Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
156 PM PDT Tuesday Jul 7 2015
Synopsis...a deep marine layer over the region will result in
widespread night and morning clouds...patchy fog and coastal
drizzle through Thursday morning. Isolated showers and
thunderstorms will also be possible Wednesday afternoon through
Thursday evening as an upper level system moves across the region.
Dry weather conditions return late in the week and into the
weekend along with near normal temperatures.
Discussion...as of 01:53 PM PDT Tuesday...the overall pattern
aloft has changed very little in the past 24 hours or so with the
middle/upper level low about 350 miles west of the North Bay coast.
Closer to the surface...the marine layer remains disorganized and
allowed for coastal stratus to mix out which gave way to mostly
sunny conditions over the region. This has allowed temperatures to
warm into the lower 70s for many coastal locations while inland
areas are generally in the 70s and 80s. Expecting the marine layer
to become better defined tonight with low clouds...patchy fog and
coastal drizzle possible tonight into Wednesday morning.
The aforementioned middle/upper level low remains on track to approach
the San Francisco/Monterey Bay area during the next 12 to 24 hours.
As a result...mid-level moisture associated with the system will
advect over the region and interact with middle/upper level
instability. While the forecast models are in good agreement with
respect to the middle/upper level pattern...they continue to struggle
on where and when any convection will occur. With that...will
continue to mention a slight chance of showers and thunderstorms
from Wednesday afternoon through Thursday evening as the system
moves over the region. Meanwhile...the marine layer is forecast to
deepen further and result in more widespread drizzle Wednesday night
into Thursday with the potential for rainfall amounts of a few
hundredths of an inch. Having said all that...cannot rule out
isolated convection developing above the marine layer and associated
coastal drizzle Wednesday morning...yet more so on Thursday morning.
The upper level low is then forecast by the models to lift northeast
and become absorbed in the mean flow Thursday night into Friday. As
a result...chances for elevated convection will diminish late in the
week with drier air aloft filtering in over the region. Will
maintain dry weather conditions Friday into next week with a broad
middle/upper level trough along the West Coast. This should bring about
a slight warming trend to inland areas with conditions along the
coast heavily weighed on the marine influence. Overall...conditions
region-wide beyond Thursday will be typical for early/middle July.
Aviation...as of 10:40 am PDT Tuesday...a disorganized...but
deep marine layer impacted much of the region this morning. Most
terminals have clear...but pesky clouds remain in sf Bay impacting
koak and ksfo. Do expect these clouds to gradually erode with in
the hour. Already seeing sign on cams and satellite. VFR this
afternoon. Marine layer will roll back in tonight with IFR
conditions. Overall conf is medium to high.
Vicinity of ksfo...MVFR through 18-19z...put 1830z for ksfo taf.
VFR this afternoon with west winds gusting to 24 knots after 20z.
Ceilings return tonight around 06z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR this afternoon. Ceilings return
shortly after 00z tonight.
Marine...as of 10:38 am PDT Tuesday...light to moderate winds
will persist along the central coast as a weak low pressure system
remains over the eastern Pacific. Winds will be gusty each
afternoon in both bays with decent onshore flow. The low pressure
system will move across the coastal waters Thursday bringing a
slight chance of thunderstorms.
... Bay until 9 PM
Public forecast: rgass
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