Winter Storm Warning 1, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1053 AM PST Wed Feb 22 2023

.SYNOPSIS...Locally strong and gusty winds continue this morning
but significantly diminished from yesterday afternoon and night
and are continuing to gradually decrease. Widely scattered
showers will continue through the day over the southern half of
our area, with just a slight chance farther north. Snow showers
will be possible above 1500 to 2000 feet, and could also be an
isolated thunderstorm and/or some small hail. As the cold upper
level low sinks down the coast from the north, widespread
precipitation is expected to develop over the area late Thursday
afternoon and evening with snow up in the hills.

&&

.DISCUSSION...as of 09:55 AM PST Wednesday...Latest satellite
imagery shows shallow convective cloud bands streaming southward
offshore, and spilling inland south of around the Golden Gate.
KMUX reflectivity data indicating scattered showers in association,
while lightning detection networks have recently been picking up
on some thunderstorms far offshore, about 200 miles SW of the
Monterey Peninsula. Winds are still gusty, though continuing to
gradually lessen. In conjunction with mid-morning temperatures
around the Bays only in the mid-40s however, it's pretty chilly
outside. The freezing level in the 12Z KOAK sounding was only 2640
feet.

Reports of measurable precipitation thus far have been very light
and largely confined to the mountains of Santa Cruz and Monterey
counties, with the largest total being 0.25 inches at the Big Sur
RAWS site. Some early morning webcam images from sites above 2000
feet in Monterey County did show a dusting of snow on the ground.
And just now received a spotter report of snow flurries at 2000
feet in the Santa Cruz Mountains.

Present forecasts appear on track, though will thoroughly review
and adjust where needed for the afternoon package following
receipt of all new model data.

&&

.PREV DISCUSSION...as of 03:31 AM PST Wednesday...Scattered
showers have been displayed on radar through the night moving
steadily from north to south over the ocean. Similar to the Great
Lakes "lake-effect", our "ocean effect" (warmer ocean temperatures
relative to the cold air mass aloft) looks to be churning as
well, as constant radar returns over the Santa Lucia Range has
been steadily producing showers overnight as well. Surface
temperatures however are still in the low 40s at the time this
discussion was posted. Many areas will drop into the 30s with the
North Bay Valleys the best chance dipping below 35 degrees, which
is why that area is in a frost advisory. Additionally the Wind
Advisory and High Wind Warning are still valid through Wednesday
afternoon. Winds at the surface are still gusting 15 to 25 mph
with higher elevations still gusting 35 to 45 mph. But this is
just a synopsis of what is currently going on. This system is only
just arriving and will continue to impact the area through
Saturday.

Key Messages:

* WINDS: As mentioned, winds are currently strong, but are
  forecast to gradually diminish Wednesday afternoon. Once winds
  subside and products are expired, attention will turn to the
  second burst of winds that is forecast Thursday night into
  Friday morning and whether new products need to be issued for
  that. These winds have also created very hazardous seas which
  results in a dangerous surf zone.
* COLD: Cold temperatures start to drop Wednesday morning with the
  North Bay Valleys in a Frost Advisory. Temperatures drop more on
  Thursday morning as the Freeze Watches have been converted to
  Freeze Warnings, with additional Frost Advisories for coastal
  areas.
* RAIN: Rain showers will continue through Saturday, but become
  more widespread Thursday night into Friday. Rain is expected to
  be light to moderate. However, heavier rain is possible as
  thunderstorms with the potential for lightning and small hail is
  forecast through the week.
* SNOW: Snow showers are forecast for areas 1,500 feet, with the
  highest confidence over the Santa Lucia Range. Winter Weather
  Advisories have been issued through Thursday morning for the first
  round of snow. However, Winter Storm Watches have been issued for
  the second round of winter impacts expected Thursday night into
  Friday which is forecast to have higher snow totals.

While the winds arrived ahead of the system on Tuesday, the
closed low at 500 mb arrives over Northern California on
Wednesday. Latest Euro ensemble has geopotential heights down to
519 dam, which remains more than three standard deviations away.
In fact, in 2001 there was a closed low aloft that brought higher
snow totals, but that only got down to around 533 dam, which just
speaks to the anomaly of this system. But returning to the winds,
high-res models are suggesting that the magnitude may diminish a
little bit earlier than currently forecast. So advisories and
warnings are still in place through Wednesday afternoon, but winds
will be monitored in case they can be ended earlier.

Convective showers have already arrived over the ocean and will
remain through Saturday. Meso-models are still projecting the
Thursday evening time frame for these showers to move inland and
become more widespread. While most of the showers will have light
to moderate rainfall, the Storm Prediction Center keeps the
coastline in the general thunder risk through Saturday morning.
Thunderstorms have the potential for locally higher rain rate.
While widespread flooding concerns do not accompany this system,
localized ponding on roadways is possible during the heavier outs
of rain. Lightning has already been detected and that risk will
prevail as well. Lastly, with the cold temperatures aloft, small
hail is also a hazard that these showers will be able to produce.

