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265
FXUS66 KMTR 242026
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
126 PM PDT Mon Sep 24 2018

.SYNOPSIS...Warm high pressure will last at least through
Wednesday and possibly into Thursday. Cooling trend starts Friday
into Saturday as an upper low approaches. There will be a slight
chance for some showers over the North Bay on Saturday.

&&

.DISCUSSION...as of 01:25 PM PDT Monday...Cool temperatures
remain at this coast this afternoon thanks to persistent low
clouds and patchy fog. Away from the immediate coast is a
different story as many locations are running a few degrees warmer
than yesterday.

Synoptically speaking surface high pressure will continue to build
across the Great Basin and a weak trough of low pressure remains
at the coast. Aloft we have a Rex Block type set up with high
pressure off the PacNW coast and low pressure off the CA coast.
The overall pattern will not change drastically through the middle
of the week.

Now for the details, given the inland high pressure and coastal
low pressure offshore flow will increase across the region through
tonight. Winds will be strongest over the N Bay Mts and E Bay
Hills with gusts 25-35 mph. The SFO-WMC gradient is projected to
be 10-12mb, which is decent, but not a major offshore flow event.
Offshore flow will keep moderate to poor relative humidity
recoveries in place through mid week as well. The combination of
wind, low relative humidity and dry fuels will result in critical
fire weather conditions. A red flag warning remains in effect
through Tuesday 5 PM. See fire section below for more details.

Temperatures will continue to warm across the region with the
offshore flow set up. Daytime temperatures will warm in the 80s
and 90s across the interior and 60s and 70s near the coast. Given
the lack of a real strong offshore flow component some lingering
night and morning coastal low clouds will remain. Temperatures
near the coast will be kept in check due to clouds. Overnight
thermal belts will be active with mild temps in the hills.

The offshore flow pattern begins to weak by Thursday as upper low
to the west begins to finally move eastward. Expect increased
onshore flow, a return of the marine layer and cooler
temperatures. Latest medium range models continue to advertise
some light showers possible over the North Bay on Saturday.
Introduce a small chance (15-20%) chance for some showers. The
moisture source could be interesting and needs some watching
between now and then. Early indications point to a typhoon
pushing northward through near Japan and drifting northward across
the Pacific. Models diverge on early next week with the GFS dry
and the ECMWF/CAN bringing rain to NorCal. At this point, forecast
will be dry, but watched closely for next Monday.


&&

.AVIATION...As of 10:45 AM PDT Monday...Compressed marine layer scoured
by unfavorable dry airmass this morning. This has prevented much
of an inland intrusion of the marine layer and kept the marine
stratus instead banked along and just inland of the immediate
coastline. The marine layer had more leeway farther south around
the Monterey Bay due to slightly more favorable air masses/wind
flow where it pushed inland over KMRY/KSNS/KWVI. Expect to see
locally patchy dense fog and VLIFR-LIFR cigs linger around lip of
the Monterey Bay through late morning into the very early
afternoon. Otherwise, expect similar setup for tonight with
unfavorable offshore flow farther north and patchy dense fog
around the Monterey Bay.

Vicinity of KSFO...VFR.

SFO Bridge Approach...VFR.

Monterey Bay Terminals...Patchy dense fog and very low ceilings
linger through remainder of morning and possibly into the very
early afternoon along lip of the bay, then VFR. Low ceilings
return tonight through late morning Tuesday.

&&

.MARINE...as of 10:41 AM PDT Monday...Split wind flow with light
winds along the coast and breezy  northwest winds in the outer
waters. Winds will continue to diminish through the day but will
remain locally breeze in the outer waters north of Point Reyes
tonight. Light mixed swell, with a predominate short period
northwest swell.

&&

.FIRE WEATHER...as of 1:25 PM PDT Monday...The biggest story
continues to be the current Red Flag Warning in effect over the N
Bay Mts and E Bay Hills. Last night many higher elevations around
the Bay Area recorded some impressive single digit to lower teen
relative humidity. Winds were not overly strong, but still some
breezy conditions over the highest peaks. Tonight will be a
different story as the offshore flow gradient increases. Overnight
humidity recovery will occur early tonight and then decrease
through sunrise as NE winds increase. The peak period of concern
will be late tonight through early tomorrow, which is common in
these scenarios, with the lowest humidity and highest winds. Winds
of 25-35 mph and RH <30% seems reasonable with most critical
areas - highest peaks of the East Bay and NE Sonoma and N Napa
counties. It should also be noted that other peaks outside of the
Red Flag Warning will see very low relative humidity, but weaker
winds. Those locations will be near critical fire weather
conditions. Winds will decrease through the day on Tuesday and
Tuesday night. That being said, despite the weaker winds relative
humidity remains low keeping heightened fire weather concerns in
place through Wednesday morning.


&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM

&&

$$

PUBLIC FORECAST: MM
AVIATION: DRP
MARINE: DRP
FIRE WEATHER: MM

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