Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1030 PM PST Wednesday Nov 25 2015
Synopsis...dry and cool weather will continue through the end of
the week. Overnight lows are forecast to drop down to near
freezing across inland areas through at least Saturday morning.
Temperatures are expected to moderate by the end of the upcoming
weekend...and especially next week when rain chances will return.
Discussion...as of 9:05 PM PST Wednesday...the cold weather
system that brought light to moderate amounts of rain to our
region over the past few days...along with a dusting of snow to
area mountain peaks...has settled into the Great Basin this
evening. This system is in the process of cutting off from the
westerlies and the forecast models agree that a cold cutoff upper
low will wobble about over the Great Basin and eastern California
through the end of the week. What this means is that the cold
airmass that this system brought into our region yesterday and
last night will remain in place for at least the next three days.
The primary forecast concern for these next three days will be
overnight low temperatures...and specifically if temperatures will
fall to freezing or below.
By 8 PM this evening temperatures had already dropped into the
low to middle 40s in most areas as skies clear. There
is...however...still some residual low level moisture in place and
dewpoints remain in the 30s. Enough low level moisture is
expected to remain in place overnight to keep temperatures from
dropping too far. Only the coldest inland areas are likely to see
temperatures drop to around freezing by sunrise Thanksgiving
morning...and probably only for a brief time. For this
reason...freeze warnings have not been issued for tonight/early
Thanksgiving morning. Instead...have re-issued a Special Weather
Statement with information on the cold weather expected for the
remainder of the week.
Northerly winds aloft will begin to increase on Thursday and these
winds will advect a drier airmass across our region by Thursday
night. This drier airmass will allow for more efficient
radiational cooling beginning tomorrow night. It's likely then
that most of our forecast area will experience their coldest
temperatures on Thursday night and Friday night. Based on current
forecast lows...freeze warnings may be needed for some of our
inland valleys tomorrow night. The North Bay valleys are one place
for concern. However...northeast winds aloft are forecast to
increase across the North Bay on Thursday night. Based on the
latest NAM and WRF model output...northeast winds in the North Bay
mountains will be in the 15 to 25 miles per hour range by late Thursday
night with gusts as high as 40 miles per hour. These winds may be strong
enough to break through the nocturnal inversion and produce mixing
into the valleys...in which case valley temperatures would not
drop as far as forecast by Friday morning. Further south...winds
are not expected to be as much of a factor and so freezing temperatures
may be more likely...especially in the interior valleys of
Monterey and San Benito counties. In addition...temperatures may drop
close to freezing in coastal locations such as Santa
Cruz and Watsonville beginning Thursday night which may warrant
issuance of frost advisories for some coastal areas. Will need to
watch how temperatures behave tonight. If they drop lower than
forecast...then freeze warnings and/or frost advisories may be
needed for portions of our forecast area tomorrow night. Temperatures on
Friday night are expected to be similar to Thursday night.
Daytime temperatures from Thanksgiving on through Saturday are
forecast to remain below normal...even with plenty of sun. Most
locations will have highs ranging from the upper 40s to middle 50s.
Satellite currently shows a weak system offshore approaching
140w. This system is already bumping up against the high amplitude
ridge in place along 130w. Indications are that this system will
undercut the ridge by late tomorrow night. Previous runs of the
GFS indicated this system would then manage to make it to the
California coast by late Saturday and bring rain to at least the
southern half of our County Warning Area Saturday night. But the 00z GFS deflects
this system well to the south of our area and keeps our area dry
through the weekend. So we will likely remove all rain chances
from the weekend forecast...depending on what the full compliment
of 00z model data shows. If that weekend system is deflected well
to our south...we may end up having one more cold night on
Saturday night...and forecast min temperatures that night would need to
be adjusted downward.
There is still a lot of uncertainty in the forecast for next
week. The GFS and Canadian show at least a potential for
relatively wet conditions...while the European model (ecmwf) maintains a dry
blocking pattern across the west. The last few runs of the GFS
have trended drier next week and so confidence is fading that we
will see a wet pattern set up next week.
Aviation...as of 10:30 PM PST Wednesday...VFR...exception is
patchy fog possible at ksts. Moderate drying taking place tonight
throughout the atmospheric column as a result radiational cooling
is a bit more optimum. Any fog patches Thursday morning should mix
out quickly. Light offshore winds developing tonight into Thursday.
Vicinity of ksfo...VFR.
Ksfo bridge approach...VFR.
Monterey Bay area terminals...VFR.
Marine...as of 9:28 PM PST Wednesday...upper low over Nevada will
be over Utah on Thanksgiving. The low will then tend to move back
westward through late week. Winds and seas will become light through
the remainder of the week and into the weekend before strengthening
late in the weekend.
... Arena to Pigeon pt 10-60 nm until 3 am
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm until 3 am
Public forecast: dykema
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