Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

508 pm PDT Wed Oct 21 2020

Synopsis

21/506 PM.

A cooling trend will continue through Sunday, as onshore flow strengthens ahead of an approaching upper low. Night through morning low clouds will continue through the weekend. Light rain or drizzle is possible from Friday through the weekend as a weak weather system crosses the region. Clearing skies with gusty Santa Ana winds are likely early next week along with a warming trend.

Short Term

(tdy-Sat), 21/129 PM.

Overall today not much different than yesterday. A little cooler down south and a little warmer up north. Marine layer clouds have been slow to clear and probably won't at some beaches. More or less the same tomorrow and Friday, but probably even less and slower clearing and cooler temps. The marine layer will deepen and get farther up the lower coastal slopes each day, and possibly up to 4000' by Friday or Saturday. With the deepening there's always a chance for some morning drizzle, especially near the foothills. Best chances for that would be Saturday. By Friday and especially Saturday clearing chances are slim for the coast and many of the coastal valleys as well so it's going to feel more like May or June than October. Temps most areas will top out in the 60s to lower 70s.

Long Term

(Sun-Wed), 21/158 PM.

Sunday will be the best chance for widespread drizzle or light rain, mainly south of Pt Conception as a weak upper low moves just south of the area and brings in additional low level moisture and lift. There's a good chance for measurable precip up against the foothills of the eastern San Gabriel Mountains and perhaps as much as a quarter inch there. Most other places will get less than a tenth and likely little to none for the northern areas. These low-based stratiform light rain events are always a little tricky but the parameters look favorable for at least some precip. If models continue to show this pattern the pops will likely need some additional boosting, especially in LA County.

Sunday night into Monday we quickly shift to a Santa Ana pattern with good cold advection and wind support aloft, though there are still enough model variations with the trough orientation and track to keep confidence low in the specifics this far out. The last couple ECMWF solutions have shifted the trough farther west providing better support for a moderate to strong Santa Ana Monday into Tuesday across LA/Ventura Counties. The GFS is a little more progressive but does have some slower members. There's a large contingent of EC members supporting 50+ mph winds in the mountains and down through the Santa Clarita Valley Monday morning so it looks like a pretty good chance of warning level winds in the mountains at least and possibly some of the higher valley areas. And with the good cold air advection we should see advisory level winds all the way down to the coast from Malibu up through Ventura County.

There's also the potential for some light backdoor precip Sunday night across the LA mountains and Antelope Valley before the stronger winds begin. And if this happens there could be some light snow accumulations down to around 5000-6000 ft.

Temps will stay on the cool side through at least Monday and how fast those recover depends largely on how fast the trough exits the area. A faster GFS solution would result in a quicker warm up Tuesday and Wednesday but the EC keeps it pretty cool through mid week so a lot of uncertainty there still.

And lastly as is typical with offshore events the far interior areas like the Antelope Valley and interior SLO County will see overnight temps dropping easily into the 30s and possibly lower so some freeze/frost headlines will likely be needed there as early as Monday morning.

Aviation

22/0003z.

At 2145Z, at KLAX the marine layer was around 2100 feet deep. The top of the marine inversion was near 3700 feet with a temperatures of 25 degrees Celsius.

High confidence in desert TAFs with VFR conditions expected. Low to moderate confidence in KPRB with a 30% chance of IFR cigs late tonight. Otherwise, moderate confidence in the coastal and valley TAF sites with an overall slight deepening trend expected, however there is still a 20% chance of VLIFR conditions for the Central Coast. Further south, IFR-MVFR forecast for the coastal sites, but there is a 20% chance that cigs remain at MVFR level.

KLAX, Moderate confidence in the 00Z TAF. MVFR cigs could arrive +/- 1 hour from the forecast time of 03Z. There is a 20% chance that cigs do not dip into the IFR category, and a 20 percent chance of east winds reaching 8 knots Thursday morning.

KBUR, Moderate confidence in the 00Z TAF. CIGs could arrive +/- 2 hours from forecast time of 05Z.

Marine

21/118 PM.

Outer Waters, A Small Craft Advisory is in effect for the northern portion (PZZ670) from this afternoon through early Thursday morning for gusty NW winds, mainly the western portion. Otherwise, winds and seas should remain below (SCA) thresholds through this weekend. There will be south-southeasterly winds developing Thursday through Friday with possible local gusts to 20 knots at times.

Inner Waters from Point Piedras Blancas to Point Sal, Winds are expected to remain below (SCA) levels through the weekend, but winds will shift to south-southeasterly Thursday and Friday with possible local gusts to 20 knots at times.

Inner Waters S of Point Conception, Winds are expected to remain below (SCA) thresholds through the weekend. Southerly winds will affect the San Pedro Channel both days where local gusts to 20 knots will be possible.

Watches, Warnings, Advisories

Ca, NONE. PZ, Small Craft Advisory in effect until 3 AM PDT Thursday for zone 670. (See LAXMWWLOX).

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