Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
100 PM PDT sun Aug 30 2015
Synopsis... a trough of low pressure will develop near the West
Coast increasing onshore flow and the marine layer through midweek.
Cooler through Tuesday...then temperatures will vary little for the
remainder of the week. Despite the cooler days...overnight lows will
be above average this week closer to the coast due to the elevated
sea surface temperatures and expected marine cloud development. The
better onshore flow will push clouds and fog well into the valleys
by midweek and keep gusty winds blowing through the desert passes.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Skies were clear and sunny once again at midday. Marine stratus was
very patchy and had mostly dissipated. Even the higher clouds just
to the north of the County Warning Area have thinned. Weak onshore trends continue
to the deserts. They were up to 3 mbs to Nevada and 6 mbs to the lower
deserts. Westerly winds were gusty again locally through the desert
passes. At noon a gust to 40 miles per hour was reported in the San Gorgonio
Coastal wind observations show the eddy may be beginning to form
already. Regardless...temperatures have responded with 24 hour drops
of 4-7 degrees common at noon. The hires models show a well
developed eddy in place over the bight by 12z on Monday. This should
help the stratus to form and spread inland across the coastal and
western valley areas by morning. Further height falls should ensure
widespread cooling across the region Monday into Tuesday.
This afternoon...the strong high pressure ridge aloft stretched from
northwest Mexico...NE across the Great Plains and over western Great Lakes
region...while a large low pressure trough covered the West Coast of
North America as far south as central California. The trough will expand
southward through Tuesday...while the ridge inches slowly eastward. This
will support the development of an eddy over the California bight overnight
and help to deepen the marine layer.
The overnight lows near the coast may remain elevated because of the
mild sea surface...but far inland...away from the marine influence
it should be cooler overnight under mostly clear skies with better
Looking ahead to the Labor Day weekend...troughing is still favored
along the West Coast by both of the latest operational runs of the
GFS and European model (ecmwf) models. The strong anchor high along 160w does not
budge. Longwave troughing near the West Coast this time of year
usually means SW flow aloft and holds the monsoon at Bay. So it
Temperatures are a bit more of a question though. The global models
show surface gradients from Nevada to san next weekend turning offshore
as a vigorous trough tracks along the Canadian border and feeds cold
air into the Great Basin. If the onshore weakens too much...or weak
offshore develops...it can make for very warm days across socal. So
confidence is low at this point regarding just how warm it will be
during the Holiday weekend. For now...the forecast is dry and
seasonably warm for early September.
301945z...coast/valleys...unrestricted visible and few-scattered clouds at or above
20000 feet mean sea level through 31/0300 UTC. 31/0300-1500 UTC...stratus
gradually developing 20 sm...to locally 30 sm...inland...with bases
900-1500 feet mean sea level and tops around 2000 feet mean sea level. Marine layer inversion
strength around 5 degree c. For coastal taf sites...confidence in
occurrence and ceiling heights/visible is moderate-to-high...with low
confidence in timing. Small chance that kont will experience low
cigs/vis. Clearing of stratus expected during the 31/1500-1800 UTC
Mountains/deserts...SW to west winds 15-25 knots with gusts 35-40 knots along
mountain ridges...through/below canyons and passes...along the Desert Mountain
slopes and into the adjacent desert foothills will create moderate-strong
uddfs and low level wind shear over and east of the mountains winds and associated
uddfs/low level wind shear will diminish by 31/1200 UTC. Unrestricted visible and few-
scattered clouds at or above 20000 feet mean sea level through Monday morning.
1245 PM...latest wind observations from the San Clemente
basin buoy show northwest winds gusting to 19 knots...with 6 foot seas.
Gusty winds will continue over the outer waters through this
evening...with gusts to 25 knots possible. A Small Craft Advisory
remains in effect through 2 am Monday for the outer waters. Winds
will weaken by early Monday morning.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
Pz...Small Craft Advisory until 2 am PDT Monday for waters from San
Mateo Point to the Mexican border extending 30 to 60 nm out
including San Clemente Island.