Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 271807
Area Forecast Discussion
National Weather Service San Francisco Bay Area
1107 AM PDT Thu Oct 27 2016
...SEVERAL ROUNDS OF RAIN EXPECTED OVER THE NEXT FIVE DAYS...
.SYNOPSIS...Rain will increases in coverage and intensity through
the day 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.
.DISCUSSION...as of 9:15 AM PDT Thursday... Satellite imagery
shows a rather interesting weather pattern, with several low
pressure features present along the Pacific coastline. Two of
these weather system will be responsible for bringing unsettled
weather throughout the area today, while one further north will
bring continued unsettled weather this weekend.
For today, a 562dm cut off low and associated surface front is
gradually shifting eastward towards the state. Further south,
Hurricane Seymour, while weakening, 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 Hurricane Seymour. Convective rain showers,
predominately of the weak variety, are lighting up our radar
scans early this morning ahead of the approaching front. Spotters
reports and rain gages indicate that so far, the bulk of these
high based (6000-9000 foot base) showers are only producing light
rainfall of a few hundredths or less. Increasing instability aloft
as the system shifts closer ashore will allow for some convective
storms to strengthen into thunderstorms by this afternoon and this
evening primarily over the southern counties.
Latest model guidance for the next 24 hours has been all over the
place with a majority of solutions speeding up versus earlier
runs. This is problematic because a more progressive weather
pattern will result in the bulk of the moisture being shunted
further southward towards San Luis Obispo/Santa Barbara rather
than Santa Cruz/Big Sur regions, which is complicating the
forecast. As a result, the timing and location of the heaviest
rain over the next day to day and half will be the forecast
problem of the day. We will take a close look at elevated moisture
transport and orographics primarily along the Big Sur region today
to address this forecast problem. 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.
Despite the model differences, most models agree in a second surge
of energy/moisture beginning later today and into tonight then
continuing into early tomorrow. Rain is then anticipated to taper
off by tomorrow afternoon. A second system (currently in the Gulf
of Alaska) is then set to sweep into the region through the
weekend bringing widespread rains to the area once again. See
previous discussion for additional details regarding the current
.PREVIOUS DISCUSSION...as of 3:30 AM PDT Thursday...After talking
about it for days, rain has returned to our CWA early this morning
with light echoes being indicated on the radar plus a few tips of
the bucket. So far amounts have been extremely light with a few
spots just getting 0.01" or so. This is actually just the start
with the main band of rain still poised to impact our region this
evening into Friday morning.
Synoptically a longwave trough is advancing toward the west coast
while a stationary front stretches from the coast of Norcal to the
southwest. As the front progresses to the east into our CWA a very
high PWs (up to 1.75" to Pebble Beach this afternoon) originally
associated with Hurricane Seymore will become entrained in the
flow and take aim on our region. This will lead to widspread
rainfall later this afternoon especially south of SF Bay due to
the moisture plume focusing on that region. Rain will continue
through the overnight hours potentially moderate to heavy at
times (rates up to 1/2" per hour in the Santa Lucia Range). At
the same time the atmosphere will begin to destabilize which will
lead to a chance for thunderstorms.
Compared to previous model runs rainfall amounts have not changed a
lot although some of the guidance has greatly decreased amounts
for the North Bay in particular for the evening and overnight
hours. In general look for 1/2" to 1.5" for many urban spots with
higher values for the Santa Cruz and Santa Lucia mountains. The
flash flood watch for the Sobranes and Chimney burn scar will be
continued due to the possibility of greater than 4" falling plus
the potential for moderate to heavy rains at time. As has been
mentioned before, winds will not be a factor although any
thunderstorms could produce locally gusty conditions.
Unsettled conditions behind the front with moisture in place will
lead to a mix of clouds, sunshine, and a few showers for the
remainder of Friday into Saturday. Highs will generally be in the
Rain will return for round two as a system moves in directly from
the west brining widespread precipitation on Sunday. Due to the
trajectory, this one will favor the North Bay with lesser amounts
the farther south you go. Rain will switch back to a few showers
on Monday. Look for lighter amounts compared to the first event
with 1/3" to 3/4" for the North Bay with generally just 1/10" to
1/3" for the remainder of our CWA. Rain will switch back to
scattered showers for Sunday night through Monday.
Monday night will bring round three as a system comes in directly
from the northwest. Unlike the first which should center over the
southern third, and the second which will focus on the North Bay,
this one should simply bring widspread light rainfall to all of
our area. Look for another 1/4" to 3/4" for many spots. Showers
will gradually dissipate on Wednesday.
Models diverge beyond Wednesday with some brining in another
system Thursday/Friday while other keep us mostly dry.
.AVIATION...as 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.
.MARINE...as of 3:15 AM PDT Thursday...Periods of rain today with
generally light southerly wind and seas. frontal boundary moves
onshore by friday morning but still light wind and seas. 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
.Tday...Flash Flood Watch...Los Padres National Forest and Santa
Lucia Range including the Soberanes
and Chimney burn scars.
PUBLIC FORECAST: DRP
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