Flooding and Strong Wind in California, November 8 to 9, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
503 PM PST Mon Nov 8 2021

.SYNOPSIS...An atmospheric river will impact the region through
Tuesday morning bringing widespread rainfall, gusty southerly
winds, and the potential for an isolated thunderstorm or two. In
wake of the passing system, warming and drying conditions look to
develop for the remainder of the week and into the upcoming
weekend.

&&

.DISCUSSION...as of 01:34 PM PST Monday...The calm continues
before the next storm to impact the region from late this
afternoon/evening into early Tuesday morning. There is high
confidence in a weak to potentially moderate atmospheric river
(AR) advecting inland over the Bay Area and Central Coast with
southerly winds increasing out ahead of the weather system. It
appears the main frontal boundary will be in the form of a warm
front resulting in precipitation developing over the North Bay and
along the coastal ranges late this afternoon. Rain will increase
in both intensity and coverage after sunset before spreading
southward across the remainder of the Bay Area and Central Coast
overnight as the deep plum of moisture makes "landfall" over the
region. The period of heaviest rainfall looks to occur over the
North Bay between 7 PM - 11 PM, Santa Cruz Mountains between 9 PM
- 2 AM, and along the Santa Lucia Range (Monterey County coast)
between 1 AM - 5 AM. While widespread flooding concerns are not
anticipated, rain rates may reach or exceed 0.50" per hour during
these periods of time. Thus, we will be closely monitoring
conditions for isolated flooding/debris flow concerns.

As mentioned above, southerly winds will strengthen late this
evening and through early Tuesday morning across the greater San
Francisco Bay Area. Winds of 20-30 mph with gusts up to or in excess
of 45 mph will be possible at times ahead of the AR. Thus, a Wind
Advisory remains in effect from 7 PM this evening through 3 AM
Tuesday morning for much of the aforementioned region. In addition
to the wind and rain, forecast models indicate enough mid-level
instability for the potential for isolated thunderstorms out over
the Pacific and even over inland areas. However, widespread
thunderstorm activity is not anticipated at this time. As mentioned
in an earlier forecast discussion; "For most areas, the bulk of the
precipitation is anticipated to occur through the dark of the night,
especially after midnight and before sunrise. Of note, is that
atmospheric river events on this trajectory tend to over perform
model data during their (initial) warm sector phase particularly in
orographically favored coastal facing slopes, so that will be worth
monitoring through the event as well."

Rainfall totals have not changed much for tonight through Tuesday
and are are projected to range from 2.00"-4.00" in the North Bay
Mountains, Santa Cruz Mountains and Santa Lucia Mountains; 1.50"-
2.50" in the North Bay Valleys; 1.00"-2.00" in the East Bay Hills
and northern Monterey Bay; 1.00"-1.50" around San Francisco and Bay
Shoreline; 0.50"-1.00" for the East Bay Hills; 0.25"-0.75" around
the southern Monterey Bay and Santa Clara Valley; and around 0.25"
or less for interior Monterey and San Benito counties. As always,
could see greater totals in the region's "wet spots" and lesser
amounts in the rain shadowed valleys. Isolated to scattered rain
showers will remain possible into midday Tuesday as the main cold
front advances through the region. However, precipitation will
diminish from north to south through Tuesday afternoon.

A warming and drying trend is then likely for the remainder of the
week as high pressure builds aloft. The CPC's 6-10 and 8-14 Day
Outlooks continue to highlight above average temperatures and below
average precipitation into mid/late November. Meanwhile, there are
some longer term ensembles showing a return to a more progressive
pattern returning toward the end of the month.

&&

.AVIATION...as of 4:00 PM PST Monday...For the 00z TAFs. MVFR/IFR
with occasional LIFR as RA/+RA moves through the area tonight. The
rain band will start impacting North Bay and KSFO terminals
around 01/02Z. RA will begin the next hour at KOAK/KLVK, and reach
KSJC by 04Z. -RA will reach KMRY/KSNS around 04/05Z. Intermittent
+RA and BR/FG will be seen through 10Z tonight. There is a chance
for a few lightning strikes around KSFO/KOAK from 04Z through
09Z, most should remain off shore but a few could reach inside the
Bay. Stronger southerly winds will be seen at all terminals
tonight, with sustained 15 to 25kt and gusts up to 35kts.
Strongest winds will be seen at KSFO/KOAK/KSJC/KSNS from 04Z
through 12Z. The main FROPA will move down the coast early
tomorrow morning, clearing out the RA and calming winds to
VRB05kts. Mostly VFR with with occasional MVFR from VCSH for
Tuesday afternoon.

Vicinity of KSFO... Moderate to heavy rain arriving early this
evening (lighter rain ~01-02z and heavier rain arriving ~04-07z).
MVFR ceilings accompany this rain with occasional FG/BR tonight.
Gusty S/SE winds 20 to 30 kts with LLWS tonight. Potential gusts
up to 35kts. Strong chance for KSFO reaching AWW criteria
tonight. Slight chance for TS to move into the Bay Area, however
best chances remain over the waters to the west. VCTS in TAF to
account for chance.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...VFR. Light winds at MRY but SE and gusty at SNS. West
to southwest winds 5 to 15 knots for the rest of today. Rain
developing this evening and tonight and MVFR cigs will make a
return. Winds to pick up tonight and be gusty ahead of the front.

&&

.MARINE...as of 03:53 PM PST Monday...Deteriorating conditions
today, as a low pressure system  approaches the Pacific Northwest
and far northern California, bringing gale force southerly winds,
larger westerly swell, steep wind driven seas and heavy rain later
today, tonight and Tuesday  morning. There is also a slight chance
of thunderstorms tonight. Winds shift to the west northwest
Tuesday morning after a cold  front passes through the waters.
Lighter winds and seas expected  mid- week through the end of the
week with diminishing moderate  period west to northwest swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Wind Advisory...CAZ006-505>512
             SCA...SF Bay from 9 PM
             SCA...Mry Bay from 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 7 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 7 PM until 3
     AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 9 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 9 PM until 3 AM

&&

PUBLIC FORECAST: RGass
AVIATION: Dhuyvetter
MARINE: Lorber

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