Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
134 PM PDT Friday Sep 4 2015
a trough of low pressure along the West Coast will bring continued
cooling today...with a deep marine layer remaining in place.
Enhanced onshore flow will bring gusty west winds to the mountains
and deserts during the late afternoon and evening today. The trough
will weaken...and high pressure will slowly build aloft...resulting
in warming and less extensive low clouds this weekend through most
of next week.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Afternoon visible satellite shows that the stratus has cleared out
of the coastal and valley areas. The 12z Miramar sounding showed a 9
degree c marine layer inversion at around 4000 feet above mean sea level...and
aircraft soundings showed the marine layer as deep as 4500 feet
above mean sea level. The strength of the inversion...in addition to how deep
the marine layer is...resulted in slow clearing again today.
Water vapor imagery shows a low pressure system over Washington and
British Columbia...with the associated trough axis sagging south
through California. Meanwhile at the surface...onshore pressure
gradients remain decently strong...at 7.7 mb from San Diego to Las
Vegas. The lowering heights aloft and enhanced onshore flow has
continued to bring cooling to the area today...as most locations are
reporting temperatures 2 to 6 degrees lower than 24 hours ago. A
similarly deep marine layer will remain in place tonight...with
stratus once again filling the coastal basin...lower coastal slopes
and Lower Mountain passes. Expect another round of gusty west winds
along the desert slopes and through/below the mountain passes late
this afternoon and evening...with gusts to 35 to 45 miles per hour...and
locally 55 miles per hour.
The trough will weaken Saturday...with a ridge to the southeast
slowly building over the area through the end of next
week...resulting in increasingly warmer days...and an increasingly
shallower and less extensive marine layer. Models have trended
stronger with the ridge for late next week...when day-time highs
could reach several degrees above normal as the 500 mb heights rise
to 590-591 dm. Any chances for monsoon/tropical moisture induced
convection looks unlikely at the moment...as we stay in mostly dry
southwest flow aloft. Although the GFS and nam12 show a little bit
of low-level moisture sneaking up from the south briefly Sunday and
Monday...it will probably not be enough to produce more than some
afternoon cumulus over the mountains. The 12z European model (ecmwf) does show some
middle-level moisture associated with a tropical system skirting our
southern areas next Thursday and Friday...while the bulk of the
moisture stays to our southwest. The GFS meanwhile remains mostly
dry through the period. With the lack of model agreement and run-to-
run consistency...have kept the forecast dry for now.
052000z...coast/valleys...mostly clear skies through this afternoon
over the entire forecast area. Between 05/0200z-0400z this
evening...stratus bases 2500-3000 feet with tops to 4000 feet mean sea level will
fill the coastal basin. Expect visible to drop below 5 sm where these
low clouds intersect terrain. Confidence of occurrence and timing is
moderate. These low clouds will dissipate and erode back to the
coast between 05/1600-1800z.
Mountains/deserts...few-sct clouds at or above fl150 and unrestricted visible
100 PM...no hazardous marine weather is expected through Wednesday.
100 PM...a long period south-southwest swell will begin to move into
the coastal waters by late Saturday morning...peaking Sunday morning
at 3 feet with a period of 20 second. Surf will build to 3-5 feet by
late Saturday...with some sets to 7 feet along south-southwest
facing beaches by Sun morning. The surf will remain elevated through
Monday with a mix of southwest and south swells. Strong rip and
longshore currents will accompany the higher surf...creating
dangerous swimming conditions over the Holiday weekend. A beach
hazards statement will be in effect beginning middle morning on
Saturday through Tuesday.
Skywarn activation will not be needed today.
California...beach hazards statement from Saturday morning through Tuesday
evening for Orange County coastal areas-San Diego County