Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 160318

Area Forecast Discussion
National Weather Service San Diego CA
818 PM PDT Sat May 15 2021

Low pressure will drop southeast over the deserts of California on
Sunday, maintaining cool conditions with gusty onshore winds over
the mountains and deserts, and a deep marine layer to the coastal
mountain slopes. Some light precipitation is likely to develop at
times tonight through early Tuesday, with the best chance on
Sunday. A ridge will arrive later Tuesday, providing dry and warmer
weather through Wednesday. Another cold, upper-level trough will
develop over the State on Thursday, setting up another round of
cooling and gusty onshore winds over the mountains and deserts.



Marine clouds were much more stubborn today over coastal areas, and
wasted no time filling rapidly inland along the coastal slopes of
the mountains late in the day. At 8 PM PDT...Clouds covered
Orange and western San Diego County, and were even seen as far
inland as Warner Springs. Our 00Z sounding had a 7.5C inversion near
3700 FT. That`s up 1100 FT from yesterday. South-southwest winds
of around 10 KTs were observed below the inversion, which is a
little ahead of schedule, based on the 00Z WRF. The sfc pressure
gradient from KSAN to KTPH was as high as 13 MBS earlier this
afternoon and remains strong this evening. Peak westerly winds of
25-35 MPH with gusts to 55 MPH have been logged near the San
Gorgonio Pass, and a Wind Advisory covers the deserts/passes.

Sunday will be the coolest day of the weekend with daytime maximum
temperatures of from 15-20F below average in the inland valleys,
while the deserts run 10-15F below average and the coastal areas run
5-10F below average. A minor forecast update was issued earlier this
evening to adjust POPS and add some fog along the mountain slopes.
The latest HRRR runs and the 00Z WRF run are looking a bit wetter
for coastal areas Sunday morning. One-quarter to one-half inch
precip amounts are forecast locally along the coastal slopes of the
mountains through Monday, while coastal and valley areas will likely
see one-tenth inch or less. Precipitation rates will be very low so
little run off is expected.

A dynamic low pressure center will track SE from Modesto CA, to
near Las Vegas by Sunday afternoon. This track will support strong
onshore flow over SoCal through Sunday, which will pull in more
moisture from the Pacific, and likely spark scattered light showers
and/or drizzle at times into Monday morning. Weak PVA and cyclonic
flow aloft will provide modest lift, while upslope flow enhances
amounts on the coastal slopes. Even as the storm center moves east
on Monday, lingering low-level moisture will be acted upon by
another weak disturbance trailing the storm in NW flow aloft, to
renew a chance for more light precip Mon night into Tue morning over
the coastal basin.

Sunday will be a chilly one for SoCal, but gradual warming will
begin on Monday as the onshore flow weakens, and some of the low
clouds break. Despite the chance for a few more light showers/
drizzle Tue morning, the warming will continue under a weak
ridge aloft and more afternoon sun.

The ridge slips by on WEdnesday as yet another cold trough deepens
along the Coast on Thursday. This will spark another round of strong
westerly winds over the mts/deserts and more cool weather to end the
work week. Some temperature moderation is likely next weekend as the
trough weakens along the Coast.


160300Z...Coast/Valleys...BKN-OVC015-030 tops 050 through 23Z Sunday
with DZ/-RA after 08Z Sunday. Only partial clearing Sunday afternoon
with a deep marine layer in place.

Mountains/Deserts...West facing mountains slopes below 6,000 ft
obscd in clouds/fog tonight and Sunday with areas of DZ/-RA.

Southwest winds gusting 30-45 kt tonight through Sunday, strongest
on the desert slopes and through the San Gorgonio Pass. MOD-STG
up/downdrafts, LLWS, and rotors possible over and east of the
mountains near KPSP.


No hazardous marine weather is expected through Wednesday.


A long period south swell from 200 degrees at 3 ft/17 sec will
produce 3-6 ft on south-southwest facing beaches of Orange County
and far northern San Diego County Sunday. Surf of 2-4 ft is expected
in San Diego County south of Carlsbad. Strong rip and longshore
currents are likely Sunday into Monday. Swell and surf lower Monday.
Another south swell arrives late Tuesday and peaks Wednesday for
another round of elevated surf and a high risk of rip currents.


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


CA...Wind Advisory until 5 AM PDT Monday for Riverside County
     Mountains-San Diego County Deserts-San Diego County
     Mountains-San Gorgonio Pass Near Banning.

     Wind Advisory until 7 PM PDT Sunday for Apple and Lucerne
     Valleys-Coachella Valley.



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