Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 191209

Area Forecast Discussion
National Weather Service San Francisco Bay Area
509 AM PDT Fri Apr 19 2019

.SYNOPSIS...A cooling trend begins today, though with inland
temperatures still well above seasonal norms. More significant
cooling is then expected over the weekend as onshore flow deepens
and strengthens and an upper level low pressure system passes
through. Can`t rule out a chance of some sprinkles or even light
rain, especially Saturday night into Sunday morning coast and
coastal hills south of the Golden Gate. A renewed warming trend
appears in store for the first part of next week.

&& of 02:53 AM PDT Friday...The taste of summer-
like warmth will be short-lived as stratus and onshore flow takes
hold. Overnight satellite imagery shows widespread stratus over
the coastal waters and move locally inland, especially the Salinas
Valley. Fort Ord profiler puts the marine layer depth at near
600-800 feet. Given the shallow nature fog is likely near the
coast. Monterey, Salinas, Half Moon Bay and Watsonville are all
reporting fog, some of it dense at times. Therefore, forecast will
include patchy dense fog over the waters and the immediate coast
this morning. Low clouds and fog will roll back to the coast by
mid morning. It`s possible that some coastal locations will hold
onto low clouds through out much of the day. The more established
marine layer will result in much cooler temperatures across the
entire Bay Area today. The biggest cooldown will be seen near the
coast and Bay Shoreline today with a 10 to 15 degree drop at some
locations. Interior locations will be cooler as well, but more
likely in the 5 degrees cooler. Highs will be in the 60s at the
coast and 70s to mid 80s inland.

An approaching trough and associated cold front will help to
further deepen the marine layer Friday night/Saturday morning.
Clouds and periods of drizzle have been added to the forecast.
Enough moisture may be present Saturday afternoon/evening to allow
for a few scattered showers with the frontal passage. Latest
models keep the best chance for showers N and E of the Bay Area.
However, hi-res models do show some showers over Santa Cruz Mts
and Big Sur Coast. Therefore, decided to add some shower activity
Saturday afternoon/evening. Confidence is low at this point, but
coverage may need to be expanded later.

Cooler weather will linger into Sunday behind the departing front.
By Monday upper level ridging will re-build over the state
starting another warming trend. It looks like this warming trend
has the potential to be warmer than yesterday, especially across
the interior. Current forecast will have a few low 90s by
Wednesday across the interior. Weak onshore flow will keep coastal
area in check, but still above normal for mid April.

The extended is very low confidence as the longwave pattern is
completely opposite depending upon model of choice. The GFS shows
a 552 DAM low and ECMWF has a 579 DAM ridge. Therefore, no really
change to previous forecast for next week.

Side note and more intersting piece of Bay Area history - Thursday
was the 113th commeration of the 1906 earthquake that devistated
San Francisco. Many buildings completely collapsed, but one of
the few buildings still standing included the Mills Building. The
Mills Building housed the U.S. Weather Bureau (now NWS)at the
time. Climate record logs for the day note - Records destroyed by
fire and earthquake. Many weather climate records for much of
Northern California were lost on that day.

&& of 5:09 AM PDT Friday...It`s VFR except VLIFR-IFR
in fog along the immediate coastline under a compressed marine layer
that is several hundred feet deep. Conditions remain ideal for more
coastal fog development this morning and 12z tafs are geared toward
persistence until by late morning when onshore winds and diurnal
warming combine to lift ceilings to marginal MVFR then VFR by noon.

The 00z WRF boundary layer and surface humidity forecasts are
under-forecasting, in some cases greatly under-forecasting actual
humidity levels at this hour, plenty of water vapor lingers today
not only along the coast, but inland as well with dewpoint temps
nearing 60 F by afternoon. 850-700 mb layer capping under ridging
persists today, however if any perturbation in the mid level
(weaknesses in capping inversion) develop there could be isolated
afternoon and/or evening convection bubbling upwards inland: near
San Benito county isolated showers/thundershower? per GFS qpf.
There`s a large amount of convective potential inland areas per
NAM/HRRR/WRF models today, but it should be mostly if not entirely
held suppressed by convective inhibition. Small to moderate Cu
could develop inland nonetheless.

Approaching trough from the northwest, onshore flow, cooling more
ambitious lower to mid level cooling late today through Saturday
will bring an inland intrusion of MVFR-IFR stratus ceilings though
as the marine layer inversion weakens and fully erodes tonight into
Saturday ceilings will lift to MVFR-VFR; patchy drizzle or light
rain is possible late.

Vicinity of KSFO...Moderate to high confidence VFR forecast based
on persistence this morning. The marine layer is compressed and
the lower to mid level flow pattern is holding nearly stationary
or unchanged for the morning hours. Once diurnal warming and
mixing increase by late morning/afternoon some stratus feed to SFO
becomes increasingly likely thus 12z taf trending toward MVFR
ceiling by early evening. Guidance forecasts west wind gradually
increasing with gusts to 20-25 knots possible by early Saturday
morning. Gusty west wind through Saturday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Fog is in for the morning, conditions are
ideal for it. Light wind. Fog begins to lift 17z-19z, but still
could be borderline IFR vsbys and cigs. VFR forecast to return for
the afternoon then stratus moves back in for the evening lowering
to IFR and possibly some patchy light drizzle forming by late

&& of 4:37 AM PDT Friday...High pressure developing off
the Pacific Northwest and northern California coast will cause
increasing northwest winds over the northern waters today and
Saturday. An upper level low will move over the coastal waters and
bays Saturday.


     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm from 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 9 AM




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