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

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FXUS66 KMTR 251725
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
925 AM PST Sat Feb 25 2017

.SYNOPSIS...A weather system dropping from north to south off the
West Coast will bring rain chances to our area today and tonight,
primarily near the coastal areas. Another system will produce
shower chances from late Sunday into Monday. Dry weather is
expected to return by Tuesday. An extended period of dry weather,
along with a warming trend, is then forecast through the remainder
of next week.

&&

.DISCUSSION...as of 09:20 AM PST Saturday...Generally quiet
weather conditions persist over the region this morning showers
off of the Central Coast earlier this morning have dissipated.
Should see dry conditions persist for the next few hours but
expecting a renewed chance of showers later in the day as an upper
level low seen on satellite imagery off of the northern California
coast drops southward across the region. This activity will
continue to shift from north to south through the night with some
accumulation of snow likely along the higher elevations (generally
above 3,000-3,500 feet) by early Sunday morning. Elsewhere, cool
conditions will persist with showers amounting to upwards of
0.10"-0.25" overnight while some may see no measurable rainfall at
all. Only minor changed were needed to the ongoing forecast in
the short-term. Please see previous discussion below for
additional details.

&&

.PREV DISCUSSION...As of 2:37 AM PST Saturday...Clouds spreading
in from the north are keeping temperatures from dipping below
where they were yesterday at this time at most locations. Current
readings are actually several degrees warmer than 24 hours ago,
with readings from the mid 30s to the mid 40s. Expect little
change in high temperatures today, with most areas warming to the
mid 50s.

Meanwhile, our Doppler radar is currently showing a few showers
moving onshore over southern Marin county and the San Francisco
Peninsula. Our spotter in downtown San Francisco reported
receiving 0.01" a couple of hours ago, as did our automated rain
gauge downtown. These showers should taper off later this morning,
per the HRRR with more showers developing mainly near the coastal
areas later this afternoon and overnight as an upper level
disturbance moves from north to south off the west coast.

IR satellite imagery shows the cold upper low moving south off of
the Oregon/California border at this time. (The current light
showers now affecting San Francisco are associated with a weak
upper level jet streak currently moving westward over the central
California coast.) Models have the upper low moving south off of
the coast with very little rain reaching the Bay Area before the
low swings inland over Big Sur by Sunday morning. Another system
dropping south could bring showers to more areas, however this
system has no moisture support, so very little in the way of QPF
is expected.

There is a third upper low currently located over 35N/145W progged
to slowly move to the east, then southeast before opening up as a
trough and moving onshore over northern Baja California by late
Monday. This system has a sub-tropical connection, and if either
of the first two systems connect with this moisture, more rain
could be produced. The Nam, the GFS and the ECMWF all have the
first low pressure system doing this, however, by the time there
is a moisture connection, the low is progged to be south of our
area, and instead, the moisture tap takes aim at southern
California on Sunday and Monday. A similar scenario is spelled
out by the Canadian model. Thus, confidence is bolstered for a
less-impactful storm scenario for our area with, instead, cool
and showery conditions with the possibility of a dusting of snow
across the higher peaks, mainly in Big Sur.

After the upper level trough moves to the east, expect a warming
and drying trend to set up beginning late Tuesday and continuing
through the forecast period. Long-range climate prediction models
keep this dry trend in place. However...the 9-10 day frames of the
ECMWF now bring zonal flow to the west with rain chances again
across portions of Northern California by late next weekend. The
GFS keeps a ridge over California at this time frame, so we`ll
see.

&&

.AVIATION...As of 3:45 AM PST Saturday...For 12z Tafs. KMUX radar
currently shows very light showers coming onshore around the San
Francisco Peninsula. These showers quickly dissipate by 15z per
latest HRRR model. KSFO has seen -RA over the past couple of
hours, with MVFR CIGs with each shower. Not very confident with
timing and coverage of shower activity today, though chances will
be low across the district. However, any shower will pose a
threat of MVFR CIGs/VIS. Otherwise, today should be mostly VFR.
Winds will be a little funky today, variable through the day, but
will stay relatively light, under 10 kt. Low to moderate
confidence on this forecast.

Vicinity of KSFO...Generally VFR, with pockets of MVFR possible,
especially with showers that occur. North Bay terminals will see
best chance of showers, but cannot rule out showers for Bay Area
terminals, especially this evening. Winds will be variable through
the day, but should remain under 10 kt.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Mainly VFR today, with showers possible.
Best chance of showers and MVFR CIGs will be this evening, though
confidence is low. Light and variable winds this afternoon.

&&

.MARINE...as of 09:20 AM PST Saturday...Low pressure sliding down
the California coast today will bring moderate to fresh southerly
winds for the outer water with resultant seas. This will exit
quickly tonight. Another system will clip the area Sunday night
with moderate to fresh northerly conditions. Monday onward
conditions look to remain moderate to fresh with possible near
gale conditions by late in the week. No significant swell trains
are expected through the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm

&&

$$

PUBLIC FORECAST: RGass/Sims
AVIATION: BAM
MARINE: BFG


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