Scientific Forecaster Discussion
fxus66 kmtr 200529
Area forecast discussion
National Weather Service San Francisco Bay area
1029 PM PDT Sat Aug 19 2017
Synopsis...a few degrees of cooling are expected over the
weekend and into early next week as an approaching trough of low
pressure weakens the area of high pressure currently over the
eastern Pacific. Coastal areas will see little in the way of
change with continue night and morning low clouds and patchy fog.
Discussion...as of 7:50 PM PDT Saturday...an upper level trough
will continue to move in from the northwest and merge with another
nearly stationary low located along the central coast. Highs today
were fairly comfortable most places with only far inland locations
experiencing warm to hot temperatures as a ridge continues to be
replaced by an upper level trough. Northerly flow continues to
bring wildfire smoke in from the north and marine based clouds and
fog are widespread over the coastal waters. Onshore winds will
usher low clouds and fog inland each night and morning this week.
Cloud coverage Monday morning will be challenging to predict much
depending upon the depth and strength of the marine layer, though
of course, the farther inland one is the better chances there are
for clear skies. Some high clouds may also be over the area during
the solar eclipse. Going forecast looks good, for additional details
please see the previous discussion.
Previous discussion...as of 1:25 PM PDT Saturday...today's stratus has
been a bit slower to mix out compared to yesterday. Visible
satellite shows clouds along entire coastline from Sonoma County
south through Monterey County, and even some clouds pushing
through the Golden Gate east into Berkeley. For the next several
hours, we have one of the two GOES-16 mesoscale sectors over parts
of the pacnw with its southern extent covering the San Francisco
Bay area. With this mesoscale sector, we are able to get detailed
satellite imagery once every minute. It's fascinating to watch
this minute-by-minute data with the clouds gradually mixing out
over the Bay combined with the continued push through the Golden
Gate gap. Check out @nwsbayarea on twitter to see it for yourself!
Synoptically-speaking an upper level low is centered just over a
hundred miles offshore from Los Angeles. This low will help to
enhance potential thunderstorm activity over the next few days for
parts of Southern California as well as the Sierra Nevada. For US
along the central coast and for the San Francisco Bay area, we're
a bit too far north to expect any sort of convective activity.
Nonetheless, the low is close enough to bring a gradual cooling
trend across interior areas of the San Francisco Bay area, San
Benito County, and interior Monterey County through the weekend.
This checks out for today as temperatures for most airports in the
area are running a few degrees below what they were 24 hours ago.
When all is set and done this afternoon, expect highs to
generally reach the 60s along the coast and the middle 70s to low
90s for interior areas. Little change is expected for highs on
Sunday along the immediate coast, but an additional 2 to 5
degrees of cooling is likely inland. Minimal day-to-day changes
are anticipated for the entire area for much of next week.
The cloud cover forecast for Monday continues to be a bit of a
challenge. If conditions permit, a partial solar eclipse can be
viewed throughout California while the total eclipse can be viewed
north in Oregon. Models remain in good agreement that the upper
level low will remain parked near Point Conception in Southern
California. Current thinking is that there will be cloud cover
along the coast on Monday morning. The big questions are 1) how
far inland will these clouds be, and 2) will areas with morning
clouds clear early enough to see the eclipse? We cannot
confidently answer these questions quite yet. Another potential
outcome (not likely, but still possible) is that the upper low
could end up mixing out the marine layer, allowing for mostly
clear conditions for much of the area. We will continue to monitor
the marine layer's behavior over the next 48 hours to see how it
responds to this upper low. The marine layer depth has trended
upward from 1,200 ft to 2,000 ft over the past day. Climatologically-
speaking, the best locations in the Bay area for clear skies on a
mid/late August morning would be interior areas of the East Bay
away from the coast, as well as the higher elevations in the Santa
Cruz Mountains, North Bay mountains, and Diablo Range. The
eclipse of the sun will begin shortly after 9:00 am PDT for the
Bay area with peak obscuration happening around 10:15 am PDT. Peak
obscuration of the sun will range from 71 percent in Monterey and
up to 78 percent in Santa Rosa.
Aviation...as of 10:29 PM PDT Saturday...the marine layer depth
has ramped up to nearly 2500 feet per The Fort Ord profiler, and
coastal stratus is spreading into inland valleys. All terminals
are expected to see MVFR/IFR cigs tonight with late morning burn
Vicinity of ksfo...VFR early, IFR cigs expected by 08z with
clearing by 18z. Afternoon sea breeze winds up to 15 kts expected.
Sfo bridge approach...similar to sfo.
Monterey Bay terminals...MVFR cigs becoming IFR cigs by 09z.
Clearing after 18z.
Marine...as of 10:24 PM PDT Saturday...moderate northwest winds
will continue across the northern outer waters tonight as high
pressure builds in from the west. Winds are forecast to decrease
below Small Craft Advisory criteria by Sunday evening. In
addition to breezy winds, locally steep wind waves will also
continue tonight and into Sunday.
... Arena to Pigeon pt 10-60 nm
Public forecast: canepa
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