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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 211736
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
936 AM PST Thu Jan 21 2021

.SYNOPSIS...Mild temperatures and dry weather conditions will
prevail through Thursday. A significant shift in the weather pattern
will bring rain showers and much cooler temperatures to the region
on Friday. After a brief break on Saturday, more widespread
rainfall is expected late in the weekend with colder, unsettled
weather likely to continue into next week.

&&

.DISCUSSION...as of 07:30 AM PST Thursday...Morning update. A
cool start to the morning for many with numerous near to sub
freezing temperature observations across the interior. Areas away
from the bayshore and/or immediate coast were able to cool
overnight under mostly clear skies and light winds. Today will be
the last of the mild and dry days before a significant shift
towards a progressive (and wet) weather pattern takes hold over at
least the next 7 to 10 days across the Bay Area and Central
Coast. At least 3 to 4 wet systems will move over or near the
region through this time frame-- with the system arriving midweek
next week looking to be the most significant. Current forecast
package looks on tracks this morning, so please see previous
discussion for more details on the overnight forecast package
currently in effect.

.PREVIOUS DISCUSSION...as of 03:01 AM PST Thursday...Today will
be the last day of dry weather conditions and unseasonably mild
temperatures ahead of a major shift in the weather pattern. Low
clouds can be seen developing off of the Bay Area coast with some
high level clouds also streaming in from the northwest. Meanwhile,
cool and dry conditions prevail over the region with temperatures
in the 30s and 40s for most urban locations. With high pressure
gradually shifting southward ahead of an approaching weather
system, daytime temperatures will cool over the northern portion
of the region by about 4 to 8 degrees yet will remain above
seasonal averages. While lower 60s will be common over the
aforementioned areas this afternoon, still expecting mid/upper 60s
to lower 70s across the Central Coast.

A much colder mid/upper level trough and associated low pressure
system will drop southward down the West Coast late Thursday into
Friday. This will result in the development of isolated to scattered
rain showers late tonight (generally after midnight) across the
greater San Francisco Bay Area (first over the North Bay and along
the coast) before spreading over the Central Coast early Friday
morning. More widespread showers are then expected through the
afternoon as the core of the mid/upper level low moves southward
across the region. Given the rapid cooling of the air mass aloft,
instability will also increase enough to potentially produce an
isolated thunderstorm or two from late Friday morning into the
afternoon. However, thunderstorms have yet to be added to the
official forecast as confidence remains low at this time. Regardless,
look for most locations to pick up > 0.10" with the potential for
rainfall amounts of 0.25"-0.50" in areas of heavier rain showers.
Daytime temperatures on Friday are only forecast to warm into the
mid 50s for most urban areas while the higher elevations will likely
peak reach the 40s.

This quick moving system will exit the region from north to south
Friday night with most precipitation ending over the Central Coast
by sunrise Saturday. With mostly dry and sunny conditions likely on
Saturday, temperatures will warm slightly, yet will generally remain
in the mid/upper 50s. The next weather system will quickly drop in
from the northwest during the day on Sunday with an increase in
cloud cover, more widespread precipitation and a reinforcing shot of
colder air (both at the surface and aloft). The speed of this system
and somewhat limited moisture tap should preclude widespread heavy
rainfall, yet totals between 0.25"-0.75" will be possible with
locally greater totals in the coastal ranges. Again, cannot rule out
an isolated thunderstorm or two Sunday afternoon/evening given the
cold, unstable air mass aloft. Snow levels are also forecast to drop
to below 2,000 feet across the Bay Area and below 2,500 feet over
the Central Coast Sunday night into Monday morning. Thus, lingering
post frontal snow showers will be likely over the region`s higher
ridges and peaks.

While a break in precipitation appears possible during the day
Monday and potentially into Tuesday morning, ensembles continue to
indicate a continued cool and potentially wetter pattern arriving by
the middle of next week.  This third system to impact the region
appears to have a deeper moisture tap and may produce more
widespread moderate to heavy rainfall over the region. Ensembles
then favor a continued cool, unsettled weather pattern through the
remainder of the month and into early February.

&&

.AVIATION...as of 9:32 AM PST Thursday...For the 18Z TAFs. All
terminals in the Bay Area are currently VFR with only high clouds
moving overhead. Along the immediate coast is a different story
with a shallow cloud deck and fog. Some lower clouds snuck through
the Golden Gate and into the City, but fortunately not to KSFO.
Through this evening forecasting VFR with typical daily winds. A
cold front to the north will gradually increasing/lowering clouds
tonight and early tomorrow. Additionally, the front will bring
some scattered showers.

Vicinity of KSFO...VFR, mainly light wind. MVFR
ceiling and showers developing by 11z Friday. Showers changing to
vc showers by 16z and thereafter on Friday. VC showers will
lingering into the afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. mainly light winds. Patchy MVFR-IFR
ceilings Friday morning. Showers arriving mid morning.

&&

.MARINE...as of 09:30 AM PST Thursday...Winds will increase later
today and tonight as a cold front arrives from the north. The
front will bring a chance for scattered showers. Unsettled weather
will continue into the weekend with additional showers. A moderate
northwest swell  prevails over the seas.

&&

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

&&

$$

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

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NWS MTR Office Area Forecast Discussion

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