Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
934 am PDT Thursday Jul 2 2015
isolated to scattered thunderstorms possible today across portions
of the mountains and deserts. Drier Friday with smaller
precipitation chances across the region. Dry on Independence
day...then slight chance of thunderstorms Sunday afternoon over
portions of the mountains and deserts with a small increase in
monsoonal moisture. Westerly flow aloft early next week will
create dry and seasonal weather...along with night/morning low
clouds over the coast and valleys.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
At 9 am PDT...water vapor satellite imagery displayed a broad upper-
level high stretching from northern California to western New Mexico...and
an upper-level around 550 miles SW of San Diego. Visible satellite
showed middle and upper-level clouds over central and northern
portions of the County Warning Area...and low-level clouds over the San Diego
County coast and valleys. The 02/1200 UTC knkx sounding displayed
a 5 degree c marine layer inversion. This is usually a weak enough
inversion for marine layer clouds to clear quickly in the morning.
However...observations are showing the cloud deck being fairly
thick...around 1200 feet...and will take a bit longer to clear this
morning and could linger into early this afternoon. The sounding
also calculated a precipitable water of 1.68 inches...with a dry
layer close to the lower-levels.
Increased diurnal heating today..combined with the moist
atmosphere...will help trigger isolated to scattered showers and
thunderstorms over portions of the mountains and deserts through
this afternoon. The main threats with storms will be
lightning...gusty winds...and isolated flash flooding. Negative
vorticity advection late this afternoon should help cut off
convection...with only an isolated thunderstorm possible over the
highest peaks of the San Bernardino Mountains early this evening. The
aforementioned upper-level high will sag south Friday...moving
our flow in the middle and upper-levels to more southerly. This will
help cut off the monsoonal moisture over the area...and limit
afternoon thunderstorm activity on Friday to the mountain peaks
and the High Desert.
The high will continue south over the weekend...continuing the
development of night/morning low clouds over the coast and valleys
and dry weather on Saturday. A slight intrusion of monsoonal
moisture to the mountains and deserts on Sunday will create the
possibility of isolated afternoon thunderstorms. Then conditions
will dry out in the middle and upper-levels as the cut-off upper-
level low moves closer to the region to our north...and maintains
onshore flow with near seasonal weather and high temperatures.
021602z...coast/valleys...ovc-bkn stratus mainly 20-30 sm inland in
San Diego County...with bases 600-800 feet mean sea level...tops around 1500 feet
mean sea level...areas of visible 3-5 sm and local visible at or below 2 sm...will likely clear
to the coastal waters through 1900 UTC. Otherwise...sct-bkn clouds
at or above 10000 feet mean sea level and unrestricted visible. Marine layer inversion of
around 5 degree c. 1900-03/0600 UTC...unrestricted visible and scattered-broken
clouds at or above 10000 feet mean sea level becoming few-sct. 03/0600-1500 UTC...stratus
gradually developing 15-25 sm inland...with bases 700-1000 feet
mean sea level...tops around 1500 feet mean sea level and areas of visible 3-5 sm. Chance that
visible could drop to 1-2 sm during the 03/1300-1500 UTC time-period for
the coastal taf sites similar to this morning...but confidence is
low. Confidence in stratus occurrence is moderate-to-high.
Mtns/deserts...chance of showers and thunderstorms during the 1800-
03/0100 UTC time-frame...mainly over the mountains and high deserts.
Thunderstorm bases will be near 8000 feet mean sea level with cumulonimbus tops to 40000-
45000 feet mean sea level. Any storms that form may produce moderate-strong
up/downdrafts...frequent lightning...and reduced visible to 3sm or less
in rain. Otherwise...sct-bkn clouds at or above 10000 feet mean sea level becoming few-
scattered through 03/0000 UTC...with unrestricted visible prevailing through
902 PM...no hazardous weather is expected through Monday night.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.