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

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FXUS66 KMTR 170426
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
926 PM PDT Thu Sep 16 2021

.SYNOPSIS...A weaker dry cold front will allow for a deepening
marine layer and cooler temperatures to end the week. A deeper,
wetter trough arrives this weekend and bring cooler temperatures,
breezy onshore winds, and a chance of rain for some. A potential
offshore wind event will develop Sunday night into Tuesday.

&&

.DISCUSSION...as of 09:25 PM PDT Thursday...Temperatures this
afternoon ranged from the upper 50s to lower 60s at the coast to
widespread 70s inland. A few of the region`s warmest spots did
make it into the 80s, yet much of the interior was 8 to 14 deg F
below seasonal averages this afternoon. The culprit was continued
onshore flow, deep marine layer and weak short-wave trough pushing
inland. As such, low clouds are rapidly spreading inland this
evening with widespread coverage likely overnight and into Friday
morning. That said, the ongoing forecast in the short-term remains
on track with no updates needed at this time. Please see the
previous discussion below for complete details on the early season
storm system forecast to bring rainfall to northern California
this weekend.

&&

.PREV DISCUSSION...as of 01:00 PM PDT Thursday...The marine layer
has deepened steadily through this morning, from 2000 feet at
sunrise, to nearly 2400 feet as of noon. This deepening is
occurring as the broader pressure is on the decline on the eve of
the arrival of a shortwave trough embedded in the deeper polar
low. Surface pressure readings are roughly down 1.8 to 2.4mb
versus 24 hours ago, while temperatures are lower and humidity is
up slightly. Latest satellite imagery shows the marine layer
remaining banked up along the immediate coastline from south of
Cape Mendocino through Baja California, and remaining across a
healthy portion of the coastal/bayside valleys. Overcast to mostly
cloudy skies are being reported at Santa Rosa, Napa, Petaluma,
Half Moon Bay, Oakland, San Francisco, and Watsonville within the
last over due to this deepening marine layer. Despite the layer
deepening, the edges of the marine layer will eventually mix out
and fade away and bring some afternoon clearing to communities
farther from the source origin along the coasts. Otherwise,
satellite imagery shows the cooler, drier air lingering beyond the
dry cold front to the northwest that will through the region over
the remainder of the day/tonight. This will consequently bring
cooler conditions to the region, with the biggest swing across the
interior of 3-6 degrees, with only 1-2 degrees of cooling along
the coast versus yesterday.

For tonight, the marine layer is anticipated to deepen further
(2500-2800+ feet) and make a deeper intrusion inland as the cool
air advection aloft should be weak enough to still allow for some
capping.

Tomorrow will see this initial, weaker shortwave lift and the
flow flatten temporarily through Friday before a deeper, wetter
wave embedded in the polar low arrives later into this weekend.
Thus, tomorrow will be marginally warmer and drier than today
after the morning clouds mix out, though temperatures Friday will
still be 3 to 12 degrees below normal.

A bigger change in the weather pattern arrives with a second
deeper wave along a vigorous upper level jet over the Aleutian
Islands late Saturday into Sunday. Ensemble data suggests highest
probability of seeing rain will be across the North Bay, with
0.10-0.20" possible in the valleys, and up to 0.30" possible along
the wetter coastal ranges. Precipitation amounts less than a .10"
are expected south of the Golden Gate, with roughly a few to
several hundredths for the Peninsula and East Bay bayshore. For
areas in the eastern interior and South Bay, the wetter scenarios
favor a few hundredths, while the drier (majority, higher
probability) favor staying dry or just seeing a trace. Saturday
night into Sunday is the current timeframe for the arrival of most
of this precipitation, from northwest to southeast, though that
could still fluctuate by about 6-12 hours until models come into
full alignment. Breezy southwest winds will accompany the arrival
of the cold front, on the order of 10-20 mph sustained, with
localized gusts in favored areas up to 35mph. Otherwise, both
Saturday and Sunday will remain on the cooler side, with
widespread temperatures in the mid 60s to upper 70s across the
region.

By Sunday night into Monday morning (and again Monday night into
Tuesday morning), a steepening northerly low level/surface
pressure gradient will enable offshore flow through the Sacramento
Valley/into the eastern periphery of the North/East Bay interior
ranges. We are still 24 hours too soon to really dial in the wind
speeds per the higher resolution models, but for now, starting to
look as if sustained N/NE winds at the higher peaks of 10-20mph
with gusts 25-40mph may be possible. The strongest winds would
occur between 3am and 10am Monday morning. Overnight humidity
recoveries are slated to be fair given the recent addition of
moisture in the area, ie as low as 35-40%, but will drop down to
the mid teens during the day on Monday at the higher interior
peaks. Model trend on these winds has come down slightly from
earlier in the week, but still warrant close watch in the event
that critical fire weather conditions present themselves. For now,
these are borderline conditions, and should may eventually
necessitate the issue of a fire weather watch. Again, fire weather
fuels remain extremely dry, and still record dry in some interior
locations. A wetting rain (in excess of >.10") could at least
temporarily abate some concerns for the upcoming offshore wind
event. The areas in question are right along the boundary of
hitting these thresholds, so again, another interesting item to
watch over the coming days as we get better data/the event
unfolds.

Beyond the immediate concerns above, a trend towards warmer
conditions should be expected into midweek as high pressure
builds. Ensemble data comes out of phase by mid to late next week.

&&

.AVIATION...as of 6:57 PM PDT Thursday...For the 00z TAFs. Marine
stratus MVFR to IFR ceilings returning tonight and Friday morning
with VFR cigs returning by late Friday morning.

Vicinity of KSFO...VFR to MVFR as stratus returns tonight and
Friday morning. VFR returns by late Friday morning.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...MVFR to IFR tonight and Friday morning. Onshore
winds. VFR conditions by late Friday morning and afternoon.

&&

.MARINE...as of 09:25 PM PDT Thursday...Northwest winds mainly
light near the coast and breezier across the outer waters,
strongest over the northern outer waters. Winds will generate
steep wind waves which may be hazardous for smaller vessels.
Chances for showers increase over the weekend as a cold front
approaches Northern California. Winds will shift to the south
southwest Saturday as the front approaches then shift to northwest
behind the front. Wind driven seas prevail at a period of about 9
seconds with a weak, long period southerly swell through the
forecast period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM

&&

$$

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

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