Weather Service Report


084 
FXUS66 KMTR 270422
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
922 PM PDT THU MAY 26 2016

.SYNOPSIS...Continued building of a ridge of high pressure along
the coast, along with low level flow progressively turning more
offshore, will result in a warming trend through Saturday.
Then for the latter part of the holiday weekend, a cooling trend
appears to be in store as the marine layer and coastal stratus
redevelop. For next week, temperatures are projected to remain
above seasonal norms inland but vary from day-to-day closer to
the coast as passing weak upper level disturbances modulate the
marine layer depth and onshore flow. Aside from possible local
coastal drizzle, dry weather conditions are expected to prevail
through the forecast period.

&&

.DISCUSSION...As of 09:15 PM PDT Thursday...Skies are mostly
clear across the district, aside from some areas of low cloudiness
over the coast and coastal slopes of San Mateo County, and in the
southern Monterey Bay region. Some afternoon cumulus did develop
over the higher terrain of northeastern Santa Clara County and
southern San Benito County, but not to the point of producing any
showers. Although a few spots on the immediate coast were actually
a tad cooler than yesterday (the high this afternoon at Half Moon
Bay was 61 today compared with 63 yesterday for example), more
inland areas generally warmed up around 4-8 degrees.

The coastal stratus is expected to fill back in and extend locally
inland overnight, though somewhat less extensively than last
night especially north of the Golden Gate. Have noted that the
north-south pressure gradient from KACV to KSFO has increased to
5.6 mb (04Z) from 3.3 mb 24 hours ago, while new NAM and local
high-res wrf model boundary layer relative humidity output continue
to restrict overnight increases over our more northern coastal
areas. Then Friday night into Saturday morning, skies look to
remain mostly clear even by the beaches, aside perhaps from some
patchy low clouds along the San Mateo County coast and in the
vicinity of the Monterey Peninsula.

The forecasted warming trend in daytime temperatures through
Saturday remains on-track. Latest model runs show continued warming
of the air mass aloft, with low-level flow first weakening out of
the west and then turning more offshore as we head into the start
of the weekend.

For Sunday and Monday, latest NAM model output indicates
progressive redevelopment of the marine layer and night and
morning coastal stratus will occur, with low cloudiness
potentially extending inland through the bays by Monday morning.
The air mass aloft however is projected to remain warm. The net
result then looks to be a cooling trend primarily areas closer to
the water beginning on Sunday, and then all but farther-inland
locations on Memorial Day. But at the latter it should remain
quite warm.

In the extended, temperatures are projected to remain above
seasonal norms inland, but vary somewhat from day-to-day closer
to the coast as passing weak upper level disturbances modulate the
marine layer depth and onshore flow. High confidence that it will
remain dry through the forecast period aside from the possibility
of some local coastal drizzle.

&&

.AVIATION...as of 6:05 PM PDT Thursday...Northerly gradients are
steepening as high pressure builds over NorCal. ACV to SFO
gradient is 5.4 mb; the NAM model still under-forecasting this
gradient by 1-2 mb but remains consistent with forecasting it to
nearly double Friday. Stratus clouds continue to pile up from the
San Mateo Coast to the Monterey Bay and Big Sur Coast while skies
are clear inland.

Onshore winds are responding to a strengthening SFO to SAC gradient
presently 4.1 mb. Area-wide dewpoint temps and coastal sea temps
are nearly identical which favors a redevelopment of stratus and
fog tonight and Friday morning. Onshore winds and marine inversion
bases from 1500 feet to 2100 feet also favor areas of stratus/fog
intrusion tonight, initially slowed by the strength of the northerly
winds and currently decreased stratus cloud coverage. This scenario
should help VFR persist into the evening for the Bay Area terminals.

Vicinity of KSFO...Moderate to high confidence VFR this evening then
lowering confidence regarding stratus return tonight based on steadily
strengthening ACV-SFO gradient. The marine layer is deep enough, and
the onshore gradient is strong enough to allow some stratus to return
however VFR is likely to persist first then tempo MVFR cigs 07Z-10Z
followed by MVFR prevailing 10Z-15Z Friday. VFR is likely to return
early Friday morning and last through the day.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR cigs gradually transitioning to IFR
cigs this evening. Low confidence VFR returns by late Friday morning
based on solid northerly gradient over the coastal waters and nearly
unchanged water vapor content in the boundary layer through the day.
When NW winds which retain moderate levels of humidity meet the local
coastal terrain of the Monterey Bay area the terrain can induce sufficient
lift to cause stratus to persist for longer periods of time.

&&

.MARINE...as of 9:12 PM PDT Thursday...High pressure will persist
over the eastern Pacific and northern California through Friday
resulting in strong and gusty northwest winds particularly over
the northern waters. Over he Memorial Day Weekend this high
pressure center will gradually weaken. Northwest winds will
continue through early next week but are not expected to be quite
as gusty.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
    .Tngt...SCA...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar
             SCA...SF Bay until 11 PM

&&

$$

PUBLIC FORECAST: Blier
AVIATION: Canepa
MARINE: Canepa


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Western Regional Climate Center, wrcc@dri.edu