Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
910 PM PDT Tuesday Jul 29 2014
the combination of weak low pressure over the Great Basin...and
strong high pressure along the U.S. Mexican border has resulted in a
drier southwest flow aloft over California. The ridge will remain
over the area with warm middle-Summer conditions and limited marine
layer clouds and fog. Higher clouds will stream off the Pacific and
dim the sun at times...mainly north of San Diego County. The high
will weaken over the area this weekend...allowing monsoonal flow to
resume. A disturbance in the flow may bring more thunderstorms to
the region by Saturday. Until then...the chance for any
thunderstorms will be slim.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
High clouds were streaming NE off the Pacific this evening..while
areas of smoke were drifting south across San Bernardino/Orange/
and Riverside County in the mid-levels. Marine clouds were just
developing off the coast around sunset. The 00z Miramar sounding had
northwest winds of 10-15kt now extending from the surface through about 7k feet.
The sounding was stable with very little available cape. Moderate
onshore surface pressure gradients of around 8 mbs were present from
ksan to the Lower Desert...supporting westerly winds of 15-25 miles per hour
with local gusts to 40 miles per hour at the windiest spots in the deserts.
Satellite imagery showed a thick sheet of middle/high clouds streaming
NE off of the remnants of Hernan. The trajectory of this elevated
tropical moisture will carry it mostly north of the sgx County Warning Area...but
some of it will clip northern areas on Wed/Thu. Under this sheet of
clouds just prior to sunset...a thin veil of smoke was evident over
the coastal waters drifting south. This smoke was likely from the
Yosemite area and is responsible for the red-tinged sunset observed
in many areas this evening.
The seasonal subtropical high centered over the Mexican border
area...will gradually build back to the north...to near The Four
Corners by Sun morning. The high is close enough to keep our
heights/thickness elevated with warm days and mild nights for the
remainder of the week. As the high builds north...both the latest
available GFS/European model (ecmwf) models drag an easterly wave north through
Mexico and over socal this weekend. The trajectory of this feature
will determine just how much moisture and instability is available
over the sgx County Warning Area for possible thunderstorms. Low probability of precipitation are in the
forecast for Sat in the mtns/deserts...but they may have to be
expanded and increased if later runs track the system farther to the
Once that wave passes...models are in agreement that a more
persistent low pressure trough will develop along the West Coast...
keeping socal dry and seasonal for much of next week. The pattern
also favors a better developed marine layer with associated
nocturnal clouds and fog west of the mountains
Convection....chances look pretty slim for any thunderstorms for the
next couple of days. However...the latest 00z NAM guidance suggests
elevated moisture and instability over the northern mountain areas
in conjunction with the moist flow originating from Hernan. Mu cape
values over the San Bernardino Mountains near 300 are indicated in the
700-500 mb layer by afternoon. Heating should be limited under the
cloud deck...but with breaks and on the edges...it may be
sufficient. Hard to call at this point so we are carrying below
threshold probability of precipitation in the current forecast. Best chance in the San
The weekend event could be interesting since both models indicate a
fairly significant wave moving across the area. The 12z European model (ecmwf) has a
track that is more to the west than the GFS...which brings the
trough north through the lower Colorado River valley. Once again we
have low probability of precipitation for convection Friday in the mountains...but there is a
possibility of activity west of the mountains by Sat morning through
Sun morning...depending on the flow and moisture advection. Too hard
to quantify at this point given the uncertainty with the timing and
development of this incipient wave.
300345z...coasts/valleys...areas of stratus...currently off the San
Diego County coast...will spread a few miles inland into San Diego
County after 07z. Timing at ksan and kcrq is low confidence...but
most likely some broken ceilings will occur there in the 08z-15z range.
Bases will be rather low...600-900 feet mean sea level...and higher coastal
terrain could be obscured. Local visible below 3 Michigan...possibly even
below 1 Michigan...could occur over higher coastal terrain and the San
Diego County mesas. In Orange County...stratus is less likely...and
ksna will most likely will remain VFR. Most areas will clear 15-16z
Wednesday. Otherwise...occasional scattered-broken clouds above 15000 feet mean sea level will
occur through Wednesday. A smoke layer around 10000 feet mean sea level will most
likely not impact visibilities near the surface.
Mountains/deserts...occasional scattered-broken clouds above 15000 feet mean sea level
will occur through Wednesday...with some cumulus with bases around 9000 feet
mean sea level Wednesday afternoon.
830 PM PDT...no hazardous marine weather expected through Sunday.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.