NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 121014

Area Forecast Discussion
National Weather Service San Francisco Bay Area
314 AM PDT Wed May 12 2021

.SYNOPSIS...The cooling trend that began near the coast on Tuesday
will gradually spread inland through rest of the work week. Also,
there will be an increase in night/morning low clouds through
Friday. Near to slightly below normal daytime temperatures will
then persist into the upcoming weekend, along with mainly dry
weather conditions.


.DISCUSSION...As of 3:15 AM PDT Wednesday...May Gray is in full
effect overnight with widespread stratus along the coast and
locally inland. Lingering upper level high pressure is keeping the
depth and inland push of the low clouds in check, but low clouds
are moving up the entire Salinas Valley nonetheless. Fort Ord
profiler holds the stratus depth at 1,000 feet. Given the slightly
compressed nature of the stratus some sensors are reporting
patchy fog, some of it locally dense and less than 1/4 of a mile.
Morning low clouds and patchy fog will linger through mid-morning
for inland areas, but the coast line will hold onto the clouds
through the day. The subtle cooling trend noticed on Tuesday will
continue on Wednesday and become more widespread with 50s/60s near
the coast and 70s/80s inland. A few far interior locations have a
chance at exceeding 90 degrees.

To round out the work week, further cooling is expected with more
widespread night/morning low clouds. High pressure overhead will
weaken and flow becomes more zonal allowing for a deeper marine
layer and stronger onshore flow. Some of the hi-res models even
suggest at some coastal drizzle, which is definitely plausible.
High temperatures will drop an additional 5-15 degrees with highs
70s/80s interior.

For the weekend, a low confidence high impact scenario. Medium
range models have been showing an upper low pivoting over the Bay
Area and tonight`s model run is no different. The overall
development of an upper low Friday night into Saturday passing
over the Bay Area has pretty decent model agreement. The upper low
will keep low clouds in place and and temperatures on the cooler
side. The one question that remains is precip. Will the upper low
kick off any showers? The 00 and 06z runs of the deterministic and
ensembles are trending toward yes. The latest EC/GFS both
generate some scattered showers Friday night and early Saturday.
The National Blend is less enthusiastic, but the ensembles do
have a few members showing precip. If precip does it occur it will
be more on the convective side with showers and possibly thunder.
At this rate, it`s still a few days out, but upper level
instability is there with high modified total totals and decent
MUCAPE. Will continue to assess the situation, but if the trend
continues showers will be added and if thunder is added fire
weather concerns will ultimately increase. Stay tuned...

After the upper low exits to the south on Sunday conditions reamin
seasonably cool with night and morning clouds heading into next


.AVIATION...As of 10:32 PM PDT Tuesday...For the 06z TAFs. Mostly
VFR as stratus moves through the Golden Gate and into the SF Bay.
growing confidence in stratus building up in the East, with a
chance of it moving back across toward KSFO. Chances of patchy fog
are possible in the North Bay, while the Monterey Bay already has
LIFR conditions that are expected to remain through the night.
Clearing is projected for 16Z for VFR condition and moderately
breezy onshore winds Wednesday afternoon. Wednesday night will
likely have another, more widespread inundation of the marine
stratus as winds subside.

Vicinity of KSFO...For the most part, just a FEW clouds expected
in the near-term, but there is growing confidence for IFR
conditions overnight tonight for lower BKN clouds around the
terminal in the early part of Wednesday morning. VFR Wednesday
afternoon with a return of moderately breezy onshore winds between
15 to 20 kts. Stratus will move back into the Bay Wednesday night
with the chance for reduced conditions.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Low clouds and LIFR conditions are
expected through the night. Conditions are expected to reduce
slightly with patchy fog possible early Wednesday morning. Clouds
are expected to begin lifting and breaking up after 16Z with
clearing after 18Z. VFR Wednesday afternoon with breezy onshore
winds. Low clouds and IFR conditions expected Wednesday night.

&& of 09:03 PM PDT Tuesday...Light to locally moderate
winds across the waters through mid to late week. Seas will
continue to subside leaving a pair of light northwest and
southerly swells. Northwest winds will strengthen towards the end
of the week as the next system approaches. Strongest winds are
expected over the northern outer waters.





MARINE: Dhuyvetter

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NWS MTR Office Area Forecast Discussion