textproduct: Los Angeles

SYNOPSIS

13/118 AM.

Dry conditions with a warming trend are expected to continue through Saturday. A strong storm system will affect the region starting on Sunday and Monday, bringing the potential for heavy precipitation with flooding and mountain snow, thunderstorms, strong winds, as well as marine and beach hazards. These conditions could last through much of next week.

LONG TERM (MON-THU)

13/347 AM.

The latest upper level analysis shows two upper lows coming through the area through Wednesday, each with multiple waves of precipitation. With a 160kt jet oriented southwest to northeast across LA County, precipitable waters now close to 1.25", and a classic cold front with diffluence aloft slamming into the state, this first system has the highest potential for generating the highest amounts or precipitation overall. The latest ensemble solutions are converging on at least 1-3 inches of rain for coast and valleys with the first system Monday and 2-5" in the mountains. We're still outside the time frame for the higher res models but based on the NAM showing 50-60 kt at 850mb out of the southeast creating a huge orographic component it looks likely that hourly rain rates will reach at least 0.75"/hour in the Transverse range between Santa Barbara and LA County and possibly as high as one inch per hour. Because of this, Monday has the highest chance for debris flows in the burn areas and overall hydrologic hazards. Also can't rule out an isolated thunderstorm or two given the upper level pattern. Snow levels are expected to remain above 6500 feet Monday but significant accumulations are likely above that.

There should be a significant decrease in rain amounts and intensities Monday night into Tuesday as low level flow shifts to the west following the passage of the first cold front, but it's always difficult to time these features 4 or more days out. There will be enough moisture for at least scattered showers through the day Tuesday but rain rates should be quite a bit less than Monday. Temperatures will be cooling aloft leading to snow levels lowering to around 5000 feet with some light accumulations possible.

The second upper level system is expected to arrive Tuesday night into Wednesday and this will bring with it much colder air aloft. Wednesday has the look of a very wacky weather day across southern California as snow levels are expected to drop to at least 3000 feet, and possibly as low as 2500 feet at times creating significant driving hazards on mountain roads, including Interstate 5, which may experience delays or closures at times. PW's drop by 50%, but this should be a very efficient rain and snow producer with periods of heavy precipitation at times and possibly even some graupel across the foothills and higher valleys. Rain amounts will be highly variable from as little as a half inch in some areas to as much as 1-3" in others.

A cold day on Thursday but at this time the forecast is for dry weather.

AVIATION

13/1103Z.

At 0930Z at KLAX, there was a surface-based inversion. The top of the inversion was 2200 feet with a temperature of 14 degrees Celsius.

For the 12Z TAF package, high confidence in CAVU conditions for all sites through the period. However, there will be a 20-30% chance of VLIFR conditions at KPRB 12Z-16Z this morning.

KLAX...High confidence in 12Z TAF as CAVU conditions are expected through the period. No significant easterly wind component is expected.

KBUR...High confidence in 12Z TAF as CAVU conditions are expected through the period.

MARINE

13/303 AM.

For the Outer Waters, moderate to high confidence in current forecast. Today through Saturday morning, high confidence in Small Craft Advisory (SCA) level northwest winds. For Saturday afternoon through Saturday night, high confidence in winds and seas remaining below SCA levels. For Sunday through Tuesday, high confidence in a combination of SCA level southerly winds and seas. There is a 50-70% chance of GALE force winds during this time period.

For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. There is a 50-60% chance of SCA level northwest winds this afternoon and evening. For Saturday through Saturday night, high confidence in winds and seas remaining below SCA levels. For Sunday through Tuesday, high confidence in a combination of SCA level winds and seas. There is a 40-50% chance of GALE force winds during this time period.

For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. Today through Saturday night, high confidence in winds and seas remaining below SCA levels. For Sunday through Tuesday, high confidence in SCA level southeast to southwest winds developing. There is a 30-40% chance of GALE force winds during this time period.

Along with the strong winds and increasing seas next week, there will be the possibility of thunderstorms Monday through Tuesday. So, this combination of adverse weather will generate hazardous marine conditions next week.

BEACHES

12/725 PM.

Surf and swell will build again at area beaches through late Friday night, and a High Surf Advisory was issued for the Central Coast. Local sets to 7 feet are possible at Ventura County Beaches.

A period of very large waves continues to be advertised by the latest swell model guidance early next week. There is a 50-70 percent chance of widespread high surf between Monday and Thursday as a combination of SW and W-NW swells affect the California coast. Sets above 10 feet will be possible across all beaches, but there is a higher chances for W-NW facing shores. There is a 20-30 percent chance of damaging sets developing between Tuesday night and Wednesday night, highest for NW facing shores along the Central Coast.

Elevated high tides as the new moon cycle approaches will coincide with the arrival of the higher surf and swell with storm system early next week. This will bring an elevated chance of coastal flooding, or at the very least, minor tidal overflows during times of the highest high tides each evening. A coastal flood advisory may be needed should swell guidance indicate similar surf and swell and locally wind-driven swell affect the beaches and coastline.

LOX WATCHES/WARNINGS/ADVISORIES

CA...High Surf Advisory in effect until 9 AM PST Saturday for zones 340-346. (See LAXCFWLOX). PZ...Small Craft Advisory in effect from noon today to 9 PM PST this evening for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PST Saturday for zones 670-673-676. (See LAXMWWLOX).