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

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433
FXUS66 KMTR 191740
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
940 AM PST Sun Nov 19 2017

.SYNOPSIS...Dry weather will continue through the end of the
weekend, with mild days and cool nights. The next weather system
will bring rain chances late tonight and Monday, mainly across
the North Bay. There is a slight chance of additional rainfall at
times in the North Bay for much of this week, but most of our
area is forecast to be warm and dry through Thanksgiving.

&&

.DISCUSSION...as of 08:27 AM PST Sunday...Despite high clouds
overspreading the Bay Area overnight and early this morning it`s a
chilly start for many locations. Overnight lows were generally in
the 30s and 40s, even urban area. Protect valleys and interior
locations had several locations dip into the upper 20s. One of the
coldest spots this morning was the Marquez RAWS in San Benito
County hitting 26 degrees.

For today, widespread cirrus is expected over the region resulting
in filtered sunshine. None the less, temperatures will be with in
a few degrees of yesterday`s max temps. Highs will be in the upper
50s to mid 60s across the North Bay (a few degrees below
normal), low 60s SF Bay (near normal), 60s to lower 70s
Monterey/San Benito (a few above normal).

No update is needed.

&&

.PREV DISCUSSION...As of 3:15 AM PST Sunday...

Today`s weather is expected to be much like yesterday`s, except
for more widespread high cloud cover. Afternoon highs will be
mostly in the 60s and winds will remain light.

Early morning satellite shows a plume of moisture over the
eastern Pacific, approaching from the southwest. Precipitable water
values in this plume are in excess of 2 inches out near 30N/140W.
Meanwhile, a frontal boundary off the coast of Washington and
Oregon is sagging slowly to the southeast. This frontal boundary
is forecast to make its way into northwest California by late
tonight, but any farther southward progression will be thwarted
by an upper ridge that will be building over central and southern
California. In any case, the models indicate there will be
sufficient warm advection acting on the moisture plume to produce
rainfall across the North Bay late tonight and Monday. Elsewhere
across our region, subsidence under the building upper ridge is
expected to inhibit the development of precipitation, although
isolated light rain may occur as far south as the San Francisco
Peninsula and the East Bay on Monday. Most models indicate that
rainfall totals in the North Bay through late Monday will be on
the order of a quarter of an inch or less. However, the 06Z NAM
has trended wetter and forecasts as much as 1.5 inches of rain in
northwest Sonoma County by late Monday. There certainly is
sufficient moisture approaching to produce these heavier rainfall
amounts, but given recent model trends of building the upper ridge
more strongly over California, these higher totals seem unlikely.
As is the often the case in situations such as this with moist
flow and warm advection under relatively high heights, there will
probably be a sharp gradient between soaking rains to the north
and no rain at all to the south. And that sharp precipitation
gradient will most likely set up somewhere near the
Mendocino/Sonoma County line.

Rainfall in the North Bay is expected to taper off Monday evening
and end by late Monday night. The upper ridge is forecast to
continue strengthening across the southwestern United States
through midweek. Meanwhile, moist southwesterly flow will continue
between this ridge and a deepening trough offshore near 145W. The
models agree that this moisture will remain completely north and
west of our area on Tuesday and Wednesday, and then perhaps sag
back south into the far northwest corner of our forecast area by
Thanksgiving. But for the most part, the upper ridge will maintain
dry and warm conditions across nearly all of our forecast area
from Tuesday through Thanksgiving. Widespread high temperatures in
the 70s are forecast during this period of time, with some low to
mid 80s likely from Monterey Bay southward.

The longer range models indicate the ridge will shift far enough
to the east late in the week to allow precipitation to gradually
spread into our forecast area between Thursday night and next
weekend. However, there are considerable model differences
regarding timing and location of rainfall late in the extended
forecast period.

&&

.AVIATION...As of 9:35 AM PST Sunday...For 18z Tafs. With the
exception of KSTS, VFR anticipated through the forecast period.
The tail end of a storm system will drop south into the northern
part of the Bay Area, lowering CIGs overnight, but should remain
above 3000 ft at all TAF terminals but KSTS. Light to moderate
showers are possible early Monday morning for the North Bay, which
will likely drop CIGs/VIS to MVFR, possibly IFR at times.
Generally light winds today, turning S/SE overnight and remaining
below 10 knots.

Moderate to high confidence.

Vicinity of KSFO...VFR. Light winds throughout the day today. CIGs
remaining above 3000 feet overnight.

SFO Bridge Approach...VFR.

Monterey Bay Terminals...VFR. Generally light winds, with highest
winds (around 10 knots) in the Salinas Valley through 19-20z,
then becoming light and variable in the afternoon.

&&

.MARINE...as of 09:23 AM PST Sunday...High pressure off the coast
of Central California will continue bring light west to northwest
winds today for the southern coastal waters. Winds will switch
out of the south through the day today for the northern coastal
waters, ahead of a frontal system. The front will stall off the
southern Oregon/northern California coast, so southerly winds will
persist.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...None.

$$

PUBLIC FORECAST: MM/Dykema
AVIATION: BAM
MARINE: BAM

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