577 FXUS66 KSGX 201603 AFDSGXArea Forecast Discussion National Weather Service San Diego CA 903 AM PDT Sat Sep 20 2025
.SYNOPSIS... Warm and dry today with some coastal clouds. Tropical moisture returns Sunday with scattered light showers, most widespread across the mountains and San Diego County. Small chances of showers continue Monday, increasing for Tuesday and possibly into Wednesday, along with a slight chance of thunderstorms. Drier weather appears to arrive Thursday and Friday. Temperatures will be mild this coming week, with decreasing coastal low clouds.
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.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...
Coastal low clouds made a valiant return overnight, except in northern Orange County. Look for lots of sunshine today for all areas after the coast clears. There is a slim layer of residual mid level moisture today, but that should only bring some cumulus clouds over the mountains this afternoon. It will be a little warmer today than it was yesterday. An upper level low pressure trough drops south off the CA coast and camps out just west of SoCal in the next few days. It draws a lot of mid and upper level moisture into our region late tonight and Sunday from a tropical disturbance southwest of Cabo San Lucas, not a tropical cyclone. The moisture is rather high, staying mostly above 12,000 feet, but enough to warrant isolated to scattered showers, but also sprinkles and virga. The moisture plume thins out a little on Monday, for a lesser but nonzero chance of showers. Meanwhile, the destabilizing environment will wreak havoc on our marine inversion so that coastal low clouds incrementally decrease each night and morning. The upper low seems to cut off and deepen, adding a little more energy to the system and also concentrating and deepening the moisture column for Tuesday. That`s when chances of measurable rain and instability increase, enough for deeper convection and a small chance of thunderstorms. This is when the range of outcomes widens. Some ensemble members rotate the low northward late Tuesday, taking the moisture with it, so that Wednesday looks dry. Other ensembles show a slower progression such that SoCal remains within the more potent moisture plume both Tuesday and Wednesday. Still other solutions keep the Wednesday moisture mostly to our west, with SoCal on the fringes. Still, a very slight chance of precipitation Wednesday. Greater certainty actually comes later next week as most solutions weaken the cutoff low over the west, north of SoCal, to bring dry weather. With a weak trough over the West, high temperatures will continue mild, a few degrees below average, through the end of the week. In the absence of the tropical moisture, the marine layer should be able to tepidly rebound and rebuild and bring more coastal clouds.
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.AVIATION... 201600Z....Coast/Valleys...Low clouds based 400-1000ft MSL sufficiently pushed 5-10 miles into San Diego County this morning with patchy low clouds in Orange County. VIS remains very low, 1-5SM within 10 miles of the coastline, locally lower to 1/2SM in some valleys and at coastal terrain. Bases will slowly rise and clear to the water by 18z, with stratus potentially prevailing at the beaches into the afternoon. Low clouds with bases 600-1000ft MSL develop and push inland again tonight after 01-02z Sun through 10z, with similar inland coverage. Low clouds look to clear quicker on Sunday, closer to 15-17z. SHRA will begin to move into the region generally after 10z Sun, with more widespread SHRA after 15z. Locally lower VIS and CIGs with any shower.
.Mountains/Deserts...VFR conditions and mostly clear skies through early Sunday. Mid and high level clouds AOA 10,000 ft MSL enter the region this evening. SHRA move into the region generally after 15z, with brief lowering of VIS and CIGs with any shower.
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.MARINE...Rain showers possible for much of the day Sunday and again Monday afternoon which could briefly lower visibly. Slight chance for a few thunderstorms over the waters overnight Monday into Tuesday afternoon. Any storm could briefly produce lightning, gusty erratic winds, and choppy seas. Otherwise, no additional hazardous marine conditions are expected through Wednesday.
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.Beaches... Thunderstorm chances return early Tuesday, peaking early Tuesday afternoon, and may prevail into early Wednesday.
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.SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.
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.SGX WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None.
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PUBLIC...MM AVIATION/MARINE...Zuber
NWS SGX Office Area Forecast Discussion