Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1109 am PDT sun Oct 23 2016
Synopsis...dry and mild weather conditions will continue through
today. 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 may bring widespread and heavier
rain to the the entire region by late Thursday. Unsettled weather
will likely continue into next weekend.
Discussion...as of 8:30 am PDT Sunday...mix of higher level
clouds and sun this morning with many locations dropping into the
40s. Highs will be in the 60s to mid 70s.
Rain will return overnight tonight for the North Bay and then down
to sf Bay Monday afternoon as a system moves in from the northwest.
Biggest question remain just how far south it will continue to
progress with some of the guidance showing it to southern Monterey
coastline by Tuesday morning. Will adjust coverage and quantitative precipitation forecast values
once the 12z solutions have arrived.
Keeping a close eye on the second system for Thursday/Friday
especially due to the European model (ecmwf) consistently indicating the potential
for heavy rain and gusty winds.
Previous discussion...early morning satellite imagery shows that
widespread high clouds continue to stream across California. These
clouds are making it difficult to detect the presence of coastal
fog, but there appears to be a narrow ribbon of fog surging
northward up the Big Sur coast at this time. Coastal temperatures
are cooling close to their dewpoints and so it's likely we will
see more widespread coastal fog develop by daybreak, with patchy
dense fog possible as well. Otherwise, expect today to be much
like Saturday, with near normal temperatures. Winds will be light
for the most part except across coastal sections of northern
Monterey County where the WRF model indicates gusty southerly
winds this afternoon.
After today the week ahead looks unsettled with a good possibility
that most of our area will see soaking rains by late in the week
as two Pacific weather systems impact California. While forecast
confidence is quite high regarding precipitation potential,
forecast confidence is much lower on specifics such as rain
timing, rainfall amounts, and the potential for strong winds. This
is due to model inconsistencies and disagreements.
The first frontal system is developing offshore just outside
130w. A potent shortwave trough currently to the northwest of the
frontal boundary near 40n/145w is forecast to catch up to the
front by this evening, enhance it, and push it eastward into
northwest California by late tonight. It's likely that rain will
spread across the North Bay on Monday and possibly a bit farther
south. But after Monday the forecast begins to get murky. Some
models, like the 00z NAM, develop rain no farther south and east
than Santa Cruz and Alameda counties by Tuesday morning while
other models, like the 00z ECMWF, spread rain across nearly our
entire forecast area by Tuesday. In any event, this initial
frontal boundary is expected to stall somewhere over our forecast
area on Tuesday and then lift back to the north as a warm front on
Tuesday night and Wednesday as the next system develops offshore.
Wednesday will probably be a dry day in between systems.
The second system is expected to dig farther to the south as it
develops offshore...allowing it to tap into subtropical moisture
by Wednesday evening. This wet system will then lift to the NE
and sweep across California by late Thursday. But here again the
models don't agree on the specifics. The European model (ecmwf) is faster,
spreading rain across our entire forecast area by late Thursday,
and generating potentially heavy rain on Thursday morning. The GFS
is slower, with most of the rain holding off until late Thursday
afternoon and evening. Also, the GFS shows the heavier rain
confined to areas form Monterey Bay southward late on Thursday
while the European model (ecmwf) extends heavier rain farther to the north. One
other difference is that the European model (ecmwf) solution develops a much deeper
surface low with this second system and would therefore result in
significantly stronger winds on Thursday and Thursday evening. For
now will highlight the potential for locally heavy rain and strong
winds later in the week and fine tune the forecast as we get
The system later in the week is expected to move through
relatively quickly which would prevent it from producing very
large rainfall totals. But it will have the potential to generate
at least a short duration heavy rain event across the Loma and
soberanes burn scars sometime between Thursday morning and
Thursday evening. This will need to be monitored closely over the
next few days due to debris flow potential in the burn areas.
A rough estimate of rain totals by the end of the week is anywhere
from 1 to 4 inches.
It appears the weather pattern will remain unsettled into next
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 08:23 am PDT Sunday...southerly winds will
increase over the northern waters this afternoon and over the
southern waters tonight 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.
... Arena to Pigeon pt 10-60 nm from 4 PM
Public forecast: Bell/dykema
marine: west pi
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