Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
907 am PDT Tuesday Jun 30 2015
high pressure over the Great Basin and low pressure off the
California and Baja California coasts will continue to direct a flow of
middle and high level monsoonal moisture across Southern California for
much of the week into early next week. This will support a chance of
thunderstorms through Thursday...mainly near the mountains and
portions of the deserts during the afternoon hours. Some showers and
thunderstorms could occur in the eastern valleys this afternoon.
Minor drying is possible for Friday and Saturday with another chance
for afternoon thunderstorms for the mountains and deserts Sunday and
Monday. Temperatures will peak today...then cool to near to slightly
below average for Friday through early next week. A shallow marine
layer and weak onshore flow will produce patchy night and morning
low clouds and fog near the coast and western valleys at times.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Morning visible satellite shows some patchy marine layer stratus
over the coastal areas...scattered-to-few middle and high clouds...and
two west-northwestward moving areas of elevated convection over
Sonora and Baja California. These areas of convection spit out an outflow
boundary which may create some brief local blowing dust in the lower
deserts as it moves through. The 14z hrrr shows the second
convective system moving into the San Diego County mountains and
deserts by 1 PM...and then sparking off some thunderstorms over the
mountain ridges. The 12z Miramar sounding shows a marine layer
inversion at around 600 feet...which is rather shallow. Thus...local
fog occurred in some of the coastal areas this morning. Stratus/fog
should clear out by late morning. The sounding also shows a
respectable precipitable water value of 1.43 inches...with 714 j/kg
of MUCAPE. However...most of the moisture is once again located
above 600 mb...which is rather high. Nevertheless...some of the hi-
res models do show some mountain convection taking place this
afternoon. With the middle and upper level winds of 20-30 knots out of the
southeast...some activity could spill into the valleys before
dissipating. Thus...with the morning update...have introduced a
slight chance of thunderstorms into the eastern San Diego and
Riverside County valleys. The WRF shows the convergence boundary
setting up over the mountain ridges by late morning...so expect any
convection to initiate there...with thunderstorms/showers then
drifting to the northwest into the eastern valleys before
dissipating. As far as temperatures go...looks like today will be
pretty warm...with day-time highs 5-10 degrees above normal...due to
the clearer skies and higher 500 mb heights.
On water vapor satellite...a large ridge remains in place over the
Great Basin...with a low far to the south-southwest of San
Diego...resulting in the continued moist southeast flow aloft.
Models show indications that a weak easterly wave could move over
the area tonight into Wednesday morning...bringing a plume of middle
and upper level moisture. Right now...this wave looks too weak to
produce measurable precipitation. However...the increasing clouds
from this middle-to-upper level moisture surge could play a factor in
inhibiting late morning/afternoon mountain convection for
Wednesday...as the cloud cover would reduce day-time heating...and
therefore decrease the instability. The clouds also will play a role
in bringing overall cooler conditions to the forecast area.
The flow becomes a lot weaker on Thursday and Friday...which would
mean that storms would be slow-moving. However...moisture is also
diminished on those days...with the GFS only showing up to 1.15
inches of precipitable water...and the nam12 showing 0.80 inches.
Cooling will continue as well as the ridge weakens over our
area...with day-time highs reaching near to slightly below normal.
The shallow marine layer will continue through Friday...with night
and morning patchy stratus and fog over the coast and extreme
For Saturday through early next week...the ridge moves over The Four
Corners region...and the flow aloft turns southerly. The GFS shows
most of the moisture during this period staying to our east...with
minimal chances for any afternoon convection...while the European model (ecmwf) is
slightly more moist...possibly enough for some afternoon
thunderstorms/showers over the mountains Sunday and Monday. Look for
temperatures to continue to be near to slightly below normal. The
marine layer should deepen and become less patchy...with night and
morning stratus over the coast and western valleys.
301550z...coast/valleys...low clouds based around 500-1000 feet
mean sea level...tops around 1500 feet mean sea level...and dissipating around 1630z.
Stratus redeveloping overnight tonight with similar bases and tops.
Confidence in occurrence of the low clouds in the ksan taf for
tonight is moderate-high...and confidence in the timing is moderate.
Slight chance of afternoon and evening showers/thunderstorms near
Mtns/deserts...cu/tcu developing with a slight chance of afternoon
and evening showers/thunderstorms. Thunderstorm tops to near 30000
feet mean sea level. Local gusty surface winds...and local visible restrictions in
heavy rain possible. Confidence in outflow winds from morning
thunderstorms over northern Mexico reaching the desert tafs is
moderate-high...so gusty winds included in the ktrm taf for this
morning. Risk of ktrm visibility falling below 1 mile in blowing
dust is very low.
850 am...no hazardous marine weather is expected through Friday.
there is a chance that Skywarn activation will be needed this
afternoon. Weather spotters are encouraged to report significant