Scientific Forecaster Discussion
fxus66 ksgx 300437 cca
Area forecast discussion
National Weather Service San Diego California
936 PM PDT Mon may 29 2017
Coastal stratus has quickly pushed into the western valleys this
evening and will continue overnight. The 00z radiosonde observation indicated a
continued strong but rather shallow marine layer. The forecast
database handles the current trends well and no adjustments were
needed this evening. One more above normal temperatures on Tuesday
for inland locations including the upper and lower deserts...then
gradual cooling commences on Wednesday with the passage of an upper
low pressure system.
a low pressure trough along the West Coast will bring slow deepening
of the marine layer into Wednesday, with nocturnal marine clouds
and patchy fog spreading a little farther into the valleys each day.
Cooler into midweek with patchy drizzle possible and only partial
clearing in some areas as the marine layer deepens. Warmer again
into the weekend under a weak high pressure ridge aloft, then cooler
early next week as a trough redevelops.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Marine clouds were slowly clearing back to the coast at midday, even
as the higher clouds spread east off the Pacific. The combination of
clouds had resulted in partial sun in many areas west of the mts
this morning, which held temps some 3 to 9 degrees f below values
observed yesterday at 11 am PDT. Quite hot in the deserts though,
with 105 in Palm Springs and 90s across the high deserts at 1 PM
PDT. Surface pressure gradients were trending a bit better onshore
today at 8 mbs ksan to the lower deserts at 1 PM PDT. Still, peak
wind gusts were only in the 20-25 miles per hour range through wind prone
A low pressure trough in the upper atmosphere along the West Coast
will deepen slightly and drift slowly east through Thursday. This
will deepen the marine layer, and produce more extensive and
persistent marine stratus throughout the coastal basin. Some patchy
drizzle is possible late nights/mornings due to the depth of the
stratus layer. The trough will lift out by Fri, allowing a weak
ridge to build into Saturday. Some reduction in marine layer depth
and stratus coverage is likely inland through the weekend under the
Another, weaker trough arrives Sunday and persists into next week,
maintaining onshore flow and a marine layer with attendant nocturnal
clouds/patchy fog west of the mts. Based on the 12z European model (ecmwf) model run,
the extent and persistence of morning stratus would be more limited
than this week for better clearing.
For temperature trends...look for slight cooling on Tue, although if
the stratus persists over portions of the coast/valleys, it could be
more significant. Greater cooling is likely all areas for Wed/Thu as
the marine layer and onshore flow peak. A coastal eddy may develop
at this time as well, limiting clearing. This will drop temperatures
back to levels that are a bit below average for early June.
Warmer by the middle of next week as heights/thickness build,
although there are differences in the global model solutions. The
European model (ecmwf) is more aggressive with the ridging along the West Coast than
both the 12z GFS/Gem.
300415z...coast/valleys...bkn/ovc stratus will continue to spread
overnight, including into west and south parts of the inland Empire.
Bases will be 1000-1500 ft mean sea level with tops to 2200 ft mean sea level and areas of
higher terrain obscurred. Clearing will be slow Tue with valleys
clearing 16z-19z with areas 5-15 miles inland from the coast
clearing 18z-21z. Some locations within 5 miles of the coast may
have broken/overcast cigs through the afternoon, possibly including ksan.
Stratus will again be widespread Tue night. Most vis will remain
above 6 miles below the cloud bases.
Mountains/deserts...unrestricted vis with few/scattered clouds at/above
20000 feet mean sea level through Tuesday evening.
no hazardous weather conditions are expected through Saturday as
winds will be mostly light with combined seas 6 feet or less from a
mix of west-northwest and south swells.
Skywarn activation will not be needed today.