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

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FXUS66 KMTR 210548

Area Forecast Discussion
National Weather Service San Francisco Bay Area
947 PM PST Mon Nov 20 2017

.SYNOPSIS...A few showers are still possible over the North Bay
tonight. High pressure will bring dry and warmer weather to the
area Tuesday through Thursday. The next chance of rain will be
late this weekend.

&& of 8:00 PM PST Monday...Rain has mostly come to
an end across our region with almost all precipitation confined to
portions of the North Bay. Totals were above forecast in a few
locations with amounts up to nearly 1" recorded. However, those
spots were in the minority. In general 1/10" to 1/4" was recorded
which was close to the forecast. For the rest of the San Francisco
and Monterey Bay Region just a mix of sun and clouds today (more
sunshine to the south) with highs in the 60s to the lower 70s. Not
completely out of the questions that a few more tips of the bucket
could be recorded tonight for northern Napa and Sonoma counties,
so will leave that mention in the forecast.

The main story remains the building ridge of high pressure which
will continue to nudge into our region from the south. At the same
time winds at the surface will become light to offshore and 850
mb temperatures will increase by as much as 7 C. These factors
will combine to bring very warm mid-November temperatures to our
area especially south of San Francisco. Highs the next two days
will be in the 70s to mid 80s for the southern half of our CWA. A
few record highs are not even out of the question on either day
with the current temperature forecast within a few degrees of
record values for many spots. For the northern half, highs will be
a bit cooler due to more cloud cover and closer proximity to the
storm track. Generally 60s to lower 70s can be expected tomorrow
and Wednesday.

Please see the previous discussion for the remainder of the
forecast for Thanksgiving into the weekend...

.PREVIOUS DISCUSSION...By Thanksgiving the ridge flattens a
little and begins to shift eastward. The weakening ridge will
allow for slightly cooler temperatures and about a 15-20% chc for
precip across the North Bay as a cold front approaches. Models
have been trending drier during this time period and tend to keep
precip confined to north of the Golden Gate. Medium range models
continue to struggle with a unified solution for Friday and into
weekend. One thing that they show is a slight chance for showers
for the far northern areas of the North Bay, but differing
details. Either way, rainfall amounts will be minimal at best
Friday and into the weekend.


.AVIATION...As of 9:47 PM PST Monday...Little change from this
afternoon`s weather, metars show VFR most places while it`s
MVFR/IFR over the North Bay. The Bay Area and north Central Coast
remain near the eastern edge of a very moist and milder flow from
the sub-tropics. Correction to my earlier statement that the ECMWF
and GFS had much higher RHs in the 850 mb to 300 mb layer tonight
compared to yesterday`s output for tonight...actually recent ECMWF
and GFS are in-line and consistent with last night`s output. Thus
a mix of cloud layers persist overnight...then mid level drying
under an amplifying 500H ridge arrives Tuesday. 06z tafs go with
this, though there`s still generally a lowering confidence in the
height of cloud bases especially Tuesday morning in the pre-dawn
hours when radiative cooling has its best potential. The air is near
saturation in the North Bay which is where the best chances for fog
are tonight.

Vicinity of KSFO...Despite an influx of milder, moister air the
higher cloud layers have prevailed this evening. Adjusted the 06z
taf to reflect slightly higher cloud bases /VFR/ with tempo group
remaining in the taf for Tuesday morning, best guess is that VFR
stands a pretty good chance of prevailing at KSFO into Tuesday as
long as radiative cooling is limited into Tuesday morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...A little better than moderate confidence
VFR persists tonight into Tuesday. Mainly light E-SE winds except
up to 10 knots KSNS Tuesday morning. Winds trending light onshore
Tuesday afternoon.

&& of 9:06 PM PST Monday...An upper level ridge will
build over California through mid-week then weaken late in the
week. Very moist, southwesterly flow extending into the Pacific
Northwest and far northern California will overlap the Bay Area,
north Central Coast and its coastal waters through late week. A
cold front will move quickly across the coastal waters and bays
Sunday into Sunday evening. Seas will be light to moderate through
Tuesday, then slightly increase mid to late week as west to
northwest swell increases.





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