Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
918 am PST Wednesday Nov 26 2014
..substantial rainfall event slated for Friday into Wednesday...
..Hydro concerns likely by next work week...
Synopsis...high pressure will bring dry weather with seasonable
temperatures through Thursday. Rain will return for Friday into
Saturday and continue into next work week. Another round of rain
will then move across our area either on Monday or Tuesday.
Discussion...as of 8:50 am PST Wednesday...little to talk about
for the first couple of days with seasonably warm temperatures along
with dry conditions forecast.
All attention is focused starting Friday evening as the next round
of rain starts to make its way through the County Warning Area. Not a lot to add
to the previous discussions, however wanted to highlight that quantitative precipitation forecast
values from wpc are quite impressive. Confidence is high for the
rain through the weekend and that another impulse will work its
way across, although the timing of the second remains problematic
(ecmwf shows a 36-48 hour break between systems while the GFS/Gem
show more of a continual event). If the GFS verifies Hydro
concerns would be much more elevated by Monday and Tuesday.
However, even if there is a break, the amount being forecast (2-4"
for urban spots with well over 1/2 a foot in the hills) will
produce some Hydro problems. Also of note are the very impressive
isentropic values off the GFS (up to 60 knots from the sw) combined
with precipitable water values up to 1.3". Other item of concern will be the
winds. Models remain in good agreement for windy conditions
Saturday night/Sunday morning. Much less agreement for Monday into
Bottom line, high confidence for a substantial rainfall event, high
confidence for the timing of the first round of rain and lower
confidence with the second round.
Will be very interesting to see what the 12z European model (ecmwf) shows.
Previous discussion...the short term forecast remains on track.
Skies are mostly clear again this morning with only some patchy
fog being reported at Santa Rosa and Concord. Temperatures and
dew-points are trending slightly higher than 24 hours ago while we
still have offshore gradients from sfo to wmc and sfo to SAC.
Upshot is another sunny and mild day in store with afternoon highs
in the upper 60s for most of the Bay area with 70s for the Santa
Clara Valley and Monterey Bay region. For once ksfo wont be
contributing to Airport delays across the country with no concern
for low clouds/fog today. However weather back east will create
plenty of air travel delays. Those traveling by car across
California should encounter no weather delays with no precipitation or
fog to worry about.
Thanksgiving weather looks pleasant with high pressure still in
control. Winds may turn more westerly by afternoon with perhaps a
few high clouds but highs still comfortably in the 60s Bay area
and some 70s for the central coast.
As has been the case the last several days were still focused on
the weekend weather event. Some of the details are becoming more
clearly defined in terms of how the models are handling the
various features over the Pacific. To cut to The Chase the big
development overnight has been the 00z European model (ecmwf) dramatically slowing
the main cold frontal passage. Will need to see if this is an
outlier but the Gem and GFS models have also trended slower.
Still looks like light rain develops in the North Bay Friday night
but stays north of the Golden Gate. Then on Saturday that boundary
will drop southward through the Bay area and into the Monterey Bay
region. It will be a warm advection rain that has origins to a
tongue of high precipitable water values tracing all the way
towards Hawaii. Precipitable water values around 1.25 inches get pointed into
the Santa Cruz Mountains on Saturday in a narrow atmospheric river
set-up. For those following along at home its the band of moisture
currently out near 35n/138w that will eventually get pointed
perpendicular to our coastal ranges. Its conceivable that it may
end up over the North Bay on Saturday or as far south as the Santa
Cruz Mountains. This could set the stage for some pretty disparate
rain totals with the initial system where several inches could
fall in the Santa Cruz Mountains with much lighter amounts for the
Bay area valleys.
That boundary will kind of wash out later Saturday night into
Sunday as the parent low well offshore starts to dig. The end
result should be continued periods of mainly light rain across
most of the district on Sunday. However with no strong dynamics or
forcing not seeing a heavy rain event for Sunday...furthermore the
flow will turn more southerly which can shut down orographics for
For the last few days the models were indicating Sunday night or
early Monday is when the worst weather would occur as the parent
low and surface front moved onshore. That timing could have been
particularly bad for return travel of the Holiday weekend. The
00z European model (ecmwf) came in around 11 PM last night with a sharply different
solution that slowed the parent low by 24-48 hours...not bringing
the main low onshore until Wednesday morning! That seems a little
suspect but the Gem and GFS have clearly slowed the timing of the
main front as well. Once again will want to see the 12z cycle to
see if the European model (ecmwf) was an outlier or trend setter. For what its
Worth the 06z GFS has the wettest conditions from 18z Monday to
00z Tuesday. So instead of impacting Sunday night its now more
into the Monday afternoon commute.
So will expect the timing to keep being altered. Good news is rain
still appears to be on track. However its somewhat disconcerting
to see the European model (ecmwf) solutions which cuts the system off from the main
jet somewhat. Complicating factors more is there does appear to
be a tap into some sub-tropical moisture. The models had been very
consistent for about 3 days so this is a little bit of a kink in
the plan and justifies keeping probability of precipitation in th chance category for much
of the extended until details get sorted out.
Aviation...as of 9:10 am PST Wednesday...no major changes from
previous taf package. VFR conditions through the period with the
exception of ksts and fog. There may be some haze in sf Bay...but
slant wise visible should be okay.
Vicinity of ksfo...VFR.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR.
Marine...as of 09:11 am PST Wednesday...north of Pigeon Point
winds will remain southerly as a storm system approaches the
region. South of Pigeon Point winds will remain out of the west to
northwest through Friday. Southerly flow will develop over all of
the coastal waters this weekend as a storm system begins to impact
the region. Unsettled weather is expected Friday into early next
week with periods of rain and gusty winds.
Public forecast: Bell
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