Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

1131 am PDT Thu Mar 28 2024

Synopsis

28/925 AM.

Light showers north of Pt Conception will end this morning, otherwise partly cloudy across southwest California today with cooler temperatures. An unseasonably cold and vigorous storm system will move into the region late Friday through the weekend with rain and mountain snow along with a slight chance of thunderstorms. A warming and drying trend will develop next week.

Short Term

(tdy-Sat), 28/939 AM.

***UPDATE***

A weak cold front moved through the Central Coast this morning and dropped around a third of an inch of rain in the Cambria area and a tenth or less to the south. That system has moved east now and skies will be clearing through the morning.

Everything still on track for the next system to arrive mid day Friday north of Pt Conception and late afternoon to early evening in the south. Latest model projections still indicating a moderate to strong system coming through, hitting hardest late Friday night through mid day Saturday, followed by showers and possible thunderstorms through Sunday, some of which could be very strong with very heavy downpours, hail, gusty winds, and possibly even a small tornado or waterspout. Rain amounts still on track as well as indicated below, a lot of which will fall in that initial frontal passage late Friday into Saturday. High snow accumulations expected in the mountains, especially above 6000 feet. A flood watch and a winter storm watch will be discussed with neighboring offices with a likely issuance this afternoon.

***From Previous Discussion***

A cooling trend will continue through Saturday as a more vigorous, stronger, and colder storm system moves into the region between Friday and Saturday. A vorticity maximum rotating around trough circulation to the north will swing around the back side and dig the trough into the region to near Point Conception by late Saturday night. The colder trough will tap into the subtropics and bring a moist air mass into region along the cold front on late Friday. PoPs trend even wetter over the previous forecast with categorical rain being mentioned across the entire area. All members of the EPS, GEFS, and CMC ensembles indicate rain developing across the region between late Friday and Saturday night. With a difluent flow pattern setting up aloft and a much colder air mass aloft, a slight chance of thunderstorms remains in the forecast between late Friday night into Sunday. Brief heavy downpours with small hail could develop. There is also enough wind shear to produce weak tornadoes or waterspouts over the coastal waters.

EPS ensemble members have trended slightly higher with precipitable water values in the latest solutions. Precipitable water value means tick up slightly to between 1.0 and 1.1 inch. There is a low-to- moderate chance that a Flash Flood Watch could be issued for late Friday and into Sunday night. Convective parameters, creating more instability, could tip the scales more in favor for a watch, but this will need to be monitored closely.

Storm total rainfall amounts remain fairly consistent, near 1.50 to 3.00 inches across the coastal and valleys areas with up to 5.00 inches possible along favored south facing mountain slopes. Most of the precipitation will arrive along the cold front between Friday and Saturday, but there is a secondary boundary that develops on Sunday in the latest solutions, which could bring additional rainfall and a higher potential for convective burst.

The latest deterministic solutions show 700 mb temperatures cooling to between -7 degrees Celsius right behind the cold front, then cooling to -9 degrees Celsius by Sunday. While the snow levels were previously advertised to remain between 5000 and 6000 feet, 700 mb temperatures suggest snow levels falling to 4500 feet quite rapidly with the cold front. This could bring some accumulating snowfall to the Tejon Pass on Interstate 5. There is an increasing concern for accumulations of snow lower than previously surmised. Almost all of the EPS ensemble members have accumulating snow for KSDB, with a range from a dusting up to 3 inches. Most EPS members show a half inch or less for KSDB. Highest confidence exists for significant snowfall above 6000 feet. A foot or two of snow above 6000 feet, impacting higher roadways such as Lockwood Valley Road in Ventura County and Angeles Crest Highway and Highway 39 in Los Angeles County, is likely. Winter weather headlines will likely be needed as we get closer to the storm's arrival and the details become clearer.

Gusty southerly winds will also develop ahead of the storm system's cold front. NAM 850 mb winds increase to between 40 and 50 knots over San Luis Obispo County on Friday and Friday night. Stronger southerly winds do spread farther south and east Friday through Saturday. A High Wind Watch was added for the higher terrain in San Luis Obispo County, and wind advisories were added for areas where winds will likely start to increase as soon as Friday. Wind advisories likely need to expanded farther to the south and east, and possibly extended by future shifts into Sunday or Sunday night.

Long Term

(Sun-Wed), 28/941 AM.

