textproduct: San Diego

SYNOPSIS

Some cooling expected for the coastal basin today as onshore flow returns. Further cooling expected across the region starting Tuesday as a low-pressure system begins to move in from the northwest. This will bring widespread light to moderate rain to the region late Wednesday into Thursday along with elevated southwest winds for the mountains and deserts.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

Santa Ana winds are finally diminishing this evening as easterly flow aloft and the offshore surface pressure gradient both continue to weaken. Easterly gusts 20-35 mph will continue along coastal slopes and through east-to-west passes through this morning. Return of weak onshore flow will allow for some cooling today, especially for coastal areas, though highs will remain 4 to 10 degrees above normal through the coastal basin.

The return of onshore flow will also herald the return of the marine layer. HREF guidance introduces patchy low clouds and FG over coastal waters starting early Monday morning. Onshore flow increases by Monday night as a closed upper low from the northwest begins to influence the region. This will allow marine layer clouds and FG to push into coastal areas and portions of the valleys.

As the upper low continues to slide southward, cooling will spread across the region with below normal highs expected from Tuesday onward. Rain chances increase from west to east on Wednesday, though the majority of guidance has trended a little slower with the onset of precipitation. Regardless, widespread light to moderate rain is expected with the most likely timeframe for precip being Wednesday night into Thursday. As the event draws closer, some details are beginning to emerge. NBM probabilities of storm total rainfall exceeding 0.5" and 1" from the coast to the mountains are about 60- 90% and 25-55% respectively. The higher probabilities are mainly focused in the mountains and portions of the coastal basin north of San Diego County. Lower rainfall totals are expected in the deserts with only 30-50% and 5-25% chances of 0.5" of rainfall in the High Desert and low deserts, respectively.

As the core of the low moves overhead on Thursday, temperatures will fall well below normal across the region. Additionally, increased mid-level lapse rates and vorticity will promote development of convective showers. There is about a 10-15% chance for thunderstorms from the coast to the mountains at this time. If thunderstorms develop, locally heavy rainfall rates will be possible.

Model solutions diverge by next weekend, but there is not much indication of any significant weather. The mean of the guidance indicates mainly zonal flow with temperatures warming back to slightly below normal through the weekend. In the long term, there is some indication that the troughing pattern will continue.

AVIATION

180600Z...Easterly offshore winds have largely subsided at the surface, though elevated easterly winds remain possible near the in the mountains and at 4000-8000ft into the early overnight hours.

Otherwise...VFR conditions expected with SCT-BKN high clouds AOA 10,000ft MSL through this evening. High clouds scatter out across the area tonight, with SKC thereafter.

MARINE

No hazardous marine conditions are expected through Wednesday.

BEACHES

Elevated surf will continue this weekend into mid-next week with multiple south swells moving in through this period. Surf increases slightly Sunday, with surf 3 to 6 feet and local sets to 7 feet along south and southwest-facing beaches through at least Wednesday.

While surf and rip currents will be elevated, daily high tides will not be particularly high, and the threat of coastal flooding is minimal.

SKYWARN

Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.

SGX WATCHES/WARNINGS/ADVISORIES

CA...Beach Hazards Statement from 9 AM PDT this morning through Tuesday morning for Orange County Coastal Areas-San Diego County Coastal Areas.

PZ...None.