Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 201721
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
921 AM PST Fri Jan 20 2017

.SYNOPSIS...A cold front is currently exiting the district but
scattered showers are expected to continue today in the unstable
airmass behind the front. In addition, isolated thunderstorms
will be possible. A few showers will then linger into Saturday
morning with another, likely wetter system arriving late Saturday
into Sunday. Given the antecedent conditions from recent rainfall,
this system will have the potential to result in additional
flooding concerns across portions of the region. Showers will then
linger behind this cold front into Monday with dry conditions
returning by Tuesday.

&&

.DISCUSSION...as of 9:21 AM PST Friday...A fast-moving but robust
cold front moved across the district overnight and this morning,
bringing moderate to heavy rains and gusty winds. Right now, the
front has exited the CWA, but there are showers behind the front
in the unstable airmass. KMUX radar is showing several impressive
echoes just off the San Mateo coast and moving across towards the
San Francisco Bay. This shower activity will persist through the
day and gradually taper off overnight. Wind speeds have decreased
and the wind advisory will be allowed to expire at 10 am, if not
cancelled before then.

By Saturday late morning/early afternoon, showers will likely be
ended with a bit of a break in precipitation for a good portion
of the region.

The next system in this series remains on track to impact the
area from late Saturday into Sunday morning. This system appears
to have a deeper moisture tap with PWAT values forecast to range
between 1 to 1.25 inches. Given this, expecting rainfall amounts
to be greater than the previous two systems which will likely
lead to a greater threat for flooding issues. Additional rainfall
amounts from Saturday into Sunday will range from 0.75 to 1.25
inches in most urban areas and upwards of 2 inches in the North
Bay Valleys. Meanwhile, the coastal ranges and North Bay Mountains
will likely see 2 to 4 inches with isolated amounts upwards of 6
inches. Thus, a Flash Flood Watch will likely be considered for
portions of the district if the forecast models remain consistent,
especially considering a few creeks went to flood stage with last
night`s storm. Along with the additional rainfall, southerly winds
will also increase and become gusty at times ahead of the frontal
boundary that will sweep through late Saturday night into early
Sunday morning.

Showers will then likely linger into Sunday night and even through
much of the day Monday as the main mid/upper level low drops
southward down along the northern California coast. By Tuesday, the
trough will shift inland while a ridge begins to develop off of
the West Coast. This ridge will maintain dry weather conditions
through the remainder of next week with temperatures near seasonal
averages.

&&

.AVIATION...As of 5:03 AM PST Friday...A very strong and fast moving
cold front is pressing eastward across the forecast area this morning
accompanied by heavy rain and wind. Pressure falls along the coastal
waters and over land have been commonly 2-3 mb per hour since late
last evening /pressure falling rapidly/. Cold unstable air is quickly
moving in this morning, expect a continuation of heavy showers and
t-storm development through this morning and afternoon. IR imagery
shows lots of open cellular clouds to our W-NW, a signature of plenty
instability to arrive today.

Vicinity of KSFO....Airport weather warning valid through 15z, may
need to extend warning if winds remain strong and gusty. Model guidance
indicates possible lull then re-energized westerly winds developing
later this morning into the afternoon. Expect downpours and possible
additional t-storms with reduced visibilities and ceilings to MVFR
possibly briefly IFR visibilities.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Strong and gusty southeast winds prior to
cold front moving through early this morning. MVFR possibly briefly
IFR visibilities in downpours and t-storms.

&&

.MARINE...as of 8:56 AM PST Friday...Strong southerly gale force
winds will continue across the coastal waters through this morning
ahead of an approaching cold front. As a result rough and
hazardous seas will continue through today. After the front passes
this morning winds will shift to the west but remain moderate and
gusty. Winds will increase again Saturday night as another frontal
system approaches. Very large swells arrive Friday afternoon with
swell heights building to 25 to 30 feet Friday night and Saturday.
Rogue waves of 30 feet or greater are possible during this
timeframe.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ006-505-507-509-511-512-517-518-529-530
             High Surf Warning...CAZ006-505-509-529-530
             SCA...Mry Bay
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar
             SCA...SF Bay

&&

$$

PUBLIC FORECAST: Sims
AVIATION: Canepa
MARINE: CW


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