Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1034 PM PST Thursday Nov 27 2014
..periods of rain...Locally heavy at times...forecast from late
Friday through the middle of next week...
Synopsis...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. Rain could be locally heavy late Friday night into
Saturday. Periods of rain are expected to continue into Sunday.
More rain...possibly heavy...is then likely around Tuesday or
Wednesday of next week.
Discussion...as of 9:05 PM PST Thursday...after a tranquil
weather day on Thanksgiving...the focus now turns to a developing
active weather pattern that will give our area periods of
rain...possibly heavy at times...beginning late Friday and
continuing through the middle of next week.
Frontal boundary near the California/or border remains nearly stationary
early this evening. Satellite shows a shortwave trough well
offshore near 40n/140w. This shortwave will move eastward
overnight and begin to kick the frontal boundary to the southeast
late tonight and into our area by late Friday. The front is
forecast to spread rain into the North Bay on Friday afternoon and
then sag slowly south into the rest of the sf Bay area on Friday
night. By late Friday night both the 00z NAM and 00z GFS slow the
southward progression of the front...and increase precipitation intensity
in the frontal rain band...as a secondary shortwave trough
approaches from the west-southwest. According to these two models rain rates
may be heavy at times from Marin County south across the sf
peninsula and into the East Bay...from late Friday night into
Saturday. There certainly is expected to be sufficient moisture
for locally heavy rain rates given the narrow plume of precipitable water values of
1.25" flowing into this system.
There won't be much of a break in the action before the next
shortwave approaches from the southwest and produces another round
of widespread rainfall Saturday night into Sunday morning. During
this time frame the NAM and European model (ecmwf) focus the heaviest rainfall
from the Santa Cruz Mountains south to Point Sur.
The models indicate a break in the action from late Sunday
through Monday night as shortwave ridging develops in advance of a
deep upper low that will have developed offshore along 140w. That
low is then forecast to move eastward and produce yet another
round of widespread rainfall across our region sometime between
midday Tuesday and midday Wednesday. The timing and location of
heaviest precipitation with the Tuesday/Wednesday system is difficult to pin down
at this point given model differences.
Recent model trends have decreased the amount of rainfall over the
next 6 days. But amounts through the middle of next week still
look impressive with totals from 3-6" expected in the coastal ranges
and from 1-3" in most urban and valley locations. Some minor
urban flooding is possible over the weekend...especially if the
frontal boundary stalls while producing heavier rain rates. But
most rivers and streams should be able to handle these initial
bursts of heavier rain. Additional heavy rain by late Tuesday into
Wednesday could potentially produce more widespread Hydro
issues...but will need to get a better picture of timing and
location of heavier rain before predicting this with much
As far as winds are concerned...there does not appear to be much
concern for strong winds through Saturday night. Southerly winds
should become locally strong and gusty by Sunday...but probably
still remain below advisory levels for the most part. Indications
are that the strongest winds during these rain events will be from
Monday night through Wednesday...with the strongest winds expected
to be over the coastal waters...along the immediate coast...and in
the coastal hills.
Aviation...as of 10:15 PM PST Thursday...rapid deterioration of
conditions expected overnight as advection fog rolls in from
offshore and radiation fog forms inland elsewhere. Coastal sites
will see advection fog... kmry is already at ovc001... and webcams
show very low clouds invading San Francisco and elsewhere along
the coast. Thus... IFR to LIFR ceilings and visbys are possible until
16-18z Friday. MVFR conditions with slick runways will accompany
frontal passage Friday afternoon to evening depending on location.
Vicinity of ksfo...advection fog and low clouds spilling into the
Bay this evening and will continue to do so ahead of the frontal passage.
Radiational fog in the east/North Bay also a concern... especially
at koak. IFR to LIFR ceilings/visbys posbl through 18z Friday. Conditions
deteriorate with frontal passage later Friday.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...advection fog already present at
kmry with LIFR conds. Do not expect any improvement until 18z Friday at
the earliest. Ksns likely to deteriorate to IFR to LIFR conds by
Marine...as of 10:02 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: dykema
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