FXUS66 KMTR 242325

Area Forecast Discussion
National Weather Service San Francisco Bay Area
425 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 04:25 PM PDT Monday...For 00z tafs. Satellite
imagery showing most of the stratus has eroded off the coast and
over land. Expect similar conditions tonight as was seen last
night with patchy stratus and patchy dense fog around the Monterey
Bay. Other areas forecast to remain VFR through the period aside
from possible patchy fog in the North Bay early tomorrow morning.
Breezy onshore winds will subside into the overnight hours before
increasing again tomorrow afternoon.

Vicinity of KSFO...VFR.

SFO Bridge Approach...VFR. Onshore winds to remain gusty through
the early evening.

Monterey Bay Terminals...VFR through the evening. Low cigs and
patchy dense fog again tonight and into tomorrow morning.


.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 

NWS MTR Office Area Forecast Discussion