Scientific Forecaster Discussion
fxus66 kmtr 161746
Area forecast discussion
National Weather Service San Francisco Bay area
946 am PST Tue Jan 16 2018
Synopsis...a few lingering showers this morning otherwise dry but
continued cloudy today. Dry and continued mostly cloudy Wednesday
as moist flow comes in off the Pacific. Next front moves in
Thursday with rain spreading north to south across the district
reaching the central coast by Thursday night. Precip will turn
showery later Thursday night into Friday as the cold upper trough
passes over the region with one of the colder airmass so far this
season. Dry but cool on Saturday before clouds increase again on
Sunday as the next front arrives with rain later Sunday into
Discussion...as of 09:02 am PST Tuesday...a dissipating system
that moved through the region overnight resulted in some light
rainfall in the North Bay with only a few hundredths in the city
of San Francisco and as far south as the Santa Cruz Mountains.
Inland and to the south, only trace amounts or no precipitation at
all reported. With many locations in the North Bay and East Bay
still reporting a few hundredths of an inch or so per hour, will
keep a slight chance of showers in the forecast through the next
few hours. However, looking to a drying trend region-wide late
this morning into the afternoon. No other updates are needed at
this time with today being mostly cloudy along with temperatures
in the 50s to 60s region-wide. Please see the previous forecast
discussion below for additional details.
Previous discussion...as of 3:38 am PST Tuesday...radar still
picking up returns over Bay area but most activity will be east of
the area by rush hour/sunrise. Coastal Sonoma County had a few
sites report upwards of half an inch at Sea Ranch on the coast and
the Oak Ridge RAWS located in the wet coastal hills. Most other
locations are reporting only a few hundredths with little or no
rain south of about the Dumbarton bridge. So by the time most
people start the day any precip will have ended. However a moist
zonal flow is set up across the Pacific and clouds will continue
to stream across the region today keeping daytime highs stuck
No changes of note for weds with a forecast of partly/mostly
cloudy skies but continued dry and seasonably mild with highs
upper 50s to upper 60s.
The next front of interest is due to arrive later Thursday. The
models have been remarkably consistent with the timing and
intensity of this system, even beyond 240 hour forecasts! High
confidence that a well organized frontal boundary will move into
the North Bay Thursday morning and then charge southward through
the Bay area and reach the central coast by Thursday evening. The
fast moving nature of the system will limit rainfall potential
with current qpf forecasts showing 0.25-0.75 on average. So this
looks like a typical/moderate winter-time storm. Winds will
briefly gust to around 35 mph during frontal passage as rain
drags down higher momentum air. The fast nature of the frontal
passage and modest rain totals should preclude any significant
Hydro issues outside of typical commute impacts and blocked storm
drains. As always will need to monitor for brief heavy rain-rates
over the North Bay burn scars, as its often the short duration
bursts versus long term heavy rain that can cause debris flows.
Precip will turn showery later Thursday night into Friday morning
as 1000-500 mb thickness values plummet to around 528 dm with 850
mb temps to around -2 celsius suggesting we should see snow on the
higher peaks of the Bay area. On Friday there will be numerous
Post-frontal showers, especially for the coastal hills that
benefit from cold northwest wind flow. 700 mb temps as cold as -13
celsius with sea surface temps in the upper 50s will allow for Big
Delta T in the lower levels. This type of pattern can produce
locally heavy rain for the Monterey coast and Big Sur hills where
the forecast will show accumulating snow above 3500 feet for any
hikers in the los padres Forest. Daytime highs on Friday will
struggle to reach 55 for nearly all areas with periodic cold rain
showers dragging down cold air. Although not in the forecast at
this time, small hail and graupel will certainly be possible as
After a cold start Saturday morning, the forecast to start the
weekend looks dry but continued cool.
Will be watching for the next system to arrive later Sunday into
Monday with another round of soaking rain forecast for the
district. Latest model trends suggest front will be fairly fast
moving and pass through sometime Sunday night. Earlier solutions
showed the boundary stalling but not seeing those signals at this
time. These systems have fairly cold air associated with them and
are progressive so Hydro issues should be minimal with beneficial
rainfall the main impact.
Aviation...as of 9:40 am PST Tuesday...wide range of Cat this
morning with ample low level moisture and passing showers - LIFR
to VFR. Given recent trends, webcams and satellite decided to keep
more pessimistic forecast through this afternoon. Some of the
guidance keeps it an all day low cig event. Conf is low. Even if
there is some lifting later, cigs and some br/hz will likely
Overall conf is low to medium.
Vicinity of ksfo...MVFR cigs through 20z with poss vsby at 5-6sm.
Cigs return tonight with some vsby again.
Sfo bridge approach...same as ksfo.
Monterey Bay terminals...lingering fog around sns through 18-19z.
Will bring cigs back tonight, but conf is low.
Marine...as of 09:39 am PST Tuesday...light to moderate winds
will prevail today as high pressure remains off the coast. Winds
will switch southerly late tonight and Wednesday as a low
pressure system develops off the British Columbia coast and
pushes the high south. Large swells will impact the beaches
through early tonight before decreasing. A larger swell with long
period will arrive Thursday.
... Surf advisory...caz006-505-509-529-530
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: rgass/rww
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