Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1033 am PDT Monday Oct 20 2014
Synopsis...a weak cold front will spread light precipitation
across portions of the region today...especially over the North
Bay down through the Santa Cruz Mountains. Dry weather conditions
will then return Tuesday with a warming trend expected through the
remainder of the week.
Discussion...as of 08:47 am PDT Monday...light precipitation is
currently being reported across much of the North Bay this
morning as a weak frontal boundary pushes inland. This activity is
expected to spread southward as the boundary approaches the
central coast later this morning and through into the afternoon.
Overall..rainfall amounts will generally remain light with up to a
few tenths of an inch across the North Bay and coastal slopes.
Many inland areas across the Santa Clara Valley southward through
the Salinas valley may receive only trace amounts or no rainfall
at all. As the main middle/upper level support stays well north of
the area... will keep thunderstorms out of the forecast for now.
However...yet cannot completely rule out an isolated thunderstorm
over the northern coastal waters or North Bay this afternoon as
instability increases slightly.
Previous discussion...as of 3:00 am PDT Monday...satellite and
analysis shows an approaching cold front about 150 miles to our
west this morning. This front, which has been the main focus of
attention for the forecast over the past few days, will advance
across our County Warning Area today and bring a chance of rain to our area.
Models continue to remain in good agreement that most of the rain
will stay well to our north with just the North Bay looking the
most likely to locally pick up more than 1/4" with a tenth at the
most around sf Bay and even less to the south. Timing brings the
best chance of rain to the North Bay around sunrise with
communities from sf down to Monterey looking at very late morning
to early afternoon. The front will advance to the east and be out
of our area tonight.
Quiet weather returns for Tuesday through Thursday as a ridge of
high pressure attempts to build into the southwestern Continental U.S..
Worth noting that the models have backed off a bit on building
the ridge and now bring almost a zonal flow into central
California. This means that highs are not expected to be nearly
as warm as we were thinking on previous shifts. Instead of upper
70s to upper 80s for inland sports by Thursday, looks more like
70s to lower 80s for most areas. Coastal communities should stay
more in the 60s to middle 70s instead of upper 60s to near 80.
With less amplitude of the ridge, the odds of another system
being able to impact our weather increases, and now a chunk of
the models bring a longwave trough near the West Coast for
Friday/Saturday and depict a long fetch of moist air moving
across. In fact, the operational GFS and Gem (plus the ensemble
mean) all bring a fairly vigorous front to the sf Bay region
during this time with notable rainfall amounts. However, the
European model (ecmwf) remains much drier and only has limited amount of rain
making into the northern part of the County Warning Area. Due to the fact that
the GFS and Gem have brought down the storm track much farther to
the south compared to previous runs and the European model (ecmwf) has flipped
from a blocking ridge, feel the best course of action is to take
a more conservative approach and bring a chance of rain for
Friday into Saturday. This will need to be closely watched and if
the European model (ecmwf) (plus the current dry navgem which only GOES out toe
saturday) flips into the wet Camp the probability of precipitation will need to be
increased quite a bit. Also Worth noting off our model spectrum
Page that the spread of solutions for Friday through all of
Saturday is considered high for all locations north of Monterey
Bay. This highlights the low confidence we currently have for the
period beyond Thursday.
Regardless of the rain, temperatures should cool around 10 degrees in
many spots by the weekend. Beyond that point the focus of the
rain shifts back to the north.
Aviation...as of 10:30 am PDT Monday...a narrow cold front is
currently moving northwest to southeast through the sf Bay area.
Ceilings and visible will be MVFR to IFR at times as the rain moves
through. Behind the front ceilings will quickly improve. Winds are
light southerly ahead of the front...briefly gusty from the west
with the front...before becoming light westerly. Low clouds will
be limited tonight though fog is likely at sts.
Vicinity of ksfo...MVFR with light rain through around 19z. Then
improving to VFR through evening. Moderate confidence. Westerly
winds the rest of the day...increasing to around 15 knots this
afternoon. Stratus possible early morning tomorrow.
Ksfo bridge approach...similar as ksfo.
Monterey Bay area terminals...ceilings becoming MVFR with light rain
possible early the afternoon as the front moves across the
terminals. Clouds clearing middle to late afternoon behind the front.
Winds turning west with frontal passage.
Marine...as of 08:47 am PDT Monday...a weak frontal system will
move across the waters on Monday morning and early Monday
afternoon. Light southerly winds ahead of the front will turn to
the northwest behind the front on Monday afternoon and increase on
Monday evening...mainly south of Point Sur. A large west-northwest
swell will arrive in the waters today and persist into Tuesday
before subsiding somewhat by midweek.
... Bar advisory for sf bar from 11 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 6 PM
Public forecast: rgass
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