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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 260339
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
839 PM PDT Sat Mar 25 2023

.SYNOPSIS...Chilly overnight and early morning temperatures with
drier conditions through Monday morning. Prepare for another round
of wind and rain for early next week with the brunt of the storm`s
impacts expected on Tuesday.

&&

...Wind and Rain Impacts Early Next Week...

.DISCUSSION...as of 08:39 PM PDT Saturday...Mostly clear skies
from Sonoma county down to Monterey.Some lightly patchy clouds are
still lingering attached to the higher peaks from the South Bay
and southward, but they are struggling to remain upright with the
setting sun. The short term impact is the moderate risk for cold
temperatures overnight. Half Moon Bay even tied their daily low
temperature record for March 25th of 35 degrees Fahrenheit, the
original set back in 1942. Another cold night forecast into Sunday
morning. No changes were made to the Frost Advisory as inland
areas will see the chillier conditions while the coastal areas
will be moderated by the ocean. Otherwise Sunday afternoon looks
to be similar to Saturday in that it will feature mostly clear and
sunny skies with similar below normal temperatures.

The rest of the forecast is in preparation for Tuesday storm. It is
looking more and more likely that it will arrive in northwest Sonoma
County Monday evening and slowly move southward through Tuesday.
There is some indications of hesitation on its southward progress
when it gets west of San Francisco, but signs point to it continuing
to move along rather than stalling and causing bigger issues. The
good news is, is that most models are suggesting that the center of
the low will be at its deepest farther offshore; otherwise, it seems
similar in set up to last Tuesday. The coastal areas will likely
receive the strongest wind gusts and most total precip, while the
valleys will remain more rain shadowed and wind sheltered. That
being said, it bares repeating that soils are saturated so it will
not take much for winds to knock down a tree and/or cause a power
outage. The cooler temperatures tonight will lower snow levels, but
thankfully there is no precip until Monday night. By then, warm
air advection will raise those snow levels above 3,500 feet. There
is the chance for some of the highest peaks to get some snowfall
with this system, but it is not expected to result in major
impacts at this time. Beyond that, look for lingering scattered
showers Wednesday and Thursday as the center of low pressure exits
southward with morning low temperatures once again cooling.

&&

.PREV DISCUSSION...as of 02:47 PM PDT Saturday...Today through Monday:
Dry weather conditions prevail over the region this afternoon
with temperatures generally in the mid-to-upper 50s with 40s in
the higher terrain. In addition, breezy northerly winds prevail as
well with gusts between 20 and 35 mph and colder air aloft
continues to advect into the region. With this, expecting little
change in weather conditions for Sunday with it starting off
chilly-to-cold. Of note, temperatures the past few mornings have
not cooled as much as previous forecast due to the aforementioned
winds keeping the boundary layer well mixed. Similar issue is
expected again tonight into Sunday morning. However, still
expecting widespread frost impacts across most interior locations
across the greater San Francisco Bay Area and Central Coast. Short
of the long, another cold night in store region-wide. As such, a
Frost Advisory remains in effect through Sunday morning at this
time. Monday morning may also be as cold if not colder depending
on the surface winds which are anticipated to diminish ahead of
our next weather system. The increased in cloud cover and moisture
will also play a role in forecast low temperatures on Monday and
therefore confidence remains moderate with the potential need for
another Frost Advisory into Monday morning as well. Individuals in
homes without power as well as the unsheltered or marginally
sheltered populations will be at risk in these cold early morning
temperatures.

Monday night through Tuesday night: A mid/upper level low pressure
system will drop out of the Gulf of Alaska and down off of the
British Columbia coastline late the weekend. The weather system will
then approach the Pacific Northwest and northern California
coastlines late Monday. This will drive a cold front into the region
on Tuesday with southerly winds strengthening ahead of the boundary
late Monday night. As the boundary moves through, southerly winds
will remain breezy with gusts between 40-50 mph across much of the
region and potentially greater than 60 mph in the favored coastal
gaps, ridges and peaks. These wind speeds, even for a short period
of time, will increase the risk for downed trees and tree limbs
resulting in isolated power outages. Wind speeds are forecast to
diminish in wake of the frontal passage by late Tuesday afternoon
and/or evening.

