Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
439 am PDT Thursday Oct 30 2014
Synopsis...cooling is expected today ahead of an approaching Pacific
cold front. The cold front will spread rain across the entire
region late tonight into Friday. Showers and isolated thunderstorms are
forecast for Friday night into Saturday morning. Dry weather will
return Saturday night and continue through the middle of next week
as high pressure rebuilds.
Discussion...as of 3:10 am PDT Thursday...after a warm day on
Wednesday, temperatures cooled nicely overnight thanks to
generally clear skies, light winds, and dew point values mostly in
the 40s. Currently most locations are in the 50s with a few
additional degrees of cooling expected.
Main weather player remains a Pacific storm system to our northwest
currently spinning around 50n/140w and an associated cold front
that will advance to our County Warning Area by tonight and bring widespread
rainfall to our entire area on Friday. Ahead of the front today
the surface flow will switch from offshore to onshore. At the same
time 850 mb temperatures will drop 2-4c. The combination of both things
will lead to cooler conditions at the surface with highs mostly in
the 70s to lower 80s. In some cases, temperatures will be 10-15 degrees
cooler than yesterday.
Clouds will move across our area by the evening as the cold front
nears and overnight rain will start across the North Bay and
spread to the south through Friday in coordination with the front.
Models are generally in good agreement with the timing and bring
the main band of rain and breezy conditions down to the sf Bay
area late morning/early afternoon...to San Jose and Monterey Bay
area for Friday afternoon. In addition, unlike the past few
events, the cold front is not forecast to wash out even as it
exits southeast Monterey County Friday evening. Therefore, rainfall
amounts could be fairly impressive for this time of year, even in
the usual dry spots in the County Warning Area. After a very brief break behind
the frontal passage, colder and more unstable air will filter in
as the main upper level low moves toward northern California.
Showers will pop up and spread across the entire area especially
for Friday evening and night. Guidance shows more than a 30%
chance of thunderstorms south of Santa Cruz County although all
sections will likely at least have a shot of seeing some lighting
or even getting small hail. Rain will tapper off on Saturday with enough
instability remaining over interior Monterey County plus portions
of San Benito County into Saturday afternoon to keep a slight
chance of thunderstorms going. By Saturday evening the rain will be
As mentioned above, rainfall amounts are expected to be impressive
for this time of year with 1/3"-2/3" likely for almost all urban
locations. For higher elevation spots locally 1-2" are possible --
even over interior Monterey County. Santa Cruz County could see
some locally impressive numbers as the models are focusing
higher quantitative precipitation forecast over that County for the second half of Friday as a
vorticity maximum rotates through.
A ridge of high pressure will slowly build back to our area going
into next week as the system departs to the east. Temperatures will warm
back up to normal to slightly above normal values by Tuesday.
Aviation...as of 4:40 am PDT Thursday...VFR conditions are
anticipated to prevail through Thursday night across area terminals.
Increasing high clouds are expected ahead of an approaching
frontal system that will move into the region late Thursday night
into Friday. Southerly winds will develop Thursday afternoon ahead
of the approaching cold front. Precipitation will begin with
arrival of front towards the end of the 30-hour period Friday
morning. Moderate to high confidence.
Vicinity of ksfo...VFR conditions and light winds are anticipated
through the the next 30 hours. Rain and MVFR ceilings possible after
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions and light winds are
anticipated through the forecast period.
Marine...as of 04:26 am PDT Thursday...weak high pressure through
Thursday will give way to an approaching storm system early
Friday. Gentle seas and generally light southerly winds are
expected ahead of the front through Thursday. Gusty northwest
winds are expected Friday as the storm system begins to move
through the area. Building seas and moderate northerly winds are
expected Friday evening into Saturday.
Public forecast: Bell
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