Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

1227 pm PDT Sat aug 19 2017

Synopsis

Expect normal to below normal temperatures and persistent coastal marine layer for the period due to a nearly stationary low pressure center offshore. There is an outside chance of showers and thunderstorms through early next week for the mountains and desert.

Short Term

(tdy-Mon)

The morning clouds made it further into the San Fernando and San Gabriel Valley areas than was expected, but are quickly retreating to the coast. Clouds are thickly along the coast from Ventura through SLO County and will be less quick to clear in those areas. Noted that SPC has a swath of LA, VTU and SBA county in a risk of thunder today and the RFC has even but some precipitation (0.01") in their forecast along the Ventura and Santa Barbara County border in the mountainous area. That all matches up well with out area with a slight chance of precipitation or thunderstorms this afternoon. Looking for clouds to start forming a little after noon.

*** From Previous Discussion ***

A weak trough of low pressure continues to spin counterclockwise over the Southern California Bight. The trough, centered between the Channel Islands and San Nicolas Island will wobble around the bight into midweek next week. The latest fog product shows a better entrenched marine layer stratus deck starting to develop this morning as the trough deepens the marine layer and strengthens onshore flow. Onshore pressure gradients are already showing close to a millibar more onshore than this time yesterday. A cooling trend will develop through the weekend, with coastal locations hovering near persistence for much of early next week.

With increasing onshore flow with the trough, the marine layer depth will deepen over the next couple of days. The marine depth currently around 1500 feet should deepen to between 2000-2500 feet by Sunday morning, if NAM-WRF solutions are correct. Possible additional deepening could take place on Monday. With a veering flow with height, some additional deepening could occur, lifting the marine layer depth to closer to 2500-3500 feet tonight and again Monday. With the favorable flow pattern through the mixed layer and some weak instability aloft with the trough, the marine layer stratus could get lifted enough to squeeze out some drizzle or light rain showers. PoPs were nudged up to near 10 percent to account for the possibility of drizzle and mentions of drizzle was added to the forecast.

Model solutions continue hint at the possibility of isolated to scattered showers and thunderstorms each afternoon and evening through Monday, and maybe into Tuesday. PoPs have been left untouched with regards to the convective activity. Favorable negative lifted index and positive CAPE values exist over the Mountains the next several days. The steering flow is quite light today and any storms that develop could be slow-moving. As a result, local flooding could develop in or near showers and thunderstorms this afternoon and evening.

Long Term

(Tue-Fri)

The trough will linger over the region into Wednesday, likely keeping an extended period of below normal temperatures, strong onshore flow in place, and a deep marine layer over the area. A threat for night through morning drizzle, and mountain and desert showers and thunderstorms could linger into Wednesday, especially with moisture and instability moving over the region into midday Wednesday.

Drier southwest flow should develop for late next week, allowing for some warming to take shape but keeping the coast and lower valleys in a more persistent onshore flow pattern. Warming trend continues away from the coast for late next week.

Aviation

19/1925z,

At 1915Z, the marine inversion was based at 2000 feet. The top of the inversion was 5000 feet with a temperature of 21 degrees Celsius.

Overall, moderate confidence in 18Z TAF package. For coastal and valley sites, the marine inversion should deepen with mostly MVFR CIGs arriving this evening and clearing slightly later Sunday.

KLAX, Moderate confidence in 18Z TAF. A return of MVFR CIGs is expected this evening and will continue through late morning Sunday.

KBUR, Moderate confidence in 18Z TAF. A return of IFR CIGs is expected around 07Z, rising to MVFR CIGS by Sunday morning.

Marine

19/900 AM,

For the Outer Waters, high confidence in current forecast. Winds and seas are expected to remain below Small Craft Advisory (SCA) levels through Tuesday. On Wednesday, there is a 50% chance of winds increasing to SCA levels.

For the Inner Waters, high confidence in current forecast. Winds and seas are expected to remain below SCA levels through Wednesday, though local gusts to 20 kt are possible through the San Pedro Channel this afternoon and evening.

Watches, Warnings, Advisories

Ca, Beach Hazards Statement in effect from 5 PM PDT this afternoon through Sunday evening for zones 40-41. (See LAXCFWLOX). PZ, NONE.

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