Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

451 am PDT Mon Oct 25 2021


25/224 AM.

A cold front moving across the area will bring gusty winds and widespread moderate to heavy rain today. Warming and drying is expected each day Tuesday through Friday with breezy offshore winds. Dry but cooler conditions can be expected next weekend.

Short Term

(tdy-Wed), 25/310 AM.

The storm is moving slower than forecast, which is good news as it means more rain and snow will fall over the Sierra Nevadas. Over the last 3 hour a strong jet streak moved into the base of the main trof and initiated a shift in the Atmospheric River into a more N/S orientation. This will turn the system into more of a classic cold front. As the front turns more the N/S the jet align more perpendicular to the front and will speed it up to the east. Steady moderate to heavy rain has set up over the NW corner of SLO county. This kind of flow pattern always brings a tremendous amount of rain to this area. Warm frontal rain will spread over the rest of the Central Coast, the SBA south coast and VTA county in the predawn hours.

Right now it looks like peak rainfall will move across the Central Coast around 700am, The SBA south coast around 900 am, Ventura County just before noon and LA county shortly after noon. Would not be surprised if the system slows down a little more than this fcst. There is quite a bit of NVA behind the main front and trof which will make for a fairly quick clearing pattern from the NW to the SE. Some showers will linger in the evening esp the mtns but most areas will be dry by sunset and all areas will be rain free by midnight.

The Flash Flood Watch for the Alisal burn scar continues. Any debris flows that occur could bring significant road closures around the burn scar. The homes in and around the area are not totally safe but the threat to structures is on the low side. Snow levels remain above 8000 feet for the vast majority of the precipitation, so snow will not be a major story.

Rainfall estimates for this storm system from tonight thru Mon evening is for about 1.00-3.00 inches across SLO/SBA Counties, except up to 7.00 inches for NW SLO County and locally up to 4.00 inches in the Santa Ynez Mtns. For VTU/L.A. Counties, 0.50-1.50 inches of rainfall is expected with local higher amounts over 2.00 inches across the coastal foothills. The highest rainfall totals will be along south and southwest facing slopes, such as the Santa Lucia Range and Santa Ynez Mtns. Rainfall intensities of 0.25 to 0.50 inch per hour are likely, with local intensities up to 0.75 inch per hour. The one exception is over the Santa Ynez Mtns where rates of 0.30 to 0.60 inch per hour are expected with the potential for up to 1.00 inch per hour.

Wind Advisories are in effect for much of the area. The winds in the advisory areas will be 10 to 15 mph stronger than a typical low end advisory. Please see the product LAXNPWLOX for all of the details. Even for those areas not in a wind advisory, it will be a breezy day.

Temperatures will come in with the typical storm values in the upper 50s and 60s across the coasts and vlys or about 15 degrees blo normals.

A very quick change in the weather on Tuesday as a ridge builds into the area. SFC high pressure will rapidly build into NV and offshore flow will quickly develop by sunrise. There will be about 3 MB of offshore flow from the north which is enough to bring some northerly canyon winds to LA county in the morning. The combination of rising hgts, mostly sunny skies and offshore flow will bring 4 to 8 degrees of warming to almost all of the area. Still max temps will be well blo normal.

Upper level ridging will continue to build on Wednesday and the offshore flow will peak. Offshore flow from the east will be near 4 mb and from the north and even stronger 6.5 mb. There should be a good north push of winds through the I-5 corridor and the SBA south coast as well as the Santa Lucias. The wind gusts should approach advisory levels. 584 dam hgts and the offshore flow will bring a hefty 8 to 12 degrees of warming to almost all areas. This will bring max temps up to near normals with quite a few readings in the low to mid 80s across the vlys.

Long Term

(Thu-Sun), 25/322 AM.

Thursday will be the warmest day of the next 7 for the coasts and vlys. Offshore flow will still be fairly strong but likely a little weaker than on Wednesday. The ridge will peak and hgts will rise to 586 dam. Max temps will respond with another healthy jump of 3 to 6 degrees. This warming will bring max temps up to 6 to 12 degrees above normal.

The offshore flow will turn onshore on Friday and this will bring 3 to 6 degrees of warming to the csts/vlys. It will be the warmest day of the next 7 for the interior where another 1 to 3 degrees of warming is in store.

