Scientific Forecaster Discussion
fxus66 kmtr 251500
Area forecast discussion
National Weather Service San Francisco Bay area
800 am PDT Thu may 25 2017
Synopsis...seasonably cool today into Friday with a deep marine
layer near the coast. Seasonable temperatures on Saturday should
trend warmer inland by Sunday and Memorial Day Monday. Aside from
the isolated coastal drizzle, dry weather forecast through the
next 7 to 10 days.
Discussion...as of 08:00 am PDT Thursday...
Only made minor tweaks to the short-term forecast with sky cover.
Satellite reveals an extensive marine layer intruding well inland
through the Delta pushing up against the Sierra foothills. Fort
Ord profiler indicates the marine layer depth has steadily
deepened over the past 24 hours to over 3500 ft this morning and
is approaching 4000 ft. There have also been some reports of
drizzle or even light rain associated with the low clouds in the
North Bay. Santa Rosa ASOS has reported several one-hundreds of an
inch since midnight.
Upper level low continues to approach the central California coast
and will help to maintain the downward trend in afternoon highs
through the end of the work week. Max temps for many inland
locations will be anywhere from 5 to perhaps as much as 15 degrees
below normal for late may. The cool weather will be short-lived,
however, as a ridge will be quick to build over the West Coast for
the Holiday weekend with temperatures rebounding back to normal or
even several degrees above normal. Guidance suggests that the
southern Salinas valley may return back to low 90s as early as
Sunday. While the inland areas ride their temperature roller
coaster, coastal locations influenced by low clouds will struggle
to see temperatures change any more than 5 degrees over the
forecast period. The only chance for precipitation over the next 7
days will be for coastal areas that may see isolated drizzle or
Previous discussion...as of 3:23 am PDT Thursday...profiler data shows
the marine layer has deepened in excess of 3000 feet with cooler
air aloft keeping the inversion weak. This has allowed deep inland
penetration of marine clouds with a finger now pushing out into
the Delta while nearly the entire North Bay is completely
inundated as well. Onshore gradient is only 2 mb from sfo to SAC
but gusts from 20 to 30 kt are showing up at Angel Island, Concord
and Travis which is a sure sign of another cool day for inland
areas with highs struggling to reach 70. In addition to the
extensive cloud cover and onshore push the models continue to show
some drizzle along the coast from Half Moon Bay to Big Sur this
morning. Forecast grids will continue to show some patchy drizzle
this morning as the sun comes up, just enough to wet the surfaces
but only expecting trace amounts.
The upper pattern wont change too much on Friday with cyclonic
flow aloft and onshore flow at the surface keeping temps on the
cool side of normal.
As is typical this time of year day to day changes can be hard to
nail down as the subtle marine perturbations can drive the daily
patterns. However the general trend of the Holiday weekend shows
near normal temperatures on Saturday then climbing back to above
normal for inland areas on Saturday and Sunday as high pressure
briefly rebuilds. All in all it looks to be a nice Holiday weekend
with dry conditions and no excessively hot inland weather. Longer
range trends portend a dry and seasonable pattern through much of
next week as we head into June.
Aviation...as of 04:45 am PDT Thursday...marine layer intrusion
across the region is now extending into the Central Valley. Bases
are much higher versus yesterday, bringing primarily MVFR to VFR
cigs to regional taf sites. Expect to see similar ceilings through
16z with mixing out by 18z-19z for Bay area sites. Stratus will
remain over the Monterey Bay taf sites through most of the day.
Stratus returns tonight. Otherwise, breezy to occasionally gusty
west to southwest winds, especially along the coast in the
afternoon and evening.
Vicinity of ksfo...breezy southwest winds through the morning
increasing and becoming more westerly by the early afternoon. MVFR
cigs mixing out between 1730-1830z. Stratus returns 05-07z Friday.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...MVFR through the day with brief pockets
of sunlight midday. Breezy onshore winds, especially in the
afternoon and evening.
Marine...as of 07:47 am PDT Thursday...increasing south to
southwest winds along the coastline today... especially along the
Big Sur coast. These winds will generate steep fresh swell in the
vicinity. Otherwise light seas. High pressure will redevelop over
the eastern Pacific just prior to the Memorial Day weekend
bringing a return to more typical afternoon and evening onshore
winds to the coastal waters and bays.
... Arena to Pigeon pt 10-60 nm
Small Craft Advisory...mry Bay from 9 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 9 am
Small Craft Advisory...sf Bay from 3 PM
Public forecast: Rowe/rww
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