Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

539 pm PDT Sun Sep 22 2019

Synopsis

22/120 PM.

Warm temperatures and locally gusty winds will continue today. After cooler temperatures on Monday, a moderate Santa Ana wind event develops and temperatures will warm significantly Tuesday and gusty winds return. A notable cooling trend begins by Thursday and there is a chance of light rain late in the week.

Short Term

(tdy-Wed), 22/223 PM.

Current satellite imagery showing high cloud shield starting to thin out across the region this afternoon, with a surge of low clouds and fog arriving to some of the Central Coast beaches this afternoon, causing a cooling trend for the immediate coastal areas north of Point Conception. Meanwhile the main weather story for today is the hot and dry conditions which coupled with dry fuels will bring elevated fire weather concerns to the region through the afternoon hours.

Weak offshore flow near the surface occurred once again this morning, with LAX- Daggett gradient peaking at -2.6 mb. This offshore component combined with a warm air mass in place caused inland temperatures to rise quickly, with warmest valley locations reaching the mid 90s and Downtown Los Angeles topping 90 degrees. Current observations at 2 pm showing winds shifting to a more southerly onshore wind direction this afternoon, with gusts 20 to 30 mph across the interior. The onshore pressure trends are expected to bring some low clouds and fog to portions of the coast tonight through Monday morning. With a shallow marine inversion, there will be the potential for patchy dense fog.

Weak upper level trough across northern and central California this afternoon is expected to become a closed low pressure system near Reno Nevada by early Monday morning. The closed low will take on an inside slider path, with better model consensus taking the closed low towards Yuma Arizona by early Tuesday morning. This low in combination with an amplifying upper level ridge of high pressure off the California coast will bring some upper level wind support for a Santa Ana wind event across Southern California late Monday night into Tuesday. While the surface offshore gradient is fairly weak (LAX-Daggett gradient peaking at around -3 mb on Tuesday morning), the upper level support will help drive this offshore wind event, with projected wind gusts of 30 to 40 mph in the mountains and foothills of LA/Ventura counties, and gusts 25 to 35 mph in the valleys. At this point, still questionable if offshore winds will reach coastal areas of Ventura county and Malibu, but if it does, winds should only be in the 10 to 20 mph range.

The Santa Ana wind event is expected to bring significant warming and drying to much of the region, with warmest valley areas now expected to top 100 degrees, while portions of the coastal plain climb into the 90s. With current forecast bumping up some of the valley temperatures for Tuesday, as well as minimum temperatures in the foothills for Tuesday night, a heat advisory may need to be considered for portions of the valleys as we draw closer to the Tuesday heat event. Some lighter offshore breezes are expected to linger into Tuesday night and Wednesday morning across the mountains and valleys, but wind speeds expected to be less than Tuesday morning due to diminished upper level wind support. Some cooling expected across coastal areas on Wednesday due to earlier arrival of the sea breeze, however inland areas will see another warm and dry day, with temperatures climbing well into the 90s. The main impact of this Santa Ana wind event will be an extended period of elevated fire weather concerns from late Monday night through Wednesday, with the potential for brief critical fire weather conditions on Tuesday when the strongest offshore winds and driest conditions are expected to occur. Please see fire discussion below for more details.

Long Term

(Thu-Sun), 22/229 PM.

On Wednesday night into Thursday morning, onshore flow and marine layer depth are expected to increase rapidly, bringing a surge of low clouds well into the lower coastal slopes. GFS model cross section showing a deep layer of moist south-southeast flow up to 4000 feet by Thursday morning, which will likely result in areas of drizzle. An unusually strong upper level trough will continue to deepen across California Thursday through Saturday. This will result in significant cooling across Southwest California during this period, along with gusty onshore winds in the mountains and deserts. In addition, long range models showing moist layer continuing to deepen from Thursday night into Saturday, which could bring periods of drizzle or light rain showers through the period. At this time, still have 20-40 percent chance of measurable light rain from Thursday night through Saturday, followed by a 20 percent chance of showers for areas north of Point Conception on Sunday.

Aviation

23/0037z.

At 2330Z, the marine layer depth was around 750 feet deep at KLAX. The top of the marine inversion was near 2000 feet with a temperature of 26 degrees Celsius.

Moderate-to-high confidence in the current forecast. Higher confidence in inland terminals, less confidence in coastal terminals. LIFR to IFR conditions will likely develop after 07Z then likely improve one category between 13Z and 15Z.

KLAX, There is a 30 percent chance of VFR conditions through the period. Any east winds should remain less than 7 knots.

KBUR, There is a 20 percent chance of MVFR conditions between 13Z and 16Z. Otherwise, VFR conditions will persist.

Marine

22/140 PM.

Moderate to high confidence in the current forecast.

For the Outer waters, expect Small Craft Advisory (SCA) level winds and seas across the northern two outer waters zones (PZZ670/PZZ673) this afternoon thru Monday evening. There is a 30-40% chance of SCA level NW winds across the southern outer waters zone (PZZ676) late tonight thru Mon evening.

Across the inner waters north of Pt. Sal, there is a 30-40% chance of SCA level NW winds during the afternoon/evening hours today and Mon.

Across the inner waters south of Pt. Conception, conditions are expected to remain below SCA level thru Thu. However, there is a 30% chance of SCA level NE winds across the eastern SBA Channel and nearshore from Pt. Mugu to Santa Monica Tue morning.

Watches, Warnings, Advisories

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

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