Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
824 am PST sun Feb 1 2015
Synopsis...high pressure will maintain dry and mild weather
across the region through the middle of the week. A Pacific
weather system will likely spread rain into the San Francisco Bay
area late in the week...with rain possibly reaching as far south
as the Monterey Bay area. Rain could be heavy at times in the
North Bay late in the week and into the weekend.
Discussion...as of 8:10 am PST Sunday...dramatic change in
temperatures across a portion of our area as nearly calm
conditions today allowed all spots to rapidly cool overnight. Of
note is Napa where the current reading to 36 degrees is 23 cooler
than this time yesterday. Nice weather is forecast for our area
today with temperatures above normal although not as much as yesterday
due to much less of an offshore component at the surface. Quiet
weather will continue into Thursday.
Main focus of the shift will be for the end of the week as a very
moist south-southwest flow is forecast to bring an atmospheric river
event into northern California and at least part of our area. Agree
with the previous shift that the biggest question is whether or
not it will be widespread and impact most of our area...or if it
will stay generally confined to norcal down to the North Bay. Will
wait for all of the 12z solutions (both operational and ensemble)
before making any possible changes to the grids. Other item of
concern will be locally strong winds at the surface. 925mb speeds
off the GFS and European model (ecmwf) indicate 40 to 55 knots around sf Bay region by
Friday as an associated cold front approaches our area.
Previous discussion...offshore flow has diminished significantly since
yesterday...allowing areas of low clouds to develop along the
coast. However...onshore flow is not yet sufficient to bring
increased low level moisture very far inland where dewpoints are
still in the 30s. Therefore...it's likely that most morning low
clouds and fog will be confined to areas near the ocean...with
only patchy fog expected this morning in the North Bay valleys and
near the Delta.
Lack of offshore flow will mean slightly cooler conditions in most
locations today. Temperatures will still be above normal...but
record highs are not likely.
Not much change is expected through midweek. An upper level ridge
will remain over California. Pacific storm systems will be moving
over the ridge and inland across the Pacific northwest and far northern California.
But rainfall with these system is expected to remain to the north
of our area through at least Thursday morning. Increasing onshore
flow by Monday night will probably mean more widespread night and
morning low clouds and fog Monday night and Tuesday morning.
Otherwise only occasional middle and high level clouds are expected.
Temperatures will remain mild.
By late in the week the upper ridge is forecast to shift
gradually east as a broad upper trough over the eastern Pacific
edges closer to the West Coast. At the same time...a plume of
very moist air with precipitable water values in excess of 1.5
inches (atmospheric river) is forecast to flow into northern
California from the west-southwest. For the past few days the medium range
models have been consistent in forecasting a potentially
significant precipitation event somewhere across northern California late
in the week and into next weekend. Where exactly the heaviest
rainfall will be concentrated is the primary forecast challenge.
The GFS has generally been forecasting the bulk of the precipitation to
fall from Sonoma County northward...with rainfall amounts tapering
off rapidly south of the Golden Gate. For instance...the latest GFS
is forecasting over seven inches of rain across northwest Sonoma
County from late Thursday through Saturday...but only about 2
inches in San Francisco...tapering down to less than a tenth of
an inch in Monterey. The European model (ecmwf) has consistently forecast more
widespread precipitation...and rainfall much farther to the south...with
only minor quantitative precipitation forecast differences from north to south. This type of warm
advection/atmospheric river pattern typically results in very
tight precipitation gradients from north to south...and so it
seems the GFS may be presenting the more realistic outcome. But
even if the GFS has a better handle...just a slight shift north or
south in the focus of the moisture plume will mean big differences
in rainfall amounts in any one area. For now it looks like the
North Bay has a reasonable chance at seeing a substantial rain
event from this...with precipitation chances and amounts falling off
significantly to the south of the Golden Gate. As far as timing is
concerned...rainfall onset could be as early as Thursday afternoon
in the North Bay...or possibly not until Thursday night. Rainfall
intensity is expected to be heaviest in the late Thursday
night/Friday time frame as a potent shortwave ripples up along the
moisture plume. The GFS forecasts a second burst of heavy precipitation
late Friday night into Saturday morning.
Given the model differences at this point...as well as lack of
strong run-to-run consistency...forecast confidence for late this
week and into the weekend remains relatively low. However...the
chance of a significant precipitation event in the North Bay is growing
Aviation...as of 3:41 am PST Sunday...high pressure positioned
over California coupled with light offshore surface flow will
maintain mostly clear skies and generally light winds at the
terminals through the next 24 to 36 hours.
Vicinity of ksfo...VFR conditions are expected to prevail through
the forecast period. Generally light winds this morning will give
way to a gentle sea breeze this afternoon...reaching 10 knots.
Confidence is high.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions are expected to prevail
through the forecast period. Generally light winds this morning
will give way to a gentle sea breeze this afternoon...reaching 10
knots. Stronger down valley winds...up to 15 knots...will be present in
the Salinas valley this morning.
Confidence is high.
Marine...as of 2:40 am PST Sunday...high pressure positioned over
the California coast will maintain generally light northerly winds
across the coastal waters through midweek. Winds will become
southeasterly middle week as a storm system approaches the West
Coast. The storm system will bring increasing
winds...deteriorating sea conditions and a chance of rain to the
waters by Thursday.
Public forecast: Bell/dykema
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