Weak troffing and stronger onshore flow will come about for the weekend. Coastal marine layer clouds will likely develop. Skies, otherwise, will be partly cloudy as some mid level clouds move overhead. Max temps will fall 3 to 6 degrees each day.



At 1000Z at KLAX, there was no marine layer nor inversion.

Low confidence in all 12Z TAFs due to cold frontal passage. Flight cat changes could be off by +/- 3 hours. Cigs and especially vis will vary frequently until 22Z-00Z. Predominant cigs will be IFR and low MVFR and vis will likely stay above 1SM. Better confidence after 00Z.

KLAX, Moderate confidence in the 12Z TAF through 14Z, then low confidence. Timing of flight cat changes could differ by +/- 3 hours. Cigs will vary frequently between BKN005 and BKN025, while vis will vary between 1-3SM through 00Z. Good confidence in an 8-10 kt east wind component from 16Z-21Z.

KBUR, Moderate confidence in the 12Z TAF through 14Z, then low confidence. Timing of flight cat changes could differ by +/- 3 hours. Cigs will vary frequently between BKN005 and BKN025, while vis will vary between 1-3SM through 00Z.


25/324 AM.

Moderate to high confidence on the storm system details. The edge of the storm system is starting to move into San Luis Obispo County, and will sweep through southwest California from early this morning through late afternoon. All areas will experience moderate to heavy rain with southerly winds at least between 20 and 30 knots some time in this period, with areas north of Point Arguello getting Gale Force gusts between 30 and 40 knots including the near shore waters. Thunderstorms are NOT expected. After the storm passes, west to northwest gusts of 20 to 30 knots will form almost everywhere this afternoon and night. These winds will generate steep short period seas on top of a large west to northwest swell pushing in. This swell will peak this afternoon through early Tuesday morning, with total wave heights between 16 and 22 feet northwest of San Nicolas Island, and between 6 and 12 feet for the inner waters from Santa Barbara to Orange County. This swell will create dangerous breaking waves near shore, including over the harbor entrance of Morro Bay and possibly Ventura.

Small Craft Advisory (SCA) northwest winds will return to their more traditional Central Coast to San Nicolas Island position Tuesday, and become more northerly on Wednesday.


25/240 AM.

A High Surf Warning is in effect for the Central Coast for this morning through the evening, with surf to around 20 feet expected. While the surf will lower after that, a High Surf Advisory will likely need to follow through Tuesday night. High Advisories remain in play for the rest of the coastline.

Moderate to high confidence in a very large west to northwest swell peaking at 15 to 20 feet with a 17 second period over the Central Coast, with about half those heights for the inner waters from Santa Barbara to Orange County. The peak of the swell will be this morning through early Tuesday morning. A swell this size will result in high surf and dangerous rip currents over nearly all coasts. Surf heights of 15 to 20 feet are expected across the Central Coast, which could bring local damage to coastal structures like piers. For west facing beaches of Los Angeles and Ventura counties, surf of 6 to 12 feet can be expected, highest near Ventura Harbor.

The entrances of Morro Bay Harbor and Ventura Harbor could be affected by large breaking waves.

Watches, Warnings, Advisories

Ca, High Surf Warning in effect until 9 PM PDT this evening for zones 34-35. (See LAXCFWLOX). Wind Advisory in effect until noon PDT today for zones 34-37-51. (See LAXNPWLOX). Wind Advisory in effect until 2 PM PDT this afternoon for zones 35-36-38-52-53. (See LAXNPWLOX). Flash Flood Watch in effect until noon PDT today for zone 39. (See LAXFFALOX). High Surf Advisory in effect until 3 AM PDT Wednesday for zones 39-40. (See LAXCFWLOX). High Surf Advisory in effect from 11 AM this morning to 3 AM PDT Wednesday for zones 41-87. (See LAXCFWLOX). Wind Advisory in effect from 9 AM this morning to 6 PM PDT this evening for zones 54-59-88. (See LAXNPWLOX). PZ, Gale Warning in effect until 6 AM PDT early this morning for zones 645-670-673. (See LAXMWWLOX). Small Craft Advisory in effect until 5 AM PDT Tuesday for zones 650-655-676. (See LAXMWWLOX).

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