Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
851 PM PDT Monday may 25 2015
Synopsis...cooler than normal conditions will continue for at
least another day along with widespread clouds. Warmer conditions
are expected during the second half of the week...especially for
inland areas. However...coastal areas will see only limited
warming as an onshore flow remains in place.
Discussion...as of 8:51 PM Monday...it was a cool Memorial Day
across the district with highs from 59 in the city to 72 at
Concord and Livermore. A deep marine layer is already surging
inland once again this evening after another day of little or no
clearing near the coast and bays. Profiler and sounding data show
a depth of nearly 3000 feet so expect nearly all locations to be
covered in clouds by morning.
In general not forecasting much change in the pattern for Tuesday.
850 mb temperatures are around 15 celsius and wont change much the next
24 hours. The synoptic pattern is cyclonic over northeast
California with a low over the pacnw at least temporarily blocking
any significant high pressure from building. So for the most part
using persistence for tuesdays forecast in terms of temperature
and for that matter cloud cover...expecting the deep marine layer
to only slowly burn back to the coast.
By midweek high pressure will begin to build and the main forecast
theme should be some noted warming for the inland valleys. Not
expecting much change at the coast with SST around 51 degrees
offshore and continued onshore flow most of this week.
By Thursday and Friday the interior valleys should warm well into
the 80s and possibly to around 90 degrees for the far East Bay. It
should feel more Summer like with the Bay area Micro-climates
kicking into gear with the more common 30 degree gradients from
the coast to the inland valleys. Should be noted that the ridge
will build and act to compress the marine layer but not induce any
offshore flow...so hot weather will be confined to those locations
outside of sf Bay I.E. The interior valleys of the
The ridge holds through Saturday. By Sunday a system approaches
from the west with a return of cooler onshore flow.
Previous discussion...as of 3:00 PM PDT Monday...clouds have burned-
off inland today although still remain fairly widespread along the
coast. Temperatures in most spots have been running close to where
we were yesterday at this time although a few pockets in the North
Bay are running 3 or more degrees cooler than Sunday afternoon.
With an onshore flow expected overnight plus a marine layer over
2500 feet and plenty of upstream clouds...look for another round
of widespread overnight clouds tonight. Patchy coastal drizzle is
also possible. Similar conditions are forecast for Tuesday and
Conditions are expected to start to improve on Wednesday as a
ridge of high pressure builds in from the Pacific. 850 mb temperatures
and 500 mb heights will rise each day at least through Friday
allowing for highs to go back to at least normal values for inland
spots. Southern inland sections of Monterey County over to San
Benito County will likely see number going to above normal
readings with highs in the middle 80s to lower 90s. At the coast the
warm up will be more limited due to the cool sea surface
temperatures plus onshore flow at the surface. On Friday highs
will range from lower 60s at some beaches up to lower 90s for the
warmest inland sections. Around Santa Clara Valley...most places
will be in the upper 70s to lower 80s. The ridge will start to
break down by next week leading to some cooler readings.
Both of the longer range outlooks...6 to 10...and 8 to 14
day...indicate warmer than normal temperatures without any substantial
rainfall. If correct it would mean that the ridge has rebuilt over
Aviation...as of 5:40 PM PDT Monday...for 00z tafs. Stratus
continues to blanket much of the coast...with latest Bodega Bay and
feet Ord profiler data indicating the marine layer depth is
remaining near 2500 feet. A bit more clearing late this afternoon
than 24 hours ago vicinity Monterey Bay terminals. On the other
hand low cloud coverage is extending inland earlier in the north
sfo Bay region. For ksfo and koak...MVFR ceilings presently prevail at
both...following a VFR window of just a couple of hours at the
former while at the latter restricted ceilings continued throughout
the day. Expect IFR to MVFR ceilings to redevelop at all remaining terminals
during the evening. Late afternoon onshore flow of 10 to 20 knots
will gradually diminish over the next several hours.
Vicinity of ksfo...latest webcam images show mostly cloudy skies
extending over the terminal from the San Bruno gap...and starting to
spill into the approach. Ceiling height presently at 1900 feet...is
expected to progressively lower to around 1200 feet by late this
evening. Current west-southwest flow of 15 knots gusting 22 knots will gradually
diminish over the next few hours.
Ksfo bridge approach...low clouds spilling in from the terminal
and should reach the San Mateo bridge within the next 1-2 hours.
Monterey Bay area terminals...MVFR ceiling redevelopment has already
occurred at kmry. High confidence will continue through the night
and Tuesday morning...and moderate confidence will lower into the IFR
range by late this evening. MVFR ceiling onset at ksns expected by
02z...then similarly continuing through the night and Tuesday
morning. Onshore flow of 10 to 15 knots will diminish during the
Marine...as of 3:00 PM PDT Monday...light to moderate west to
northwest winds persist over the central California coastal waters
over the next 24 hours. Winds weaken and back towards the west
through midweek then veer back towards the northwest and increase
later in the week. Predominately gentle seas through the period.
Public forecast: rww
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