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

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FXUS66 KMTR 221802
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1002 AM PST Sun Jan 22 2017

.SYNOPSIS..A strong cold front pushed through the region last
night and early this morning. Convective showers will gradually
fill in today with thunderstorms expected by the afternoon and
evening. Strong and gusty winds, small hail and brief heavy
downpours will be possible within these showers and thunderstorms
this afternoon. Showers will then continue through Monday before
drying conditions return Tuesday afternoon.

&&

.DISCUSSION...as of 09:30 AM PST Sunday...A vigorous cold front
swept through most of the region this morning and is currently
over southern Monterey and San Benito county. The strongest winds
occurred overnight, with a peak gust of 79 mph reported over Big
Sur. Winds have now weakened significantly in the post frontal
environment. As a result, the high wind warning and wind advisory
have both been cancelled as of 9am. The heaviest rain also
occurred overnight, with the highest precipitation accumulations
coming over Big Sur and the Santa Cruz mountains (picking up 2 to
3.5 inches since midnight). Lower lying urban locations picked up
.25 - 1.25" since midnight. Due to the overnight unsettled weather
and saturated antecedent conditions from weeks of excessive
rainfall, numerous reports of nuisance flooding, mud and rock
slides, and downed trees have been received. A flash flood watch
remains in effect through 6 PM this evening. Be extremely
cautious if traversing the area today, especially over back
country roads.

The main forecast focus is now on the unstable post frontal
environment and the potential for widespread rain showers and
embedded convection. Isolated to scattered thunderstorms will be
possible today, especially by the afternoon into the evening
hours. The Storm Prediction Center has placed our region under a
marginal risk of severe thunderstorms, a rare issuance, as a
strong upper level jet spreads into the coast - which highlights
the magnitude of todays convection. Copious amounts of small hail
that can cover roadways, slightly larger hail than normal, intense
heavy downpours, erratic gusty winds, and lightning are all
possible with thunderstorms later today. Eyes will be glued to the
radar today as the ingredients for a brief spin up are also there
( 0-1km total shear over 10m/s, 0-6km total shear over 35m/s, LCLs
below 1500 ft, and sufficient cape and helicity) for later today.
Rain showers will then persist through tonight and Monday, before
tapering off into Monday night. Additional precipitation
accumulations will be difficult to pin down to the hit or miss
nature of these convective showers, however, it will be possible
to see precipitation accumulations for low lying areas range from
.5"-1.5" and 1.5-3" for higher terrain between now and Monday
night.  More importantly are the intense rain rates which occur
with stronger convective storms that can overwhelm urban storm
drainage, burn scars, and smaller waterways.  Additional ponding of
water and nuisance flooding should be expected today and tomorrow
as a result.

See the previous discussion for additional details on the current
forecast.


.PREVIOUS DISCUSSION...as of 03:45 AM PST Sunday...A strong cold
front remains just off the coast early this morning and continues
to produce light to moderate rain across the region along with
strong, gusty winds. Rainfall rates have increased a bit over the
past few hours across the North Bay and along the coastal ranges
with 1/3 to 1/2 inches per hour. With this, have seen some slight
rises in area creeks, yet no major reports of urban issues at this
time. Do expect moderate to locally heavy rainfall to continue,
especially from the Santa Cruz Mountains southward along the coast
as deeper moisture advects inland to the Central Coast ahead of
the frontal passage. Wind speeds also remain strong and gusty this
morning with many reports of gusts exceeding 60 MPH over the
ocean, along the coast and in the higher elevations. Thus, will
keep the High Wind Warning and Wind Advisory posted until the
front passes through the region. Widespread rain will then taper
off from north to south through the remainder of the morning with
many areas across the North Bay potentially seeing a break in
precipitation after 7 AM or so while lingering rain continues
along the Central Coast. By late morning, the region can expect a
brief break in precipitation before another round of showers
return in the unstable post-frontal environment.

The SPC has put the central California coast in a Marginal Risk for
severe weather this afternoon as the exit region of a strong upper
level jet spreads into the coast. With this, deep shear and unstable
post-frontal environment, cannot rule out a line of convection that
could produce small hail, damaging wind gusts and the potential for
a brief/weak tornado. The HRRR and other short-term, high resolution
models generally show this line of convection impacting just about
the entire San Francisco Bay Area as well as the Central Coast. In
addition to this treat, brief heavy downpours will likely pose a
flash flood threat as well. Once this passes through early this
afternoon, showers will linger into tonight and through a good
portion of Monday as main mid/upper level low off of the Pacific
Northwest coast shifts southward down the California Coast.

By Tuesday afternoon, the main upper level trough shifts inland and
a ridge of high pressure begins to build over the eastern Pacific.
This will allow for dry weather conditions to return to the region
with clearing skies possible Tuesday afternoon/evening. The forecast
models continue to show a weak disturbance passing inland to our
north Wednesday/Thursday. However, this system should be mainly dry
and any precipitation will likely stay north of the region. Could
see an increase in mid/high level clouds however. Regardless, much
of the region should remain dry through the late week and into the
upcoming weekend. This will be much needed after very wet conditions
the past several weeks. Temperatures by the weekend may even climb
above seasonal averages region-wide as high pressure builds over the
West Coast.

&&

.AVIATION...As of 10:01 AM PST Sunday...Winds have decreased
behind the front, and showers are lightening up. Good chance of
instability showers or thunderstorms this afternoon so have put
VCTS in all TAFs.

Vicinity of KSFO....SW-W winds today with gusts up to the 20 knot
range by late morning or early afternoon. Air mass conducive to
heavy showers through today, isolated t-storms this afternoon and
evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Air mass conducive to heavy showers and
thunderstorms, especially later today and tonight.

&&

.MARINE...as of 09:00 AM PST Sunday...Winds are decreasing behind
a strong cold front that moved across the waters overnight. Gale
force winds have given way to small craft advisory category winds.
Cold and unstable air will move in today through Monday with
showers and thunderstorms developing. Large westerly swell will
persist through Monday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
 .Tday...Flash Flood Watch...CAZ006-505>513-516>518-528>530
         High Surf Advisory...CAZ006-505-509-529-530
         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...Pigeon Pt to Pt Piedras Blancas 10-60 nm
         SCA...Pt Arena to Pigeon Pt 10-60 nm
         SCA...Rough Bar Advisory for SF Bar
         SCA...Mry Bay
         SCA...SF Bay until 3 PM


&&

$$

PUBLIC FORECAST: DRP
AVIATION: Sims
MARINE: Sims


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