Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

943 pm PDT Sun aug 1 2021

Synopsis

01/827 PM.

Expect fair weather through the period, with high temperatures five to ten degrees above normal through midweek. Many coastal areas will have morning low clouds and dense fog. Gusty afternoon and evening winds and lower humidities will elevate fire risk.

Short Term

(Sun-Wed), 01/941 PM.

The latest satellite imagery shows an upper-level ridge centered near the California-Mexico border. Stratus clouds continue to be noticeably absent across the area this evening, and low clouds and fog are likely to be minimal tonight and into Monday morning. The best coverage, if any, will be along the Central Coast. Warmer summertime ocean temperatures and a thin marine layer depth will continue to make stratus formation difficult south of Point Conception.

500 mb heights will climb over the next couple of days and 950 mb temperatures warm. A warming trend will develop through Tuesday or Wednesday. Dangerously hot conditions cannot be ruled out at this time as temperatures warm to between 10 and 15 degrees above normal away from the coast. A heat advisory or excessive heat watch could be needed for a few areas, especially across the valley, mountain, and desert locations of Los Angeles County, and the interior portions for the remainder of the area.

An update will be issued shortly to tweak high temperatures over the next several days and add partly cloudy conditions for the coastal areas for late tonight and into Monday morning.

***From Previous Discussion***

Forecast-wise, main issue will be the winds, more specifically Sundowner winds across southern Santa Barbara county. High resolution models indicate increasing northerly offshore surface gradients each evening. Additionally, the models indicate a little increase in upper level wind support each evening. So for tonight, will expect wind to generally remain below advisory levels with only local gusts 40-45 mph in usually windy spots (Refugio, Gaviota, etc). However for Monday night and Tuesday night, wind speeds look to be a bit stronger and more widespread with a good chance of advisory-level winds. Based on the gradients, the strongest winds will remain from Gaviota westward.

Otherwise, the forecast for the short term will be rather typical for early August. The moderate onshore gradients will generate gusty southwesterly winds each afternoon/evening across interior sections. With the upper high building overhead, the marine inversion will remain quite shallow and any stratus/fog should remain confined to the coastal plain each night with a bit more widespread coverage each night. Given the shallow nature of the inversion, some of the fog will be dense. Other than the stratus and fog issues, skies should remain clear through the period.

As for temperatures, coastal and coastal valley areas can expect Monday to be the warmest day with some slight cooling Tuesday and Wednesday (due to bit more marine influence). For interior sections, expect a slight warming trend Monday/Tuesday then some slight cooling on Wednesday.

Long Term

(Thu-Sun), 01/155 PM.

For the extended, 12Z models continue to be on the same synoptic page. At upper levels, high will remain over the area Thursday and Friday, but will be pushed eastward Saturday and Sunday as a broad trough develops over the West Coast. Near the surface, moderate to locally strong onshore gradients will prevail.

Forecast-wise, no major issues through the extended period. Given the upper level flow pattern, there is no concern about any monsoon moisture or convective precipitation. So, the main issues will be the marine layer stratus and temperatures. Inversion should remain quite shallow through Friday with stratus/fog confined to the coastal plain. For Saturday/Sunday, the inversion may deepen a bit and stratus/fog could push into some of the lower coastal valleys. Other than the stratus/fog, skies should remain mostly clear through the period. As for temperatures, expect a gradual cooling trend for all areas as the upper level weakens and is replaced by the trough.

Aviation

02/0200z.

At 23Z, the marine layer depth was near 500 feet deep at KLAX. The top of the inversion was at 2200 feet with a temperature of 28 degrees Celsius.

High confidence in the current forecast for terminals away from the coast. Moderate confidence in the current forecast for coastal terminals.

For coastal terminals, less confidence in flight categories, higher confidence in timing. There is a moderate (30-50 percent) chance of VLIFR to LIFR conditions between 08Z and 16Z.

KLAX, VFR conditions will likely persist through the period. There is a 40 percent chance of LIFR conditions between 10Z and 16Z. No wind impacts are expected at this time.

KBUR, VFR conditions are expected through the period. No wind impacts are expected at this time.

Marine

01/943 PM.

High confidence in very short period and choppy seas continuing over most of the coastal waters through most the week. Heights will be fairly small through today, then build some through the week. A noticeable long period south swell will continue to build in through Monday.

Moderate-to-high confidence in low-end Small Craft Advisory (SCA) wind gusts around 25 knots late Monday night from Point Conception to San Nicolas Island. There is a 40 percent chance for SCA level gusts across the western portion of the Santa Barbara Channel this evening, increasing to 50 percent Monday evening, and a 30 percent chance for the nearshore waters off Los Angeles and Orange Counties both evenings. These winds should shift focus more to the Central Coast through San Nicolas Island Tuesday. By Wednesday or Thursday, there is a moderate chance for Gale Conditions. These winds will further amplify the persistent short period seas.

Watches, Warnings, Advisories

Ca, NONE. PZ, Small Craft Advisory in effect until 3 AM PDT Tuesday for zones 673-676. (See LAXMWWLOX).

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