Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

931 pm PST Wed Jan 17 2018

Synopsis

One more day of above average temperatures is in the forecast for Thursday. A weak storm system will push a cold front through Southern California on Friday, bringing cooler temperatures. Light showers will also be possible Friday into early Saturday. Drier weather will prevail Saturday afternoon into early next week, as afternoon high temperatures slowly return to near average values.

Discussion

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

At 9 PM, Ridging remained in control over California with a trough of low pressure beginning to dig south into the Pacific Northwest. In SoCal cirrus clouds above 20,000 ft MSL were on the increase ahead of the troughs approach. At the surface winds were light with weak offshore surface pressure gradients. The Bight and coastal areas had very limited low cloud cover.

Not much in the way of notable weather for tonight and Thursday. A few areas of patchy low clouds or fog may form near the coast after midnight, but little to no impact is expected for the morning commute. We will see one more warm day on Thursday, with afternoon high temperatures running 5-12 degrees above seasonal averages.

.PREV DISCUSSION,

An upper-level trough will swing across the region from the aforementioned low Friday through early Saturday. Not much moisture or instability is available with the trough, however light showers (mostly over and west of the mountains) are expected as the system moves through. Around 0.05-0.25 inches of precipitation is expected, with highest amount over and near the mountains. Isolated locations in the San Diego County mountains could receive near 0.50 inches. High-res models show showers developing first over northern areas mid-Friday morning, and into southern areas by late morning. Drizzle may develop earlier before showers move through. The morning commute in San Diego and Riverside Counties should not be greatly affected at this time. The front should exit the region by late Friday afternoon, then post-frontal showers are possible over portions of San Diego County Friday night through early Saturday morning. Snow levels start around 7500 ft, then crash to around 4000 ft by Friday night. Most precipitation should end before snow levels drop, therefore snowfall amounts are expected to be fairly light, from a trace to 0.5 inches 4500-6500 ft, and 1-2 inches above 6500 ft. Westerly wind gusts of 35-45 mph will develop across the mountains and deserts Friday, with isolated gusts to 60 mph along desert mountain slopes. Highs will lower 5-10 degrees below normal Friday.

Northwest flow aloft will help keep highs below normal through Monday, then weak ridging will increase high temperatures to around 5 degrees above normal by next Wednesday. Next week looks dry at this time.

Aviation

180400z, Coast, Patchy dense fog and low clouds are expected within 3 miles of the coast tonight, with bases 300-500 ft MSL and tops to 800 ft MSL. Local vis AOB 1/2 mile is possible at times. KSAN, KCRQ or KSNA could be impacted by low cigs/vis at times between 06Z and 15Z, but the timing is uncertain.

Valleys/Mountains/Deserts, SCT clouds AOA 20000 ft msl with unrestricted vis through Thursday morning.

Marine

A shallow marine layer will persist through tonight, likely resulting in patchy dense fog at times. Visibility over portions of the coastal waters could drop to 1 NM or less.

A storm system moving through California will bring combined seas of 10 ft or more and periods of gale-force winds late Friday through Saturday. A Gale Watch has been issued for that time period.

Beaches

The dominant west-northwest swell from 280 degrees at around 6-8 feet/13-14 seconds will gradually lower through Thursday, with surf heights likely falling below 7 feet Thursday morning. However, another large west-northwest swell from 280 degrees moves in Thursday night, peaking at 11 feet/18 seconds Friday, and then gradually lowering late Saturday into Sunday. This will bring another round of high surf, with average surf heights of 5-9 feet and sets 10-12 feet. Strong rip currents will be a hazard throughout the period. A High Surf Advisory remains in effect until 8 PM Saturday.

Watches, Warnings, Advisories

Ca, High Surf Advisory until 8 PM PST Saturday for Orange County Coastal Areas-San Diego County Coastal Areas.

PZ, Gale Watch from Friday afternoon through Saturday evening for Coastal Waters from San Mateo Point to the Mexican Border and out to 30 nm-Waters from San Mateo point to the Mexican Border Extending 30 to 60 nm out including San Clemente Island.

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