Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
951 am PST Monday Jan 26 2015
Synopsis...temperatures cool slightly for the start of the work
week as high pressure over California weakens and shifts eastward.
A weak storm system will then move into Southern California
tonight into Tuesday...bringing a slight chance of showers to
the central coast while the Bay area stays dry. Dry and seasonable
weather forecast Wednesday through Friday with a warming trend
Discussion...as of 9:16 am PST Monday...middle and high clouds are
increasing across the region as moisture from Southern California
continues to slowly increase. In terms of fog the Livermore
Airport is still reporting some dense fog that should erode
shortly. After mild to warm weather over the weekend the
combination of increasing clouds and some airmass cooling will
ultimately lead to a subtle cooling trend for the start of the
work week. The main weather feature of interest continues to be a
circulation off the Southern California coast. 12z data spreads
some showers northward later this evening into Tuesday morning.
Most of the precipitation will stay south and east of our district. The
best chance for showers looks to be over Monterey and San Benito
counties early Tuesday morning. Current pop forecast has this
covered fairly well and will only plan on minor adjustments for
the afternoon package.
The pattern stays benign this week with zonal flow or some weak
troughing. This will keep skies partly to mostly cloudy with
morning fog in the valleys closer to the Delta. Temperatures will
run near normal with fairly mild days and nights.
Another warm weekend is looking likely as the European model (ecmwf) builds a
strong ridge over the Bay area again Saturday and
Sunday...although record warmth looks less likely for the central
coast at this time. That ridge will break down by next Monday as
the calendar turns to February. The waiting game will then
continue as we look for some type of a pattern change. The GFS
continues to play its same game of hinting at rain of substance
about 2 weeks from now.
Previous discussion...as of 3:46 am PST Monday...the center of the
middle/upper level high that resulted in record breaking temperatures
across portions of the region this weekend will continue to shift
further inland today. Meanwhile...an upper level low pressure
system currently southwest of the Southern California coast will
shift northward during the next 12 to 36 hours. This feature has
already brought an increase in middle/high level clouds over the
region this morning with deeper moisture forecast to advect
northward through Tuesday. The latest forecast guidance continues
to show light precipitation spreading across Monterey and San
Benito counties late tonight and may reach as far north as the
Santa Clara Valley/East Bay by Tuesday morning. It still appears
rainfall amounts will remain light as the low begins to transition
into an open wave and becomes sheared apart while approaching the
central coast on Tuesday. Most areas that do receive rainfall will
likely only pick up a few hundredths to one- tenth of an inch.
However...cannot rule out rainfall amounts up to one-quarter of an
inch within heavier showers that may develop over the higher
elevations. A few showers may linger into Tuesday afternoon before
the middle/upper level support and associated moisture shift inland.
Another middle/upper level trough and associated low pressure system
is forecast to follow behind the aforementioned system and
approach Southern California by midweek. Unfortunately...the
model generally agree this system will be too weak and
disorganized to bring precipitation to our region. Thus...will
maintain dry weather conditions beyond Tuesday with lingering
middle/high level clouds streaming across central California. Given
only slight cooling aloft from these two systems...temperatures
across the region will likely remain above seasonal averages
through the work week.
The upper level ridge off of the West Coast is then forecast to
nose into northern and central California late in the week. This
will maintain dry weather conditions and result in another
warming trend across the forecast area through the upcoming
weekend. With this...January 2015 will likely go down in the
record books as the driest on record for many (if not most) sites
across the San Francisco/Monterey Bay area.
Aviation...as of 9:50 am PST Monday...patchy fog in the valleys
will clear by late morning. A weak weather system moving into
Southern California will spread considerable middle and high level
clouds across the region through Tuesday. Mostly VFR conditions
are expected except local IFR conditions in fog in the valleys
late Monday night and early Tuesday. Winds will be light
easterly...transitioning to light westerly this afternoon and
light southerly tonight.
Vicinity of ksfo...VFR. Light offshore winds winds this morning
becoming light variable this afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR. Light winds.
Marine...as of 9:00 am PST Monday...east winds over the coastal
waters will turn to the southeast later today and tonight as weak
low pressure develops offshore. High pressure will build off the
northern California coast by midweek...resulting in light to
moderate northwest winds during the second half of the week.
Public forecast: rww
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