Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

228 pm PST Thu Feb 20 2020

Synopsis

20/855 AM.

It will be sunny today with above normal temperatures. Clouds will increase on Friday and it will be a few degrees cooler. An approaching storm will bring showers to the area Friday night and Saturday. Dry weather returns Sunday into next week.

Short Term

(tdy-Sun), 20/205 PM.

Tonight we will get a break from the marine layer clouds as offshore flow continues to be strong enough to keep clouds away. However, there is a small chance of clouds over the southern portion of the LA County coast by the early morning. Friday will be another sunny day otherwise with light offshore flow in the morning. As a cutoff low approaches, the winds will shift from the southeast in the afternoon. Temperatures will be warm with high temperatures a few degrees above normal, but cooler than today.

Models are indicating the cutoff low is now remaining far offshore on Friday, then making its approach to Southern California on Friday night. Now timing is a little later than previously thought, with the low center around 40 NM south of the Channel Islands by around sunrise. Because of its more west track, the low will entrain a little more moisture and bring a pretty solid chance of rain to the region. Increasing southerly flow bringing in moisture from the ocean will also help rain chances, especially over south-facing slopes. There will be a chance of rain starting Friday evening mainly for points north of Point Conception (especially higher terrain), then a little lesser chance in Ventura and western LA Counties. After midnight on Saturday showers are likely everywhere.

The low will continue moving eastward into LA County by Saturday afternoon, then accelerate towards Las Vegas by early evening. As the low moves eastward, rain chances over western areas will start to decrease by the afternoon into the evening. Wraparound moisture will continue the chance of rain over eastern LA County through the evening.

Overall, total rainfall will be pretty light, with amounts ranging from around a tenth to a quarter inch over the coasts and valleys. The mountains will see a bit more, especially southern facing slopes which will get orographic enhancement, from a quarter to half inch. With the low directly overhead and good instability indicated by high-res models, have added a slight chance of thunderstorms to the coastal waters south of Point Conception and southern portions of Ventura and LA Counties late Friday night through Saturday morning. Therefore there may be some localized rain totals above these amounts.

Snow levels will lower to 5500 feet locally down to 5000 feet. 1 to 3 inches of new snow can be expected mainly at resort level. Therefore not expecting snow-related impacts to the major mountain passes as they will likely have mainly rain.

After the low exits, dry NW flow will set up on Sunday and skies will clear out. Temperatures will rebound a few degrees but still remain a little below normal.

Long Term

(Mon-Thu), 20/226 PM.

The dry NW flow aloft that set up on Sunday in the wake of the low will continue early next week and a ridge will begin to build back in. It will be a return to dry and sunny weather with temperatures above normal for next week.

On Monday there will be some decent northerly flow, then will shift to northeasterly Tuesday through Thursday. The offshore flow will help temperatures rise close to 80 degrees over the valleys of LA and Ventura Counties mid to late week. The GFS and the EC have been going back and forth on how strong winds will be and which day will have the strongest winds. At this point it looks like Tuesday will be slightly stronger.

Aviation

20/1750z.

At 1730Z, the marine layer depth was around 400 ft deep at KLAX. The top of the inversion was near 1100 ft with a temperature near 17 deg C.

Hi confidence in the 18Z TAFs with mostly VFR conditions expected through Fri morning. However, there is a 50% chance of MVFR vsbys due to mist at KLGB from about 09Z=16Z Fri.

KLAX, Hi confidence in the 18Z TAF with VFR conditions expected through Fri afternoon. Any east to northeast winds Fri morning will remain less than 10 kt.

KBUR, Hi confidence in the 18Z TAF with VFR conditions expected through Fri morning

Marine

20/115 PM.

For the Outer Waters, moderate to high confidence in the forecast. Winds and seas should remain below SCA levels through Saturday, then there is a 70% chance SCA level winds and seas Saturday night through Monday, and a 50% chance of SCA level wind gusts at times on Tuesday. There is also a 20% chance of Gale force wind gusts between Sunday and Monday.

For the Inner Waters north of Pt. Sal, moderate to high confidence in the forecast. Winds and seas will remain below SCA levels through Tuesday, except for a 50 percent chance of SCA wind gusts at times Sunday night.

For the Inner Waters south of Pt. Conception, moderate to high confidence in the forecast. Winds and seas will remain below SCA levels through Tuesday, except for a 50% chance of SCA level W-NW winds across western portions of the SBA Channel Sat night, and a 20%-30% chance of SCA gusts Sunday into Sunday evening for the same area. There is also a 20%-30% chance of SCA gusts for western portions of the southern inner waters zone (PZZ655) Saturday night.

In addition, a cold upper level low pressure system is forecast to move through the coastal waters late Friday night and Saturday morning. This system will bring a slight chance of thunderstorms to the waters S of Point Conception during the period. Any thunderstorms that develop would bring the potential for brief downpours, locally gusty winds and choppy seas, small hail and dangerous cloud-to- ocean lightning. Conditions will also be favorable for the formation of waterspouts.

Watches, Warnings, Advisories

Ca, NONE. PZ, NONE.

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