Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

117 pm PDT Wed apr 21 2021


A weak low pressure system will move through Southern California through Thursday with gusty mountain and desert winds, more clouds, and periods of light rain or drizzle along west of the mountains. There is also a small chance of isolated, heavier showers over the higher mountains on Thursday afternoon. It will be much cooler through Thursday, then warmer into the weekend as a weak ridge develops aloft. A stronger Pacific storm system will arrive early next week with a chance for more widespread and heavier rain and mountain snow.


For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,

Clouds prevailed west of the mountains at midday, although there were plenty of breaks providing some Sun. Little if any rain shower activity was observed at the noon hour. Most reporting sites were considerably cooler than midday yesterday. Moderate to strong onshore sfc pressure gradients across the region were supporting stronger winds through the passes. Peak gusts were over 60 MPH there, but peak gusts to around 40 were more broadly prevalent in the desert.

Low pressure is centered over SW NV at midday. It will be trailed by a weak trough aloft through Thursday. Scattered, light showers will likely redevelop and continue along and west of the mountains ahead of the trough tonight, shifting toward the mountains on Thursday. Instability and moisture will still be sufficient for isolated, heavier showers to develop over the mountains on Thursday afternoon. Aside from that, rainfall totals through Thursday night are expected to remain below one-quarter inch, except near the coastal mtn slopes where isolated heavier amounts are possible. A few light showers may redevelop Thursday night west of the mountains, but after early Fri morning, fair and dry weather is expected through the weekend.

A weak ridge will drift over SoCal this weekend, suppressing the marine layer and weakening the onshore flow. This will set up SoCal for modest warming under fair skies as temperatures return to about average levels.

An anomalous, late season Spring storm still has it's sights set on SoCal for Monday. Precipitation will spread south through the northern half of the State on Sunday, reaching the south by Monday morning, based on current trends. All of the ensemble members of the ECMWF are wet, while about one-third of the GFS members remain dry. Most of the Canadian members are wet too, so pretty good odds for precipitation. It could be substantial as well, with the models showing a decent tap on some Pacific moisture. And given the depth of the saturated layer, some of this should make it into the deserts. Depending on the speed of the system, some precipitation could continue into Tuesday.

For the latter half of next week, many of the ensemble members of the various global models show warming, but it doesn't last, as heights begin to fall along the West Coast again. The trend is for cooling the following week to end the month.


212000z, BKN-OVC low clouds with bases 3500-4500 feet continuing through this evening. Coastal mtn slopes below 5000 feet remaining obscured in low clouds/fog through Thursday morning. Clouds lowering to 1800-2500 feet after 03Z Thursday. Areas of -DZ/-SHRA possible again tonight through Thursday morning over and west of the mountains, with vis reduced down to 2-5 miles.

Southwest to west winds with surface gusts 30-40 kt will continue through this evening along the ridges/desert slopes and east of San Gorgonio Pass, weakening tonight into Thursday morning. Moderate up/downdrafts and areas of LLWS will be possible over/east of the mountains.


No hazardous marine conditions are expected through Sunday.


A south swell from 180-190 degrees of 3-4 ft with a period of 16-20 seconds will reach the beaches of Southern California Friday through Sunday. This swell could bring elevated to possibly high surf, especially to the south-facing beaches of Orange County, as well as strong and dangerous rip and longshore currents at all beaches. Occasional sets of 6-7 feet will be possible at south facing beaches of Orange and far northern San Diego Counties, with surf of mostly 3-

Watches, Warnings, Advisories

Ca, Wind Advisory until 10 PM PDT this evening for San Diego County Deserts-San Gorgonio Pass Near Banning.


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