Scientific Forecaster Discussion
fxus66 kmtr 230554
Area forecast discussion
National Weather Service San Francisco Bay area
1054 PM PDT Sat Jul 22 2017
Synopsis...temperatures will begin to moderate on Sunday as a
weak upper level trough begins to develop off our coast. A more
pronounced cooling trend appears on tap for the first part of next
week, with a deepening marine layer and increased onshore flow.
But then by the latter part of the week, high pressure looks to
build back in again resulting in a return to warming temperatures.
Aside from some possible coastal drizzle, no precipitation is
presently expected in our area through the forecast period.
Discussion...as of 9:00 PM PDT Saturday...warming occurred at
nearly all locations throughout our forecast area today as high
pressure aloft strengthened over California. The sharpest increase
in temperatures occurred in the East Bay valleys and locally
across interior portions of San Mateo County. In these areas highs
today were as much as 15 degrees above those of yesterday. A few
locations in the East Bay reached 100 this afternoon, as well as
several locations across interior Monterey County and San Benito
A cooling trend will get underway on Sunday for much of our
forecast area, as high pressure aloft begins to retreat to the
east and a weak upper low develops off the northern California
coast. Winds aloft have already turned southerly, and surface
winds are also expected to gradually shift to the south by Sunday,
and then increase on Monday as a surface low develops just off
the coast of the North Bay. Cooling on Sunday will be most
pronounced in areas that have greatest exposure to marine air
influence under southerly flow (e.G., North Bay valleys, Big Sur
coast, Santa cruz). Valleys farthest inland and locations in the
hills above 1000 feet will remain very warm on Sunday and
experience only slight cooling. Cooling will spread farther inland
and to higher elevations on Monday as the marine layer deepens.
Meanwhile, locations with terrain to the south (e.G., Monterey,
san jose) will see less cooling the next few days due to downslope
Low clouds had cleared from the coast from Monterey Bay northward
by early evening. Skies are currently mostly clear except for a
band of high clouds moving from south to north through the Bay
area. Models indicate that low clouds and patchy fog will develop
along much of our coastline by late tonight, but clouds/fog
should mostly remain confined to coastal areas. Then, as the
marine layer deepens by Sunday night, low clouds will begin to
develop farther inland.
Mid and upper level moisture to our south is forecast to advance
northward over the next 24 hours. However, the flow aloft is
expected to have enough of a westerly component over our area to
deflect the bulk of this moisture to our east. Therefore, there is
virtually no chance that high-based convection will develop in
our area anytime soon, except for a very slim chance in the far
southeastern portion of our forecast area.
Temperatures for most inland areas are expected to be slightly
cooler than normal for Monday and Tuesday. By midweek the upper
low near the northern California coast is forecast to move off to
our east, allowing upper level ridging to once again strengthen
over our area. This will mean a warming trend for most areas
during the second half of the week.
Tropical storm hillary has recently developed off the southwest
coast of Mexico. This storm is forecast to track to the northwest
and, according to some GFS ensemble members (and operational 18z
gfs), remnants of this tropical system may end up approaching the
California coast by day 10.
Aviation...as of 10:30 PM PDT Saturday...upper level high over
the southwestern US extending to California brought a very warm
airmass to the district. The marine layer is shallow at around
1000 feet. The east-west component of the onshore gradient is
weaker than yesterday. As a result stratus is expected to be
confined to the coastal areas with little inland penetration
except through the Golden Gate to Oak. Clouds in northern mry Bay
expected to spread into southern mry Bay 08-10z.
Vicinity of ksfo...VFR. West winds to 15 kt through 04z.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR. IFR for cigs/vsby after 08-10z
clearing by 17z.
Marine...as of 10:45 PM PDT Saturday...high pressure over the
eastern Pacific will shift to the northwest as a thermal trough
extends towards the central coast of California. Winds will begin
to decrease for most of the coastal waters. The northern outer
waters, however, will remain gusty through the weekend.
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...sf Bay until 11 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm until 3 am
Public forecast: dykema
aviation: west pi
marine: west pi/S Sims
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