Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

846 pm PDT Sun aug 1 2021


Building high pressure aloft will bring hot and dry weather inland this week. Marine layer clouds and fog will likely be scarce until midweek when the marine layer will slowly begin to recover. The heat will peak Tuesday and Wednesday for most areas including the coast, where temperatures will be moderated by some chilly ocean upwelling and moderate sea breezes. A cooling trend will set in for the latter half of the week. Prospects for any more monsoonal activity will have to wait until next week.


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

Skies were clear over SoCal this evening. We still have some moderate onshore sfc pressure gradients accelerating westerly winds near the passes (25 MPH with gusts 35-40 MPH). Our 00Z Miramar sounding had a weak inversion topping an 1100 FT deep mixed layer. This will likely fall back to near the sfc tonight as winds drop off. There could be some patchy fog, but indications are that few if any low clouds will form. No evening forecast updates are planned.

From previous discussion,

As we head into a new work week, high pressure currently over the Rockies will continue its westward expansion, becoming anchored over the Colorado River basin on the CA/AZ border Tuesday and Wednesday. 500 mb heights look to peak near 595 dam - not quite as impressive as recent 600 dam highs - but 850mb temperatures near 31-32C combined with drier surface conditions should allow temperatures to climb excessive heat levels for some areas, particularly inland. Latest guidance has continued to trend gradually warmer, which has been the case over the past day or two. Thus, have posted an Excessive Heat Watch for the deserts Tuesday-Wednesday to highlight the potential for Excessive Heat, as well as a Heat Advisory Monday through Wednesday for the Inland Empire and mountains.

Moderate to high Heat Risk will primarily be driven by anomalously warm overnight lows (generally 10 to 15 degrees warmer than normal), rather than daytime highs which will range generally 4 to 8 degrees above normal. Expect highs to reach 100-105 for the Inland Empire, and likely exceed 115 F in the lower deserts quite easily. There, lows between 85 and 90 are anticipated during the first half of the week. For those living in and visiting coastal areas, the marine influence will substantially reduce the heat impacts, with high temperatures at the beaches unlikely to reach 80 F any of the days. Per numerous Coast Guard reports today, sea surface temperatures at the beaches are running between 58 and 62 degrees, noticeably cooler than a few days ago, and anomalously cool for this time of year. This may be one reason for the persistent coastal fog the past few days, and will certainly help mitigate the heat in those areas. The building high pressure will compress the marine layer however, so overnight and morning marine layer clouds should be less patchy, or even disappear altogether, as we start the work week. High pressure will weaken after Wednesday, allowing temperatures to fall back closer to seasonal normals and the marine layer to strengthen its grip west of the mountains. No monsoonal activity appears likely through the next week. Beyond that, there are some indications of a more favorable pattern for increased moisture heading into next week, and perhaps even some interaction with E-Pacific tropical activity, but it's too early to say anything more concrete.


20320z, Coast/Valleys, Patchy low clouds and fog are possible between 10-15Z along the San Diego County coast and extending up to 5 mi inland. Bases will be 300-600 ft MSL with tops to 700 ft MSL and local vis below 1 mile in fog. Low confidence in occurrence of clouds/fog at any of the coastal airports, including KSAN and KCRQ. After 15Z, mostly clear with unrestricted vis through Monday evening.

Mountains/Deserts, Mostly clear with unrestricted vis through Monday evening


Patchy dense fog is possible again late tonight and early Monday morning with visibility less than 1 nm, mainly near the coast and up to about 15 nm offshore. Wind gusts to 20 kt are possible in the outer waters each afternoon through Wednesday. Otherwise, no hazardous marine conditions are expected through Friday.


A couple of south swells of 2-3 feet at 15-17 sec will bring surf of 3-5 ft and a high rip current risk to the south facing beaches of Orange County Tuesday and Wednesday.

Watches, Warnings, Advisories

Ca, Heat Advisory from 10 AM Monday to 8 PM PDT Wednesday for Riverside County Mountains-San Bernardino County Mountains- San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Mountains.

Excessive Heat Watch from Tuesday morning through Wednesday evening for Apple and Lucerne Valleys-Coachella Valley-San Diego County Deserts-San Gorgonio Pass Near Banning.


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