Scientific Forecaster Discussion
fxus66 ksgx 251112
Area forecast discussion
National Weather Service San Diego California
312 am PST Sat Nov 25 2017
a slowly deepening marine layer will bring a chance for some dense
fog near the coast this morning, then farther inland tonight into
Sunday morning. The marine air will accelerate inland on Monday,
as an upper-level trough swings overhead, accompanied by gusty
westerly winds over the mountains and deserts, and the chance for
some scattered, light rain showers along and west of the mountains.
Cooler this weekend, then much cooler on Monday. Winds will turn
back offshore Tuesday morning, and be locally gusty below the passes
and canyons, driving the marine air back out to sea. The drying will
help daytime highs to rebound midweek. Another chance for some
precipitation arrives late next week.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Satellite imagery through 3 am PST showed an expanding area of low
clouds/fog over the coastal waters in the California bight. Much of this is
hanging just offshore, but could well drift onto the beaches before
dawn. However, little progress inland is expected. The surface
pressure gradient remains weak and winds are light with peak gusts
below 20 miles per hour.
A strong high pressure ridge centered sough of San Diego remains
firmly in control of our weather today, but will begin to move east
tonight, weakening the offshore gradient. The marine layer is
already rebuilding over the coastal waters and should be deep enough
tonight to be drawn inland over coastal areas. This layer has a good
chance to become fog tonight into Sunday morning, and may become
dense in some areas.
The high will drift farther east on Sunday into Monday as a
shortwave trough charges southeast toward the central California coast. This will
drive stronger onshore flow and deepen the marine layer
sufficiently for scattered showers to develop west of the
mountains. Over and east of the mts, strong and gusty westerly
winds will develop, possibly requiring a Wind Advisory near the
major passes and along the desert slopes. Any precip that develops
will be light. Much cooler on Monday.
The shortwave trough will be racing east on Tue morning, allowing
winds to swing back offshore and become gusty through and below the
passes and canyons for several hours before diminishing. Weak
ridging aloft develops in the wake of the trough for modest warming
and seasonally mild conditions through midweek.
There is a chance for more Santa Ana conditions during the latter
half of next week. This is dependent on the track of a shortwave
trough swinging over the northern rockies. Model differences are
sufficient to keep confidence low that strong offshore winds
will materialize over socal, but will need to be monitored. Here the
06z GFS is more aggressive, while the CMC and latest European model (ecmwf) runs show
less amplification, and hence less wind this far south.
Later in the week...forecaster confidence fades further as models
suggest the development of a temporary block along the West Coast.
The Block allows a cut-off low to form well offshore of San Diego.
The 06z GFS and 00z European model (ecmwf) operational runs bring the low over socal
next weekend with sufficient moisture and strength for widespread
precipitation...perhaps the first of the season? Typically cut-off
systems are slower than forecast models would like to move them, so
the slower 00z European model (ecmwf) model is preferred at this time, and the latest
06z GFS run is trending that way. Low probability of precipitation are in the forecast for
250945z...coast...stratus with bases at or below 500 ft mean sea level and areas of vis
1 sm or less possible along portions of the San Diego County coast
and up to 5 sm inland mainly 12-16z. Impacts most likely at ksan,
with only small chances of ceilings/vis restrictions at kcrq and ksna.
Fog may continue along the immediate coast through the afternoon,
then spread 5-10 miles inland after 26/05z. Bases will remain below
800 ft mean sea level with widespread vis 2 sm or less.
Valleys, mountains and deserts...few-sct clouds above 20000 feet mean sea level
and unrestricted vis through Sun morning.
areas of dense fog continuing over the coastal waters through the
morning. Visibility may be reduced to 1 nm or less at times. Vis
will improve during the late morning, but dense fog will be possible
again this evening through Sunday morning.
A trough of low pressure moving through will bring northwest winds
gusting 20 to 25 kt over the outer waters late Monday morning
through Monday evening. Combined seas will build to 7-9 ft over the
outer coastal waters Monday afternoon into Tuesday afternoon as
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.