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

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FXUS66 KMTR 011632
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
932 AM PDT SAT OCT 1 2016

.SYNOPSIS...Cooler than average temperatures will persist across
the region today ahead of an approaching upper level system. As
this system pushes inland on Sunday, chances for rain showers will
increase with the potential for afternoon and evening
thunderstorms, especially across the San Francisco Bay Area.
Another system will bring chances for rain showers to the region
from late Monday into Tuesday morning. Drier conditions and a
slight warming trend is then forecast for the second half of next
week.

&&

.DISCUSSION...as of 09:30 AM PDT Saturday... Satellite imagery
shows an upper level trough positioned off the Pacific coastline,
with the core of the upper low offshore of Seattle/Vancouver,
while the base of the trough is offshore of central California.
A surface dry cold front preceeding this upper level feature moved
through the area yesterday/overnight and has brought cooler
temperatures, breezy to gusty winds, and a mixed out marine layer
over the last 24 hours. Temperatures are generally running 3-6
degrees, but up to 20 degrees, cooler this morning as a result of
continued cold air advection from the northwest and predominately
clear skies overnight. Expect fair weather today as the upper low
takes a bit of hiatus due to the lack of any significant vorticity
pulses transversing the trough in our vicinity. Onshore winds will
prevail today, albeit weaker than yesterday, however daytime highs
for most locations will actually be warmer than yesterday by a few
degrees.

The upper low resurges tomorrow to dominate our local weather.
This comes as a result of the core of the upper low entering our
region, bringing with it cold unstable air and more energetic
dynamics aloft. These factors will combine to bring cooler
temperatures, increased winds, and the first rain of the season
for some locations. Surface temperatures are anticipated to be
10-15 degrees below seasonal normals tomorrow, with -24C at 500MB,
-5C at 700MB, and 5-6C at 850MB. Winds should be similar to those
experience on Friday, with the strongest winds over north/east bay
ridge tops and along the immediate coast. Precipitation will come
in the form of isolated to scattered rain showers, predominately
located over the Greater Bay Area. Short term models also indicate
the potential for some showers to stray further south over the
Santa Cruz mountains, and possibly into the Monterey Bay. That
said, the bulk of the precipitation is expected over the
North bay, where around a tenth of an inch of rain is expected,
but locally higher amounts of up to a quarter of an inch are
possible. Furthermore, convective parameters are bringing
sufficient cape and instability into the region to warrant a
mention of isolated thunderstorms for Sunday primarily over the
north/east SF Bay.

The upper low then shifts further inland on Monday, allowing for
cold air to settle in. Monday should be just as, if not colder
than, Sunday. By Tuesday, the upper low exits the area and
transitionary northwest flow will reign aloft. By Wednesday,
an eastern Pacific high pressure system will build into the state
and bring moderately warmer temperatures. By the end of the week,
medium to long term models show a low pressure trough digging into
the state. The overnight forecast package looks in good order, so
no changes are planned before the afternoon forecast package is
sent out. See previous discussion for more details.


.PREVIOUS DISCUSSION...as of 03:28 AM PDT Saturday...Mostly clear
skies prevail this morning across inland areas where temperatures
range from the upper 40s to upper 50s. Patchy low clouds can be
seen on satellite imagery just off of the coast with a few clouds
developing inland as well. Cloud cover has been less extensive as
previously thought given the well mixed marine layer. With this,
any cloud that develop through sunrise will likely burn-off
through the morning hours and give way to mostly sunny conditions
this afternoon. Temperatures today will be similar to those
yesterday afternoon as a mid/upper level trough remains off the
West Coast.

Meanwhile, off of the British Columbia coast, a mid/upper level
low continues to deepen and is forecast to drop southward down the
Pacific Northwest coast and into northern California late tonight
into Sunday morning. As this system approaches the region,
isolated to scattered rain showers will develop offshore and push
into the North Bay region by early Sunday morning. Showers will
then be possible as far south as the Monterey Bay region as the
system pushes inland to our north. The forecast models do show
enough instability to generate a few thunderstorms over the
northern half of the region as well by Sunday afternoon and
evening. Temperatures will also cool further on Sunday as the
system advects cooler air across the region. Thus, afternoon
temperatures will likely remain in the 60s for most urban areas
with cooler conditions in the hills/mountains. While lingering
showers will be possible Sunday night, most locations will see
drying conditions from Sunday night into Monday morning as the
system shifts inland.

The forecast models show another system with precipitable water
values up to around 1.25-1.50" advecting into the region as
another short-wave disturbance pushes across the region late
Monday into early Tuesday. While this system is forecast to weaken
as it approaches the coast, showers will again be possible over
the region from Monday afternoon into early Tuesday morning.

A drying and warming trend is then forecast to develop by the
middle of the upcoming week with temperatures forecast to rebound
back to near seasonal averages by Wednesday. Little change is
forecast for the latter part of next week as a weak short-wave
ridge builds along the coast. Temperatures will generally remain
near seasonal averages with ongoing dry weather conditions. Late
next week, the models do show another trough impacting the Pacific
Northwest, yet precipitation looks to stay well to our north at
this time.

&&

.AVIATION...as of 3:52 AM PDT Saturday...Patchy morning low clouds
will dissipate during the day, otherwise VFR. A deeper upper level
trough moves eastward across northern California late tonight and
Sunday with showery weather reaching the forecast area after 12z-
18z Sunday.

Vicinity of KSFO...VFR. Gusty west winds this afternoon and evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR.

&&

.FIRE WEATHER...as of 03:28 AM PDT Saturday...Decent humidity
recoveries observed over much of the region this morning, with the
driest conditions over the higher peaks around the Soberanes Fire.
In addition, northwest winds remain the strongest over that region
at around 10-15 MPH with gusts around 15 MPH while wind speeds are
slightly weaker over the Loma Fire. Winds will be locally breezy
again today and on Sunday yet not as strong as those on Friday.
Meanwhile, an upper level system will push inland to our north on
Sunday and bring chances for rain showers to the region with the
potential for a few thunderstorms, mainly over the San Francisco
Bay Area. Chances for wetting rains will be best over the North
Bay with only a few hundredths of an inch or so from the Santa
Cruz Mountains southward. Another chance for showers will return
Monday night into Tuesday morning as another system impacts the
region. Cooler than average temperatures will persist through
early next week along with modest humidity recoveries.

&&

.MARINE...as of 09:04 AM PDT Saturday...An upper level trough will
move east across northern California this weekend bringing a
slight chance of showers mainly to the northern coastal waters. A
small mixed swell with a long period southerly swell will continue
through the period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm from 12 PM
             SCA...SF Bay from 12 PM
             SCA...Mry Bay from 12 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

PUBLIC FORECAST: DRP
AVIATION: BAM
MARINE: BAM
FIRE WEATHER: RGass


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