Rain will increase in coverage and intensity late Monday night over
the North Bay and through the San Francisco Bay Area early Tuesday
morning. An early look into the high-resolution models indicates the
potential for the development of a narrow cold-frontal rainband
(NCFR) as the boundary progresses southward through the Central
Coast. That said, periods of moderate to locally heavy rainfall are
expected on Tuesday along the main frontal boundary which will
result in an increased risk for urban and small stream flooding,
ponding of water on roadways, and shallow mudslides/rockslides.
There is also a 10-20% probability of thunderstorms on Tuesday as
the air mass becomes more unstable just along and in wake of the
frontal passage. At this time, the North Bay has the greatest
potential for thunderstorm development from late Tuesday morning
into the afternoon. Scattered to numerous rain showers will persist
in wake of the frontal passage as the core of the mid/upper level
low lingers off of the Northern California coast. Rainfall amounts
are forecast to range from 0.75-1.75" in the North Bay and other
coastal areas of the Bay Area/Central Coast, 1.50-3.00" in the North
Bay Mountains and coastal ranges, and 0.50-0.75 for most inland
urban areas.  Meanwhile, snow levels will range between 2,500-3,500
ft Monday night into Tuesday morning before rising to above 3,500 ft
during the bulk of the precipitation. Thus, snow accumulations are
likely in the highest peaks of the Bay Area and Central Coast, yet
should have minimal impacts to the general public.

Tuesday night into Thursday: The mid/upper level low will begin to
shift southward and traverse the central California coast on
Wednesday. This will keep the potential for isolated to scattered
rain showers and high elevation snow showers into Wednesday if not
early Thursday morning. The eventual track of this system will
determine exactly when conditions begin to dry out region-wide.

Thursday night and beyond: Drier conditions develop by Friday.
However, overnight/morning temperatures will turn colder once again
as cold air settles in over the region. Forecast confidence
decreases for the next weekend and into early next week. However,
there is good agreement with a the pattern over the West Coast
remaining troughy with cool and potentially unsettled weather. Stay
tuned...

&&

.AVIATION...as of 4:55 PM Saturday....For the 00Z TAFs. VFR
through the TAF period. Another late afternoon/evening with gusty
northwest/onshore winds. Similar to last night, winds will ease by
midnight. Lighter winds and clear skies to start the day on
Sunday before stronger winds develop again in the late afternoon.

Vicinity of SFO...VFR. Northwest winds with gusts around 30 kt
through 06z before diminishing overnight. Light northeast flow may
develop again Sunday morning. Northwest winds return by Sunday
afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through the TAF period. Northwest
winds with gusts 15-20kt cont through 06z tonight before
diminishing overnight. Light variable flow early Sunday before
increasing northwest again Sunday afternoon. The KMRY terminal
may see MVFR cigs briefly overnight with higher likelihood for
clouds south of the terminal.

&&

.MARINE...as of 08:21 PM PDT Saturday...Strong high pressure to
the north and low pressure to the south will maintain strong
northerly over the coastal waters. Gusty, gale force winds will
continue through early Sunday, especially along the immediate
coast south of Point Sur. Larger swell mixed with gale force winds
will create hazardous seas up to 15 feet in the outer waters. A
low pressure system develops over the eastern Pacific bringing
rain and wind to the coastal waters and bays early to mid next
week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Frost Advisory...CAZ503-504-506-510-512>518-528-529
             SCA...SF Bay until 3 AM
             SCA...Mry Bay until 3 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM

&&

$$

PUBLIC FORECAST: RGass/DK
AVIATION: MM
MARINE: MM

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