Small Craft Advisory 15, NWS San Diego, CA

Area Forecast Discussion
National Weather Service San Diego CA
925 PM PDT Thu May 4 2017

.SYNOPSIS...
Strong high pressure will bring another warm day to the region
today. A low pressure trough moving into the west coast will
deepen the marine layer and bring cooling Friday and Saturday. A
cold low pressure system from the north will arrive this weekend
and bring strong winds Saturday along with some light showers,
then much cooler weather with periods of scattered to numerous showers
and isolated thunderstorms Sunday through Tuesday. The system
will move east Wednesday and allow fair weather to return.

&&

.UPDATE...

The Pops/Wx grids were tweaked a bit for Sunday and Sunday night to
increase the pops a bit, especially over the mountains. The
significant, late-season storm system is still on track to bring
sensible weather impacts, beginning Saturday, with the brunt of the
precipitation potential Sunday through Monday morning. No other
significant changes were made to the forecast at this time.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

(Previous discussion issued at 118 PM)

Afternoon visible satellite imagery shows marine layer stratus
trying to sneak up the southern San Diego county coast from Baja
Mexico, while some flat cumulus clouds have formed over the
mountain ridges. The 12Z sounding from Miramar shows a 10 deg C
inversion at around 1200 feet, which indicates a deeper marine
layer than yesterday morning. We continue to trend better onshore
flow today, with the San Diego to Las Vegas pressure gradients at
1.0 mb onshore, whereas they were 0.7 mb offshore 24 hours ago.
Today should be another warm day across the region, with slight
cooling west of the mountains as the upper level ridge starts to
move east, although high temperatures should still be 5-15 degrees
above normal. The marine layer should deepen a little further
tonight with strengthening onshore flow and lower 500 mb heights
moving into the region, so expect marine layer clouds to move into
the coast and valleys tonight/Friday morning.

The weather will continue to cool Friday as a trough develops
along the west coast and the upper level ridge moves east. The
marine layer will continue to deepen as well. On Saturday, the
trough deepens further and a closed low starts to split off from
the trough and drop down into Central California. The marine layer
will likely be up to 5000 feet deep, and with onshore flow
continuing to get stronger, there may only be partial clearing of
marine layer clouds during the day Saturday, with potentially a
strong coastal eddy helping to produce some marine layer drizzle.
Onshore pressure gradients from San Diego to Las Vegas peak
Saturday evening at 15-17 mb, with 850 mb winds of around 20-30
kt. This will result in strong west winds along the mountain
ridges, desert slopes, through/below the passes/canyons and into
the adjacent deserts. Widespread gusts of 40-50 mph will be
possible in those areas Saturday afternoon and evening, with 60+
mph gusts in the windiest locations. With continued cooling, highs
will likely lower to 5-15 degrees below normal.

The closed low will strengthen further and dip south into Southern
California and become cut-off from the jet stream. Deep moisture
up through 400-500 mb will move in with the low, along with most
unstable cape of around 100-400 j/kg and 500 mb temperatures
within the core of the low falling to around -25 deg C. This, in
addition to lift provided by the low, should create showers and
isolated (to maybe scattered) thunderstorms on Sunday. The cold
air will result in the snow level lowering to 4000 feet Sunday
morning, and then increasing to around 5500-6500 feet by late
Sunday.

Model differences are apparent starting Monday, with the GFS
keeping the low over Southern California before moving it east
into Arizona by late Monday and resulting in a dry Tuesday.
Meanwhile, the ECMWF and Canadian models move the low south of the
border, perhaps bringing offshore winds and wrap-around
moisture/precipitation. On Tuesday, the ECMWF and Canadian then
show the low moving northeast from Baja into Arizona, with wrap-
around showers continuing over Southern California. Since the GFS
solution is the outlier and the ECMWF has had a better handle on
this low throughout the week, will lean more towards the ECMWF
solution with the forecast. It does not look like we dry out until
Wednesday as the low moves east and a ridge moves into the area.
Storm-total precipitation will be highly variable from one
location to the next, due to the showery nature of the
precipitation. Generally, 0.25 to 1+ inches is possible across
the region, with a trace of snow down to 4000 foot elevation, 1-3
inches of snow from 5000-7000 feet, and 4-6 inches possible above
7000 feet. Instability still exists on Monday, so thunderstorms
will be possible that day as well. The ECMWF and Canadian actually
show a lot of instability on Tuesday (surface based CAPE of
300-700 j/kg), so have added a slight chance of thunderstorms
during the day Tuesday. Any heavier convective
cells/thunderstorms could produce locally heavy rain and possible
urban/small stream flooding. The cold system will result in highs
falling to 10-20 degrees below normal Sunday and Monday with
slight warming Tuesday and Wednesday.

&&

.AVIATION...
050340Z...Coasts/Valleys...Stratus with bases 1000-1500 ft MSL
spreading 20 miles inland overnight. Local vis 3-5 miles where
stratus nears higher terrain in the valleys. Stratus clearing to the
coast 17-19Z, then moving back inland after 06/23Z.

Mountains/Deserts...Mostly clear with unrestricted vis through
Friday evening.

&&

.MARINE...
No hazardous marine weather through Friday. A late season storm will
bring west-northwest winds with gusts of 20-30 knots late Saturday
into Sunday along with building seas. Showers are likely Sunday into
Monday with  the potential for a few thunderstorms. Precipitation
will begin to taper off Tuesday.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.

&&

PUBLIC...Harrison/JMB
AVIATION/MARINE...SS