Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
825 PM PDT Monday Jul 27 2015
Synopsis... high pressure will build westward this week...bringing
warmer and more humid weather as monsoonal flow returns. High clouds
will increase...while the marine layer shrinks back toward the
coast...and there will be a chance of showers and thunderstorms
beginning on Wednesday. A few may stray west of the mountains at
times...but the best chance will be over the mountains and deserts
in the afternoons and evenings. The chance for showers will continue
into early next week. A shallow marine layer will continue areas of
night and morning low clouds and fog along the coast.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Satellite imagery early this evening showed marine stratus gathering
offshore....otherwise skies were clear. The 00z Miramar sounding had
a weak inversion based near 2100 feet. This was about 500 feet lower
than this morning. The surface pressure gradients were locally 6-8
mbs from San Diego to the local deserts...continuing to support some
local wind gusts of 40-45 miles per hour through and below the San Gorgonio
Pass. The I-8 grade into Imperial County reported gusts to 35 miles per hour as
Expect the marine stratus to return to coastal areas later
tonight...but since the layer will be decreasing...it will not
extend as far into the valleys. The clouds should clear quickly on
Tuesday morning with temperatures expected to rise a few degrees higher
than today. Onshore gradients will weaken and turn offshore from the
Great Basin. This should allow the high deserts to warm to around
the century mark...and the inland Empire temperatures to climb well into
the 90s under mostly sunny skies.
Only minor tweaks to the ongoing forecast. The latest operational
runs of the GFS/European model (ecmwf) build the subtropical high westward toward the
for corners area late in the week. Temperatures and humidity will
rise as heights build over the area...and the flow aloft becomes
southeasterly. Clouds will develop...thicken...and build into
scattered showers and thunderstorms...possibly lasting into early
next week. The first chance of showers comes Wednesday.
Varying nuances in the model moisture and height fields from run to
run make it difficult to predict which days...and what areas will be
most susceptible to heavy rainfall and strong thunderstorms. But the
monsoon pattern unfolding looks typical for late July and August
across the SW with precipitable waters increasing to 1.5-1.8 inches...even perhaps
as high as 2 inches for a time over the lower deserts. Absent of any
organized synoptic lift...thermodynamics will play the major roll
for thunderstorm development over the mountains and deserts each
afternoon...with some drift into the valleys possible before
dissipating. The most difficult activity to nail down is nocturnal
and elevated...or propagates westward from strong convection earlier
in the day over Arizona/the lower Colorado River valley/Sonoran Desert.
This type of action has a better chance to run to the coast and over
the coastal waters.
Onshore flow is maintained through this event...so a shallow marine
layer is expected to persist over coastal areas with the sea breeze
It is difficult to call the end to this monsoonal flow based on the
current model solutions. The European model (ecmwf) slowly dries out the region
starting on Sunday. The GFS is slower with the drying and maintains
the higher-level moisture through the middle of next week on
southerly winds aloft. For now chance/slight chance probability of precipitation are in the
forecast through the extended period over the mts/deserts.
Aviation... 280250z...coast/valleys...bkn-ovc stratus will
gradually develop 15-20 sm inland through 1500 UTC...with bases
around 1100-1500 feet mean sea level and tops around 1700-2100 feet mean sea level. Expect
areas of visible 3-5 sm along higher coastal terrain and inland mesas.
For the 00z ksan...kcrq and ksna tafs...confidence in stratus timing
is low...with moderate confidence in ceiling heights/visible and high
confidence in occurrence. Moderate confidence that kont will not be
impacted by broken or overcast ceilings...with maybe a few hours of 3-5 sm visible
due to haze/br during the 1200-1600 UTC time-period. Stratus likely
clearing to the coastal waters during the 1500-1900 UTC time-period.
1900-29/0300 UTC...unrestricted visible and sky clear. After 29/0300 UTC...bkn-
overcast stratus gradually developing 15-20 sm inland with bases and tops
about 200 feet mean sea level lower than tonight.
Mtns/deserts...mostly sky clear and unrestricted visible will prevail through
Marine... 750 PM...there is a slight chance of showers and
thunderstorms at times over the coastal waters Wednesday night
through Friday morning due to possible disturbances moving through
the region. Otherwise...no hazardous marine weather is expected
Beaches... 750 PM...there is a slight chance of lightning occurring
at the beaches sometime between Wednesday night and Friday
morning...though exact timing is difficult to pin down at this
point. Any lightning activity that does occur can pose a danger to
Skywarn... Skywarn activation is not requested. However weather
spotters are encouraged to report significant weather conditions.