Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 272209

Area Forecast Discussion
National Weather Service San Francisco Bay Area
309 PM PDT Thu Oct 27 2016


.SYNOPSIS...Rain will increases in coverage and intensity through
the evening continuing into early Friday morning with periods of
heavy rain likely, especially for the coastal ranges of Monterey
County. Showers will taper off Friday afternoon. Unsettled weather
expected to continue Saturday before another front arrives from
the northwest with rain likely late Saturday night and Sunday.
Halloween afternoon and evening looks mostly dry with another
system possible by late Monday into Tuesday.

&& of 3:00 PM PDT Thursday... Satellite imagery
shows a rather interesting weather setup for this afternoon, with
three low pressure features present along the Pacific coastline.
Two of these weather systems have brought unsettled weather to the
area through the day and will continue to do so into this evening
and overnight into tomorrow, while one further north in the Gulf
of Alaska will bring continued unsettled weather this weekend.

A 566dm cut off low and associated surface front is presently
situated around 500 miles west of southern California and is
slowly shifting eastwards towards the state this afternoon. Latest
indication is that the surface front has also stalled where there cut
off low`s cold front approached the warm air mass of Tropical
Storm Seymour. Further south, Tropical Storm Seymour, while
continuing to weaken from being a category 3 hurricane yesterday,
is tapping into the moisture laden tropical air to its south and
advecting it northwards towards the state. Our forecast area
presently resides between these two features in an area where the
energy from the cutoff low is able to interact with the
overabundance of moisture exceeding 200% of normal from Seymour.

Due to the atypical weather pattern, we are seeing a flip of our
typical storm behavior. Instead, we are seeing widespread to
numerous convective rain showers preceeding the frontal passage.
So far today, these rain showers have been predominately higher
based (with bases at 6000-1000 feet) and have struggled to bring
in much, if any, precipitation to the surface. That said, an area
of enhanced convection and cold cloud tops is evident on infrared
satellite imagery just upstream of the region this afternoon where
numerous lightning strikes have also been reported. These cold
cloud tops are an indication of taller storms/stronger convection
and likely increased rainfall intensities. Thus, we should begin
to see much greater rainfall accumulations along the coast over
the next few hours. Additionally, further to the north, our more
typical stratiform frontal precipitation has brought more than an
inch to a few select locations in the north bay. This frontal
boundary has stalled out as of now but should slowly shift
southward through the next 18 hours and bring additional precip
accumulations. Generally speaking, most model guidance has also
suggested that the heavier precipitation will come no earlier than
5pm this evening and then persisting through the evening and
overnight. Rain is then anticipated to taper off by tomorrow

Model guidance was all over the place today, with a vast majority
of the models coming in half or even a third as wet as their
previous runs did. As a result, one of the main changes to the
forecast was in our quantitative precipitation forecast. Seeing
models diverge on a solution within 12-24 hours of the event is
increasingly rare as computation power increases. As a result,
much of the short forecast fell back upon radar/satellite trends
as nearly all models also failed to initialize as we had expected.

Currently, a flood watch remains in effect over coastal Monterey
county in anticipation of rainfall accumulations in the 2-4"
range, which could be sufficient in generating flash flooding and
debris flows over the burn scars from the Soberanes and Chimney
fires in that region. Winds are not expected to be much of an
impact with todays system as a result of the complicated weather
pattern upstream.

Looking ahead, the next system (currently in the Gulf of Alaska)
will arrive descend into the region midway through the weekend
with another round of widespread precipitation. A weaker system is
then set to arrive later Monday night, presently later than most
trick or treaters would be roaming about. Short wave ridging will
then set up by mid week next week leading to gradually improving

&& of 11:00 AM PDT Thursday...For 18z tafs. Despite
rain in the area, cigs expected to remain above 3000 ft through
the afternoon. The only exception is for the North Bay, where
KSTS is currently reporting MVFR cigs. Cigs are expected to lower this
afternoon, as more rain expected through tonight; MVFR cigs and
vis can be expected. Winds today will be southerly/southeasterly
and light, 10 kt or less. Low confidence on cigs and cig timing.
High confidence on winds.

Vicinity of KSFO...Light south/southeast winds today. Periods of
-shra through the day, with MVFR cigs by 03-04z. However,
confidence is low on timing. Any heavier pockets of rain could
possibly drop vis to IFR levels. High confidence on winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR through late afternoon. Any cigs
will remain above 3000 ft. Light southeast winds this afternoon,
with higher speeds through the Salinas Valley. Cigs will drop
tonight, around 02-03z. Low confidence on timing. Periods of light
rain will be possible through the afternoon, but will be more
widespread tonight and through the overnight hours. Overall
confidence on forecast is low.

&& of 02:32 PM PDT Thursday...Periods of rain today with
generally light southerly wind and seas. weak frontal boundary
moves onshore by friday morning but light winds and seas will
prevail. south winds increase saturday ahead of the next cold
front that will arrive from the nw. rain develops with the next
front saturday night into sunday with a return of gusty west to
northwest winds by sunday afternoon behind the cold front along
with rapidly building seas.


     .Tngt...Flash Flood Watch...Los Padres National Forest and
                                 Santa Lucia Range including the
                                 Soberanes and Chimney burn scars.




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