Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
431 am PDT sun Sep 21 2014
Synopsis...sligthly warmer weather is forecast over the next
few days. Otherwise little change is expected with a relatively
deep marine layer resulting in widespread night and morning low
clouds. A Pacific storm system will likely bring rain to the
northern part of the San Francisco Bay area from late Wednesday
into Thursday. Rain may extend as far south as the Monterey Bay
area by Thursday. Showers are possible from Thursday afternoon
into Friday evening.
Discussion...as of 3:50 am PDT Sunday...an upper low has
continued to track very slowly across central California
overnight and is currently centered near Fresno. Isolated showers
are wrapping around the north and northwest side of the low and a
few showers continue to move into the North Bay at this hour.
There was even an isolated lightning strike detected near the
northern tip of Napa County just after midnight. The models did a
very poor job of forecasting the convection that made its way into
the northern and eastern portions of our area late yesterday and
overnight. Only the most recent run of the GFS (06z) appears to
have caught on. The models agree that the low will begin to move
quickly to the east-northeast and out of our area within the next few
hours...ending all precipitation chances by sunrise. But will monitor
closely for signs of isolated showers lingering beyond 12z.
The marine layer remains deep early this morning and we will
probably see low clouds clear rather slowly once again today.
Temperatures today are expected to be close to what they were
yesterday...with inland areas remaining slightly below normal and
coastal areas holding near normal. As the upper low moves off to
the east...a shortwave ridge will develop over California by late today.
That ridge will then amplify over the next few days in response to
a deepening upper trough upstream over the eastern Pacific. But
the ridge will also be moving slowly off to the east and so we
probably won't see more than just slight warming the next few
days. Nights will continue to be balmy...especially as the upper
trough approaches Tuesday night (similar to the situation that
occurred this past Wednesday night ahead of an approaching cold
front when overnight lows were in the middle and upper 60s). Have
warmed overnight lows above guidance...but may not have gone warm
The models have trended slower in moving the cold frontal rain
band through our area around midweek and this seems reasonable given
that the trough will be deepening as it approaches the coast. It
now looks like rain will begin in the North Bay on Wednesday
but not make it south and east through the rest of the
sf Bay area until late Wednesday night or even Thursday morning.
Have adjusted the pop grids accordingly. The southern extent of
the frontal rain band is expected to reach to about the Monterey
peninsula on Thursday morning. Thereafter...the GFS and European model (ecmwf)
agree quite well in producing only isolated light showers as the upper
trough moves through from late Thursday through Friday evening.
The Gem model...on the other hand...indicates much more moisture
with the trough and this model spreads another round of
widespread rain across our area on Friday and Friday evening. Have
kept rain chances going through Friday night...but confidence in
the forecast decreases considerably after Thursday evening given
model differences. There is currently no expectation for
thunderstorms with this system...but this will need to be looked
at more closely as we draw nearer to the precipitation event.
Rainfall totals with this system look to be mostly around a
quarter of an inch or less except up to a half inch or more in the
coastal mountains...especially in the North Bay. These totals are
based on the GFS and European model (ecmwf) guidance and higher amounts would
occur if the 00z Gem scenario were to verify.
Aviation...as of 04:30 am PDT Sunday...widespread stratus under a
fairly deep marine layer this morning. The onshore gradient has
strengthened while the northerly gradient remains near neutral so
similar to slightly later burnoff forecast for today. Local high
res model indicates possible early return this evening.
Vicinity of ksfo...MVFR ceilings this morning clearing around 18z.
West winds...increasing to around 15 knots in the afternoon.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings this morning. Kmry to
clear early afternoon...slightly earlier at sns.
Marine...as of 2:15 am PDT Sunday...generally light winds and
seas will continue over the coastal waters into the beginning of
next week. By Wednesday longer period northwest swell is set to
move into the waters along with stronger winds due to an
approaching low pressure system.
Climate...autumn officially begins (autumnal equinox) at 7:29 PM
PDT on Monday September 22.
... Bay north of the Bay Bridge from 3 PM
Public forecast: dykema
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