Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

230 am PDT Thu Sep 21 2017

Synopsis

A trough of low pressure will drop into Southern California today and linger through Saturday. Today it will bring cooler and cloudier weather, some showers from the coast to the mountain crests, and areas of strong winds in the mountains and deserts. As the trough departs over the weekend, offshore flow will develop to bring clear skies and a warming trend next week.

Discussion

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

A big trough of low pressure, quite unseasonal for the last day of summer, will drop into Southern California today. Clouds have already filled the coastal basin pretty much to the brim, that is, to the mountain crests. A couple spots on the coastal slopes have already begun to measure a few hundredths of rain. Later this morning, a more organized band of legit showers will form along a surface wind discontinuity over the ocean, meaning some uplift will be generated along a line where southerly winds meet with westerly winds. This band of showers will bring light rainfall: below a tenth inch for most everywhere, but some foothill locations such as near Crestline could harvest a third inch or more. Winds strengthen this afternoon and evening. Strongest winds will occur through mountain passes and east into adjacent deserts with top gusts over 45 mph. Temperatures will be in the low 70s for most of the low elevations west of the mountains today, 15-20 degrees below average for the inland valleys. Residual moisture and onshore flow will keep skies partly cloudy and temperatures well below normal for Friday. A secondary shortwave within the big trough over the West will slow down the trough such that it won't really move out until Sunday. The surface flow will gradually go offshore and that will help clear the area of clouds over the weekend. Northeast winds will get started Sunday and appear each morning through at least Tuesday. Local gusts below Cajon Pass and in the Santa Ana Mountains could exceed 40 mph each morning, but overall this will not be a widespread or strong offshore event. This fall weather comes right on schedule as if the atmosphere knew it was time to start acting like fall. Temperatures will climb each day next week, topping 90 in a few spots by Monday, with 90s more common in the following days. Clouds will be hard to come by next week. The clear skies and dry air will allow temps to drop at night, providing some relief from the hot days.

Aviation

210900z, Coast/Valleys/Coastal Slopes, widespread stratus over the coast, valleys, coastal slopes and mountain passes, with highly variable bases of 2500-4000 ft msl, tops of 5000-6000 ft msl, areas of -DZ/isol -SHRA producing local vis/cigs of 2-5 sm/1000-1500 ft msl, and mountain obscuration of coastal slopes below 6000 ft msl, will continue for the most part through 22/0000 UTC, although cloud coverage may become BKN-SCT after 1800 UTC with -DZ/-SHRA likely ending by 2100 UTC. 22/0000-1500 UTC, BKN-OVC stratus filling the coast, valleys and coastal slopes, with bases 2500-3500 ft msl, tops 4500-5000 ft msl and mountain obscuration of coastal slopes below 5000 ft msl.

Mountain Ridges/Desert Slopes/Deserts, Mostly SKC with P6SM vis through tonight. Through 1800 UTC, west winds 15-25 kt with gusts 35 kt with mod-stg uddfs/llws. Winds will strengthen after 1800 UTC with gusts to 45 kt possible below the passes, on desert slopes, and near the desert foothills, resulting in stg uddfs/llws as well as surface wind gusts at KPSP and KTRM possibly reaching 35 kt at times.

Marine

A Small Craft Advisory will be in effect for the outer coastal waters this afternoon through late tonight as a trough of low pressure creates moderate to strong onshore flow. Northwest wind gusts of 25 kt are likely in the outer waters, while the inner waters will see gusts mainly in the 15-20 kt rang (which is below Small Craft Advisory criteria). At the same time, there will also be short-period seas of 4-7 ft. Although winds will decrease early Friday morning, another increase in winds is expected late Friday afternoon with wind gusts in the outer waters mainly around 20 kt. Lighter winds and smaller seas are forecast over the weekend.

Watches, Warnings, Advisories

Ca, Wind Advisory from 11 AM this morning to 1 AM PDT Friday for Apple and Lucerne Valleys-Coachella Valley-Riverside County Mountains-San Bernardino County Mountains-San Diego County Deserts-San Diego County Mountains-San Gorgonio Pass Near Banning.

PZ, Small Craft Advisory from 11 AM this morning to 1 AM PDT Friday for Waters from San Mateo point to the Mexican Border Extending 30 to 60 nm out including San Clemente Island.

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