Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
636 PM PST Tuesday Mar 3 2015
Synopsis...a building ridge of high pressure along the West
Coast will result in dry weather conditions and a slight warming
trend through the upcoming weekend.
Discussion...as of 02:24 PM PST Tuesday...a middle/upper level ridge
off of the West Coast will slowly strengthen and build inland
through late week as the long-wave trough axis over Southern
California shifts eastward. As a result...850 mb temperatures are
forecast to warm above 10 degrees c and bring surface temperatures
into the 70s for most locations by Thursday. These warm and dry
conditions are then forecast to persist through the upcoming
weekend and likely into early next week as the ridge remains
parked over much of the region. With weak offshore flow possible
over inland areas this weekend...a few spots may even reach or
exceed the 80 degree mark. With limited moisture in the boundary
layer...do not expect much in the way of stratus during the late
night/early morning hours through early next week.
The medium/long range models continue to show the middle/upper level
ridge shifting eastward and weakening by the middle of next week.
This pattern change would allow for an upper level trough to impact
the Pacific northwest and bring widespread rainfall to the San
Francisco/Monterey Bay region Wednesday into Thursday of next week.
Temperatures will also cool over the region during the latter half
of next week as a result. Will continue to monitor this possible
pattern change for next week...stay tuned.
Aviation...as of 6:20 PM PST Tuesday...the California coastal trough will
very likely shift just offshore by early Wednesday morning. Local
onshore winds this evening will trend to neutral or briefly weak
offshore during this time. The larger scale wmc-sfo gradient
increases modestly to approx 8 mb by Wednesday morning per today's
and last evening's NAM model runs. This favors VFR for the Bay
area terminals for the period.
Am less certain about the low cloud cover in the Monterey Bay area
for tonight as partial low cloud cover is converging on this area
as of this writing. The WRF model run shows an eddy circulation
developing over the Bay waters by midnight with a trend toward southeast
winds developing over the northern Salinas valley very early
Wednesday morning. As a result of this southeast wind the low clouds will
likely move out over the waters Wednesday morning. Also a southeast wind
flow is consistent with what we usually see in offshore patterns
even if they are only brief...and it is expected to be brief.
Weak to locally moderate onshore winds return all areas by Wednesday
afternoon as the NAM shows the trough moving back inland...but we
could see a similar pattern of brief offshore wind developing once
again very late Wednesday night into Thursday morning.
On a side note: coastal buoy water temperatures have cooled off a little
more in the last 1 to 2 weeks presently ranging in the middle to upper
50s north to south...still a little above normal but cooler. We may
be seeing more in the way of coastal low clouds and/or fog through
this week with possible weather related implications for area terminals
due to this cooling and further strengthening of the upper level
Vicinity of ksfo...VFR. Onshore winds less than 20 knots will
continue to subside by later this evening. Light and variable wind
trends toward light NE wind Wednesday morning. Westerly winds return
by middle to late Wednesday afternoon. Moderate to high confidence.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...may need to amend the 00z kmry and
ksns tafs to include more low cloud cover tonight/Wednesday morning.
Low confidence forecast overall especially with clearing times
Wednesday morning. Cloud cover as well as clearing will be highly
dependent upon strength of the eddy circulation if it even develops
overnight. Much improved chances for VFR during the day Wednesday
but low clouds may not be that far away from the terminals this
time tomorrow evening.
Marine...as of 02:24 PM PST Tuesday...high pressure situated off
the West Coast will maintain northerly winds along the central
coast. Locally gusty winds will be possible north of Point
Reyes...especially the outer waters. Seas are expected to be
moderate and gradually decrease through the week.
... Arena to pt Reyes 10-60 nm until 3 am
Public forecast: rgass
Visit US at weather.Gov/sanfrancisco
Follow US on facebook, twitter, and youtube at: