Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA
FXUS66 KMTR 261752
Area Forecast Discussion
National Weather Service San Francisco Bay Area
1052 AM PDT Wed Oct 26 2016
.SYNOPSIS...Dry weather today with seasonably mild temperatures.
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 looks dry then another
system possible by Tuesday.
.DISCUSSION...as of 8:30 AM PDT Wednesday...Calm before the storm
with all eyes on a system that will bring rainfall back to our
region for tomorrow into Friday. Some of the 12Z model runs are in
and continue to highlight Monterey County as the location for the
main focus of moisture in our CWA (coastal PW values peak over
1.50" Thursday night in both the NAM and GFS). Therefore, a flash
flood watch remains in effect for the Sobranes burn scar for
Thursday afternoon into Friday afternoon. Based off the guidance
feel that high rainfall rates at this time look unlikely for the
Loma burn scar so will keep that out of the watch. Also worth
noting that for the Chimney scar amounts also appear to be quite a
bit less than the Sobranes area.
Previous guidance suggests the potential does exist for a few
thunderstorms Thursday night into Friday for most of our region.
Will add that to the forecast if it continues to be shown in the
.PREVIOUS DISCUSSION...Main weather focus will be on rain event
for Thursday into early Friday. In the short term its another mild
start as the area remains under broad southerly flow ahead of a
longwave trough offshore. Skies have cleared for most of the
district but fog product shows some fog over the ocean with Half
Moon Bay and Monterey now showing fog and low clouds that should
only impact the Highway 1 commute this morning. Otherwise the
North Bay will get a chance to dry out after yesterdays rain with
sunny and mild weather south of the Golden Gate. Sea surface temps
have warmed into the upper 50s to around 60 degrees while last
nights 00z koak sounding shows tpw around 1.1 inches giving the
air an almost muggy feel.
Anyway after a dry Wednesday, things will change quickly on
Thursday. There`s a longwave trough off the coast with plume of
moisture pointed north of Cape mendocino. A jet streak rounding
the base of the trough will allow a tongue of moisture extending
back towards the equator to surge northward. This plume ties all
the way back towards Hurricane Seymour which is currently a cat
four hurricane over the eastern Pacific. Models show this juicy
airmass getting dragged northward and pushed up against the
coastal hills. The ecmwf has been showing similar solutions for
about one week now. Latest scenarios suggest some showers or
light rain will begin as early as Thursday morning but then begin
to increase in coverage Thursday afternoon. The area most likely
to see heavy rain will be the Santa Lucias of Monterey county
where the 132,000 acre Soberanes burn scar is located. Have issued
a Flash Flood watch for the potential of heavy rain rates to lead
to debris flows or flash floods. The heavy rain looks to start
late Thursday afternoon or evening.
As the core of the upper low ejects towards the Golden Gate nearly
all areas should see some periods of moderate to heavy rain during
the overnight hours of Thursday into Friday morning. Unlike the
boundary that hung up in the North Bay earlier this week, this
system will quickly eject inland giving only a short window of
moderate to heavy rain. Most areas that didn`t see the heavy rain
earlier this week should see some with the Thursday/Friday system
and consider clearing drains and gutters now in anticipation of
this early season rain event. Orographics should favor southwest
facing hills but given the juicy airmass and light winds expect
even the normally dry valley locations to see decent rains from
this event. Rain totals from 1-3 inches for the hills with
0.25-0.75 for the valleys is expected. Locally higher amounts
possible for the hills above Big Sur. May also need to consider a
watch for the Loma fire burn scar if confidence increases for
heavy rain that far north as well.
Showers Friday morning will taper off by midday with isolated to
scattered activity expected Friday afternoon. A break in the
weather Friday night through most of Saturday. Models then bring
another system in, this time from the more traditional northwest
Rain looks likely late Saturday night into Sunday, this time
heaviest in the North Bay again with a strong cold frontal passage
that will bring in noticeably cooler air by Sunday night.
Right now it looks mainly dry and seasonable for Halloween. Then
the models show another weak weather system possible for Tuesday.
Will be interesting to tally up the October rainfall totals at the
end of the month in what could turn out to be a very wet October
for many climate sites.
.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.
.Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
PUBLIC FORECAST: Bell/RWW
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