down to the surface at least in the North Bay
with APC gusting to 30 kt at 12Z. Models indicate wind mixing down
into SFO and OAK during the morning with gusts possibly as high
as 25-30 kt. Low level wind shear is still possible where winds
are still light at the surface but as more mixing occurs it should
minimize low level wind shear. Winds aloft decreasing in the
afternoon but may pick up again tonight bringing another round of
low-level wind shear.

Vicinity of KSFO...VFR. North winds 10-15 kt increasing to 15-20
kt gusting to 25 kt after 17Z then holding steady through the

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Southeast drainage winds 5-10 kt
this morning becoming north to northwest in the afternoon.

&& of 4:22 PM PST Thursday...A long period northwest
swell continues through this morning before decreasing in the
afternoon and evening. Currently coastal buoys report 13 to 15
foot waves with a 16 to 17 second period. This could produce
breakers of 18 to 24 feet with largest breaking waves at northwest
facing beaches. In addition, enhanced coastal erosion of the
summer beach profiles will continue in earnest with this large
swell. Anomalously cool water temperatures and turbulent,
unpredictable seas will lead to hazardous conditions along the
coast. It is recommended that individuals stay off rocks and
coastal jetties if visiting the coastline and remain vigilant of
their surroundings until a safe distance from the coast.

&& of 04:09 AM PST Thursday...Gusty northwest winds
will decrease and switch to northeasterly tonight and Friday as
high pressure builds into the Pacific Northwest. Gusty winds will
generate steep fresh swell hazardous for small craft vessels. A
large northwest swell train will start to diminish today. Another
long-period moderate northwest swell arrives late Friday.


     .Tngt...High Surf Advisory...CAZ006-505-509-530
             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...SF Bay




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Prepared by Weather at: Thu Nov 26 06:30:02 PST 2020
From the National Weather Service