Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

1034 am PST Sun Dec 9 2018

Synopsis

09/915 AM.

There will be elevated winds, cooler temperatures, and a slight chance of precipitation on the north slopes of the Ventura and Los Angeles County mountains Monday night and Tuesday. Another slight chance of precipitation will develop across the region next weekend. Otherwise dry and slightly warmer conditions will prevail.

Short Term

(tdy-Tue), 09/932 AM.

A 581-583 dm H5 dirty ridge will prevail over srn CA today. Plenty of hi clouds will move over the area from the W today with filtered sunshine at times. The LAX-DAG gradient at 16Z was -3.8 mb, but with minimal upper level support, the offshore flow this morning was generally weak with local gusts up to 30 mph noted in the L.A. County mtns and up to 20 mph for the wind-prone vlys and foothills of VTU/L.A. Counties. The offshore breezes will persist til about midday then turn weakly onshore this afternoon at least for coastal areas. It will still be a mild day despite the hi clouds thanks to the offshore flow. Highs in many of the inland coastal areas and vlys should reach the upper 60s to lower 70s, which is about 2-8 deg above normal for this time of year.

The upper level ridge is forecast to move E of the region tonight with a broad positively-tilted upper level trough approaching from the ern Pac. This generally weak upper level trof is expected to move into central CA on Mon then thru srn CA Mon night into early Tue. There are some model differences with the evolution of the trof, with the NAM cutting it off into and upper level low near the srn NV/CA border by late tonight, while the EC cuts off the upper low over swrn CA late tonight into Tue morning then moves the upper low S of the region thru the day on Tue. Meanwhile, the GFS remains progressive with the upper trof thru Tue. Even with the cutoff over swrn CA in the EC, the lack of moisture precludes any pcpn from developing over the area. The GFS likewise forecasts no pcpn over the area. The NAM does predict north slope showers later Mon night and early Tue. All the models show a northerly gradient tightening up Mon night into Tue, with gusty N winds likely from the SBA County S coast and mtns thru the L.A. County mtns and some of the adjacent vlys.

Some offshore flow will persist tonight into Mon morning with partly to mostly cloudy skies expected thru the day as additional hi clouds push into the area. Partly cloudy skies as a whole should prevail Mon night with clearing expected Tue, except the N mtn slopes will have mostly cloudy skies and a slight chance to chance of rain and snow showers late Mon night into Tue morning. The showers forecast for southern L.A. County for this time period is no longer expected and will be removed from the zones with a morning update.

In addition, gusty N winds will develop Mon night and persist into Tue, with the strongest winds likely from the SBA County S coast and mtns to the I-5 corridor in the L.A./VTU County mtns. It looks like near advisory level wind gusts up to 35 to 40 mph will be possible for these areas.

Temps across the region are expected to have a cooling trend for Mon and Tue, with highs by Tue falling to a couple of degrees below normal for many areas. Temps in the warmest inland coastal areas and vlys are forecast to be in the upper 60s to low 70s on Mon, then fall into the mid to upper 60s on Tue.

Long Term

(Wed-Sat), 08/121 PM.

Forecast confidence is fairly high for the first two days of the long range, but quickly falls later in the week. Ridging and some rebound in heights will bring more Sun and a warming trend on Wednesday. There is a potential for a significant Santa Ana wind event Wednesday night through Thursday due to a trough moving across the central Rockies. It appears we will be far away from cold advection, but offshore pressure gradients do indicate 4-5mb between LAX-DAG which is likely to generate advisory level winds in many areas. Temperatures will push back into the lower 70s by Thursday in the valleys/coasts due to the downslope flow.

Details in the latter part of the week get more fuzzy as models attempt to bring a more southern latitude low pressure area toward southern CA and Baja. The Euro even phases with a northern latitude trough moving across the PacNW. For now, precipitation chances look minimal as the main system passes well south of our area, but went ahead of used climatology PoPs due to the uncertainty in model solutions. Friday does look rather cloudy with some breaks by Saturday, but another system is approaching northern CA which could spill additional high clouds over southwest CA. Overall, the best storm track seems to be aligning more to our north for the next two weeks which may result in less organized precipitation and/or weak cut-off systems for southern CA.

Aviation

09/1834z.

At 1748Z, there was no marine layer at KLAX.

Moderate-to-high confidence in the current forecast. There is a chance of LIFR conditions at terminals north of Point Conception after 08Z Monday. Elsewhere, VFR conditions are expected through Monday.

KLAX, High confidence in VFR conditions throughout the period. Any east winds will remain less than 7 knots.

KBUR, High confidence in VFR conditions throughout the period. No wind issues are expected at this time.

Marine

09/937 AM.

High confidence in winds staying below Small Craft Advisory levels through tonight, except for a 20 percent chance of Small Craft Advisory level winds from Point Conception to San Nicolas Island. High confidence in increasing NW flow Monday through Wednesday, along with short period seas on top of a building long period NW swell. Hazardous marine conditions are likely by Monday across the coastal waters with Small Craft Advisory conditions in the outer waters near certain from the Central Coast to San Nicolas Island. There is a 60 percent chance of Small Craft Advisory winds and seas for the western portions of the Santa Barbara Channel and Santa Monica Basin in the afternoon and evening hours.

Watches, Warnings, Advisories

Ca, NONE. PZ, NONE.

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