Swell Matrix

Area Forecast Discussion

National Weather Service Los Angeles/Oxnard Ca

416 am PST Mon Jan 18 2021

Synopsis

18/414 AM.

There will be weak offshore flow today and temperatures will begin to cool down. Very strong northeast winds will affect the area late tonight through Wednesday morning, especially across Los Angeles and Ventura Counties. It will likely remain dry except for possible showers over the eastern San Gabriels on Tuesday. There will be a widespread chance for light rain Friday into Saturday with significant cooling.

Short Term

(tdy-Wed), 18/313 AM.

Today will be the calm before the (wind) storm. There is about 2 MB of offshore flow from both the north and the east. There will be some weak canyon winds this morning but nothing near advisory levels. A little bit of energy, currently over the Bay Area, will move over Srn CA today. It will not do much to today's weather save for a little bit of cirrus but it will set the stage for tomorrow. The lowering hgts and weaker offshore flow will bring 4 to 8 degrees of cooling to the area today.

Tonight the little trof that ran overhead during the day will move off to the west of Baja. Simultaneously a cold upper low will retrograde out of NV. The upper flow will turn to the NE through at least 700 MB it will be accompanied by a grip of cold air advection. A cold dome of sfc high pressure will build into NE Nevada and the offshore LAX-DAG grad will increase to somewhere between 6 and 8 MB. 850 MB upper support will be about 50 KT at midnight and 65 kt shortly after sunrise. All of these ingredients will combine to produce warning level gusts between 60 and 70 mph (70 to 80 mph in the mtns). High wind warnings have been issued for much of LA and VTA counties starting at 10pm tonight and continuing through the predawn hours of Wednesday morning. Many areas not covered by the high wind warnings will see advisory level winds. Please see the product LAXNPWLOX for details. These winds will also dramatically increase the fire danger - please see the fire weather discussion below for all of the details.

The upper low will retrograde over LA county during the day Tuesday and it will bring some clouds with it and somewhat strangely it will bring just enough moisture, instability and dynamics to create a slight chc of showers over the eastern LA mtns. The highest elevations might see an inch maybe two but because of the cold air advection some areas as low as 3500 feet may see a dusting.

The massive cold air advection will completely overwhelm the compressional heating and max temps will fall 10 to 15 degrees. Cst/Vly temps south of Point Conception will be in the mid to upper 60s. There will be less cold air advection across the Central Coast and temps there will rise into the lower 70s.

On Wednesday the upper low will be well to the SW of LA. The offshore flow will be similar in strength to Tuesday but there will be less upper support and much less thermal support. There will likely be advisory level gusts across much of LA and VTA counties. The lack of cold air advection and the continued offshore flow will bring 5 to 10 degrees of warming to the entire forecast area.

Long Term

(Thu-Sun), 18/312 AM.

Both the EC and the GFS agree that a substantial change in the weather is coming in the xtnd period.

On Thursday a little pop up ridge moves over Srn Ca in between the departing upper low and a cold upper low moving out of the Pac NW. There will still be some offshore flow but it will be 2 to 3 MB weaker than it was on Wednesday. Looks for some sub advisory canyon winds in the morning. The coasts and vlys will cool (The Central Coast may see 8-10 degrees of cooling) due to the reduced offshore flow while the interior warms as the airmass warms.

On Friday an upper low moves down the coast and into the San Joaquin Vly in the evening. On Saturday the low is forecast to open up into a pos tilt trof with the axis oriented from the center of NV to Srn VTA county. The upper flow will be quite weak over SoCal with the strongest winds to the NW SW and S. Deterministic Mdls as well as their ensembles all agree that this system will bring a some showers to the area on both days but there is no agreement on exactly where or when. There is no moisture tap and the system will be pretty moisture starved so rain rates will be low but some areas (notably the eastern San Gabriels) could receive quite a few hours of rain over the 60 hour period and end up with over three quarters of an inch of rain. it produces will be quite light. For now will broad brush slgt chc to chc pops and partly to mostly cloudy skies. This is a cold system and snow levels will be on the low side, but as with the rainfall amounts any snowfall that occurs will be light.