There's a small chance the upper low will linger into early Monday across LA County, otherwise dry and warmer conditions are expected next week with highs back into the 70s across the valleys and inland coastal plain areas by Tuesday.

Aviation

28/1830z.

At 18Z at KLAX, there was no marine layer nor inversion.

Good confidence in TAFs through 08Z, then moderate confidence in all coastal TAFs and LA Valley TAFs. High confidence in desert TAFs There is a 30% chance of CIGs remaining VFR after 08Z for KSBA, Ventura, and LA TAFs coastal/valley TAFs. Timing of flight cat changes may be off by +/- 3 hours.

KLAX, Moderate confidence in TAF. Timing of flight cat changes may be off by +/- 3 hours with a 30% chance of conds remaining VFR through the period. There is a 20-30% chance of an east wind component of 8-10 kt from 13Z-18Z Sat.

KBUR, Low to moderate confidence in TAF. There is a 30% chance that conds remain VFR through the period.

Marine

28/836 AM.

Fairly complex marine forecast, with rapidly changing conditions, and moderate confidence thru the weekend

For today thru Fri morning,

Small Craft Advisory (SCA) level W-NW winds are expected to develop this morning in the SBA Channel affecting PZZ650 and eastern portions of PZZ673, then across the remainder of the waters this afternoon. Winds will drop below SCA levels in most areas late tonight, except early Fri in the SBA Channel and the southern inner waters. However, seas will remain above SCA levels in the outer waters and the inner waters N of Pt Sat thru Fri, so a SCA will remain in effect for those areas into Fri afternoon.

From Fri afternoon into the weekend,

Winds will shift to S and increase ahead of a cold front Fri. In the northern two outer waters zones (PZZ670/PZZ673) and in the inner waters N of Pt Sal, winds may reach gale force Fri afternoon thru Fri night, so a GALE WATCH is in effect. SCA level S-SW winds are likely Sat/Sat night, with SCA level seas thru early Sun.

In the southern outer waters, high end SCA level S winds are expected Fri afternoon thru Sat morning, with a 40% chance that winds will remain at SCA levels thru Sat night. Seas will remain at or above SCA level thru early Sun.

In the SBA Channel and in southern inner waters, SCA level SE-S winds are likely (60% chance) Fri evening thru Sat morning, with a 40% chance that winds will remain at SCA levels thru Sat night.

Conds are expected to be below SCA level across all waters Sun afternoon thru Mon.

There is a 20-30% chance of thunderstorms late Fri night in northern areas and across all of the waters Sat thru Sun.

Beaches

28/836 AM.

On W and NW facing beaches on the Central Coast, high surf is expected to develop this morning, then continue thru at least Sun morning and possibly thru the day Sun. Surf heights of 8 to 12 ft with local sets to 15 ft are expected.

On W facing beaches of Ventura County, high surf is expected to develop this afternoon, then continue thru at least Fri afternoon. Surf heights of 5 to 7 ft with local sets to 8 ft are expected. There is a 40-50% chance that high surf will continue thru the weekend, and the High Surf Advisory will have to be extended.

On any beaches with a westerly exposure on the south coast of SBA County, and on the beaches of L.A. County, surf of 3 to 6 feet is expected later today thru the weekend. There is a 50% chance that surf will reach High Surf Advisory levels Fri thru Sat, especially on the beaches of L.A. County late Sunday into Monday.

Strong and dangerous rip currents are expected on all beaches thru the weekend.

Watches, Warnings, Advisories

Ca, High Surf Advisory in effect until 9 AM PDT Sunday for zones 340-346. (See LAXCFWLOX). Wind Advisory in effect from 3 PM Friday to 6 AM PDT Saturday for zones 340-341-343-344-346-347. (See LAXNPWLOX). High Wind Watch in effect from Friday afternoon through late Friday night for zones 342-345. (See LAXNPWLOX). High Surf Advisory in effect from 3 PM this afternoon to 3 PM PDT Friday for zone 354. (See LAXCFWLOX). PZ, Small Craft Advisory in effect from 3 PM this afternoon to 3 PM PDT Friday for zone 645. (See LAXMWWLOX). Gale Watch in effect from Friday afternoon through late Friday night for zones 645-670-673. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PDT Friday for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM this afternoon to 9 AM PDT Friday for zone 655. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM this afternoon to noon PDT Friday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect until 3 PM PDT Friday for zone 673. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM this afternoon to 9 AM PDT Sunday for zone 676. (See LAXMWWLOX).

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