Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1056 PM PDT Tuesday Aug 4 2015
Synopsis...a modest warming trend is forecast for Wednesday and
Thursday as high pressure builds across the region. Cooler
temperatures return late in the week along with a slight chance of
showers and thunderstorms Thursday night and Friday as an upper
level low approaches the region from the west.
Discussion...as of 8:00 PM PDT Tuesday...subtropical moisture
streaming in from the southwest resulted in widespread middle and
high level cloud cover today. In addition...a weak shortwave
trough tracked across our region today and produced isolated light
rain showers across the San Francisco Bay area...mainly the North
Bay. Brief light rain fell as far south as San Francisco
Airport...but most of the precipitation evaporated before reaching
the ground and only trace amounts were recorded. Radar and
satellite data indicate all light precipitation has moved off to the
north and east of our area...and dry weather is expected for the
remainder of the evening.
Cloud cover held inland temperatures well below normal today
while warmer-than-normal sea surface temperatures helped elevate coastal
temperatures as much as five degrees above normal. The result was
unusually isothermal temperatures across our region today with
most locations reporting highs in the 70s.
More typical summertime conditions are expected to develop
tomorrow and Thursday as a shortwave ridge develops over
California. Look for inland temperatures to warm as much as ten
degrees tomorrow and then a few more degrees on Thursday. Coastal
temperatures will probably remain near persistence or warm just a few
degrees over the next two days as a marine layer redevelops.
There presently is no marine layer...but expect one to redevelop
by late tonight as the airmass becomes more stable. Areas of
coastal low clouds will likely develop by sunrise Wednesday
morning and then continue through Thursday.
An upper level low currently centered offshore near 24n/137w is
forecast to track gradually to the east-northeast over the next few days and
is projected to move inland across our area on Friday. Both the
NAM and GFS forecast scattered light precipitation across the sf
Bay area by Thursday night as the low approaches. The models have
been trending earlier with the arrival of this system. In
fact...the 00z NAM forecasts rain as early as 03z Friday (8 PM
Thursday evening). A forecast update this evening included adding
shower and thunderstorm chances starting on Thursday evening...as
opposed to late Thursday night as had been forecast. Model
consensus now is that most precipitation will end by Friday
morning...except perhaps for some lingering showers across the
northeast portion of our area. For the remainder of Friday...and
on through the upcoming weekend...we can expect dry weather along
with warming temperatures.
Aviation...as of 10:45 PM PDT Tuesday...a few areas of low clouds
are redeveloping near Point Reyes and middle/high clouds with the
sub-tropical jet stream continue to stream NE over the Bay area
and central coast. Otherwise VFR conditions prevail this evening.
The marine layer is absent on The Fort Ord and Bodega Bay profilers.
There's a little bit of warming actually showing up near the surface
due to proximity to warmer coastal waters. A gradual warming in the
lower to middle levels is forecast tonight and Wednesday as a ridge
develops over the area. However due to a lack of temperature
difference from the normally cooler ocean waters to warmer lower
levels stratus and fog coverage will likely be at a minimum during
Vicinity of ksfo...low to moderate confidence MVFR ceilings will redevelop
Ksfo bridge approach... similar to ksfo.
Monterey Bay area terminals...VFR conditions with mainly high clouds.
Light winds. Low to moderate confidence IFR redeveloping tonight.
Marine...as of 9:52 PM PDT Tuesday...mainly light winds will
continue over the coastal waters...but northwest winds will tend
to increase once again by late in the week. An upper level low
pressure area over the eastern Pacific will move over the coastal
waters Thursday into Friday with some shower or thundershower
activity possibly arriving by late in the day Thursday.
Public forecast: dykema
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