Rip Current in California, October 8, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
256 PM PDT Mon Oct 8 2018

.SYNOPSIS...Offshore gradient weakens through the day. Onshore
flow returns tonight and into tomorrow, promoting a cooling trend
through mid week. A weak offshore wind event is possible later
this week.

&&

.DISCUSSION...as of 3:00 PM PDT Monday... The synoptic scale
pattern features a 594dm 500mb high pressure ridge 1000 miles west
of the coast and a 559dm 500mb dry low pressure trough over the 4
corners region. The upper low recently descended southward over
the Sierra Nevada in an "inside slider" pattern, which are common
during the fall and are primarily known for driving strong
offshore winds. A year ago, a similar "inside slider" event
sparked the onset of the North Bay fires that impacted Napa and
Sonoma counties. The current event sparked red flag warnings and
wind advisories in anticipation of gusty, dry offshore winds, and
observations from last night and through this morning confirmed
this occurred, with wind gusts ranging from 55 to 75 mph reported
numerous times along the higher terrain of the Napa/Yolo and
Napa/Lake county lines. Dry, gusty offshore winds also brought
extreme fire weather conditions to the Diablo Range and local
areas of the Santa Cruz mountains. Model guidance suggested these
winds would begin to back off by mid morning and diminish
significantly by the late afternoon/early evening. So far, this is
on track, with just a handful of ridges and highest peaks
reporting gusty offshore winds. The San Francisco to Winnemucca
pressure gradient is often used as a proxy for the strength of the
offshore flow and also shows the decline over the last 6 hours,
down to 4.8MB from 8.0MB over the 375miles that separate them (a
40% drop). Breezy offshore winds are forecast to persist along the
Yolo and Lake county line of Napa into this evening and possibly
thru sunrise Tuesday, however, the widespread offshore wind event
is rapidly coming to a close.

Temperatures this afternoon are running a few degrees warmer than
the same time yesterday largely due to the warm start they
received this morning and the continued offshore flow suppressing
the onshore flow and marine layer. Dew points are up as well as
water vapor imagery shows a large pocket of dry air associated
with the ridge exits the area and offshore winds begin to subside.
Short term model data continues to suggest the possibility of a
southerly surge developing later today and pushing northward
tonight. We are closely watching the development of any stratus on
the eastern flank of an eddy just off of the northern San Luis
Obispo coast early this afternoon in anticipation of this
occurring. Typically, wed like to see a more widespread stratus
field already developed to the south of the Big Sur coast to feed
a southerly surge, and that has yet to develop. Instead, an area
of moisture/stratus is pooling along the border of the
Monterey/San Luis Obispo county line as of around 245pm. This pool
of moisture may not be sufficient to bring a southerly surge
about, so confidence is currently on the low end that this will
occur in short order. A deeper marine layer may be allowed to
develop as soon as the aforementioned dry slot exits farther south
and offshore winds weaken further.

Elsewhere, onshore winds are beginning to make a resurgence along
the coast and just inland. Since there is little to no marine
layer currently along the coast and a broad area of warmer, dry
air just offshore, the onshore winds will not bring an immediate
surge of cooler, moister air inland. It will take a little bit of
extra time to advect in a cooler, moister airmass once the winds
have shifted direction. Tomorrow and the next couple of days are
forecast to fall an onshore regime, with cooler temperatures,
onshore sea breeze by the afternoon, higher humidities, and
morning clouds. Local WRF model shows onshore winds returning to
the extreme inland areas by late tomorrow afternoon/early
evening.

Models tend to agree that another dry disturbance will rapidly
descend into California as an "inside slider" Wednesday night
into early Thursday. The offshore gradient is forecast to be
significantly weaker than the current/previous "inside slider" as
the offshore ridge has retrograded farther off the coast and the
upper disturbance is weaker overall. At this time, no red flag
warning or wind advisory is expected later in the week, however,
we will continue to monitor this event in case of any change in
the model data.

The high pressure ridge reverses course, amplifies, and shifts
closer to coast for Friday, this weekend, and into early next week,
promoting a warming/drying trend and a suppressed marine layer
near the coast. The only potential snag in the forecast model data
is the presence of a tropical system (Sergio). Recently, a
different tropical system (Rosa) introduced a relatively moderate
to large degree of uncertainty into the system that brought stark
run to run model difference regarding the last rainy storm system
that arrived at the start of the month.

&&

.AVIATION...as of 10:34 AM Monday...Offshore flow will bring
clear VFR conditions to the area today. Winds aloft will decrease
Monday afternoon as offshore flow weakens. A southerly stratus
surge could set up overnight for stratus returning to the
Monterey Bay area TAF sites.

Vicinity of KSFO...VFR. North winds becoming westerly to 15 kt
after 21Z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Return of stratus overnight tonight
but low confidence.

&&

.FIRE WEATHER...As of 2:21 AM Monday...Offshore winds in the
hills and mountains of the North and East Bay are underway this
early morning, bringing critical fire weather concerns to these
areas. Wind gusts between 45 and 60 mph have been observed at
Mount Saint Helena (4300') in Napa County through the evening and
overnight hours. Elsewhere in the hills, wind gusts have been
mostly between 20 and 35 mph. Poor recoveries have been observed
in relative humidity values overnight with the higher elevation
sites holding around 25 to 45 percent. The combination of the
continued gusty offshore winds and a very dry airmass will
maintain critical fire weather conditions into Monday morning. A
Red Flag Warning remains in effect until 9 am Monday for the North
Bay Mountains, East Bay Hills and Diablo Range, the Santa Cruz
Mountains, and the East Bay Interior Valleys.

Winds are expected to subside by late Monday morning, but very dry
air will remain across the district through Monday, especially in
the hills.

Onshore flow is forecast to return by Tuesday and this will result
in a cooling trend on Tuesday and Wednesday, with much better
humidity recoveries expected by Tuesday night.

Longer range forecast calls for another offshore wind event late
in the week with warm and dry conditions expected to develop again
by Friday. Locally gusty northerly winds are possible late
Thursday night into Friday morning, although winds are not
expected to be as strong as the current offshore wind event.

&&

.MARINE...as of 8:15 AM PDT Monday...Northerly winds will begin
to decrease today as high pressure over the eastern Pacific
weakens. However moderate northwest winds can still be expected
over the northern outer waters through midweek. A southerly surge
will travel up the coast Tuesday reaching the Sonoma coast by
Wednesday. Southerly swell from Hurricane Sergio will impact the
area mid to late in the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: DRP
AVIATION: Sims
MARINE: Sims
FIRE WEATHER: Dykema/Rowe

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