517 FXUS66 KSGX 102035 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 134 PM PDT Mon Aug 10 2020 .SYNOPSIS... Building high pressure over the Southwest will bring warmer weather this week and eventually restrict any nocturnal marine clouds and fog to coastal areas and offshore. Moisture aloft will bring more high clouds midweek, but showers are not expected. A heat wave is expected to develop inland by Friday and continue into early next week. It will be warmer over coastal areas as well, but sea breezes will keep temperatures more moderate. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Marine clouds cleared inland areas rather quickly today, but remained stubbornly along the immediate coast until midday. The onshore sfc pressure gradients were trending more onshore to the high deserts, and supporting westerly winds in the 15-25 MPH range with a few higher gusts in the wind-prone desert/mtn/pass areas. Some marine clouds/patchy fog may not clear beaches completely this afternoon, but are on track to be much less extensive tonight into Tuesday morning. Fog could become more of an issue in the coming days as onshore flow continues under a strong, low inversion. Low pressure off the Central Coast and high pressure over the Rio Grande Valley will continue a dry SW flow aloft over SoCal into Tuesday, then gradually transition to a more moist flow aloft as moisture spinning off of T.S. Elida is entrained into the flow. The bulk of this moisture will be high in the atmosphere, but some will be felt as low as mountain top, helping to boost relative humidity a bit there. No convection is expected from this moisture since it will be quite stable, but virga and some sprinkles are possible. The clouds may shave a few degrees off of max temps midweek, but the lower levels look to stay quite dry over the deserts. Along the coast, the nocturnal marine cloud coverage will shrink to coastal areas and may be totally absent some days. So the extensive coverage inland we have become accustomed to this month is about to end. The tropical moisture looks to clear the area late Thursday, as the high pressure center to the east builds west over SoCal. A strong 500 MB center of 596+ DM should be over the Lower Colorado River Valley this weekend. With an abundance of dry air expected at lower levels, temperatures will rise, especially inland away from any marine influence. Heat Risk calculations indicate Warning level inland by Friday, so a Heat Watch will be needed soon to cover the end of the week, but the persistence of the high and a look at the ECMWF ensemble runs suggest the heat may last well into next week. The high aloft is forecast to drift north with time and become more east-west oriented. This would allow for continued onshore flow and gradual cooling by the middle of next week. It also creates a more favorable pattern for easterly waves to graze SoCal with more moisture and maybe even some Monsoon activity. Still a long way off, but looks like some potential is there as stability associated with the upper high diminishes after the middle of the month. && .AVIATION... 101930Z...Coast/Valleys... Areas of low clouds were lingering at the beaches at 1930Z. Low clouds will begin to move inland around 03Z Tue. Shallower cloud coverage is expected tonight into Tue AM compared to last night. Bases will be 600-1000 ft MSL. Patchy fog could develop near the coast with areas of reduced vis where low clouds intersect terrain. Clearing is expected 15-17Z Tue. Mountains/Deserts...Mostly clear skies with unrestricted vis through Tue AM. && .MARINE... Periods of gusty northwest winds close to 20 kt in the outer waters near San Clemente Island will occur each afternoon and evening through Wednesday. Areas of patchy fog could develop over the coastal waters late tonight into tomorrow morning. Otherwise, no hazardous marine weather through Saturday && .BEACHES... Today through Wednesday, an increasing south-southwest swell will bring above average surf and a higher risk of rip currents. 3-5 foot surf with sets to 6 feet in Orange County and northern San Diego County. South of Carlsbad in San Diego County, surf will be 2-4 feet. && .FIRE WEATHER... The prevailing onshore pattern will continue locally gusty afternoon through late evening westerly winds, and hold temperatures near to slightly below average. Strongest wind gusts of 25-35 MPH will be near and below the passes. Lowest daytime humidity of 10-15% will be on the lower desert slopes and into the deserts each day this week, while mountain top RH climbs with the influx of tropical moisture from the south. No wetting rain or thunderstorm activity is expected at this time, however a few sprinkles are possible over the higher elevations. At low elevations, the low RH combined with the locally gusty westerly winds will enhance fire weather conditions late each day. After Wednesday, heat will build inland and restrict the marine layer and associated higher RH to immediate coastal areas and offshore. By Friday through early next week, high temperatures are forecast to be from 5 to 10 degrees above the warm August averages. The onshore pattern should continue though, with gusty afternoon sea breezes eventually reaching the desert passes each day. && .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/FIRE WEATHER...10 AVIATION/MARINE/BEACHES...CO