Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 262143
Area Forecast Discussion
National Weather Service San Francisco Bay Area
243 PM PDT Wed Oct 26 2016
.SYNOPSIS...Rain starts to develop by midday tomorrow then
increases in coverage and intensity late Thursday afternoon
through early Friday morning when periods of heavy rain are
likely, especially for the coastal hills in Monterey County.
Showers Friday morning will taper by afternoon. A break in the
weather for Saturday before another front arrives from the
northwest with rain likely later Saturday night into Sunday.
Halloween afternoon and evening looks mostly dry with another
system possible by late Monday.
.DISCUSSION...as of 2:30 PM PDT Wednesday...Looking around our CWA
this afternoon you certainly would not know that a return to wet
conditions will be back in the San Francisco and Monterey Bay
Region starting tomorrow. Abundant sunshine helped push up
temperatures past values from yesterday with many spots currently
in the upper 60s to mid 70s. Even many locations near the coast
have been well into the 60s to lower 70s.
Satellite shows hurricane Seymore (now down to a category 3)
slowly progressing to the north around 800 miles south-southwest
of Cabo San Lucas. Moisture associated with this system will
become entrained in the flow and progress toward our region by
tomorrow. PW values will increase to over 1.60" to the Monterey
coastline by Thursday as a frontal boundary approaches from the
NW. Rain will advance to the coast by tomorrow morning and then
spread to the east through the day. In general, amounts will be
fairly light from this initial round of rainfall.
Look for rain to increase in coverage plus amounts as the main
moisture plume advances to the coast. At the same time an upper
level low will move to near the Monterey or San Luis Obispo coast
around Friday morning. This will lead to destabilization of the
atmosphere with a chance for higher rainfall rates plus
thunderstorms. Based on the convective parameters and the pattern,
biggest concern is for the southern quarter of our CWA especially
for Big Sur to the south. A flash flood watch remains in effect
and now specifically includes the Chimney Fire burn scar.
Rainfall totals have been adjusted upward from the overnight
package for most spots especially for Monterey County. Totals by
Friday afternoon will likely range from 1/2" to 1" for many urban
spots to more than 4" for the Santa Lucia Range. Thankfully winds
will not be as much of an issue compared to previous events.
Rain will switch to showers with a few forecast to be around into
Saturday. By Saturday an upper level low and surface low will move
toward the NorCal coast directly from the west. Unlike the first
system, the location of the low will bring higher rainfall amounts
to the North Bay with 1-2" likely in parts of the North Bay
Mountains. Rainfall will be less for the remainder of our region
with another 1/3 to 2/3" around SF Bay with small amounts for
inland spots to the south. Santa Cruz Mountains could pick up
another inch. Rainfall will generally end late on Sunday with just
a few showers forecast for Monday.
In many cases, we will be fortunate with the first two systems and
both of them will focus the bulk of rainfall on different parts of
our CWA. That should mitigate some of the flooding potential.
For those with Halloween plans, Monday should stay mostly dry for
the afternoon and evening before a third system quickly moves through
Monday night into Tuesday. This feature will be moving in from the
NW with the center of the low expected to go over Monterey County
or San Luis Obispo County. Due to the speed of the system plus PW
values not exceeding 0.95" through the period rainfall amounts
should be less than the first two systems.
Model solutions then diverge for the second half of next week
with some brining in a fourth system while the others indicate dry
weather. CPC 8 to 14 day outlook does favor wetter than normal
conditions, so active weather remains in the cards at least
through November 9th.
.AVIATION...as of 10:55 AM PDT Wednesday... Generally widespread
VFR across all TAF sites this morning. Winds are generally light
and out of the south early this morning ahead of an approaching
frontal boundary. Winds will increase and veer towards the
southwest to west through the day as the onshore gradient
increases. Ceilings will begin to deteriorate and rain chances
will increase towards the end of the TAF package as the frontal
boundary shifts further southward. Periods of moderate to heavy
rain are possible with the arrival of the frontal boundary. MVFR
ceilings will begin to impact the northern taf sites tomorrow
morning. Brief reductions in visibility down to 2 to 4 statue
miles will be possible with any moderate to heavy rain.
Vicinity of KSFO... South to southeast winds late this morning
will veer towards the southwest into the afternoon. West to
southwest winds will peak around 10 knots with a stray gust to 16
to 18 kt possible in the afternoon. A few high clouds will begin
to shift over the airport during the early afternoon. VCSH begins
around 12Z with MVFR around 18Z THU. High confidence.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals... A stratus circulation in the Bay will
wash out through the morning leading to generally improving
conditions and clearing skies. Models are struggling around the
Monterey with some showing clearing through the day and others
showing IFR. Given current satellite trends... seems more like to
push TAF towards the drier solutions. Thus, VFR expect by 1830-19Z
for all locations and through the day with IFR-LIFR cigs forming
overnight. Low confidence.
.MARINE...as of 10:42 AM PDT Wednesday... A pair of approaching
storm systems will maintain southerly winds through tomorrow.
Winds are anticipated to increase today as the front approaches
and then gradually weaken and then shift towards a split flow
towards the end of the week. The strongest winds are anticipated
over the northern outer waters and along the Sonoma coast. Seas
will gradually taper off through the week as the bulk of the wave
energy misses the coastline.
.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
PUBLIC FORECAST: Bell
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook and twitter at: