Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
346 PM PST Thursday Nov 27 2014
..periods of rain forecast from late Friday through the middle of
..potential heavy rain...Hydro concerns...and strong and gusty
winds next week...
Synopsis...high pressure will maintain dry and mild weather
through today. Rain will develop in the North Bay by
late Friday and press south through most of the rest of the region
Friday night into Saturday. Periods of rain are expected to continue
into Sunday. More rain...possibly heavy...is then likely around
the middle of next week.
Discussion...as of 01:45 PM PST Thursday...active weather pattern
through the entire forecast as a series of disturbances are
forecast to impact the Bay area.
Moist westerly flow aloft continues to stream middle-high level clouds
over the Bay area. Despite the filtered sunshine temperatures
around the region are somewhat mild for this time of year. In
fact...Pinnacles is currently sitting at 85 degrees.
Synoptically speaking...high pressure over socal continues to
weaken and flatten out as a trough digs deeper over the epac. The
deepening trough will be the first storm to bring rain to the
region. Farther upstream...another trough/low pressure is slowly
developing near the Aleutian Islands. This developing system will
be the rain maker Sunday into next week.
the cold front to the west will March toward the Bay area
increasing clouds tonight and Friday. Models remain in pretty good
agreement in developing rain over the North Bay initially Friday
afternoon/evening and then slowly moving southward toward sf
Friday night/Saturday morning. Monterey Bay area southward will
see rain late Saturday morning into Saturday afternoon. Rainfall
amounts with the cold front are expected to be 1-2" over the North
Bay with generally 1/4"-3/4" for most urban areas and southern area.
Things get interesting Saturday night if you believe the GFS/ECMWF.
The impulse near the Aleutians really deepens off the Monterey/Big
Sur coast pointing a decent moisture plume toward the coast. Precipitable waters
are near 1.25 inches...not bad at all. That being said...a decent
push of moderate to potentially heavy rain will be possible south
of sf over the Santa Cruz mts through the Big Sur coast. Rainfall
amounts will be on the order of 1/2-1" for many urban spots with
1.5-2.5" around the Santa Cruz Mountains plus coastal ranges in
Monterey County. If the forecast pans out...this will have to be
watched as it may cause Hydro concerns around the pfieffer burn scar
near Big Sur. The bulk of the rain will end by Sunday
evening...but a few lingering showers will be possible. In
addition...locally gusty southerly winds can be expected through
the weekend. Strongest winds will be along the coast and higher
lots of uncertainty remains for the long term as medium range
model disagreement remains. As stated on the previous
discussion...the GFS is still the wettest and most progressive
early next week. The European model (ecmwf) on the other hand is drier and brings
in the rain almost 24-36 hours later (tuesday night instead of
Monday night). Given the dry vs wet forecast...will not change
current forecast and hope that model consensus increases between
now and then. Current forecast will keep the chance of rain Monday
Regardless of timing differences next week...rainfall totals
through the entire event look pretty decent. Definitely not a
drought buster...but much needed rainfall anyway. Rainfall totals are
expected to range from 4-7" over the North Bay plus coastal ranges
of Monterey and Santa Cruz counties with generally 1.5"-3.5" for
many urban and valley locations. Locally higher amounts are
possible in both cases.
Aviation...as of 3:40 PM PST Thursday...dry airmass continues
over the area through the remainder of Thursday. Upper low will
push onshore later Friday and bring -ra and deteriorating ceilings from
north to south beginning around 00z Sat for the North Bay.
Vicinity of ksfo...VFR. Hazy conditions today with light winds.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR.
Marine...as of 03:40 PM PST Thursday...weak high pressure off the
Southern California coast will persist with light winds expected
across the coastal waters through tonight. Southerly winds will
develop Friday as a frontal system approaches the area. These
southerly winds will increase Sunday or Monday as another storm
system approaches California.
Public forecast: mm
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