Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
949 PM PST Thursday Nov 20 2014
Synopsis...scattered showers will continue across the southern
portion of the region through about midnight Thursday night. Dry
weather is then forecast from late Thursday night through most of
Friday. The next storm system will spread rain across the region
from north to south from late Friday through midday Saturday. Dry
weather will return by Sunday and continue through Thanksgiving
Discussion...as of 9:00 PM PST Thursday...today's 00z Oak
sounding illustrates just how unstable the airmass was late this
afternoon behind a departing cold front...showing an Li of minus 3
and a cape value of 780 j/kg. Widespread shower activity was
observed across our area from late afternoon through early
evening. The most intense shower activity was over the coastal
waters between the Golden Gate and Monterey Bay...inland across
southern San Mateo County...and locally over and near San
Francisco Bay. In these areas isolated thunderstorms also
developed. Rain rates were heavy in these showers and
thunderstorms. The La Honda RAWS station...located south of
Highway 84 in southern San Mateo County...picked up nearly 7
tenths of an inch of rain over two hours late this afternoon.
Shower activity continues to taper off from north to south at the
present time...with nearly all convective activity now from Santa
Cruz County southward. Also...intensity of these showers has
decreased. The 00z NAM indicates all shower activity will end by
about midnight and this looks good given recent radar trends. A
forecast update from earlier this evening included a removal of
rain chances for most areas after midnight tonight. Forecast
update also added patchy fog for portions of the north and East
Bay valleys overnight and into Friday morning. The visibility at
the Sonoma County Airport has already dropped to 1/4 mile.
Friday is shaping up to be a partly cloudy and dry day. The models
agree that the next system...currently developing off the coast of
the Pacific northwest...will not begin to spread rain into our
forecast area until late Friday afternoon...and only then across
the northern reaches of the North Bay. Rain is then expected to
spread slowly south across the North Bay to near San Francisco by
late Friday night and then south to the Monterey Bay area by late
Saturday morning. This timing is somewhat slower than previously
thought and the forecast update from earlier this evening included
a reduction of probability of precipitation for Friday afternoon and Friday night. This
Friday night/Saturday system looks similar in strength to the one
that is currently exiting our area...although it is expected to
have somewhat more moisture to work with. Rain totals from late
Friday through late Saturday are forecast to be in the quarter to
half inch range across most of the sf Bay area and south into Santa
Cruz County...except locally an inch or more in the North Bay
mountains. Farther to the south...in Monterey and San Benito
counties...rainfall totals are expected to mostly be less than a
quarter of an inch.
Precipitation is forecast to end by late Saturday evening and the
second half of the weekend will be dry. Dry and mild weather is
then forecast to continue through at least Thanksgiving day as the
persistent West Coast ridge once again dominates our weather.
Aviation...as of 9:49 PM PST Thursday...for 06z tafs. Last shower
activity currently over kmry and ksns and should be ending
overnight. Some 1/4 mile fog has already developed at ksts and
will likely stay in place through early morning hours. Cloud
heights in general throughout the Bay area are above 3000 feet.
Tafs will account for some lowering of ceilings by sunrise but then
medium confidence for VFR by 18z Friday for Bay area terminals.
Next storm moves into the North Bay late Friday night and wont
impact sf Bay until Saturday.
Vicinity of ksfo...expecting ceilings 3000 feet and higher overnight
then lowering to MVFR by 12-14z Friday before mixing out by 17-18z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...lingering -shra until around midnight
then slow clearing overnight into Friday morning. Some concern for
patchy fog around sunrise but wont show in taf for now.
Marine...as of 09:42 PM PST Thursday...light winds with a few
lingering showers overnight. On Friday southerly winds will
develop again ahead of a stronger system passing to our north.
That front will drop into the waters Saturday morning with rain
and increasing winds. Behind the front stronger northwest winds will
develop Saturday afternoon into Sunday along with building northwest
Public forecast: dykema
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