Rip Current in California, December 24, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1020 AM PST Tue Dec 24 2019

.SYNOPSIS...A weather system will bring widespread rain showers to
the region from late this evening into Christmas morning. Lingering
showers will be possible over the Central Coast into early Thursday
morning before conditions dry out region-wide from north to south.
Cooler, drier conditions are then likely through Saturday before
another weak system approaches late in the weekend.

&&

.DISCUSSION...As of 10:00 AM PST Tuesday...Water vapor imagery
shows one trough exiting the state this morning as the next one
approaches. As a result, some light showers are drifting along the
immediate coast from Point Reyes southward. These showers are
light in nature, with just a few locations picking up a hundredth
here and there. The other forecast concern this morning is with
the patchy fog in the North Bay Valleys. This fog is locally
dense, and with temperatures still hovering around the freezing
mark, patchy freezing fog remains a concern as well. At this time,
Santa Rosa is the only one left reporting 32 degrees, and expect
conditions to gradually improve over the next hour.

Otherwise, our main focus for today will be with the developing
rain late tonight and for Christmas day. Although isolated
showers will be possible throughout the day today along the
immediate coast, widespread rain will spread inland along the
north coast around 10 pm, and then gradually spread south and
east through the night and into the early morning hours of
Christmas. This rain and higher elevation snow will for the most
part be light. However, some convection is possible with this
line, with locally moderate to heavy precipitation possible at
times. Cannot rule out a lightning strike or two as well. Although
the threat of showers will continue through the afternoon, there
could be a break period before the next system slides south along
the coast tomorrow afternoon and evening. This secondary system
will bring additional rain and higher elevation snow to San
Francisco and areas southward. In fact, Monterey County will
likely see the best chance of showers continuing through the night
and into Thursday morning as some wrap around showers will be
possible as well. Snow levels vary quite a bit, but should drop to
between 3500 and 4000 feet. Think some accumulation will be
possible, but more favored to the San Lucias and Gabilan Ranges.
Will take a closer look at this for the afternoon package, as we
may need to consider winter products. Palmer

&&

.DISCUSSION...As of 03:29 AM PST Tuesday...It's a chilly start to
the morning with temperatures in the 30s and 40s across the
entire region. The coldest spots at this hour are in the North Bay
Valleys were surface temperatures are in the lower 30s with
patchy fog (dense at times). Thus, cannot rule out patchy frost in
these locations through sunrise. Meanwhile, scattered to broken
low level clouds have returned over areas from coastal San Mateo
County inland into the East Bay and then southward. Offshore, weak
returns are showing up on KMUX Radar and are likely picking up on
very light rain showers that have developed as moisture beings to
advect toward the region. Aside from an isolated shower or two
near the coast this morning, most of the region will remain dry
through at least the first half of the day.

Rain chances increase by late this evening and especially so during
the overnight hours as an approaching weather system drives a cold
front into the region. As the frontal boundary approaches from the
west, look for more organized rain showers to develop offshore late
this afternoon reaching the coastline likely after sunset. Isolated
thunderstorms will also be possible overnight given increased
instability associated with this system. Thus, given the convective
nature of these showers, rainfall may become moderate to locally
heavy at times as showers move inland through about sunrise
Christmas morning. Showers are then forecast to diminish in coverage
by midday as the cold front shifts to the east and dissipates.
Rainfall amounts will likely range from a few tenths of an inch to
around one-half inch through midday Christmas with locally higher
amounts possible within heavier showers and/or thunderstorms.

An upper level low in wake of the passing cold front will drop
southeastward offshore on Christmas Day. The general thinking is
that most precipitation during the day will remain offshore as well.
However, chances for precipitation may increase over the Central
Coast later in the day and into Thursday morning as the center of
the low pressure system begins to shift closer to the southern
California coastline. The greatest potential for additional
widespread rainfall will be over Monterey and San Benito Counties.
Additionally, this system is forecast to be cool (yet not extremely
cold) with snow levels falling to between 3,500-4,000 feet tonight
into Christmas Day. This would likely result in some snowfall
accumulation in the region's higher terrain, especially over the
higher ridges and peaks of the Central Coast.

