Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
840 am PDT Tue Oct 25 2016
Synopsis...North Bay showers will decrease this morning with
only isolated showers this afternoon while dry conditions will
continue for the central coast. Brief dry spell then tonight
through Wednesday with southerly winds ahead of the next front.
Rain redevelops on Thursday and may be heavy at times Thursday
evening as the frontal boundary moves through. Showers linger
Friday then another possible storm Saturday evening into Sunday.
Discussion...as of 08:36 am PDT Tuesday...light rain continues
over portions of the region this morning, mainly from the Santa
Cruz Mountains northward across the North Bay. Meanwhile, areas
of the Santa Clara Valley remain dry with rainfall amounts in
other locations generally around a few hundredths of an inch per
hour or so. This activity is forecast to diminish in coverage
through the morning hours as the weak frontal boundary over the
region lifts northward and becomes washed out. With that said,
mostly dry conditions will return to the region by late afternoon
and evening. For our region, the best chance of light rain
overnight will be over far northern Sonoma County and points
northward. The ongoing forecast for today remains on track and no
major updates are needed at this time.
Note: kmux radar will be offline briefly this morning sometime
between 8:45 am PDT and 11:00 am PDT for calibration.
Previous discussion...as of 4:13 am PDT Tuesday...showers continue
at this hour, mainly across the North Bay and down the peninsula
into the Santa Cruz mtns. Storm totals for the North Bay so far
are fairly impressive with 1-3 inch storm totals. Much lighter
amounts of rain fell south of the Golden Gate. Current
shower/light rain activity will continue to diminish through the
morning hours with just isolated North Bay showers this afternoon.
Then a brief lull in the weather overnight into Wednesday with
some south winds and partly cloudy skies ahead of the next weather
This system will arrive on Thursday. Still plenty of model
differences but general idea is there for another moderate rain
event that should favor areas south of the Golden Gate including
the central coast while also bringing more rain to the North Bay.
The Thursday system will be coming in from the southwest with a
different trajectory. Initial rain showers forecast to reach the
coast around sunrise Thursday morning. Bands of rain will then
slowly increase in intensity and rotate onshore by Thursday
afternoon. The Santa Cruz mtns and Santa Lucia Range of Monterey
County appear poised to see the heaviest rain under this type of
set-up. Perhaps one change of note now is that the the main upper
low will move through the region faster thus minimizing some of
the concern for flash flooding over the burn scar areas. Both the
06z NAM and GFS bring the upper low onshore overnight Thursday
into Friday keeping the best dynamics and associated heavier rains
south of the parent upper low. Expect about a 2-4 or 3-6 hour
window of moderate to heavy rains for the coastal hills on
Thursday night when several inches of rain could potentially fall
while even the usual rain-shadowed valleys should see decent rain
as the cold core upper low and sfc Boundary Pass through. Some
Post frontal showers will continue Friday morning but dissipate
through the day.
Both the GFS and ECMWF are now showing another potent system
Saturday night into Sunday morning. The trajectory of this system
will be a more typical NW flow that will bring the heaviest rains
to the North Bay and coastal hills. Unlike todays system the
weekend system should have a well defined cold front that will
help to produce another round of decent rainfall with lingering
showers into Sunday afternoon. Should this all pan out it could be
quite a wet October for some of our climate sites as its unusual
to see such an active pattern so early.
Low confidence looking towards Halloween with latest solutions
showing dry and seasonable weather during the day with possible
showers by Monday night, especially in the North Bay. Pattern
looks to remain active next week with best chance for rain in the
North Bay as active Pacific jet takes aim at the Pacific northwest
southward towards Cape Mendocino.
Aviation...as of 5:15 am PDT Tuesday...MVFR cigs with isolated
showers moving through the area. Light rain has fallen mainly in
the northern sfo Bay area, North Bay and the Santa Cruz
Mountains. Rain will decrease as the frontal system pulls north
out of the area. Conditions improving to VFR after 18z.
Southeast winds 10-12 kt will shift to more southerly in the
Vicinity of ksfo...MVFR through 18z. Southeast winds 10-12 kt
shifting more southerly in the afternoon.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR with rain and low clouds staying north
of the mry Bay area.
Marine...as of 08:36 am PDT Tuesday...Small Craft Advisory
conditions for hazardous seas will continue across the coastal
waters today. Light south winds are expected today then increase
briefly on Wednesday before decreasing on Thursday. Another low
pressure system is expected to impact the coastal waters Friday
... Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: rgass
aviation: west pi
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