Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

1018 pm PDT Thu Sep 21 2023


21/1017 PM.

Night through morning low clouds will continue into early next week but they will be less extensive. There will be a slight warming trend over the weekend with some valleys reaching the lower 80s by Sunday and Monday. Gusty northwest winds may impact the coastal waters and nearby Central Coast at times next week with a return to below normal temperatures expected.

Short Term

(Thu-Sun), 21/1016 PM.


Clouds covered much of the area this morning, as the marine layer remains very deep, around 5000 feet. High temperatures trended 3 to 10 degrees colder over the interior, and the area still remains an impressive 12 to 27 degrees below normal. This should begin to change as heights rise and an upper level low moves in from the southwest. This low may bring another chance of drizzle to the area, but lower confidence so waiting to add it to the forecast for Friday.

Tonight, clouds are slow to redevelop over the area, perhaps due to the cold air advection up north. Clouds mainly remain trapped against the south facing slopes, but these areas appear to be eroding over time. Therefore, will be quite the challenging forecast overnight. Still expecting clouds to come into the coasts and valleys, but likely won't be as widespread as last night.

Southwest winds over the Antelope Valley and foothills grew gusty today, in the 25 to 35 mph range with gusts peaking around 55 mph. Winds have decreased tonight, so letting the wind advisory expire. However, some local gusts to 50 mph are possible over the next few hours. Expecting these winds to become light overnight. For the next few days, not seeing much in the way of winds aside from the typical breezy conditions for the Antelope Valley each afternoon and perhaps some 20-30 mph north winds for western portions of the Santa Ynez range.

***From Previous Discussion***

Cold upper level trough digging southward across Nevada and eastern California today. This resulted in a deep marine layer pattern to around 4500 feet, along with strengthening onshore flow(as of 130 pm, the LAX-Daggett pressure gradient has increased to +6.9 mb). This combination generated some drizzle and areas of light rain last night into this morning, mainly on south facing slopes and foothills. As cooler air filters into the region this afternoon, the marine inversion has weakened, resulting in some breaks of low cloud coverage.

With the passage of the upper level trough to our north, strengthening onshore flow, and deep marine layer pattern, we have seen stronger onshore winds across the interior today, especially the Antelope Valley and adjacent foothills where a wind advisory is in effect through 9 pm this evening. Wind gusts of 45 mph will be common across the Antelope Valley floor, with gusts up to 55 mph in the foothills. The main story today will be the cool temperatures and persistent clouds due to onshore flow and a trough axis moving through southwest CA. Max temps throughout most areas will only be in the mid 60s to lower 70s. These temperatures are 10 to 20 degrees below normal and are more typical of a day in early December. Smoke has been reported in our northern counties where air quality is rather poor (PM2.5 values near 150 at times) and appears to be originating from northern CA fire areas and caught up in northwest flow on the back side of the trough.

HREF model data supports the idea of increasing high clouds on Friday, due to moisture from minimal strength Tropical Storm Kenneth currently located nearly 1000nm southwest of Baja. Model cross sections showing the thickest high cloud coverage across LA/Ventura counties on Friday afternoon and evening. With some lift being noted at the upper levels, could even see some sprinkles across LA County during this time, with a 5-10 percent chance of measurable light rain.

The low clouds are expected to be slightly less extensive Friday and will potentially clear sooner due to weaker onshore flow. Some warming is expected across most areas on Friday with warmest inland areas approaching 80 degrees. A gradual warming trend will continue across inland areas through the weekend, with the marine layer depth expected to lower gradually through the period.

Long Term

(Mon-Thu), 21/227 PM.

Sunday and next Monday will be the warmest days within the next 7 days as heights build slightly and the inversion become more shallow. A large low pressure system moves across the northern Pacific toward the Pac Northwest during this time frame with a rather significant cold front entering western Oregon and northwest CA on Monday. No precipitation is expected to survive the trip this far south next week, however, this storm system is likely to drop temperatures back down for the remainder of the week and bring bouts of gusty northwest to northerly winds to the area. The NBM and GEFS data suggest gusty northwest winds over the coastal waters by Tuesday and this will likely kick in our evening and nighttime Sundowners for the south Santa Barbara County mountain range and canyon areas. Then EPS data shows the potential for some type of offshore flow by mid-week with a few ensemble members reaching 40-50 mph wind gusts through the typical Santa Ana corridors. However, the bulk of the ensembles and the overall averages do not indicate much of a risk of this occurring next week. The bottom-line will be below normal temperatures will persist through the last week of September with little significant weather impacts overall.



At 0024Z, the marine layer depth was 4300 ft, with an inversion top at 6100 feet with a max temperature of 16 C.

Moderate confidence in the TAF package. Cigs may be off by one category through the period due to the layered and variable cloud deck that may scatter out and reform frequently. There is a 30% chance of MVFR cigs remaining through the day. Additionally the potential return of cigs tonight may be off by +/- 2 hours.

KLAX, Cigs may be off by one category through the period due to the layered and variable cloud deck that may scatter out and reform frequently. High confidence that any east wind component will be under 5 kts.

KBUR, Cigs may be off by one category through the period, and return of cigs tonight may be off by +/- 2 hours.


21/832 PM.

Moderate confidence in current forecast. Extended the current Small Craft Advisory (SCA) for the Outer Waters north of the Channel Islands through late tonight due to SCA level seas around 10 feet.

Local SCA level winds with choppy seas are possible (20% chance) in the afternoons and evenings today and Friday from Point Arguello south to San Nicolas Island and into the western portion of the Santa Barbara.

Then there is a 50-60% percent chance of SCA levels winds between Saturday and Sunday for the waters southwest through northwest of the Channel Islands and into the western portion of the Santa Barbara Channel. There is a moderate-to-high chance of SCA seas and winds Tuesday onward, including a 20% chance of Gale Force winds for the outer waters.

Watches, Warnings, Advisories

Ca, NONE. PZ, Small Craft Advisory in effect until 3 AM PDT Friday for zones 670-673. (See LAXMWWLOX).

Visit this site often? Consider supporting us with a $10 contribution.
Learn more