Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
831 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 County Warning Area (coastal precipitable water 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 6:15 am PDT Wednesday...based on metars and
satellite there is a shallow layer of low clouds along the coast
with low vsbys at mry. The clouds are spreading into the Golden
Gate and may briefly impact Oak. Clouds not expected to be deep
enough to spread over the coastal hills into sfo. Therefore VFR
expected for sfo and the approach. But will have to watch for
deepening of the layer which might allow some clouds into sfo.
Vicinity of ksfo...VFR expected but confidence is not high.
Possibility of some clouds spreading over the coastal hills into
sfo. Variable winds becoming southwest to 10 kt in the afternoon.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...IFR conditions have spread into the mry
Bay area. It now appears to be a later clearing of 17-18z.
Light southeast winds in the morning switching to westerly in the
Marine...as of 5:00 am PDT Wednesday...a 995 mb low centered 300
miles west of Crescent City is moving northeast. This system will
keep southerly winds over the area with the strongest winds over
the northern waters. Another low will develop over the Southern
California offshore waters on Thursday and move into the central
waters Friday morning however winds are not expected to be
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Public forecast: Bell/rww
aviation: west pi
marine: west pi
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