Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
411 PM PST Monday Nov 30 2015
Synopsis...a weak weather system passing through the Bay area
this evening may generate some light sprinkles or rain showers.
High pressure returns Tuesday and Wednesday with dry and
seasonable weather. A fast moving cold front will arrive Thursday
bringing a round of rain and wind to the Bay area. Dry weather
quickly returns Friday and Saturday before the next chance of rain
arrives Sunday or Monday.
Discussion...as of 2:52 PM PST Monday...an old rule of thumb in
wet Winters is that even weak systems generate rainfall. That
seemed to be the case today as a weak boundary moved through
bringing some rain showers...albeit very light to portions of the
greater Bay area. Rain totals today were from a trace to around
0.05 on average. Radar still showing some returns and have kept
slight chance sprinkles in the forecast for this evening. Cloud
cover today kept highs cool but the blanket of clouds should keep
overnight lows in the 30s and 40s so will not be issuing any
High pressure noses in Tuesday and Wednesday. Will have to make sure
boundary layer moisture doesn't get locked in place with a layer
of stratus clouds. Its the time of year with the low sun angle
where moisture trapped in the valleys can be hard to mix out
leading to forecast busts of high temperatures. For now will go with
highs on Tuesday in the upper 50s and lower 60s...then warming a
few degrees Wednesday.
All attention is on Thursday system which models have been honing
in on for days now. Just issued a Special Weather Statement to better advertise the
event. Latest trend has been to slow the main front as the upper
trough digs. Its still a few days out and timing and intensity
could change but for now it looks like a Thursday midday to
evening event. Rainfall should be 0.25-0.75 on average with
lowest amounts south of Salinas and locally higher amounts in the
coastal hills. Upper trough may tilt negative and bring some south
winds that are less favorable for orographics. Nonetheless pretty
high confidence for a fast moving and breezy frontal passage.
Rain will quickly turn to showers Thursday evening with all
precipitation expected to end by sunrise Friday. High pressure to bring
dry conditions then for Friday and Saturday.
Overall pattern looks to remain fairly active with another chance
of rain by Sunday or Monday and hints of another system beyond
Aviation...as of 4:00 PM PST Monday...a weakening frontal
boundary will continue to move through the region this afternoon
and evening resulting in a slight chance of an isolated shower.
VFR conditions are expected to continue with ceilings at or above 5000 feet.
The one exception to the rule will be ksts that will likely see
MVFR ceilings after 04z followed by LIFR ceilings around 400 feet after
10z as a result of lingering low level moisture. Winds will be
light offshore or locally variable tonight.
Vicinity of ksfo...VFR conditions through tonight. Mainly high
ceilings and light offshore flow.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions through tonight. Mainly
high ceilings and light offshore flow.
Marine...as of 02:38 PM PST Monday...light to moderate winds will
continue through midweek over the coastal waters. Winds will
increase Wednesday night and Thursday as a storm system approaches
the region. A longer period swell will also impact the coastal
waters tonight. The period will gradually decrease through the
week...but a larger swell will arrive late in the week.
Public forecast: rww
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