Area Forecast Discussion
Issued by NWS San Diego, CA
FXUS66 KSGX 230443
Area Forecast Discussion
National Weather Service San Diego CA
930 PM PDT Sat Apr 22 2017
A shallow marine layer will develop tonight and bring areas of fog,
locally dense, near the coast late tonight and early Sunday. A low
pressure trough will move through California Sunday and bring much
cooler weather west of the mountains and increasing winds in the
mountains and deserts. The marine layer will deepen Sunday night and
bring low clouds into the valleys then. Monday will be the coolest
day under the trough with rather strong winds in the mountains and
deserts. High pressure over the East Pacific will very gradually
expand towards California the second half of next week, which will
bring a warming trend and less of a marine layer.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
Skies were mostly clear this evening as low clouds so far were
struggling to form. Low clouds/fog were patchy along the northern
Baja coast and will gradually spread north along the So-Cal coast
tonight, especially with models showing an eddy developing. There
will likely be dense fog for awhile with the onset of the stratus
and fog, gradually shifting from near the immediate coast after
midnight to the mesas and higher coastal terrain by sunrise Sunday.
With the rapidly moving trough through California Sunday, replacing
the ridge from today, there will be much stronger onshore flow and
the resulting cooling, 15 degrees or more in some areas west of the
mountains. Westerly winds could gust over 40 MPH from the mountain
crests east into the desert slopes and locally in the deserts by
late afternoon Sunday.
The trough for Sunday will shift east, but another wave will move by
to the north Monday as the trough becomes more of a long-wave
trough. This will result in tighter height gradients at 700 and 500
MB and winds aloft will be rather vigorous, 50+ knots over some
locations at 700 MB. Local WRF is showing some mountain wave
activity late Monday/Monday night to complement the MSLP gradients
and the winds aloft. Local gusts could exceed 60 MPH in the normally
windy locations, such as San Gorgonio Pass and the desert mountain
slopes, with a lot of areas likely exceeding 40 MPH with gusts. A
wind advisory will almost certainly be needed Monday
afternoon/night. Monday will be the coolest day with a lot of 70s in
the valleys and barely above 90 in the lower deserts. After Monday,
we will be in northwest flow aloft due to the longwave trough over
the interior west and a building high over the East Pacific which
could partially nudge into So-Cal. This will bring a gradual warming
trend and a shallower marine layer. Models hint at the western US
trough deepening late in the week which could bring windy conditions
again in the mountains and deserts, possibly followed by offshore
flow next weekend, but no precipitation is expected as the moisture
will stay well to our north and east.
230345Z...Coast/Valleys...Primarily P6SM vis and SCT clouds AOA
20000 ft msl through 06Z, with a small chance for local SCT clouds
blo 1000 ft MSL and 2-5 sm vis near the coast. Aft 06Z, low
clouds/fog likely to increase in coverage and gradually spread 5-10
sm inland, with bases/tops starting out around 200-400 ft msl/600 ft
msl and likely rising to 500-800 ft msl/1000 ft msl by 12Z. Expect
areas of vis 1/4-1 sm, mainly near the coast before 12Z, then
impacting mainly the mesas/higher coastal terrain after 12Z.
Confidence in low cig/vis occurrence at TAF sites is moderate, with
lower confidence for the timing. Low clouds/fog likely to clear to
the coast by 17Z.
Mountains/Deserts...SCT clouds AOA 20000 ft MSL and unrestricted vis
Patchy dense fog will likely develop over the coastal waters late
tonight. By sunrise, the marine layer could deepen enough so that
fog will no longer be a hazard over the waters.
Northwest wind gusts may approach 25 kt in the outer waters Sunday
evening as a trough of low pressure moves into the Great Basin. A
better chance for northwest winds gusting to 25 kt and possibly 30
kt will occur Monday evening, resulting in possible small craft
advisory conditions in the outer waters.
Gusty northwesterly winds may continue at times through the end of
the next work-week.
A southwest swell around 3 ft/15-17 seconds, will create elevated
surf of around 3-6 ft with occasional 7 ft sets today, and possibly
a little higher on Sunday as a new northwest swell builds as well.
The above normal surf will also create favorable conditions for
strong rip currents and longshore currents. A Beach Hazards
Statement remains in effect through Sunday.
Elevated northwest swells will continue through most of next week,
potentially reaching high surf advisory criteria at times.
Skywarn activation will not be needed on Sunday.
CA...Beach Hazards Statement through Sunday evening for Orange County
Coastal Areas-San Diego County Coastal Areas.