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

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FXUS66 KMTR 241733
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
933 AM PST Fri Nov 24 2017

.SYNOPSIS...Areas of patchy dense fog in the North Bay and Oakland
shoreline this morning. Above average temperatures today with near
record temperatures in a few locations. Pattern change arrives
Saturday night into Sunday with cooler, wet weather.

&&

.DISCUSSION...as of 8:45 AM PST Friday... Patchy dense fog is
being reported throughout the North Bay, with visibility down to
around a quarter mile at Napa, Novato, Santa Rosa, as well as
farther southward near the Oakland shoreline. This shallow fog
will burn off through mid morning as the sun rises and helps to
mix out the nocturnal inversion.

Temperatures are generally running similar to yesterday morning,
with mid to upper 50s throughout the area. Above average
temperatures are forecast to extend through tomorrow, with
temperatures generally 5 to 10 degrees above normal today. The
South Bay will be up to 10 degrees above normal today and San
Jose, forecast to reach 73, will fall just shy of their record
high of the day (75 set in 1959). These above average temperatures
are the result of a high pressure ridge that built into the
region from the Baja California region.

The next significant shift in the weather will come tomorrow night
into Sunday as a weak to moderate low pressure trough aligns with
a wet 1.4-1.8 inch precipitable water plume off the coast. The
cold front ahead of the upper level feature will begin to bring
rain to the North Bay overnight Saturday into Sunday, before the
frontal boundary and moisture plume eventually sweep southward to
include the San Francisco and Monterey Bay regions. There is a bit
of model uncertainty regarding amounts at this point, however,
the alignment of the wind field and precipitable water plume to be
orthogonal to the higher elevation terrain means that the higher
elevations will once again see more rain than the lower vallies.
For now, higher coastal hills and mountains could see from 1 to 2
inches, while lower vallies will see lesser amounts in the range
of 1/3 - 7/8", with rain shadowed valleys even lower. The main
forecast challenge of the day will be deciphering the timing,
intensity, and amounts of this system.

See the previous discussion for more details on the current
forecast. Next forecast with updated numbers will be issued early
this afternoon.

.PREVIOUS DISCUSSION...As of 5:43 AM PST Friday...The tail end of
a very weak atmospheric river with IVT values decreasing to less
than 250 kg/m/s is drifting southward across the area this
morning. This is part of the wet weather over the Pacific
Northwest in recent days. Unseasonably mild weather this morning
will keep going a little longer with temperatures forecast to
reach about ten degrees above normal again today, record highs are
not expected.

A little cooling is arriving at the 925 mb and 850 mb levels based
on the NAM output with a slow rate of cooling seen on the profiler
data. Residual subtropical air with dewpoint temperatures in the
50s this morning combined with light winds and a gentle rate of
cooling is resulting in patchy dense fog over the North Bay in
particular. With the arrival of a dry cool frontal boundary this
afternoon and tonight it`ll become cooler yet, at least a little
closer to normal for late November, everywhere overnight into
Saturday morning. 40s will be common over inland valleys Saturday
morning and lower 50s by the water; current forecasts may be a bit
mild with Saturday morning lows and may need to be adjusted
downward a little.

With peak intrusion of cooler, drier air Saturday warmer air aloft
then develops along with an increasingly moist southwest flow aloft
by late in the day. This moist flow reaches the North Bay first then
slides southward to the Central Coast late Saturday night into Sunday.
On the synoptic and planetary scales a stationary long wave ridge
remains over the west coast compounding with occasional strengthening
of a short wave ridge which will strengthen once again Saturday.
Stronger westerlies over the eastern Pacific with synoptic and planetary
scale long wave troughing will advance toward this massive ridge
this weekend setting the stage for wet and windy weather the main
focus being the North Bay, East Bay hills/mtns and Santa Cruz Mountains.

A hefty plume of precipitable water with values approaching 1.50" per
recent GFS and WRF model output would be at seasonal max values on
all Oakland soundings from 1948 to 2014 if it reaches here, which is
what the WRF is forecasting. There`s pretty good agreement between
the GFS and NAM IVT forecasts indicating a moderate to borderline
strong atmospheric river (AR) Saturday night into Sunday, with
possible lingering AR effects Sunday into Monday morning over East
and South Bay areas. IVT values 500-600 kg/m/s rise to 700-800 kg/m/s
over the Bay Area Saturday evening into Sunday morning. Similar
values then overspread the Santa Cruz Mountains early Sunday morning
to early-mid Sunday afternoon. All of this progresses to the Big Sur
coast, but will be moving faster Sunday due to an upstream kicker low
pressure trough in strengthening westerlies.

What`s a bit confusing at this time is that the ECMWF, GFS (and
NAM too) are indicating only about 1 inch QPF in the wettest spots
late this weekend where orographic forcing will be strongest in
areas mentioned above. Additionally the WRF forecasts a core of
40-50 knots of southwest winds over the highest terrain of Santa
Cruz county and in the East Bay from 1 am to 8 am Sunday. Thus if
model qpf increases in subsequent output and the WRF continues to
highlight strong gusty winds as mentioned flash flooding may occur
on recent wildfire scars and winds may be close to advisory levels
later this weekend. Sunday night into Monday a cold core low moves
in destabilizing the atmosphere with increasing mucape in the several
hundred j/kg range; isolated thunder not in forecast but a possibility
late Sunday night into Monday morning.

Made some adjustments to the grids pops, qpf follows GFS more
closely for instance. Also adjusted higher terrain winds upwards a
bit, but not to wind advisory levels. Need to see if model qpf
changes toward higher amounts before issuing a flash flood watch.
Stay tuned, the potential is there for quite an unsettled weather
pattern this holiday weekend.

Then.. models are in decent agreement indicating dry weather returns
from midday Monday through the remainder of next week. The long wave
ridge becomes nudged eastward to the Desert SW Sunday-Monday then
retrogrades back toward California mid-late next week.

&&

.AVIATION...As of 9:30 AM PST Friday...For 18z Tafs. Interesting
start to the day with very shallow, but dense fog impacting
numerous airports. Latest trends on cams/satellite/metars all
indicate the fog burn off will be occurring shortly. Therefore,
TAFs valid at 18z will not have any mention of fog. VFR this
afternoon and overnight for most locations. KSTS will likely have
fog again tonight. Models suggest some fog from MOSguidance, but
local WRF shows much drier RH at the surface. Will have to watch
closely, but didn`t include any dense fog tomorrow morning at
other airports.

Moderate confidence.

Vicinity of KSFO...VFR and light winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Light W/WNW winds this afternoon,
5-10 kt.

&&

.MARINE...as of 09:27 AM PST Friday...High pressure to the north
will maintain moderate to locally strong northerly winds over the
coastal waters through Saturday morning. Winds will initially be
strongest south of Point Sur. An  approaching storm system,
currently over the Gulf of Alaska, will  turn winds southerly and
increase by mid Saturday morning. A  secondary and more robust
storm system will affect the coastal  waters Sunday evening and
through the day on Monday. Winds may  approach Gale Force for
parts of the coastal waters with this  system, though forecast
models have been inconsistent at this time to say with high
confidence. Northwest swell will increase by  Sunday morning.
Chances of rain begin late Saturday afternoon.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

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

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