Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
835 am PDT Sat Sep 20 2014
a slow moving area of low pressure off the coast will continue
cooler weather and a deep marine layer through Sunday. It may also
spark a few thunderstorms over the higher mountains and deserts of
southwest California this afternoon. The low pressure area will move
inland on sun...and then exit the region next week as high pressure
rebuilds across the Southland. The deep marine layer will thin under
the building high...and it will become progressively warmer through
midweek as high temperatures climb back above average. Afternoon sea
breezes will continue to provide relief along the coast...but
elevated sea surface temperatures will limit the cooling...and keep
it mild overnight. Much cooler by the end of next week as a stronger
trough arrives with gusty onshore winds.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Marine stratus was extensive once again this morning west of the
mountains and offshore. The 12z Miramar sounding had a very weak 2
degree c inversion based near 2700 feet. The saturated layer was about
700 feet thick with the top around 3000 feet. Weak onshore surface pressure
gradients continued to the east but were trending neutral to Nevada.
Latest visible satellite imagery showed middle-level clouds streaming
north-northwest across the deserts. Knxp had a 10k feet ceiling at 15z.
Looks like another fine late Summer day is shaping up across the
region. The weak inversion should allow the marine clouds to clear
by midday...and the deep marine layer and onshore flow will hold
daytime temperatures a few degrees below average. The only potential
fly in the ointment is the slight chance of thunderstorms over the
higher mountains. Slightly warmer on Sunday...and dry...with less
extensive marine clouds as the upper low weakens inland.
A cut-off low was spinning off of Point Conception this morning...
within a broad ridge of high pressure over the eastpac and western
U.S. The low will drift toward the coast today and then pull NE on
sun ahead of a stronger middle-latitude trough. Models are keying in on
a vortmax swinging around the south side of the low this morning. It
is forecast to lift NE today...providing instability...and the
potential for showers/thunderstorms over the northern mountain/desert areas of the
County Warning Area. The limiting factor will be moisture. Blended tpw imagery does
show some moisture drift from the east ahead of this feature...but
it is unclear at this time whether or not it will be sufficient to
fuel thunderstorm development. Latest model runs are not generating any
precipitation...but based on pattern...slight chance/chance probability of precipitation look
appropriate for the afternoon and early evening. Dry lightning
strikes cannot be ruled out either due to limited moisture.
Once the upper low is drawn into the westerlies...high pressure will
rebuild with the overnight GFS/ec runs showing 590+ dm 500 mb
heights over socal by midweek. This will bring warmer days and above
average high temperatures again...but not as hot as earlier this
week. The upper-level ridge is forecast to remain rather flat...so
the thermal trough at the surface should remain inland...providing
onshore flow and milder marine air along the coast.
Even though autumn officially begins Tuesday evening...Summer weather
will be the rule much of next week. But by the end of the week...
the weather may be more seasonally appropriate. The overnight GFS/
European model (ecmwf) runs show the coldest trough of the season developing over the
eastpac and moving through California next weekend. This will bring below
average temperatures...and may bring rain to northern parts of the
state. However...the warmer than normal surface temperatures will
buffer the cooling from the trough this far south...especially near
the coast. The trough will also produce gusty winds over the
mountains and deserts.
201500z...coasts/valleys...low clouds with bases 2000-2300 feet mean sea level
and tops to 3500 feet mean sea level now cover most of the coastal areas and
inland valleys. All taf sites west of the mountains are affected. Low
clouds should clear to the coast between 17z and 19z. Expect low
clouds to return to coastal airports after 03z sun...spreading 20-30
miles inland overnight. Similar bases and tops expected tonight.
Mountains/deserts...scattered with bases at or above 8000 feet mean sea level this afternoon.
There is a slight chance of showers/thunderstorms over the mountains
and high deserts this afternoon as well. Any storms and cumulus will
dissipate by 02z sun. Otherwise...clear with unrestricted visibility
800 am...no hazardous marine weather is expected through Wednesday.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.