Area Forecast Discussion
Issued by NWS San Diego, CA
FXUS66 KSGX 302023
Area Forecast Discussion
National Weather Service San Diego CA
123 PM PDT Sun Apr 30 2017
High pressure off the coast will drift slowly east through Thursday,
continuing fair and warm weather inland, while onshore flow and a
slowly building marine layer bring cooling and areas of nocturnal
marine clouds and fog to coastal areas. For next weekend, low
pressure will develop along the West Coast for much cooler weather.
Depending on the track of the system, showers could develop at times
Saturday through Tuesday.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
Once again this afternoon, skies were completely clear across the
region. It was another very warm afternoon inland where temperatures
were well into the 80s, with some low 90s over the southern deserts
at 1 PM PDT. Surface pressure gradients were about neutral to NV and
weak onshore to the lower deserts. Trends were onshore. Mostly light
winds were reported.
Through 1 PM PDT there has been no indication of marine cloud/fog
development over the coastal waters, even though onshore flow is
increasing. Surface dewpoints were quite low, but were drifting
upward with the sea breeze. Hires models show there is a chance that
fog/low clouds will develop overnight and may affect coastal areas
by morning, but confidence is low. So for now, the clouds/fog are
qualified in the zone forecasts with the word "patchy".
A strong high pressure ridge over the EastPac will drift slowly
eastward this week, keeping skies fair and the days warm. Warmest
in the deserts. Thursday looks to be the hottest, when daytime highs
will be some 7 to 12 degrees F above average. Closer to the coast,
onshore flow will prevail, and increase as the interior warms. This
will keep the marine layer in place, with seasonal temperatures and
areas of nocturnal clouds and fog, possibly remaining near some
beaches in the afternoons by midweek.
Forecast confidence drops rapidly into the end of the week with the
various 12Z runs of the CMC/GFS/ECMWF not delivering any clarity in
their respective solutions. At this point it is quite certain that
the flow over the hemisphere will become highly amplified, resulting
in some type of block forming over North America. Pick a model and
run, and the resulting sensible weather for SoCal can range from dry
and breezy to widespread precipitation. At this point, confidence is
highest for cooling with below average daytime temperatures. And in
terms of weather; slight chance POPS are in the forecast for Sunday,
which betters climatology by around ten percent.
302000Z...SKC and P6SM vis all areas through 01/0300 UTC. 01/0300-
1600 UTC...low stratus/dense fog may gradually develop 5-10 sm
inland. If it does develop, bases would likely be around 200-500 ft
msl, with tops around 500-700 ft msl and vis AOB 1/4-1 sm.
Confidence in low stratus/dense fog occurrence is low at the moment.
Most likely time-period for low stratus/dense fog to impact KSAN,
KCRQ and KSNA is during the 01/0900-1400 UTC time-period. Elsewhere,
P6SM vis and SKC will continue.
Patchy dense fog is possible tonight into Monday morning over the
coastal waters, resulting in visibilities of 1 NM or less. Otherwise,
no hazardous marine conditions are expected through Friday.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.