Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1044 PM PDT Wednesday Jul 23 2014
Synopsis...expect a significant warming trend as we head in to
the latter part of the week...peaking out on Friday...as the air
mass warms and surface high pressure temporarily builds inland to
our north. This will prove short-lived however...especially areas
closer to the coast...as the marine layer and more onshore-
oriented flow redevelop over the weekend. Next week looks to be
warm and dry...with inland temperatures especially staying well
above seasonal norms.
Discussion...as of 8:30 PM PDT Wednesday...an unseasonably strong
upper level low pressure center is moving onshore over the
Washington state coast. Behind it surface high pressure will build
inland through the Pacific northwest and into the northern Great
Basin. The net result for US will be lower-level flow turning more
offshore beginning late tonight. NAM model boundary layer relative humidity
output indicates this will erode the marine layer and stratus
even from the coast itself by late morning on Thursday. At the
same time models remain in excellent agreement in their
projections of significant warming and drying of the overlying air
mass above...with 850 mb temperatures increasing about 3 degree c each of
the next 2 days.
Only shorter-term concern is about how warm it could potentially
get on Friday. At this point do not expect any heat advisory
thresholds to be broached. That said...in some places it may not
miss by much and it is not outside the realm of reasonable
possibilities for downtown San Francisco to reach the middle
80s...San Jose and Santa Rosa the middle 90s...and Concord/Livermore
the lower 100s. This is warmer than what's in our current
forecasts...and although consistent with new 00z NAM MOS
guidance have not yet received enough of the 00z model output to
start making significant changes two days out...so will leave
that decision to the midnight shift. Might note that in a way
this is an early first hint or harbinger of the approach of the
change in seasons...to late Summer/early autumn when Post-frontal
high pressure systems more commonly build inland to our north
leading to offshore flow and clear skies over our area.
For the upcoming weekend...it looks like a return to a cooling
trend as surface flow returns to being onshore and the marine
layer redevelops. That said however...500 mb heights are projected
to remain high...around 590 dm...and 850 mb temperatures warm...in the 24
to 25 degree c range. So expect the marine layer to be pretty shallow
and thus limited in its degree of inland influence.
Output from several of the models earlier in the day...and
especially the 12z and 18z runs of the NAM...were indicating that
another round of upper level subtropical moisture would move into
our district from the south in the Saturday night/Sunday morning
time frame. But now the just rec'd new 00z run of the NAM has
significantly backed off on this. So at this point nothing more to
do than continue to monitor over subsequent model cycles.
In the extended...not a lot of change is expected as longer range
model output indicates 500 mb heights will remain high and 850 mb
temperatures warm...but at the same time onshore flow and at least a
shallow marine layer along the coast. Thus in general expect near
to a bit warmer than seasonal norm temperatures along the coast...while
well-above average temperatures continue inland. And with the center of
the upper level high projected to remain more or less parked in
The Four Corners area...will need to continue to watch for
possible intrusions of upper level subtropical moisture.
Aviation...as of 10:40 PM PDT Wednesday...surface to low level
humidity will trend lower overnight and Thursday as northwest winds bring
in drier air. Night-time cooling and lingering humidity will keep
at least a few patches of low clouds going on the immediate coast
otherwise mostly clear to clear skies should prevail elsewhere.
Vicinity of ksfo...VFR.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...a few patches of low clouds are possible
overnight otherwise VFR is forecast.
Marine...as of 01:47 PM PDT Wednesday...high pressure building
over the eastern Pacific coupled with a thermal trough developing
over the California interior will result in increasing winds
across the coastal waters beginning this afternoon and evening.
The strengthening winds will result in deteriorating sea
... Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Public forecast: blier
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