Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

1044 am PST Fri Dec 15 2017

Synopsis

Very dry and warm conditions will persist today. Some cooling is expected this weekend as a trough moves through the region. Northerly winds will return on Saturday, turning into another Santa Ana wind event late Saturday night into Sunday. Gradual warming early next week with weaker offshore winds expected.

Short Term

(tdy-Sun)

Forecast on track today, with similarly warm conditions compared to yesterday and mostly light winds during the day as we quickly turn from offshore flow to light onshore. This light wind regime will allow smoke from the Thomas fire to become more concentrated and spread more into Ventura and possibly west Los Angeles County. Updated the forecast to better reflect that. Northerly winds for tonight into Saturday are still on target to strengthen, including Sundowner winds over Santa Barbara County and the Thomas Fire. These winds should stay just below wind advisory however.

***From Previous Discussion***

Synoptically, An upper level disturbance will ride down the eastern side of the Eastern pac upper ridge through California tonight into Saturday. Earlier models had this shortwave becoming cutoff across the southern SoCal Bight early Saturday morning, but the GFS, ECMWF and the high resolution NAM-WRF now quickly move this digging trough towards San Diego or Baja before it becomes cutoff Saturday morning into So Arizona. Areas to the south will have the best chance of seeing a few showers develop, or off the coastal waters of Orange County and San Diego. But this is still a slight chance. With the height falls, clouds should develop tonight across LA and VTU Counties then clear out by Saturday afternoon.

The main story for Saturday will be the northerly winds developing behind the frontal passage. Sundowner winds are expected to develop by late tonight into Saturday morning. By late Saturday into Sunday the pattern shifts to a Santa Ana. For Ventura/LA Counties north to northeast winds will start up during the day Saturday and increase overnight and peaking Sunday morning with solid advisory level winds and possibly local gusts to 60 mph in the mountains as good upper support and cold air advection will enhance this wind event Sunday. The additional cold air coming in Saturday night and Sunday will more than offset any warming from the downslope flow so highs at lower elevations expected to top out in the lower 70s.

Long Term

(Mon-Thu)

Both the GFS and ECMWF remain in good agreement with large scale features through the Thursday. Offshore winds will be rather weak both Monday and Tuesday with a gradual warming trend. By Wednesday, another upper level trough begins to drop out of the PAC NW and become an inside slider as it moves into the Great Basin on Thursday. Both models then retrograde the upper low towards northern Baja by Thursday night. By Wednesday night into Thursday morning, northerly winds develop affecting the Santa Barbara Mtns and adjacent south coast as well as the I-5 Corridor, then turn offshore into another Santa Ana event. The placement of the upper low over nrn Baja and more cold air advection could bring another strong Santa Ana Wind event. OF course, this scenario is still several days away, and models can flip-flop like your buttermilk pancakes, so stay tuned.

Aviation

15/1755z,

At 1709Z, there was a weak surface-based inversion at KLAX. The top of the inversion was near 1100 ft with a temperature of 22 deg C.

Generally hi confidence in the 18Z TAFs. VFR conditions are expected at most of the airfields thru Sat morning. However, there is a 10-20 percent chance low clouds may move into KLAX or KLGB Sat morning with LIFR/IFR cigs. For KSBA, CMA and OXR, there is a 40-50 percent chance that smoke from the Thomas Fire may reduce vsbys to MVFR at times thru Sat morning. Low level wind shear will be possible at KSBA aft 12Z Sat due to gusty N canyon winds in the vicinity.

KLAX, Generally hi confidence in the 18Z TAF with VFR conditions expected thru Sat afternoon. However, there is a 10-20 percent chance low clouds may move into the airfield Sat morning with LIFR/IFR cigs.

KBUR, Hi confidence in the 18Z TAF with VFR conditions expected thru Sat morning.

Marine

15/830 AM.

Across the outer waters, fairly good confidence that NW winds will increase to SCA levels this evening, then to gale force conditions by late tonight. There is a 30% chance that seas will build to SCA levels as early as this afternoon, at least across the northern two thirds of the outer waters. Gale force winds will continue through late Sat night, then SCA conds will continue for a few hours thereafter. Seas will possibly remain above SCA thresholds for hazardous seas through around noon Sunday.

Across the inner waters north of Point Sal, moderate to high confidence that winds and seas will reach SCA level winds from late tonight and continue through late Saturday night. There is a 20-30% chance that SCA conds will arrive before midnight tonight.

Across the inner Waters south of Point Conception, winds and seas are expected to be below SCA levels through Tue, except for a 40-50% chance of SCA level NE winds late Sat night into Sun morning from Ventura to Anacapa Island south to Santa Monica, and possibly from San Pedro to Catalina Island.

There will be areas of smoke from the Thomas fire over portions of the coastal waters today, mainly from the waters west of Ventura County through the Santa Barbara Channel and over the outer waters. Visibilities could locally drop to 2 NM or lower at times today.

Watches, Warnings, Advisories

Ca, High Surf Advisory in effect until 9 AM PST Sunday for zones 34-35. (See LAXCFWLOX). Fire Weather Watch in effect from late Saturday night through Sunday evening for zones 244>246-253-254-288-547. (See LAXRFWLOX). Red Flag Warning in effect from 2 AM to 10 PM PST Saturday for zone 252. (See LAXRFWLOX). PZ, Small Craft Advisory in effect from 3 AM Saturday to 3 AM PST Sunday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect from 9 PM this evening to 3 AM PST Saturday for zones 670-673-676. (See LAXMWWLOX). Gale Warning in effect from 3 AM Saturday to 3 AM PST Sunday for zones 670-673-676. (See LAXMWWLOX).

Share this forecast!