Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
749 PM 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 7:40 PM PDT Wednesday...partial cloud cover
can be seen over the immediate coast while skies are mostly clear
to clear over the remainder of the area. Temperatures today were
near average for early September.
Deep cold core middle level low over British Columbia will continue
to move steadily south-southeast through the western North America
longwave trough tonight through early Saturday. Per recent model
forecasts 500 mb heights over the Bay area will lower late tonight
through Friday morning with heights becoming more typical of middle-late
fall. As the low takes a trajectory mostly over land it'll remain
mostly moisture starved therefore wet weather is not predicted
here. However...some temperature guidance starting to indicate...the
North Bay counties especially...northern counties could see some
lows in the 40s Saturday morning. Small handful of upper 30s possible
Napa and interior Sonoma county? 850 mb temperatures will cool off...approx
2 to almost 3 Standard deviations colder from the Bay area and central
coast north and east to the Sacramento Valley and northern Sierra
Nevada. Clear skies and lighter...decoupled surface winds late
Friday night/Saturday morning may set up our first brief fall-like
chill...the chilliest temperatures over North Bay valleys Saturday
morning. Once the low moves to the east of the forecast area
Saturday 500 heights will recover to the upside and northerly low
level winds will bring a return to milder/warmer temperatures for
the remainder of the Holiday weekend. Between now and then however
area temperatures will likely be below average possibly by several
degrees in some cases. Sea temperatures have recently cooled back to
September average over the northernmost coastal waters due to
persistent northwest flow and resultant upwelling. Air mass modification
with low reaching norcal is likely to be minimal. As per usual the
trajectory of the middle level low will be highly dependent on the
jet stream...any slight shift east will make a difference as to
how much cooler air reaches the area. Seeing moderate to high
consistency in the models so confidence is the same. Warmer
weather returns either late Saturday and/or Sunday...including the
Holiday. For the remainder of next week it'll be dry with daily
temperatures back to readings more typical of early-middle September.
Will take a look at temperature guidance again. Forecast update likely for
low temperatures Friday and Saturday mornings.
Previous discussion...as of 3:00 PM PDT Wednesday...as expected slightly
cooler air due to a deeper marine layer and stronger westerly
flow has settled across our area today with many locations running
3 to 6 degrees less than this time yesterday. Currently looks like
70s to lower 80s for most spots with the warmest readings for inland
spots plus southern Santa Clara. Clouds are forecast to reform
tonight and spread through coastal locations plus most inland
areas. Lows in most spots will be in the 50s to lower 60s. A few
upper 40s will be possible over interior Monterey County.
Synoptically a longwave trough is forecast to remain over the
western Continental U.S. Through Saturday before finally lifting to the
north and east. The northerly flow aloft will switch to more zonal
(westerly) over the weekend which will help to warm temperatures
again across our area through next week. By Sunday many inland
spots will be back in the middle 80s to middle 90s. Closer to the coast
most places will see 70s to lower 80s. The warmer readings
should hold through all of next week as a ridge of high pressure
rebuilds from the south. Rain is not expected the next ten days
as the storm track is forecast to stay well to our north.
No weather related hazards anticipated through next week.
Aviation...as of 4:45 PM PDT Wednesday...the marine layer has all
but scattered out this afternoon making way for mainly clear
skies across the region. The exception being patchy stratus over
San Mateo and Marin counties. Patchy stratocumulus is also making
its way through the San Bruno gap and over ksfo at this hour.
While stratus is forecast to return tonight over the Bay area we
will likely see broken to scattered ceilings over ksfo and possibly
over koak through this afternoon/early evening. Models are hint to
the return of the marine layer overnight however there is low
confidence with regard to timing. Have pushed back the onset time
a bit but no major changes. Moderate afternoon and evening
onshore breezes. Moderate confidence.
Vicinity of ksfo...VFR conditions expected through about 04z with
MVFR ceilings from 04z through 18z Thursday. Have included a tempo
group between 00z-04z for scattered conditions. West winds near
20 knots with gust to around 25 knots will continue through through this
evening then gradually ease overnight.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR ceilings will redevelop between
01z-03z this evening. MVFR/IFR ceilings expected overnight and through
most of Thursday morning. Moderate onshore winds will ease
Marine...as of 2:44 PM PDT Wednesday...an upper level trough
will deepen along the West Coast through Friday. This will result
in increasing northwest winds over the coastal waters tonight and
Thursday. Brisk northwest winds will then continue into Friday
before gradually decreasing over the weekend.
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 3 am
Small Craft Advisory...sf Bay until 9 PM
Public forecast: canepa/Bell
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