Cooler for sure with 8 to 10 degrees of cooling slated on Friday as the hgts plummet and then an additional 2 to 4 degrees of cooling on Saturday which will bring max temps only into the lower 60s across the coasts and vlys which is 5 to 8 degrees blo normal.

On Sunday the trof will slowly move out and skies will clear and the threat of rain will end in the afternoon. Any additional rainfall or snowfall will be minimal. Max temps will rise but will still be 3 to 5 degrees blo normal.

Aviation

18/1208z.

At 0719Z at KLAX, there was no marine layer. There was a surface- based inversion with a top at 1500 feet at 22 degrees C.

High confidence in CAVU TAFs.

KLAX, High confidence in CAVU TAF. Good confidence that an east wind component will remain below 5kt through 18Z.

KBUR, High confidence in CAVU TAF. There is a 20 percent chance of 15KT easterly winds 15Z-21Z.

Marine

18/208 AM.

High confidence that Small Craft Advisory (SCA) level hazardous seas over 10 feet will continue through at least Tuesday afternoon across the waters along the Central Coast southward to San Nicolas Island. There is also a 30% chance of SCA level northwest winds late this afternoon across the northern portion. High confidence that SCA level northeast winds will develop by late tonight and last through at least late Tuesday night north of Point Conception. There is a 30-40% chance that winds may exceed Gale force.

High confidence in gusty northeast winds and choppy short-period seas across all the inner waters inside the Southern California Bight and out to all the Channel Islands, San Nicolas Island, and Catalina Island late tonight through Wednesday morning. The strongest winds are expected Tuesday morning through afternoon. A Gale Watch will be in effect from late tonight through Wednesday morning with winds likely peaking between 30 and 40 kt with local gusts to 50 kt. Choppy wind-generated waves of 5 to 8 feet are also expected. These conditions are expected to impact the eastern Channel Islands and Catalina Island. East facing harbors, such as Avalon Harbor and other east facing harbor entrances across the Channel Islands will be affected with strong winds and steep seas. These conditions are hazardous to boaters, and there may be damage to boats that are not anchored properly. Mariners should avoid these areas, or remain in safe harbor until conditions improve.

Beaches

18/139 AM.

A long period west to northwest swell will move through the coastal waters today through Tuesday, resulting in high surf and dangerous rip currents on all west and northwest facing beaches. Surf will peak today then slowly subside on Tuesday. There is a chance that surf will subside slower than expected and Surf Advisories may need to be extended past Tuesday night.

For the Central Coast, large breaking waves of 15 to 20 feet will be likely today along favored northwest-facing beaches, then slowly subside on Tuesday. There could be some local sets greater than 20 feet today.

For the Los Angeles and Ventura County beaches, large breaking waves of 6 to 10 feet will be likely today along west-facing beaches, with some higher sets across Ventura County beaches. The west-facing beaches of the Santa Barbara County South Coast will likely see breaking waves of 5 to 8 feet. The surf will slowly subside on Tuesday.

Watches, Warnings, Advisories

Ca, High Surf Advisory in effect until 10 PM PST Tuesday for zones 34-35-39>41-87. (See LAXCFWLOX). High Wind Warning in effect from 10 PM this evening to 4 AM PST Wednesday for zones 40-41-44>46-53-54-88-547. (See LAXNPWLOX). High Wind Warning in effect from 4 AM Tuesday to 4 AM PST Wednesday for zones 51-52. (See LAXNPWLOX). Fire Weather Watch in effect from this evening through Wednesday morning for zones 234>236-251-252. (See LAXRFWLOX). Fire Weather Watch in effect from late tonight through Tuesday evening for zones 240-241-244>246-253-254-288-547-548. (See LAXRFWLOX). PZ, Small Craft Advisory in effect until 3 AM PST Wednesday for zones 645-670-673. (See LAXMWWLOX). Gale Watch in effect from late tonight through Wednesday morning for zones 650-655-676. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PST Tuesday for zone 676. (See LAXMWWLOX).

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