Scientific Forecaster Discussion
fxus66 ksgx 072109
Area forecast discussion
National Weather Service San Diego California
109 PM PST Sat Dec 7 2019
moist southwest flow aloft will bring periods of light rain and high
elevation snowfall this weekend. A dry northwest flow aloft will
develop on Monday with the passage of a weak trough and developing
high pressure aloft. Prevailing high pressure over the west through
next weekend will will bring an extended period of fair, dry, and
warmer weather. A large west swell will bring high surf this
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
At 12 PM PST...radar and rain gages continued to show scattered
light rain across socal. The most persistent rain has been on the
coastal mtn slopes where upslope flow is aiding lift. Rain rates
were light (under 1/10 inch/hr.) Infrared satellite imagery well out
across the eastpac did not show any large, organized areas of
enhancement in this moist stream of air aloft. Probability of precipitation do remain high
though, due to the near saturation aloft and the continued weak
disturbances expected to traverse the area over the next 24-36
hours. Rainfall totals are posted and headlined on our website.
The sfc pressure gradients (ksan to the deserts) was trending weakly
onshore. There were some areas near the mts/high deserts with
sustained winds of 20 miles per hour or more but most areas had much lighter
wind. Southwest winds may strengthen a bit more through Sunday,
before turning more from the west later on Sunday. Peak gusts of 40
to 50 miles per hour are possible on the mtn ridges and desert slopes.
The moist, Pacific flow currently in place over socal will continue
into tonight before getting squeezed to the south on Sunday by a
trough that will drop south across the state. Periods of light rain
and high snow levels will prevail this period, with the greatest
rain amounts on the coastal slopes where weak upslope flow will
On Sunday, the approaching trough from the north should organize
another period of precip over our northern areas, while upslope flow
prevails over the Riverside and San Diego County mountains, helping
to enhance amounts of rainfall there. Positive vorticity advection with the approaching
trough will be muted due to the highly positive tilt at which it
will move over socal. Still, the cold air aloft and increased sfc
convergence should spark a round of more organized pcpn beginning
Sunday afternoon over northern portions of the County Warning Area. The snow level
will begin to drop over these areas and bottom near 6000 feet Monday
morning. Snow could begin to accumulate above 7k feet late Sunday as
the bulk of the precip winds down. This could result in a few inches
of snowfall near and above 7500 feet late sun/Sun night.
Meanwhile, most of the significant rainfall accumulation will be on
the coastal slopes of the mountains (1-2"), while lesser amounts
accumulate at lower elevations (0.5" or less). These are total
amounts spread out over 48 hours, so runoff will be very limited,
reducing any significant flood risk.
Looking ahead to next week...models show a highly amplified ridge
along the West Coast on Tuesday, behind the trough, which gets
flattened by weak shortwaves next week, but maintains over socal,
supporting fair, dry, and warmer weather through the week. Toward
the end of the week, temperatures could be some 5-10 degrees above
072025z...coasts/valleys...bkn-ovc cigs with bases 2500-3500 ft mean sea level
and -ra through Sunday. Broken-overcast cigs 1000-1500 ft mean sea level and vis 3-5 sm
during -ra. -Ra could continue at times through Sunday evening.
Mountains...-ra likely at times through sun. Cigs and vis will be
lower during -ra. Low clouds will obscure terrain above 3000 ft mean sea level
through the forecast period. West winds with gusts to 35 kt over the
mountains will bring weak-moderate up/downdrafts and low level wind shear over and
east of the mtns after 12z sun.
Deserts...isolated to scattered -ra this afternoon and evening. Cigs
around 5000 ft mean sea level and vis 3-5sm possible with -ra.
scattered rain showers at times through Sunday. Combined seas of 6
to 8 ft are expected this afternoon through Sunday evening.
a high surf advisory remains in effect through 4 am Monday for long-
period west to northwest swells of 14 to 18 seconds. Swells will
build today, peak Sunday morning, and subside by Monday morning.
Surf of 6-8 ft is expected in San Diego County and 4-7 ft surf is
expected on beaches prone to west-northwest swells in Orange County.
California...high surf advisory until 4 am PST Monday for Orange County
coastal areas-San Diego County coastal areas.