Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
130 PM PDT sun Aug 28 2016
high pressure to the southwest will expand into California through
Tuesday, lowering the marine layer, and bringing much warmer
weather. Cooler again by Wednesday through the upcoming Labor
Day weekend, as a low pressure trough develops south along the West
Coast. Areas of fog are more likely tonight and again Monday night
in the western valleys and coastal areas as the marine layer lowers.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Today...despite a modest increase in mid-level moisture, a stable
atmospheric profile and subsidence held cumulus development in the
mts to a minimum. At 1 PM PDT, skies were sunny across the region,
except along the immediate coast south of La Jolla where marine
clouds have been very slow to dissipate. Otherwise, most of the bight
has cleared. Surface pressure gradients were weakly onshore to the
lower deserts, and had come back to neutral from ksan to SW Nevada. Peak
wind gust reports were under 25 miles per hour.
The pattern...a high pressure ridge centered off the coast, will
expand over socal through Tue, then succumb to a developing long
wave trough along the West Coast Wed through the end of the week.
Moderate easterly flow in the mid-levels will turn southerly by Wed,
possibly allowing a minor mid-level moisture intrusion, before drying
SW winds develop with the upper-level trough. Otherwise, dry,
westerly flow will prevail aloft with seasonal conditions through the
first week of September.
Models/guidance...the 12z global model runs (gfs/Gem/ecmwf) are
trending toward agreement on the strength and development of the West
Coast trough this week, so confidence is building in a dry and
seasonally cool period beginning around midweek, and lasting through
the Labor Day weekend.
Weather/concerns...the marine clouds will be limited again overnight
into Mon morning. Warmer most areas Mon/Tue, with the marine layer
decreasing further and fog becoming more of a factor in the western
valleys, and possibly along portions of the coastal strip. High
temperatures Mon/Tue will be from 5 to 10 degrees f above average
most areas, making for very warm to hot afternoons. See below for
further discussion on fire weather.
Despite a brief window of opportunity around mid week, ahead of the
deepening trough, when additional moisture and instability arrive,
pops do not reach threshold for inclusion in the zone forecast
product. Models generate no precip either.
281951z...coast/valleys...low clouds based around 1200 feet mean sea level
with tops around 2000 feet over the coastal waters will
push inland later this evening. Visibilities locally 3-5sm in fog
and haze where low clouds intersect high coastal terrain. Risk of low
clouds reaching kont tonight is low so will keep low clouds west of
kont. Confidence in low clouds reaching kcrq tonight is moderate so
will continue to indicate low cloud development for tonight.
Mountains/deserts...mostly clear skies with unrestricted vis through
no hazardous marine weather is expected through Friday.
relative humidity will be dropping through Tuesday over the mountains as high
pressure builds aloft and temperatures rise. Min relative humidity values of
around 15 percent are likley in the afternoons. Over the mountains,
winds will be east to northeast late nights and mornings 10-15 mph,
becoming westerly 15 to 20 miles per hour each afternoon/evening. Higher gusts
are likely at times near the passes.
The heat and marginally low relative humidity each afternoon/evening will create
elevated wildfire concerns, however guidance at this time suggests
the lack of strong winds and sustained low relative humidity will keep most of the
area from reaching critical fire weather conditions this week.
Skywarn activation will not be needed tomorrow.