Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

605 am PDT Sat Jun 10 2023


10/444 AM.

A low pressure system moving over the region will bring a cool, showery, and unsettled weather pattern through the weekend and into Monday. An unstable air mass with the low pressure system could bring isolated thunderstorms to the region through Sunday or Monday, especially during the afternoon and evening hours over the mountains and across the interior portions. Onshore flow should remain intact for much of next week and keep night through morning low clouds and fog in the forecast.

Short Term

(tdy-Mon), 10/444 AM.

The latest water vapor imagery shows an upper-level trough of low pressure off the Central Coast of California early this morning. A chaotic cloud pattern exists this morning across the region as a lot of middle and high level clouds are streaming over the region with the a low-level stratus deck beneath. The area is entering the cyclonic flow portion of the cut off low pressure system. Some shower activity can be seen on the latest radar mosaic moving north and east over the coastal waters towards southwest California.

A cool period with cloudy, showery, and unsettled weather will commence today and through the weekend. The latest model solutions and forecast ensemble members are continuing to lean wetter with the forecast and the forecast increases PoPs over the previous one. PoPs already lean wetter than NBM values and toward high- resolution multi-model ensemble members. EPS, GEFS, CMC ensemble members all indicate a vast majority of solutions indicating light rainfall developing across the region between today and Monday. Mentions of light rain or drizzle remain for this morning, then the air mass will start to destabilize this afternoon.

The latest EPS members shows precipitable water value means increasing to just over 1 inch at KLAX and close 1.2 inches at KVBG later today. These values range between 150 and 225 percent of normal for this time of year, or two to three standard deviations above the mean. 850 mb mixing ratios hone in above 8 g/kg across portions of the area, or between three and four standard deviation above the mean. With 850 mb mixing ratios exceeding 8 g/kg, pattern recognition would suggest more than ample moisture to work with for the possibility of showers or rain. The moisture looks to hang around and remain in place through the weekend and plenty of opportunity to interact with the instability with cutoff trough.

The forecast has mentions of showers and a general blanket of isolated thunderstorms from this afternoon through the weekend, but there are some periods where the rain could turn more stratiform, especially across the Los Angeles Basin tonight and into Sunday morning. There is plenty of instability to work with as a series of vorticity maximums will move over the region through the weekend in the trough circulation. NAM-WRF solutions have a large swath of negative lifted index values, K-index values greater than 30 degrees, MUCAPES greater than 600 J/kg over the mountains, and across the interior and northern portions of the area through this afternoon. The focus will shift to the southern portions tonight into Sunday. CMC ensemble members continue to hold tight to CAPE value means nearing 1200 J/kg at KSDB on Sunday, but it will certainly be higher over the mountain ranges. Enough instability is now in the forecast period to possibly continue some isolated thunderstorms into Monday.

While the steering flow is a bit stronger today, the flow lightens on Sunday. The Ventura and Santa Barbara County mountains seems to be the focus over the coming days where the highest PoPs are placed, but the San Gabriel range could see some activity starting late tonight and into Sunday. NCEP high-resolution FV3 solutions suggest the possibility of heavy rainfall developing over the Ventura County mountains on Sunday with two frames indicating 2+ inches of precipitation near Lockwood Valley. While this model is in an outlier and could be too heavy in terms of QPF, the model is picking up on the potential for heavy rainfall on Sunday afternoon and evening. High-resolution multi-model ensemble solutions give a 10-15 percent probability of rainfall rates greater than 1 inch per hour over the Ventura County mountains on Sunday and a 5-10 percent chance of greater than 1 inch per hour over the backcountry of Santa Barbara County today. Confidence is low in these solutions as these solutions may encapsulate convective schemes improperly. Given the impacts, there is a low-to-moderate risk for showers and thunderstorms producing flash flooding over the next couple of days. The messaging remains the same. For anyone planning outdoor activities this weekend, when thunder roars, go indoors. Gusty erratic winds, brief heavy downpours, small hail, and dangerous lightning are possible with any of storms the next couple of days.

Long Term

(Tue-Fri), 10/444 AM.

The trough should exit the region between Monday and Tuesday, but onshore flow should remain in place over the coming days as broader troughing reinforces along the West Coast. While the forecast keeps a mention of warming and better clearing each day, it should noted that strong onshore flow will likely remain in place through the week. NAEFS solutions suggest KLAX-KDAG and SMX-KBFL surface pressure gradients remaining strongly onshore into Thursday.

Another weak cutoff trough will approach the area on Wednesday. If NAEFS and ENS standardized anomalies are correct, this trough has the potential to have more moisture with it. Precipitable water and mixing ratios anomalies approach four standard deviation above average for this time of year. The only difference is the trough appears to be more compact and not as deep. Height and temperatures anomalies indicate a system not as cold. PoPs above NBM values remain in the forecast over the mountains for Wednesday afternoon and evening.

There are some hints of weakening onshore flow for the third week of June. NAEFS surface pressure gradients dive toward the end of the period with GFS deterministic runs introducing offshore flow during that week.



At 12Z, the marine layer was about 2000 feet deep with a top around 4000 feet and temp of 13C. However, there were a couple of different moist layers above to 15000 feet.

It was generally clouds across the region this morning. Most areas had MVFR cigs, but there were local IFR cigs, and widespread IFR to VLIFR cigs in the foothills and mtns. Skies should stay mostly cloudy today, but cigs may lift into the VFR category in some areas. The exception will be in the mtns and foothills, where IFR conds or lower will likely persist. There will be a chance of showers especially later today. There is a slight chance of thunderstorms, mainly this afternoon in northern areas and all sections tonight. KLAX, Lowish to moderate confidence in the 12Z TAF. Cigs will likely vary between BKN-OVC015-035 through the period. There is a 30 percent chc that conds will remain MVFR or lower thru the entire period. There is a 30% chance of showers later today into tonight with a 10-20% chance of a thunderstorm near the airport tonight. No significant easterly wind component is expected.

KBUR, Lowish to moderate confidence in the 12Z TAF. Cigs will likely vary between BKN-OVC015-035 through the period. There is a 30 percent chc that conds will remain MVFR or lower thru the entire period. There is a 30% chance of showers later today into tonight with a 10-20% chance of a thunderstorm near the airport tonight.


10/604 AM.

For the coastal waters, generally high confidence in the forecast the winds and seas will remain mostly remain below Small Craft Advisory (SCA) levels thru Wed. However, there is a 30% chance of SCA level winds across the outer waters this afternoon/evening, and a 20% chance of SCA level winds this afternoon/evening north of Pt Sal, in western portions of the SBA Channel and near Santa Cruz Island.

There is a slight chance of thunderstorms across the coastal waters thru Sun. Any thunderstorms that develop may produce brief heavy rain, gusty winds, small hail and dangerous cloud to surface lightning. Also, waterspouts are possible.

Watches, Warnings, Advisories


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