Hail and Strong Wind in California, March 15, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
449 AM PDT Mon Mar 15 2021

.SYNOPSIS...After last night's cold front passage, scattered
showers behind the front will impact the region today, along with
low snow levels and a slight chance of thunderstorms with small
hail. Winds have switched to the west and northwest behind the
front and are expected to increase, becoming locally strong and
gusty through this morning, especially near the coast. Cold
temperatures and frost is forecast for Monday night and early
Tuesday. Dry weather appears likely on Tuesday and Wednesday. Rain
chances redevelop on Thursday and unsettled weather may continue
into the weekend.

&&

.DISCUSSION...as of 03:32 AM PDT Monday...We've been watching the
radar all night and it's been easy to see the progression of the
cold front. Southwest winds ahead of the front have been pushing
rain echos from the SW to the NE while the northwest winds behind
the front have been pushing small convective showers from the NW
to the SE. As of 2:30 am, the frontal boundary was draped from
southern Monterey County near King City up towards Sacramento.
There was some decent rain that fell across Monterey County
including over the Carmel and River burn scars, but the highest
rain rates we observed overnight were 0.12"/hour which is well
below debris flow criteria. Rain amounts so far with this system
have ranged from a few hundredths (mainly in the far eastern
portion of our CWA) with 0.5-1" along the coast and over coastal
mountains. Additional rain today will be sporadic depending on
where convective showers develop. NAM and HRRR Hi-res models
suggest that the best potential for shower activity will be over
our coastal waters and between SF to the southern portion of
Monterey County for inland areas where we could see an additional
0.1"-0.5" of rain...the higher end falling over terrain south of
Monterey Bay.

Looking at satellite, the center of the upper low is spinning a
little northwest of Redding, CA. Behind the cold front, we see a
vast area of cellular clouds which indicates the colder airmass
that is approaching our region. That cold air aloft will make the
environment unstable and may cause the convective showers to
develop into thunderstorms today. If that occurs, we expect the
main impact will be small, quick-moving storm cells with brief
moderate to heavy rain and the potential for small hail. This cold
air will also significantly lower snow levels down to around 1000
ft, however, snow accumulation will be limited based on where
showers develop. As advertised in prior forecasts, the best chance
for decent accumulation will be across the higher elevations of
the Santa Lucia Mountains with 6-7 inches as a max snow amount
forecast.

Winds will be an additional factor today as the northwest flow
strengthens through late morning. Strongest winds remain over the
ocean and along coastal areas. SFO has been reporting gusts up to
25 mph so far and that could increase up to around 35 mph as the
morning progresses. Wind Advisory remains in effect for coastal
and bay shorelines through 11 am.

Shower activity and the stronger winds should diminish this
afternoon. By evening we should have dry weather with lighter
winds and clear skies which brings us to our final concern...frost
for Tuesday morning. Have issued a Frost Advisory for the North
Bay Valleys, southern Salinas Valley, and Hollister Valley. In
these areas, any agriculture and budding grapevines could be at
risk as temperatures drop into the upper 20s to low 30s during the
early morning hours of Tuesday (3-9 am). Wednesday morning could
also have a chilly start with some areas getting into the low 30s,
but it won't be as widespread as Tuesday morning.

Looking ahead, Tuesday and Wednesday daytime highs will see a
warming trend ranging mid 50s to mid 60s as dry weather continues
with weak ridging aloft. By Thursday morning, the NAM, ECMWF, and
GFS models are showing the potential for another frontal system to
move into our CWA. Extended models have that front continuing to
impact us through Friday followed by some clearing on Saturday.
Then more unsettled weather possible on Sunday. Confidence isn't
high on the details as models haven't come into good alignment,
yet.

&&

.AVIATION...as of 04:50 AM PST Monday...For the 12Z TAFs. The
cold front has passed and is moving to the southeast, leaving
scattered showers across the Bay Area and the Central Coast.
Conditions are mostly VFR as SCT cigs appear at 3000 ft, with
BKN/OVC much higher aloft. Winds remain out of the northwest, with
several terminals receiving stronger gusts between 20 to 25
knots. Showers can be expected around the region through 00Z with
decreasing clouds and lifting cigs into Monday afternoon. The main
impact to terminals today will be the gusty, post-frontal winds
that are expected around the region. Models indicating that most
terminals will see winds subside after 03Z Monday night, with
clearer skies and VFR conditions.

Vicinity of KSFO...Scattered showers around the area with brief
periods of rain as they pass over the terminal through Monday. Brief
MVFR cigs may be possible during those period of rain, but otherwise
VFR is expected with ceilings mostly above 3000 ft. Winds are strong
with gusts currently around 25 kts, expecting them to increase
Monday late morning to up to 32 kts - close to airport weather
warning criteria for winds and will be monitored. Showers look to
diminish after 00Z with winds remaining onshore and breezy through
the end of the TAF period.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Cold front is gradually exiting and
introducing onshore winds that will bring blustery conditions to
Monterey Bay and the Northern Salinas Valley, along with early
morning showers. Can expect cigs/vsbys lowering to MVFR at times as
showers linger through the early afternoon. Winds will lessen
through Monday evening and skies will gradually clear through the
overnight.

&&

.MARINE...as of 02:33 AM PDT Monday...As the cold front has
passed over the waters, scattered showers continue and are
expected through Monday, with a slight chance for an isolated
thunderstorm. The bigger impact to mariners will be the strong and
gusty, gale force northwest winds that are forecast to continue
on Monday, generating very steep combined seas of 15 to 20 feet at
roughly 10 to 12 seconds. The Sonoma Coast north of Point Reyes
is expected to develop the strongest wind gusts up to 50 knots
through noon on Monday. These very steep seas and gusty winds can
capsize small vessels and bring other hazardous conditions across
the seas. Both winds and seas are expected to subside late on
Tuesday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ006-505-508-509
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SRW...Pt Arena to Pt Reyes 0-10 nm until 9 AM
             GLW...SF Bay until 3 PM
             GLW...Mry Bay until 3 PM

&&

PUBLIC FORECAST: Bingaman
AVIATION: DK/Diaz
MARINE: DK

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