163 FXUS66 KSGX 012057 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 200 PM PDT Mon Jun 1 2020 .SYNOPSIS... High pressure aloft will build into the southwest through Wednesday and bring a warming trend, including more hot weather in the deserts. Temperatures could reach 110 Wednesday and possibly Thursday in the lower deserts. Upper level low pressure southwest of San Diego will bring high clouds at times most of this week and possibly a few showers around Friday. The marine layer low clouds and fog will be patchy early this week during the nights and mornings but could increase and extend farther inland late in the week. Cooler weather will also return late this week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... High clouds were moving over this afternoon with temperatures mostly a little above seasonal normals. There should not be much stratus or fog tonight, partly due to the high clouds drifting over. High pressure aloft over the central US will gradually shift southwest towards AZ/NM by Tue and remain there through about Thu. A deep closed upper low about 600 miles west of San Diego will oscillate until around Thu Night/Fri when it is likely to move through SoCal. The upper high will be associated with 850 MB temperatures as high as around 27 deg C Wed/Thu in the deserts, and most ensemble solutions have surface high temperatures around 110 Wed and nearly that Thu. An excessive heat watch has been issues for the lower deserts for Wed/Thu. Some temperatures will likely reach the mid to upper 90s, possibly even 100, by Wed in hottest locations west of the mountains, with milder conditions near the coast. A possible eddy developing around Thu could bring substantial cooling west of the mountains then, but the deserts will likely not cool until the passage of the upper low Fri, though the NBM 1-D point viewer does have a greater range of possible temperatures Thu versus Wed in the lower deserts, but with most solutions close to those from Wed and a few that are a few degrees lower. The upper low will likely move over the region around Fri, but it will have very limited moisture, unlike the cutoff low from a few days ago. The instability is also limited, and will most likely be elevated versus surface-based, though models struggle 4 days in advance with determining any elevated convection and right now are not showing any strong signals. For now, we will go with just a slight chance of showers over most of our forecast area. Ensemble members show only a few hundredths of an inch of precipitation. Temperatures will of course be lower late in the week but could increase again early next week after the low departs. && .AVIATION... 012015Z...Patchy low clouds will likely return to the coastal areas tonight between 06Z and 15Z Tuesday with bases around 1000 feet MSL. However, cig/vis impacts at TAF sites are uncertain. Expect clearing to the coast between 14Z and 16Z Tuesday. Elsewhere, variable high clouds AOA 20000 feet MSL and unrestricted vis through Tuesday. && .MARINE... Northwest winds gusting 20-25 kt are likely in the outer waters this afternoon and evening, mainly near San Clemente Island. No hazardous marine conditions are expected Tuesday morning through Friday. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Excessive Heat Watch from Wednesday afternoon through Thursday evening for Coachella Valley-San Diego County Deserts. PZ...Small Craft Advisory until 1 AM PDT Tuesday for Waters from San Mateo point to the Mexican Border Extending 30 to 60 nm out including San Clemente Island. && $$ PUBLIC...Maxwell AVIATION/MARINE...PG