Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
217 PM PDT sun Oct 23 2016
..two rounds of rain slated to impact our region this week...
Synopsis...a Pacific weather system will approach the northern
California coast tonight and bring rain chances to the region from
Monday through Tuesday. The best chance of rain will be across the
north. Another weather system will likely bring widespread and
heavier rain to the the entire region by late Thursday continuing
into Friday. Unsettled weather will likely continue into next
Discussion...as of 2:00 PM PDT Sunday...comfortable albeit cool
temperatures across our entire County Warning Area today with a mix of sun and
clouds. Generally spots are running 2 to 4 degrees cooler than
Saturday with temperatures in the 60s to lower 70s. Also Worth
noting a southerly surge that is currently bringing clouds and
cooler conditions around Santa Cruz.
Rain will slowly advance into the northwestern portion of our
region late tonight as a frontal boundary associated with a low
west of the pacnw continues to track to the east. Models have
only slightly changed since the past couple of days although in
general timing has slightly slowed. Now looks like rain gets into
the North Bay near sunrise and down to sf Bay starting either
Monday afternoon or evening (could start as drizzle before
changing over to rain). Precipitation will likely not advance
much past Santa Cruz as the front stalls out and dissipates.
Rainfall amounts still look like up to 2" for the North Bay with
generally 1/4"-1/2" around sf Bay and the Santa Cruz Mountains.
Lower amounts can be expected for areas farther to the south.
Rain will mostly come to an end by Tuesday evening with mostly dry
conditions through Wednesday night before a second stronger
system moves back into our region. This is one that the European model (ecmwf) has
been showing over the past five days and latest runs have little
change. Synoptically a longwave trough will set up to our west
with an associated surface low that will pull in subtropical
moisture. The low will rapidly deepen and approach the northern
California coast as a cold front moves through. Although there is
still some disagreement with the details, in general there is good
agreement that widspread rain can be expected Thursday into
Friday. Periods of moderate to heavy rain are possible although at
this time there is a wide range of solutions for where the main
plume of moisture will be focused. In light of the European model (ecmwf)
consistency decided to heavily weight the forecast toward its
solution. Rainfall amounts are very tricky due to the uncertainty
of the main focus of moisture, although would expect some of the
coastal hills to get 2 to 4". Southerly winds will also be on the
increase for Thursday and Friday as the gradient tightens. Higher
elevation spots could see gusts well over 30 miles per hour.
Unsettled weather will likely continue beyond the second system
with a moist unstable flow across our entire region.
Aviation...as of 11:02 am PDT Sunday...for 18z tafs. A trough of
low pressure has mixed out the marine layer and has filtered in
drier air at the surface for most areas this morning. VFR should
prevail through the day for the Bay area terminals, while
MVFR/IFR cigs return as early as 01-02z this evening, perhaps
earlier, for the Monterey Bay terminals. Visible satellite
currently shows a southerly surge of stratus trying to push into
Monterey Bay already. Forecast models bring in MVFR cigs later
tonight for the Bay area. Westerly winds will turn more
southerly/southeasterly by late tonight.
Vicinity of ksfo...VFR this afternoon. MVFR cigs return around
05z, give or take a couple of hours. West to southwest winds will
hover around 12 kt this afternoon, then becoming southeast and
gusty after 06z. Gusts over 20 kt are possible after 12z Mon.
Forecast models suggest rain showers in the area, around or after
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR through 00z. A finger of stratus
showing up on satellite could impact the mb terminals late this
afternoon or early this evening. Kwvi already reporting broken IFR
cigs with this feature occurring. Generally light to moderate west
winds this afternoon, sustained around 12kt at ksns. Winds shift
out of the south/southeast early tonight, which will aid in bring
the MVFR/IFR cigs in.
Marine...as of 02:15 PM PDT Sunday...southerly winds will
increase over the northern waters through tonight and over the
southern on Monday as an upper level system approaches the
Pacific northwest. Unsettled weather conditions and moderate
southerly winds will continue through early next week as a series
of storm systems push inland to our north.
... Pt to pt pinos 0-10 nm from 3 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 3 am
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 3 am
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 9 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 9 PM
Public forecast: Bell
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