A Frost Advisory is currently valid for the North Bay Valleys due
to the risk of temperatures dropping down below 35 degrees
Fahrenheit. But that is just the beginning. The Freeze Watch for
Thursday morning has been converted to a Freeze Warning for all
interior zones, while Frost Advisories have been issued for
coastal zones. Morning low temperatures are currently forecast to
be at their coldest Thursday and Saturday mornings where freezing
conditions are expected with low temperatures into the upper 20s.
Most records around the area from past events are down in the mid
to low 20s. Widespread records are not expected in this event, it
is not out of the question for certain locations. For records
around the area, consult the CLIMATE section listed below.

Ocean temperatures over the Monterey Bay are around 52 degrees
Fahrenheit and the 12Z Oakland balloon is measuring -3 degrees
Celsius at 850 mb. With significant fetch from the ocean and
strong northwest winds present, there's no wonder as to the radar
returns over the Santa Lucia Mountains as a result from the "ocean
effect". While rain showers are possible, it is likely with that
snow is falling at the higher elevations. Hopefully fire cameras
in the morning when the light is out will be able to capture
pictures of them blanketed in white. Snow showers will continue
through Saturday morning; however, the current showers are only
the first event resulting in multiple Winter Weather Advisories
being issued. However, Winter Storm Watches have now been issued
for the threat of additional snow showers expected Thursday night
into Friday. By all indications, this timeframe looks to have the
potential for higher snow amounts falling. Additional products
will likely need to be issued. If that was not enough of an
impact, winds will increase along with this second round of snow
fall which can potentially reduce visibilities.

Ensemble models are still suggesting that showers taper off on
Saturday, but exact confidence on timing is still hard to have
confidence in. Nevertheless, the cold airmass will remain.
Temperatures are still not expected to recover until Monday
morning when low temperatures have the best chance to return to
the 40s.

The long range guidance still has another upper level trough
descending down the Pacific coastline early next week, with
cluster analysis in agreement which adds to the confidence for
continued unsettled weather of the end of the month and into early
March. However, ensemble models are still all over the place in
the exact placement, timing, and accumulations for the event. For
now, rain remains in the forecast for the Monday/Tuesday timeframe
next week with a low to moderate confidence in the event.

&&

.AVIATION...as of 10:50 AM PST Wednesday...For the 18Z TAFs. VFR
through the TAF period. Winds gain strength through the afternoon
becoming breezy to gusty across the region. Spotty showers will
move along the coast through the day with some moving slightly
inland and affecting some of the terminals. Expect some reduction
in winds into the late night as showers begin to move more inland
increasing rain chances.

 Vicinity of KSFO...VFR through the TAF period. Strong and gusty
westerly winds will continue through the late night, peaking around
20 kts with gusts around 30 kts. Rain chances will be spotty through
the day but increase into early Thursday as winds become breezy
again.

KSFO Bridge Approach...Similar to SFO.

Monterey Bay...VFR through the TAF period. The afternoon offers
gusty westerly winds that remain strong well into the evening. Winds
will reduce but stay moderate into the night, but become light and
variable in the late night into early Thursday. Showers will stay
spotty through much of the day, but chances increase into early
Thursday morning as southwest winds increase.

&&

.CLIMATE...
Minimum Temperature Records
                        02/22      02/23      02/24      02/25
San Francisco D'town...38, 1890...38, 1890...40, 1891...38, 1887
SFO Airport............34, 1951...35, 1955...35, 1955...35, 1953
Oakland D'town.........38, 2019...36, 1974...34, 1987...36, 1987
Santa Rosa.............24, 1920...28, 2022...26, 2022...24, 1915
Kentfield..............27, 1920...26, 1955...29, 1955...29, 1920
Napa...................29, 1955...28, 1949...25, 2022...28, 2022
Livermore..............29, 1976...21, 1942...26, 2022...26, 2022
Redwood City...........30, 1939...32, 1975...30, 2022...31, 2022
Half Moon Bay..........30, 2006...30, 2022...28, 2018...29, 2022
San Jose...............27, 1897...24, 1897...26, 1897...28, 1894
Salinas Airport........30, 1975...30, 2018...30, 2018...25, 1935
King City..............24, 1953...24, 1984...24, 2022...23, 1974

&&

.MARINE...as of 08:44 AM PST Wednesday...Expect northwesterly
gale force winds and steep, hazardous seas into the afternoon.
Hazardous marine conditions from wind wave driven seas persist
through the week. Scatter showers and the slight chance for
thunderstorms is expected for the remainder of the work week. A
brief break is expected over the weekend before the next rainy
system arrives early next week.

&&

.BEACHES...High Surf Advisory in effect for all coastal areas
(except the northern Monterey Bay region in Santa Cruz County)
through 4 pm Wednesday. A large northwest swell will put northwest
facing beaches at the highest risk. Expect large breaking waves
of 20 to 25 feet causing dangerous conditions along the beach and
in the swimming/surfing areas close to the beach.

A Beach Hazards Statement is in effect for the northern Monterey
Bay region in Santa Cruz County through 4 pm Wednesday. Expect
large breaking waves of 8 to 12 feet causing dangerous conditions
along the beach and in the swimming/surfing areas close to the
beach.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Surf Advisory...CAZ006-505-509-530
             Wind Advisory...CAZ502>504-506-508-510-512>516-528-529
             Winter Weather Advisory...CAZ503-512-514-517-518
             GLW...Mry Bay until 3 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm until 3 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: Blier/DK
AVIATION: Murdock
MARINE: Murdock

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