Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1048 am PDT Wednesday Sep 2 2015
Synopsis...dry weather will continue through next week.
Temperatures will continue to cool through Saturday as an upper
level trough digs southward into the Pacific northwest. Warmer and
drier conditions return over the weekend and early next week.
Discussion...as of 8:30 am PDT Wednesday...thanks to a much
deeper marine layer (now over 2000 feet) plus an onshore flow at
the surface...cloud cover is much more substantial today compared
to recent mornings.
Current forecast appears to be on track. No substantial updates
Previous discussion...generally quiet weather will continue for
the Bay area with seasonably cool weather through Saturday
followed by a gradual warm up over the Holiday weekend into early
Overnight satellite imagery continues to show a diffuse marine
layer trying to develop with patchy stratus along the coast and
locally inland. Sodars and the feet Ord profiler put the marine
inversion 1500-2000 feet this morning. Therefore...will keep the
low clouds and patchy fog this morning. Model guidance also
suggests patchy drizzle will also be possible. This maybe the
result of a passing shortwave trough currently visible on water
vapor to the SW of Point Conception.
The more notable weather item today will be the cooling
trend and slightly below normal temperatures...especially inland. The
aforementioned shortwave trough and a longwave trough over the
Pacific northwest will aid in dropping 850mb temperatures (by 3c) and lowering 500mb
heights over the region. The end result will be a cool down of 5
to 8 degrees over Tuesday/S maximum temperatures...especially away from the
coast. After the shortwave exits to the east later today...the
broad longwave trough will keep cool conditions in place as we
head into the Holiday weekend. Patchy night and morning clouds
will still be possible the next few days. Temperatures will
generally be in the 60s to near 70 along the coast and 70s to near
90 inland. Overnight lows will remain somewhat mild and mostly in
the 50s...a few pockets in the 40s is not out of the question.
By this weekend...a Pacific northwest low is forecast to swing through the
northern part of the state bringing a slight chance of rain well
north of the state. As the system exits high pressure will
develop inland with a thermal trough along the coast. Weak to
moderate offshore flow is forecast to develop Sunday into Labor
Day. The offshore flow will bring warmer temperatures with little
to no marine layer.
Still no sign of precipitation. The latest CPC 6-10 day calls for near
normal precipitation with above normal temperatures.
Aviation...as of 10:45 am PDT Wednesday...an upper trough will
gradually deepen along the West Coast and result in a continued
deep marine layer and moderate onshore flow. Low ceilings will mostly
clear by 18z Wednesday...but then begin to redevelop near the
ocean shortly after 00z...and into sf Bay by 04z. Expect MVFR ceilings
to develop over most terminals by late evening with local IFR ceilings
late tonight and early Thursday. Moderate afternoon and evening
onshore breezes. Confidence moderate.
Vicinity of ksfo...VFR conditions expected through about 04z with
MVFR ceilings from 04z through 18z Thursday. West winds increasing to
near 20 knots by late afternoon and continuing through middle evening.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions should persist
through most of Wednesday...with MVFR ceilings redeveloping shortly
after 00z Thursday. MVFR/IFR ceilings then expected overnight and
through most of Thursday morning. Moderate onshore breezes.
Marine...as of 9:08 am PDT Wednesday...an upper level trough off
the Pacific northwest will deepen and move inland on Saturday.
This will result in increasing northwest winds over the northern
coastal waters beginning this afternoon and over the southern
waters by Thursday. Brisk northwest winds will continue into the
... Arena to pt Reyes 0-10 nm from 2 PM
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 2 PM
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm from 2 PM
Small Craft Advisory...sf Bay from 2 PM
Public forecast: Bell/mm
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