As the aforementioned upper level system continues to push southward
on Thursday, conditions will dry out region-wide from north to
south. A short-wave ridge is then forecast to build over the region
and maintain dry weather conditions through at least Saturday of the
upcoming weekend. This will also result in mainly clear sky
conditions allowing for overnight conditions to drop into the 30s
and 40s for most locations through late week. Cannot rule out a few
hours of freezing temperatures in the region's coldest inland
valley/higher elevation locations as well. Daytime temperatures will
likely top out in the 50s each day this week with a slight warming
trend (upper 50s) late in the week as precipitation and cloud cover
moves out of the region. While confidence in the details remain low
at this time, the forecast models bring another weak system down
from the northwest either late Saturday or Sunday. RGass

&&

.AVIATION...as of 10:20 AM PST Tuesday...Patchy freezing fog
continues in the North Bay, but observations show visibility
beginning to improve. Expecting low cigs and reduced vis IFR-MVFR
to hang around the North Bay for much of the period ahead of an
approaching weather system. VFR conditions to prevail elsewhere
into this evening with FEW-SCT lower cigs between 2000-4000 ft in
addition to high level clouds around 20,000 ft. Pushed back the
timing on the arrival of heavy showers until later this evening
based on the latest runs of the HRRR and the local WRF. At this
moment, expect showers to begin across the North Bay late this
afternoon before progressing southward to the Bay Area this
evening and reaching the Monterey Bay Region later tonight. As
showers get closer, expect cigs to lower to MVFR with patchy IFR
cigs possible. Rain showers will then continue through the taf
period. Vis may lower during heavier showers. Low confidence on
exact cig heights.

Winds will become southerly for all locations by this evening
before southeast winds are expected to increase around the Bay
Area early tomorrow morning and locally down the Salinas Valley.
The latest models are showing southerly winds exceeding 10 kt with
higher gusts starting around 11z-12z tomorrow AM. Timing may
change as we get closer. Guidance has southeast winds continuing
until early tomorrow evening, around 02z-03z and outside the
current taf window.

Vicinity of KSFO...VFR with FEW to SCT cigs around 2000-4000 ft
and a BKN layer at around 5000 ft. High clouds also continue at
20,000 feet. Would not be surprised to see those lower cigs
occasionally go BKN later this morning, but at this time expecting
them to remain mainly SCT. Cloud layers expected to lower this
evening with the arrival of showers. MVFR cigs forecast to prevail
overnight and through the rest of the taf period. Guidance shows
SE winds increasing around the Bay Area tomorrow morning around
11z-12z. The latest local WRF indicates gusts of 15 to 20 kt will
be possible. MOS guidance as well as other models suggest SE winds
will lower in speed by the afternoon but will remain in that
direction until around 02z-03z tomorrow evening.

SFO Bridge Approach...Same as KSFO.

Monterey Bay Terminals...A few showers remain in the vicinity per
latest radar imagery. Expecting borderline VFR/MVFR conditions
with cigs around 3,000-5,000 ft before lowering overnight as
showers arrive. Heavier showers may reduce visibility. At this
time, looks like the heaviest precip will arrive late tonight and
early tomorrow morning. SE winds to prevail overnight and tomorrow
morning. Locally higher SE winds around 15 kt forecast for the
Salinas Valley tomorrow morning.

&&

.MARINE...as of 08:04 AM PST Tuesday...Light to locally moderate
northwest winds will persist today across the coastal waters along
with some scattered showers this morning. Winds will turn
southerly beginning this evening as a quickly moving upper level
trough moves through late this afternoon and tonight. This system
will bring rain along with a slight chance of thunderstorms over
the southern waters tomorrow. A long period northwest swell will
arrive tomorrow and build through Thursday. A second long period
northwest swell will then move through this coming weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...NONE.

&&

PUBLIC FORECAST: Palmer
AVIATION: AS
MARINE: AS

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