Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
536 PM PDT Tuesday Sep 1 2015
Synopsis...aside from the possibility of coastal drizzle tonight
into Wednesday morning...dry weather conditions will persist
through the upcoming weekend. Temperatures will cool through late
week as an upper level trough digs southward into the Pacific
northwest. Warmer and drier conditions return for the upcoming
Discussion...as of 02:02 PM PDT Tuesday...coastal stratus
continues to hug the coastline from Point Reyes southward down
the Big Sur coast as a relatively deep marine layer remains in
place. Expecting the low clouds to spread back inland late in the
afternoon or early evening and persist through Wednesday morning.
Meanwhile...a weak middle/upper level short-wave disturbance can be
seen approaching the region from the southwest. As this system
moves across the central coast late tonight...drizzle appears more
likely along the coast and even into the East Bay by Wednesday
morning. Cannot rule out a few locations picking up a few
hundredths of an inch.
The aforementioned disturbance will then become absorbed by the
long-wave trough dropping southward across the Pacific northwest
by Wednesday afternoon. This approaching trough will also result
in a cooling trend region-wide Wednesday into Thursday. Lingering
boundary layer moisture should keep some coastal clouds across the
region into Thursday morning before a stronger middle/upper level
trough digs into the Pacific northwest/northern California late
Thursday. This will result in a cooling trend into Friday as 850mb
temperatures cool to around 12-13 degrees c. Breezy northerly
winds can also be expected on Friday in wake of the frontal
passage. The cooler and drier air mass advecting in from the north
will likely allow overnight/morning temperatures to drop into the
40s across the hills/mountains during the latter half of the week.
Elsewhere...overnight lows in the lower 50s will be possible away
from the coast.
By the weekend...the middle/upper level trough will shift inland and
eject toward the northeast. This will result in slight warming on
Saturday with a more robust warm-up (back above seasonal averages)
expected Sunday when the thermal surface trough shift toward the
coast. This should also allow for at least weak offshore flow to
develop over the inland hills/mountains through the weekend. Given
the dry fuels over the region...will need to continue to monitor
this pattern in the coming days. Temperatures then hold steady
into early next week along with dry weather conditions as a broad
upper level trough remains along the West Coast.
Aviation...as of 5:25 PM PDT Tuesday...patchy coastal stratus
seen from southern Marin County to northern Monterey County
otherwise VFR. The forecast area is on the boundary between very
weak upper level SW and northwest winds causing a narrow but long band of
cirrus seen on the visible imagery.
A fairly decent smx-sfo pressure gradient is persisting since late
last evening...presently 2 mb. Northwest winds over the inner coastal
waters are tending to weaken because of this south-north directed
gradient. Onshore winds will persist this evening however...driven
by a slowly increasing sfo-SAC gradient presently 2.4 mb.
Stratus and fog are expected to increase tonight with increasing
inland intrusion...but mainly late tonight into Wednesday morning.
The GFS model is consistent with forecasting a fairly vigorous
low-level trough over the epac to slide southward over the coastal
waters tomorrow. Increasing boundary layer humidity overnight into
Wednesday morning coupled with lower level cooling will likely
enhance stratus cloud cover possibly causing a few localized
pockets of drizzle.
Mesoscale model predicts 3-4 mb acv-sfo gradient Wednesday into
Wednesday night but given back-drop of the middle-week weather
pattern would not be surprised if it became a bit stronger. If it
does west-northwest winds will tend to be a bit gustier than advertised and
skies will tend to clear out faster from north-south Wednesday evening
Vicinity of ksfo...VFR. Increasing marine layer depth forecast during
the period. MVFR ceilings beginning early Wednesday morning...but less
confident about how long ceilings will last at ksfo Wednesday. Few-sct014-015
forecast late morning through afternoon. Model guidance indicates
gusty westerly winds Wednesday afternoon and evening sustained and gusts
up over 20 knots.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...SW-W wind flow into the Monterey Bay
with stratus clouds gradually approaching the terminals this
evening. IFR ceilings forecast to develop 03z-06z...IFR prevailing
overnight. MVFR ceilings late Wednesday morning with partial clearing by Wednesday
Marine...as of 02:02 PM PDT Tuesday...a weak surface pressure
gradient will maintain generally light winds over a good portion
of the coastal waters through Wednesday afternoon. Moderate winds
are however expected to persist over the northern outer coastal
waters through tonight. The surface pressure gradient is expected
to increase for the second half of the week...resulting in
... Arena to Pigeon pt 10-60 nm until 2 am
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm until 2 am
Small Craft Advisory...sf Bay until 8 PM
Public forecast: rgass
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