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

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FXUS66 KMTR 161243
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
443 AM PST Sun Dec 16 2018

.SYNOPSIS...A potent system will cross the Bay Area and Monterey
Bay regions through the Sunday hours. Look for widespread rain,
heavy at times, and strong gusty winds out of the south ahead of
frontal passage. Showers and a few thunderstorms will be possible
in the wake of the front, followed by clearing and drying
conditions. Large waves will being impact the coastline late
Sunday through Tuesday.

&&

.DISCUSSION...as of 03:00 AM PST Sunday...Several weather impacts
the next 24-36 hours with gusty winds, moderate to heavy rain,
possible thunderstorms and very large surf as a potent storm
sweeps through.

A quick look at the satellite imagery over the Eastern Pacific
pretty much says it all - a solid low pressure circulation
spinning in the Gulf of AK with a frontal structure extending
southward. Behind the front is the nose of a 160kt jet further
amplifying the upper trough. Impressive cooling cloud tops near
the base of the trough as well.

Now for the details closer to the Bay Area. KMUX radar imagery
continues to show light showers over the coastal waters and
locations north of the Golden Gate. These showers are associated
with warm advection ahead of the cold front and isentropic lift as
seen on the 295k. Rain gauges over the North Bay the last six
hours have been a few hundredths to a tenth. There have been a few
jack pots like Venado at over 0.5". Expect the shower activity to
pick up the next few hours as the cold front currently off the
coast slowly tracks eastward. One trend on the forecast has been
to slow the onset of rain south of the Golden Gate until later
this afternoon. In fact, some guidance keeps areas south of the GG
dry through almost 00Z Monday. Did not go that extreme, but did
trend drier. The heaviest rain is still ontrack to accompany the
actual cold front later this afternoon. HRRR model timing brings
heavy rain into the North Bay 20-21Z before moving southward this
evening and early tonight. This period of intense rainfall could
lead to some minor flooding/ponding. Rain will quickly end from N
to S behind the front late tonight and early Monday. No real
change from an Atmospheric River stand point either. Latest output
remains ontrack with decent moisture available to put this AR in a
moderate category. Outside of some potential minor flooding this
will be a mostly beneficial rain to the region.

Rainfall totals will range from 1-3 inches across the North Bay
and coastal mountains and 0.5 to 1.25 inches in the populated
areas from the Golden Gate south. Some of the higher terrain of
the North Bay could see locally greater than 3-4 inches as
moderate rain falls most of the day Sunday.

As for for potential thunderstorms, decided to leave the low end
chance in the forecast with the fropa, but confidence has
decreased. Models have backed off the instability late today and
tonight, but upstream obs show lightning over the EPac near the
front and digging jet. SPC still has the region in a general
mention for thunder.

One final concern today will be the increasing southerly winds.
Winds will continue to increase across the region be strongest
over the higher terrain and coastal locations. Gusts of 20-40 mph
are already being seen and a few isolated gusts up to 50 mph will
be possible. Winds will be strong and gusty, but not widespread
enough to warrant a wind advisory at this time.

Drier weather returns Monday into Tuesday for most of the Bay Area
with building high pressure. However, a few weak disturbances push
into far NorCal keeping some lingering showers over northern areas
of the North Bay. A warming trend will develop by midweek with
warmer 850mb temps and building high pressure. Another chance for
light rain develops on Friday with a weak system moving through
the region.

&&

.AVIATION....as of 03:20 AM PST Sunday...Main aviation impacts
continue to be focused on the approaching storm system. One change
to the current taf package from the previous one was to slow the
onset of some of the moderate to heavy rain. Otherwise, biggest
impacts with deteriorating conditions will be later this afternoon
and into the evening with the passage of the cold front. Steady
precip will quickly end from N to S behind the front with only a
few lingering showers late tonight. However, low level moisture
will keep lowered cigs in place.

Winds will also be an issue this afternoon with gusty conditions
develop with the front. SE winds with gusts up to 30 kt and wet
runways will likely lead to some delays.

Vicinity of KSFO...VFR conditions expected into Sunday morning.
Fropa has slowed to near midday Sunday with gust southerly winds
and moderate to brief heavy rain during that time frame. Winds may
gust up to 30 kt ahead of the front. Lingering showers possible
overnight behind the front.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR thru early this afternoon and then
lowering cigs with rain late this afternoon.

&&

.BEACHES...as of 04:44 AM PST Sunday...Dangerous conditions are
expected today and Monday as very large surf will impact the
coastline. Long period forerunners will arrive Sunday morning
increasing the risk for sneaker waves and rip currents. The period
will diminish as the swell height increase through the day today
before peaking this evening through early Monday. Peak swells of
17 to 21 ft at 19 to 21 seconds will be possible. Buoy 006 SE Papa
is currently showing the swell train moving through - swell 31.8
ft at 19 secs. JASON Satellite waveheight data shows wave heights
of 36-38 feet. Large breaking waves of 25 to 40 ft will be
possible at west and northwest facing locations, with breakers up
to or exceeding 50 ft at favored break points along the coast
today. A High Surf Warning remains in effect from 9 AM Sunday
through 9 PM Monday along the entire coastline.

&&

.MARINE...as of 03:19 AM PST Sunday...Southerly winds will
increase ahead of an approaching frontal  system leading to steep
fresh swell and hazardous sea conditions.  A very large, long
period WNW swell will begin to move into the  coastal waters this
morning, with significantly building seas by  this evening through
Monday morning. The swell will taper off  late Monday into
Tuesday, however, a different large northwest  swell train builds
Wednesday into Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Surf Warning...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...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...Mry Bay from 11 AM
             SCA...SF Bay from 10 AM



&&

$$

PUBLIC FORECAST: MM
AVIATION: MM
BEACHES: MM
MARINE: Canepa

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