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

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FXUS66 KMTR 251121
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
421 AM PDT Mon Sep 25 2017

.SYNOPSIS...Warm offshore pattern will continue at least through
Wednesday with mild nights and breezy winds in the hills. Some
coastal cooling possible by Thursday with a gradual cooling trend
back to more seasonable temperatures by next weekend but with a
continued dry weather pattern.

&&

.DISCUSSION...as of 3:37 AM PDT Monday...Classic September
offshore weather pattern is forecast for much of the upcoming work
week. Synoptic pattern features northerly wind flow aloft with a
thermal trough at the surface over coastal California while the
center of the surface high remains well offshore. Current surface
gradients include 4 mb northerly down the coast with about 6 mb
offshore from the Nevada Deserts towards the coast. This is
occurring with mostly clear skies to start the day. Winds in the
hills are generally light this morning with lows from the mid 50s
to mid 60s on average, mildest where the winds are strongest.
Humidity values remain under 30 percent for many of the hill
locations so Red Flag Warnings will continue for the North and
East Bay hills through at least Wednesday.

Expect warm to locally hot weather today through Wednesday under
offshore wind regime. However, low dewpoints will keep heat
indices and impacts to a minimum along with longer early fall
nights allowing for night time cooling, even for buildings without
AC. To be clear the magnitude and overall set-up of this heat
event is not nearly as extreme as what was observed at the start
of Sept.

For tonight, the models are showing another burst of gusty
northeast/east winds, strongest over the Napa and East Bay hills.
Further drying of the airmass will persist right through Tuesday.
The combination of hot day time temps, dry offshore winds and
prolonged periods of low humidity values has prompted the Red
Flag Warning to run through Weds. No appreciable rain has fallen
and fuels remain critically dry so use extra precaution with any
ignitions sources including nigh-time warming fires.

By Weds we`ll lose some of the offshore/adiabatic warming but no
strong onshore return is forecast so will likely squeeze another
warm to hot today, even near the coast. By Thursday the ridge will
finally weaken and allow for some cooling but temperatures to
remain on the warm side of normal through late week and likely
into the weekend as well with no rain in sight.

&&

.AVIATION...As of 4:30 AM PDT Monday...Upper level ridge off the
California oast extends into the Pacific Northwest. Offshore
winds at 925 mb keeping the area clear of stratus. A shortwave
riding over the top of the ridge will bring in some cirrus clouds
from the north.

Vicinity of KSFO...VFR. West winds to 15 kt after 22Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR.

&&

.FIRE WEATHER...As of 3:37 AM PDT Monday...Red Flag Warnings in
effect through Weds afternoon for the North and East Bay hills.
Temps will warm up and RH values dry out again today. Expect
stronger offshore winds in the hills tonight with continued
lowering of humidity into Tuesday. Models show another burst of
gusty northeast winds in the north and east bay hills Tuesday
night into early Weds. Winds will finally ease by Weds afternoon
but by then the fuels will be very dry with low humidity and
likely the hottest day of the week with widespread readings into
the upper 80s and 90s. We are near the peak of Bay Area fire
season and this is a classic critical fire weather pattern.

&&

.MARINE...as of 3:00 AM PDT Monday...High pressure off the
California coast extends northeast into Vancouver Island. This is
keeping moderate northwest winds over the outer waters. Winds will
decrease Tuesday as a thermal trough over the California interior
shifts to the coast.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm

&&

$$

PUBLIC FORECAST: RWW
AVIATION: W Pi
MARINE: W Pi
FIRE WEATHER: RWW

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