Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

505 am PST Sat Jan 28 2023

Synopsis

27/304 PM.

Much cooler temperatures are expected over the weekend as an area of low pressure begins to move over the area, especially starting late Sunday. There is a chance of light rain and mountain snow Sunday and Monday, with low snow levels. Dry and cool weather is expected Tuesday through Thursday of next week.

Short Term

(tdy-Mon), 28/448 AM.

Low clouds were widespread on the Central Coast, and the Santa Ynez and Salinas Valleys early this morning. Areas of low clouds have also developed across coastal sections of L.A. County, and will likely spread into portions of the San Gabriel and San Fernando Valleys this morning. It even appears that some low clouds could push into coastal sections of Ventura County and possibly even the south coast of SBA County this morning.

Skies should be mostly sunny to partly cloudy after any low clouds dissipate later this morning, though the high res WRF is actually showing clouds returning to the Central Coast by late afternoon. Gusty NW winds across southwestern SBA County were below advisory levels and will continue to diminish this morning. With lowering heights and thicknesses, a return to onshore flow and an increased marine influence, expect a fairly decent amount of cooling today, especially west of the mtns, where max temps will likely be 5 to 10 degrees lower than they were on Friday.

A fairly vigorous short wave trough will dig southward through the Pac NW this afternoon, then into far northern CA and Nevada tonight. Increasingly cyclonic flow and falling height will cause the marine layer to deepen significantly, and expect widespread low clouds in all coastal and valley areas by late tonight, likely even reaching the coastal slopes. With the rapidly deepening marine layer and increasingly cyclonic flow, some drizzle or light rain may develop late tonight or Sun morning, especially in the foothills, particularly in L.A. County.

The upper trough will pinch off into a closed upper low near San Francisco Sunday afternoon according to the operational runs of the GFS and the EC, while the WRF keeps it a positively-tiled open trough. Sharp height falls will help to promote decent lift through the deepening moist layer across the region on Sunday, and there will be an increasing chance of rain in the afternoon. Gusty southwest to west winds due to increasing onshore gradients may be close to advisory levels in the Antelope Valley and through the Highway 14 Corridor Sunday afternoon. Max temps will take another large drop on Sunday, and will likely be 8 to 12 degrees below normal in most areas. Snow levels will likely drop to between 4000 and 4500 feet late in the day.

The upper low will drop southward Sun night, reaching a position just west of Point Conception by daybreak Monday, then depending upon which model is correct, it will continue to drop southward during the day Monday (according to the operational run of the GFS), or it will move southeastward into the CA bight (according to the operational run of the EC). The WRF, which keeps is an open trough shows the trough axis moving across the region late Mon morning.

The GFS and the EC, both of which show some over-water trajectory, give the system a chance to pick up a limited amount of moisture from the Pacific late Sun night and Mon morning, and hence more in the way of precip across the region. It certainly does not look like a major precipitation producer by any means, but the GFS and EC show more widespread rain and higher totals than does the WRF. Given that almost all ensemble members (GFS and EC) show measurable rain in Downtown Los Angeles, later shifts will almost certainly have to bump precip chances up into the likely category in most areas for Sun night/Mon morning.

Snow levels will likely drop to between 2500 and 3500 feet late Sun night and Mon morning, which would bring travel problems to Interstate 5 and Highway 14 in L.A. County, and to Highway 33 in Ventura County. Snow will likely fall in the foothills surrounding the Antelope and Cuyama Valleys, and possibly to the valley floor. Snow could also affect the higher peaks of the Santa Ynez Range.

On average, expect one tenth to a third of an inch of precipitation across the region, except one third of an inch to three quarters of an inch across the eastern San Gabriel mtns. There could be 1 to 3 inches of snow above 3500 feet, except 4 to 8 inches across the San Gabriel mtns. Winter Weather Advisories will likely be needed for portions of the region. Showers will decrease in coverage from NW to SE across the region Mon afternoon, with showers possibly lingering across southern and easter L.A. County Mon evening. Monday will be a very cold day across the entire, with even the "warmest" locations likely having highs in the 50s.

Long Term

(Tue-Fri), 28/501 AM.

Clouds will decrease Monday night, and in most areas it will clear early enough to make for a very cold night, with frosts or freezes likely in the valleys and even some interior coastal zones. Low level gradients will turn offshore, so there might be enough wind in locations that typically get NE winds to keep temps up there. It will be another very chilly day on Tue.

A weak ridge will build into the region Wed and persist into Thu. Rising heights and thicknesses, along with weak offshore flow will allow for some warming each day, though max temps should remain below normal, and it will be cold Tue night and Wed night. Skies will be clear through Thu morning, but high clouds will likely increase during Thu as the ridge axis moves east of the region.

A trough moving through the Eastern Pacific will flatten out as it moves into the West Coast Thu night or Fri. This could bring some light rain to northern areas by late Fri, but most ensemble members keep it dry region-wide.

Aviation

28/1216z.

At 1030Z, there was no marine inversion at KLAX, but there was a shallow moist layer up to 1500 feet.

Overall, low to moderate confidence in 12Z TAF package. North of Point Conception, high confidence in LIFR/IFR conditions this morning, but moderate confidence in timing of dissipation (could be +/- 2 hours of current forecast). South of Point Conception, low confidence this morning for coastal/valley TAF sites with a 20-40% chance of /MVFR/IFR CIGs developing at KSBA, KOXR, KCMA, KBUR and KVNY and 70-90% chance of MVFR CIGs at KLAX, KSMo and KLGB. High confidence in CAVU conditions for deserts.

For this evening and overnight, high confidence in return of MVFR CIGs for all coastal/valley sites, but only moderate confidence in timing (could be +/- 3 hours of current forecasts).

KLAX, Moderate confidence in 12Z TAF. Dissipation of current MVFR CIGs may be +/- 2 hours of current 17Z forecast. For this evening and overnight, high confidence in return of MVFR CIGs, but only moderate confidence in timing of return (could be +/- 3 hours of current 02Z forecast). No significant easterly wind component is expected.

KBUR, Moderate confidence in 12Z TAF. There is a 30-40% chance of IFR/MVFR CIGs 13Z-17Z. For this evening and overnight, high confidence in return of MVFR CIGs, but only moderate confidence in timing of return (could be +/- 3 hours of current 06Z forecast).

Marine

28/416 AM.

For the Outer Waters, moderate to high confidence in current forecast. Today through tonight, high confidence in Small Craft Advisory (SCA) level winds. For Sunday and Sunday night, there is a 50-70% chance of the SCA level winds continuing. For Monday through Wednesday, high confidence in winds and seas remaining below SCA levels.

For the Inner Waters north of Point Sal, high confidence in current forecast. Today through Wednesday, winds and seas are expected to remain below SCA levels.

For the Inner Waters south of Point Conception, high confidence in current forecast. Today through Wednesday, winds and seas are expected to remain below SCA levels. For this morning, an eddy circulation will produce southeasterly winds of 5 to 10 kt with isolated gusts to 15 kt possible.

Watches, Warnings, Advisories

Ca, NONE. PZ, Small Craft Advisory in effect until 6 AM PST Sunday for zones 670-673. (See LAXMWWLOX). Small Craft Advisory in effect until 9 PM PST this evening for zone 676. (See LAXMWWLOX).

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