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

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FXUS66 KMTR 260552

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1052 PM PDT Tue Apr 25 2017

.SYNOPSIS...A weak system passes inland to the north dragging a cool
front southeastward through the Bay Area later Wednesday. This will
result in light rain mainly from the Santa Cruz Mountains northward
Wednesday. A warming and drying trend will then get underway beginning
Thursday and likely continuing into the upcoming weekend as high
pressure builds over the eastern Pacific. Breezy northerly winds are
also possible from Thursday into Friday, especially in the hills.

&& of 9:05 PM PDT Tuesday...Temperatures are generally
a few to several degrees milder than they were this time last evening.
Cloud cover has been variable through the evening and winds are from
the west to northwest.

Cloud cover will increase tonight associated with an appreciable plume
of precipitable water tracking E-SE across the eastern Pacific and
entering the California coastal waters; values near 1.30 inches by
early Wednesday morning as forecast by the GFS model is fairly close
to Oakland`s April all-time maximum (1.36 inches) for all Oakland
soundings in the period 1948-2014. A nearly stationary, strong and
closed mid-upper level high is centered over the Pacific straddling
the 30N latitude between approx 130W and 150W longitude. We`re located
under the eastern extent of this high while a zonal jet stream enters
the Pacific Northwest. If this much above normal amount of moisture
streaming over the Pacific was coupled with moderate-strong dynamics
we`d most certainly be looking at an appreciably wet day coming up,
but this is not quite the case as the primary mechanism at work will
be a cool frontal boundary sweeping into the moist air enhancing upward
vertical motion and resulting in mainly light rain. The eastern extent
of the ridging currently developing over the Bay Area will be briefly
weakened by this cool frontal passage Wednesday. Gusty/breezy NW winds
develop Thursday-Friday. The pattern turns warmer and dry late this
week and weekend with steadily building 500 mb heights as the eastern
Pacific ridge steadily moves toward NorCal.


.PREV of 01:20 PM PDT Tuesday...Deep moisture aloft
continues to advect inland across the region this afternoon and
has resulted in widespread mid/high level cloudiness. All of this
is in advance of a weak mid/upper level trough that will push
inland into the Pacific Northwest late tonight into Wednesday.
This system may bring some very light precipitation to the
northern portion of our region, mainly from the Santa Cruz
Mountains northward early Wednesday morning. Cannot rule out some
lingering showers into the early afternoon from the East Bay Hills
down into the Santa Lucia Mountains, yet chances for widespread
measurable rainfall remain minimal as this system lacks needed
mid/upper level support as it pushes inland to our north. With
this passing system, temperatures will be seasonably cool and
temperatures will range from the lower 06s coast to middle 60s
inland through Wednesday. Some of the warmer, interior locations
of the Central Coast may warm into the lower 70s.

A warming trend will then get underway by Thursday as dry conditions
return along with mostly sunny skies. As a stronger mid/upper level
low drops southward out of Canada into the Rockies and high pressure
develops offshore late in the week, temperatures will warm into the
70s inland. In addition, breezy to locally gusty winds will develop
due to the tightening pressure gradient from Thursday into Friday.
Further warming of the air mass aloft will result in lower 80s
during the afternoon hours this weekend while onshore winds near the
coast keep temperatures in the 60s to 70s.

Temperatures moderate slightly by early next week as the mid/upper
level ridge weakens as a system pushes inland into the Pacific
Northwest. With that said, the main storm track will remain well
north of the region and dry weather conditions are likely into the
beginning of May.

&& of 10:45 PM PDT Wednesday...Biggest challenge with
the TAFs is the uncertainty with ceiling heights. High res models
seem to advertise higher boundary layer RHs this evening compared
to the observations. Current thinking is terminals will be
VFR/MVFR for the period with the best chance for -RA north of the
Bay Area. Westerly winds have subsided since sunset and will
increase again Wednesday afternoon.

Vicinity of KSFO...Latest 5 minute observations have brought a
SCT-to-BKN016 deck over the terminal. Expect VFR/MVFR ceilings to
persist through the period. Westerly winds will increase by
Wednesday afternoon. Confidence on the ceilings is low.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions at the present with
light winds since sunset. Increasing clouds will likely bring
MVFR ceilings to terminals on Wednesday.

&& of 10:29 PM PDT Tuesday...Strong winds continue to
impact the coastal waters south of Monterey Bay this evening.
Winds will increase across all waters beginning Wednesday night
and continue into the end of the week as surface pressure
gradients increase.


     .Tday...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm




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