NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 252331

Area Forecast Discussion
National Weather Service San Francisco Bay Area
431 PM PDT Tue Sep 25 2018

.SYNOPSIS...Warm offshore flow will persist in the hills and for
inland valleys at least through midweek. The coast and coastal
valleys will still be under the influence of cooler onshore flow
with a marine layer in place. A cooling trend for all areas Friday
into the weekend as a mainly dry upper trough approaches the
region. Some indications for rain chances across northern California
by early next week.

&& of 12:30 PM PDT Tuesday... A Rex block centered
approximately 900 miles WNW of San Francisco is acting as a 1750
mile meridional (north to south) barrier in the atmospheric flow.
Water vapor imagery shows moisture wrapping over the north of the
barrier through the Gulf of Alaska before rotating into the
Pacific Northwest. At the same time, a band of subtropical
moisture associated with the upper low (which forms the southern
half of the Rex block) is rotating counterclockwise into
Southern/Baja California south of the Rex block. Meanwhile, a dry
slot of air is positioned over the San Francisco and Monterey Bay
area in between the moister air rotating into the western CONUS
from both the south and northern flanks of the Rex Block.

At the most simple level, a Rex block consists of a high pressure
positioned north of a low pressure. Both the high and low pressure
circulations present in the current Rex block will play a role in
the weather through the next several days. The high pressure
portion of the Rex block has and will continue to result in an
increase of offshore winds, warmer temperatures, dry inland
conditions, and a compressed marine layer near the coast as it
wobbles over Northern California. The Rex block breaks down late
in the work week, freeing up the low pressure system to slide over
Northern California through the early weekend. This will bring a
cooling trend, increased onshore flow, and even a slight chance of
some very light rainfall over the extreme northern stretches of
Sonoma county. The medium to longer term models are in fairly good
agreement of this scenario playing out, albeit, with the bulk of
any rainfall remaining farther northward over extreme northwest
California. Any accumulations will be measured in the low
hundredths range.

A red flag warning remains in effect for the North and East Bay
hills and ridgetop above 1000 feet through 5pm this afternoon.
Earlier indications hinted that the offshore winds may cease by
the mid to late morning, however, there are still a handful of
ridge tops and mountain passes with sustained winds 15 to 20 mph,
gusting 20 to 30 mph. Mount Diablo was our windiest location over
the last 12 hours, with sustained speeds in the low 30mph range
and gusts into the low 40mph range. These dry, gusty offshore
winds have been steadily decreasing over the last few hours and
should drop below criteria level through the afternoon. Relative
humidity readings are not dropping to levels quite as low as
yesterday, due in part to entraining of some moister air from the
north and south. As of noon, a very small number of stations are
reporting in the upper single digit range, while many more are
reporting in the lower 10s. Very low minimum humidity values are
forecast to persist into Thursday morning.

Low model to model and run to run model consistency beginning
early next week. The GFS and EURO both agree that the weather
pattern will remain progressive and unsettled for early next week.
Both models feature a cool, wet trough arriving behind the
remnants of the Rex block features by Monday morning in addition
to an eastern Pacific tropical system moving into the vicinity of
the region. Neither model seems to know what to make of these two
features. The 00z EURO was extremely wet and progressive for our
region, however, the 12Z Euro has slowed down and keeping the
system corralled along the coastline, rather than directly over
the San Francisco Bay area. The GFS is playing a completely
different tune, shunting the upper low and moisture farther south,
over Southern California. None of these solutions will likely be
the one that actually plays out due to the unknown variable of the
tropical system lingering farther offshore. That said, have
nudged chance of rain into the slight chance category for early
next week. Expect to see the forecast change rapidly as both the
approaching trough arriving from the northwest and developing
tropical cyclone arriving from the southwest come into better

&& of 04:31 PM PDT Tuesday...For 00z tafs. Marine
layer remains at around 1000 to 1200 feet in depth with stratus
clinging to most of the coastline. VFR through the afternoon and
evening before low clouds return tonight. Expecting similar
conditions overnight as was seen yesterday with low cigs at most
sites expect for KSJC and KLVK. Similarly, patchy fog will be
possible again for the Monterey Bay and North Bay sites.

Vicinity of KSFO...VFR with winds gusting to around 20 kt through
the early evening. Lower cigs and possibly vis may return tonight
between 09z-12z Wednesday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR through the evening. IFR/LIFR cigs to
return around 06z-08z tonight with possibly reduced visibilities


.FIRE of 12:30 PM PDT Tuesday...Red flag warning will
remain in place through 5pm for the North and East Bay hills. Main
concern is the continued presence of breezy to gusty offshore
winds primarily across the Diablo range. The offshore winds have
steadily decreased through the late morning and early afternoon
and are expected to be cut off later today. That said, very dry
air will remain in place, especially just above the marine layer,
through as late as Thursday morning. Fuels will also remain very
dry ahead of any rain early next week.

&& of 02:38 PM PDT Tuesday...Weak to occasionally
moderate west to northwest winds will prevail over the coastal
waters through the week as an upper level ridge remains of off the
Pacific Northwest Coast. Low pressure will then replace the ridge
and result in the development of weak southerly winds by this






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NWS MTR Office Area Forecast Discussion