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

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530
FXUS66 KMTR 231752
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1052 AM PDT Sat Sep 23 2017

.SYNOPSIS...High pressure building near the West Coast will result
in dry weather, along with a warming trend, through at least the
middle of next week.

&&

.DISCUSSION...as of 08:53 AM PDT Saturday...Cool start to the
morning region-wide with overnight lows generally in the 40s away
from the immediate coast where low/mid 50s were common. Looking
for today to be the start of a warming trend that will extend into
the upcoming work week. Under a sunny sky, temperatures will warm
into the 70s to near 80 inland with upper 60s likely at the
coast this afternoon. The ongoing forecast remains on track and
no updates are needed at this time. Please see the previous
forecast discussion below for additional details.

&&

.PREV DISCUSSION...As of 3:30 AM PDT Saturday...Clear skies
across our entire CWA this morning along with a squashed marine
layer and a moderate offshore flow. Temperatures are generally
running a few degrees cooler than yesterday thanks in large part
to drier air and light winds. One exception is Napa where speeds
are still just enough to keep them currently at 50. Look for parts
of the North Bay to drop into the upper 30s (40s will be common)
with the remainder of our CWA generally in the 40s. Low 50s will
be mostly confined to the coast.

Temperatures will quickly rebound as abundant sunshine will
combine with a synoptic pattern that will feature the longwave
trough overhead starting to progress to the east. In addition, 850
MB values will increase 1-2C by the afternoon. Highs will
generally be in the 60s at the coast with 70s to lower 80s
inland. Winds will mostly be from the north or northeast with
gusty conditions possible over parts of the North Bay. That has
lead to an issuance of a red flag warning starting this afternoon
for the North Bay Hills. Please see the fire weather section
below for more information.

Temperatures will continue to warm each day for Sunday into
Wednesday as a ridge near the coast builds into our region and
850 MB values go up each day. The flow should remain offshore
while the marine layer will be non-existent. This will translate
into the warm weather making it down to the beaches. By Tuesday
highs will be in the 80s at most coastal spots with 80s and 90s
inland. Beyond that the range in solutions greatly increases
although the majority of the models keep very warm readings going
at least into Friday.

The big challenge for the forecast is just how warm will it get
both at the coast and inland. 850 mb temperatures and 500 mb
heights are not that impressive and anomaly values are generally 2
standard deviations or less, so did not feel comfortable going
with extremely high numbers that some of the ensemble members are
suggestion (such as 90s to the coast). The current forecast keeps
the heat threshold in the low category at the worst, however if
the forecast trends upwards that could easily move to the moderate
level. Anyone with outdoor plans or at a location without air
conditioning should keep a close eye on the forecast.

Longer range guidance favors warmer than normal conditions
continuing all of the way into the middle of October. No
precipitation is expected.

&&

.AVIATION...As of 10:50 AM PDT Saturday...Widespread VFR through
the duration at all TAF sites. Breezy onshore winds by the
afternoon, especially along the coast. Models all showing a few
low marine clouds attempting to sneak into the Monterey region
around sunrise tomorrow, however, confidence is low.

Vicinity of KSFO...VFR. West winds 14-16 kt after 22Z.

SFO Bridge Approach...similar to KSFO.

Monterey Bay Terminals...VFR. Some low clouds possible around
sunrise.

&&

.FIRE WEATHER...As of 3:30 AM PDT Saturday...A warming and drying
trend will start this afternoon and continue at least into at
least Tuesday morning. In addition to the warm temperatures there
will be lowering humidity values and periods of gusty northeast
winds across the North Bay hills, especially across the Napa
county hills and the hills of northern Sonoma County. Fine fuels
will continue to dry given the lack of marine air and humidity.
Climatology also favors a greatly chance for critical fire
weather conditions for the weekend and next week.

&&

.MARINE...as of 10:50 AM PDT Saturday...High pressure offshore
will bring light to breezy north to northwest  winds today and
through the coming days. The northwest swell train  that first
arrived several days ago will continue to weaken.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Mry Bay from 2 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 2 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 2 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

PUBLIC FORECAST: RGass/Bell
AVIATION: DRP
MARINE: DRP
FIRE WEATHER: Bell/RWW

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