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Area forecast discussion 
National Weather Service San Francisco Bay area
901 PM PDT Sat Apr 18 2015

Synopsis...a persistent marine layer along the coast will
result in cooler temperatures near the ocean while inland temperatures
remain above average through Sunday. Weak upper level weather
disturbances will bring cooler weather...along with slight shower
and thunderstorm the region for much of the upcoming

&& of 9:00 PM PDT Saturday...a slightly deeper
marine layer and increase in onshore flow resulted in cooler
conditions near the ocean...around sf Bay...and in the coastal
valleys today. The inland valleys and hills remained warm...with
afternoon highs as much as ten degrees above average for middle-

Evening water vapor satellite imagery shows a weak upper level
disturbance dropping south along the West Coast. This disturbance
triggered isolated thunderstorm activity across the higher
mountains of northwest California and the far northern Sierra Nevada this
afternoon. In the the weak upper level disturbance
continues to drop south appears that moisture and
instability necessary for convection initiation will only occur
across the higher mountains of northern and central California...and not
over our forecast area. The one exception may be the higher
mountains of southern San Benito County where the 00z NAM
forecasts afternoon and evening convective precipitation late tomorrow
and again late on Monday. Current forecast includes a slight
chance of thunderstorms in that area for late Monday...but not
tomorrow. May need to add thunderstorm chances for southern San
Benito County late tomorrow if other 00z model data supports the
00z NAM in this regard. The chance for convection over our
southernmost mountains appears higher on Tuesday when the 18z GFS
forecasts widespread late afternoon and early evening
precipitation across the higher terrain of Monterey and San Benito

Fort Ord profiler data shows the marine layer has been deepening
since this afternoon...likely in response to the approaching upper
level disturbance. A deeper marine layer should result in
more widespread low cloud cover on Sunday morning...and cooler
conditions for those inland areas that remained warm today. The
cooling trend will continue into Monday. Inland areas will be much
cooler by Tuesday when an upper trough approaches from the
northwest and triggers an increase in onshore flow. That upper
trough may also trigger scattered shower activity across our area
on Tuesday and Wednesday...although models once again indicate
that most precipitation around midweek will remain over the higher mountains.

The medium range models continue to indicate the possibility of
more widespread precipitation later next week as a system with
potentially more moisture moves in from the northwest.

&& of 9:00 PM PDT Saturday...the marine layer was
deepening on Saturday evening and terminals near Monterey Bay were
experiencing IFR conditions. Onshore flow remains relatively weak
and so low clouds thus far been slow to develop in sf Bay. Given
the increasing depth of the marine layer feel it's likely there
will be IFR/MVFR ceilings around much of sf Bay by late tonight.
A deeper marine layer should mean later clearing on Sunday...but
both the NAM and WRF models show light easterly flow developing
near the top of the marine layer which may hasten clearing on
Sunday morning. Therefore...confidence is low regarding clearing
times on Sunday morning.

Vicinity of ksfo...VFR conditions likely through about 08z with
IFR/MVFR ceilings likely to develop sometime between 08z and 12z. Low
ceilings expected to clear by 18z Sunday morning. Confidence low
regarding timing of low ceiling onset and clearing. Light west winds
overnight and into Sunday morning...increasing to about 15 knots
Sunday afternoon.

Sfo bridge approach...similar to ksfo.

Monterey Bay area terminals...IFR ceilings through about 17-18z Sunday
with VFR conditions thereafter. Light westerly winds.

&& of 8:30 PM PDT Saturday...northwest winds and seas
will continue to subside through tonight and into Sunday.
Relatively light winds and gentle sea conditions will then
continue through Monday. Northwest winds are forecast to increase
on Tuesday and Wednesday as low pressure develops over the Great
Basin and high pressure strengthens offshore.


Mtr watches/warnings/advisories...
... Arena to pt Reyes 10-60 nm until 9 PM



Public forecast: dykema
aviation/marine: dykema

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