Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

502 pm PDT Wed apr 21 2021

Synopsis

21/500 PM.

A cooler air mass will remain in place through Thursday as strong onshore flow and a deep marine layer bring extensive low clouds to the area. Clouds will struggle to clear from the coast and valleys each day. Drizzle or light rain will be possible at times through Thursday. A slight warming trend is expected late in the week, then a storm system could bring rain and mountain snow to the area later Sunday into Monday.

Short Term

(tdy-Sat), 21/109 PM.

Pretty typical deep marine layer day today with a distinct reverse clearing pattern where the clouds clear around the Channel Islands and at some beaches but it remains pretty cloudy over most of the land. Haven't had any reports of drizzle or light rain this afternoon and will keep the forecast dry through the evening. However, a second upper low is rapidly approaching from the north and will bring additional cold air aloft tonight into Thursday. The low is actually tracking a little farther west than earlier models had suggested, and the combination of that with cooling aloft which will further weaken the inversion may actually result in less clouds and lower precip chances Thursday morning. Will leave the forecast as is for now with low pops and pretty solid cloud coverage but there's definitely a scenario where it's less gloomy than forecast.

Otherwise, the forecast looks on track. Regardless of cloud cover temps will be several degrees below normal again Thursday with breezy conditions in the interior due to a strong 8-9mb LAX-DAG gradient.

Friday and Saturday a weak upper ridge develops between weather systems. While this sounds optimistic, the reality is that we'll still have a strong onshore flow in place pulling in that cold ocean air, and the warming aloft from the ridge will actually help solidify the marine layer (though at a lower depth) and we could be looking at low clouds lingering at the beaches much of the day. Inland areas should still warm up at least a few degrees but minimal warming expected closer to the coast.

Long Term

(Sun-Wed), 21/123 PM.

A late season trough will hit the West Coast Sunday into Monday. The GFS appears to be the one blinking first this time as it has now significantly shifted towards a wetter scenario, though still not all in like the ECMWF continues to be. In any case, the trends and ensembles certainly favor at least some light rain across southern California during this time and the latest forecast reflects those trends with pops in excess of 60 percent across most of the area. Amounts likely to be in the quarter to half inch territory with some light snow accumulations in the mountains mainly above 6000'. Though there are the usual timing differences the ensemble means have remained pretty consistent with some light rain developing across northwest SLO County by noon Sunday, then spreading south into Ventura/LA Counties Sunday night into Monday morning. Some of the ECMWF solutions slow down the forward movement as some additional energy drops into the trough, which likely would help sustain a longer period of precip, but most of the solutions push the precip out by noon Monday.

Dry weather to follow the rest of the week with a decent warming trend by around mid week as a high pressure ridge sets up.

Aviation

21/2358z.

At 2300Z, the marine inversion at KLAX was based around 2400 feet. The top of the inversion was near 6200 feet with a temperature of 6 degrees Celsius.

Overall, moderate confidence continues for coastal and valley TAFs, with high confidence in desert TAFs. A deep marine layer and strong onshore flow will bring MVFR cigs to sites north of Point Conception, with MVFR to VFR cigs further south. The onset time of cigs may vary +/- 2 hours from TAF times. For Thursday afternoon, sites are not expected to clear with BKN VFR cigs forecast, but there is a 30% chance that clearing will occur.

KLAX, Moderate to high confidence in the 00Z TAF. For this evening, high confidence in return of MVFR CIGs, but timing could differ by +/- 2 hrs from TAF time of 04Z. There is a 40% chance of clearing after 20Z Thursday, otherwise BKN VFR cigs expected through 03Z.

KBUR, Moderate to high confidence in the 00Z TAF. High MVFR CIGs will transition to VFR but lower confidence in timing. 30% chance that cigs will clear Thu afternoon.

Marine

21/143 PM.

For the Outer Waters, moderate to high confidence in current forecast. High confidence in Small Craft Advisory (SCA) level winds continuing through this evening with winds and seas remaining below SCA levels Thursday and Friday. On Saturday, there is a 40% chance of SCA level winds across PZZ673/676. On Sunday, high confidence in winds and seas remaining below SCA levels for all the Outer Waters.

For the Inner Waters north of Point Sal, high confidence in current forecast. Through Sunday, winds and seas are expected to remain below SCA levels.

For the Inner Waters south of Point Conception, high confidence in current forecast. Through Sunday, winds and seas are generally expected to remain below SCA levels. The only exception will be across the western half of the Santa Barbara Channel where there is a 20% chance of SCA level winds through this evening. Also, east to southeast winds with gusts to 15 kt are expected this morning.

Beaches

21/142 PM.

The first south swell of the Spring season has an expected arrival time of Friday and will continue through the weekend. The swell heights will be in the neighborhood of 3 to 4 feet, and along with a period of around 19 seconds should produce breakers of 3 to 6 feet, mainly on south facing beaches. There is a 30 percent chance of breakers reaching the minimum criteria of a High Surf Advisory (7 feet), but for now a Beach Hazards statement seems to be the most likely result.

Watches, Warnings, Advisories

Ca, Wind Advisory in effect until 10 PM PDT this evening for zone 59. (See LAXNPWLOX). PZ, Small Craft Advisory in effect until 9 PM PDT this evening for zones 670-673-676. (See LAXMWWLOX).

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