Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

544 pm PDT Mon Mar 18 2024

Synopsis

18/1248 PM.

Skies will be partly cloudy and it will be a little warmer today. There is a slight chance of afternoon and early evening showers today and Tuesday in Los Angeles, Ventura, and Santa Barbara Counties, mainly over mountain areas. Dry conditions are expected the rest of the week before a weak system brings wind, light showers, and cooler weather for the weekend.

Short Term

(Mon-Thu), 18/111 PM.

Synoptically, the upper-level low that drove the wind event last Thursday is still in the area, now centered over the CA-AZ border in a location favorable for transporting moisture into the CWA and introducing a slight chance of afternoon and evening thunderstorms. There is already activity to our east, near San Bernardino. This feature will stay in place over night and begin to move to the east by Tuesday afternoon, leaving the CWA largely under an Eastern Pacific ridge midweek. By the weekend, another upper-level low will approach the area, this time from the northwest. It will drop a front down the west coast which will pass through the CWA on Saturday and Sunday. By Monday the area will again be under the EPac ridge.

Weak offshore flow and abundant sunshine will bring warming to most areas today, with max temps possibly getting close to 80 degrees in some of the warmer locations in the valleys and interior portions of the coastal plain.

An upper low which was centered to the northwest of Phoenix early this morning is drifting slowly southwestward this afternoon. There is moisture associated with the upper low despite it's inland position. Northeasterly flow aloft will transport increasing deep moisture into the region today, with the best moisture in L.A., VTU, and SE SBA Counties. There is instability over the region today as the strong March Sun warms the surface, and a cold pool aloft approaches from the east. There are favorable convective parameters over southern CA today, with a broad area of lifted index values between -3C and -5C from SBA County thru L.A. County. Surface based CAPE values are forecast to be between 500 and 1000 J/kg from SBA County southward, and locally up to 1300 J/g in SBA County. Convection is most likely to initiate in the mountains this afternoon, but with strong NE flow aloft, showers/tstms could drift into valley and coastal areas, and eventually into the coastal waters later in the afternoon. The main threat from thunderstorms will be strong wind gusts up to 50 mph, as evaporative cooling due to precipitation falling into dry air at low layers helps to accelerate the air toward the surface. The other main threat from storms will be isolated dry lightning. With so much dry air near the surface, there is a chance of isolated dry lightning strikes. There is also a slight chance of hail as colder air aloft moves into the region. Convective activity should decrease fairly quickly after sunset as the atmosphere becomes more stable.

Skies should become mostly clear tonight, and there could be some patchy low clouds and fog in coastal areas late tonight.

The upper low will drift slowly southward tonight, then eastward on Tue, moving back into Arizona in the afternoon. There will still be some residual moisture and instability Tue, with another slight chance of showers/tstms for Tue afternoon/evening for L.A., VTU and eastern SBA Counties. Once again, convection will likely initiate in the mountains, then spread south and west of the mountains due to northeasterly flow aloft. Max temps Tue should be similar to those today in most areas, generally a few degrees above normal.

The upper low will likely continue to move east Tue night and Wed, finally losing its influence on the weather in the forecast area. Expect areas of night thru morning low clouds and fog in coastal areas Tue night/Wed morning, then mostly sunny skies by afternoon. Max temps may drop a bit Wed as onshore flow returns.

There will likely be areas of night/morning low clouds in coastal and some valley areas Wed night/Thu morning. The ridge will weaken and flatten with a large upper low moving toward the West Coast. Max temps will likely still be above normal Thu.

Long Term

(Fri-Mon), 18/115 PM.

The flow aloft will shift from zonal on Thu to southwesterly Fri. There may be low clouds again Thu night/Fri morning. Increasing mid and high level moisture pushing into the region will likely bring an increase in clouds to all areas Fri, along with some cooling, bringing temps close to normal levels.

Most ensemble members show measurable rain in most of the area this weekend, but there is uncertainty with respect to amounts and timing. In general the highest chances and rain amounts are in northwest SLO County where up to 0.5" could fall over the weekend. Most other areas will be between 0.1 and 0.25" except for the Antelope Valley which will likely remain rain free.

By Monday the system should be through the area and the CWA will see mostly clear skies and warming temps under the eastern Pacific ridge.

Aviation

19/0044z.

At 00Z at KLAX, there was no marine layer nor inversion.

Low confidence in TAFs due to uncertainty in low clouds as well as isolated showers with a 10-20 percent chance of TSRA through 03Z focused from LAX to LGB. There is a 30-50 percent chance of MVFR to IFR cigs/vsbys around 12Z for KSBA and south including KLAX and a 40 to 60 percent chance elsewhere except less than 10 percent chance for KPMD and KLGB.

KLAX, Low confidence in TAF through 17Z. There is a 10 percent chance of a thunderstorm through 03Z. There is a 30 percent chance of MVFR to IFR cigs/vsbys between 10-17Z and a 50 percent chance after 03Z. Outside of any showers and thunderstorms, any easterly winds should remain less than 7 knots.

KBUR, Moderate confidence in TAF through 16Z. There is a 10 percent chance of a thunderstorm through 03Z. There is a 10 percent chance of MVFR to IFR cigs/vsbys between 10-17Z.

Marine

18/210 PM.

There is a 10-20 percent chance of thunderstorms across the coastal waters south of Point Arguello and into the southern California bight this afternoon and this evening. Any thunderstorm that forms will be capable of produce dangerous cloud to surface lightning, locally gusty and erratic winds, and brief heavy showers or small hail that reduces visibility.

For the waters southwest through northwest of the Channel Islands and along the Central Coast, there is a 40-60 percent chance of Small Craft Advisory (SCA) level winds each afternoon and evening through Wednesday evening. The highest chance of SCA level winds will be for the northern waters today and Tuesday, expanding to north of San Nicolas Island on Wednesday. There is a 30-40 percent chance of SCA level winds for Thursday and Friday, then widespread SCA conditions are likely over the weekend.

Inside the southern California bight, there is a 20-30 percent chance of Small Craft Advisory (SCA) level winds each afternoon and evening through Wednesday evening. Wind and seas should remain below SCA levels for Thursday and Friday, then there is a high-to-likely chance of SCA level winds and a moderate-to-high chance of SCA conditions over the weekend.

Watches, Warnings, Advisories

Ca, NONE. PZ, Small Craft Advisory in effect until 9 PM PDT this evening for zones 645-670-673. (See LAXMWWLOX).

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