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Area forecast discussion 
National Weather Service San Francisco Bay area
410 am PDT Tuesday Apr 21 2015

Synopsis...weather conditions over the region today will be a
near repeat of those on Monday with a deep marine layer in place.
A slight inland cooling trend is then expected through late week
as weak upper level disturbances move through the state. These
weak weather systems will result in a slight chance of showers at
times through the remainder of the week...primarily over the
mountains.

&&

Discussion...as of 04:10 am PDT Tuesday...the presence of a
relatively deep marine layer once again allowed for low clouds to
develop well inland early this morning. As a
result...temperatures are similar to 24 hours ago...generally in
the low/middle 50s. A weak upper level low remains to our south and
will move into Southern California today while a middle/upper level
trough approaches the Pacific northwest. With little change in
the overall pattern aloft...weather conditions will be similar to
those yesterday. While inland areas are expected to see clearing
by late morning/early afternoon...many coastal locations will
once again remain beneath low cloud cover through much of the
day. In addition...the forecast models continue to suggest the
best chances for convection today will be well inland over the
higher terrain...with just an outside chance over southern
Monterey and San Benito counties.

The approaching trough to our northwest will push inland over
northern California through midweek and bring with it a slightly
cooler airmass. This too should help break down the marine layer
to some extent by Wednesday...especially over the northern half
of the region. Meanwhile...the NAM and other higher resolution
models maintain deep boundary layer moisture across much of the
central coast. Regardless...daytime temperatures will cool to
near seasonal averages region-wide through late week. With broad
middle/upper level troughing forecast over the state through late
week...cannot rule out a few showers over the inland
hills/mountains. Yet most locations will remain dry through the
remainder of the work week.

A short-wave disturbance is then forecast to sweep through on
Friday as the main upper level trough finally shifts inland. This
will cool temperatures by a few degrees for Friday with another
slight chance of showers over higher elevations across the
central coast. Otherwise...this system will likely pass across
the region without much fanfare.

The medium range forecast models now generally agree in weak
ridging over much of the state this weekend into early next week.
As a result...will keep generally dry conditions region-wide
along with inland warming for much of the upcoming weekend. The
next middle/upper level system is then forecast to push inland to
our north Sunday into Monday...mainly only impacting the Pacific
northwest and northern California. Thus...will maintain a dry
forecast through the extended.

&&

Aviation...as of 10:57 PM PDT Monday...high clouds from
thunderstorms that developed over the Sierra this afternoon have
moved over our area this evening putting a wrench in forecast.
For one low clouds were obscured by the high clouds making it
difficult to see what was happening at the surface and two...the
high clouds hampered the inland progression of the clouds near
the surface. Have pushed the onset of the stratus back an hour or
two overnight. Borderline IFR/MVFR ceilings expected through the night.
Clearing time is expected to be similar to Monday with low clouds
scattering out around 19z-20z. Moderately high confidence.

Vicinity of ksfo...VFR ceilings will transitions to borderline
IFR/MVFR ceilings around 08z-09z tonight. Clearing is anticipated
around 20z Tuesday morning. West to southwest winds 5 to 10 knots
expected through tonight. Winds will increase to around 15 knots with
the afternoon sea breeze. Moderately high confidence.

Sfo bridge approach...similar to ksfo.

Monterey Bay area terminals...VFR ceilings will transitions to borderline
IFR/MVFR ceilings around 08z-09z tonight. West to southwest winds
around 5 knots expected through overnight. Ceilings will lift to MVFR
around 19z-20z Tuesday morning but are not forecast to clear.

&&

Marine...as of 04:10 am PDT Tuesday...strong northwesterly winds
are expected today across the coastal waters as high pressure
builds over the eastern Pacific. Waters north of Point Reyes will
see the strongest winds with gale warnings in place for late this
afternoon. Seas will rapidly build on today in response to these
winds. Short period northwesterly swells are expected by tonight.

&&

Mtr watches/warnings/advisories...
... Arena to pt Reyes 10-60 nm until 3 PM
glw...pt Arena to pt Reyes 10-60 nm from 3 PM
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 3 PM
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from noon
Small Craft Advisory...sf Bay north of the Bay Bridge from 9 am

&&

$$

Public forecast: rgass
aviation/marine: CW

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