textproduct: Los Angeles

SYNOPSIS

11/238 PM.

Other than a few lingering showers around this afternoon, expect dry conditions and a warming trend for late this week 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 (SUN-WED)

11/238 PM.

Medium-range model guidance is continuing to suggest increasing probabilities for a much cooler pattern featuring multiple rounds of precipitation starting Sunday-Monday. This would be in conjunction with a longwave, high-amplitude trough developing offshore and along the Pacific Coast vicinity, and eastward. Uncertainty inherent to the forecast regarding weather conditions and related impacts remains significant. However, in general, numerical models are offering above-average consensus regarding the longwave pattern over the local area for this distant of a time range. While the details remain uncertain, there is growing concern for heavy precipitation and cool-temperature impacts in the extended outlook starting Sunday and continuing throughout much of next week.

At this time, numerical models are gaining consensus on the most energy coming through Sunday through Monday, in the form of a complex, deep trough perhaps containing multiple low centers. While exhibiting some characteristics similar to the system that affected the local area this past Tuesday night, the upcoming Sunday-Monday system appears to be associated with considerably colder 500-mb temperatures, perhaps as low as around -30C. Some model solutions indicate a prominent shift in the tilt of the parent upper trough to negative during the emergence of the energy, characterized by a 500-mb speed maximum perhaps upwards of 100 kt. The surface reflection of these upper features is comparatively more diffuse owing to the broader upper troughing, though intense low-level mass responses and atmospheric momentum should undoubtedly be boosted by the intense ageostrophic circulation around the jet streak. So, while the complex interactions between upper low centers is highly uncertain and will influence mesoscale-driven impacts, the large-scale pattern is appearing increasingly conducive for convection to spread across much of the area, accompanied by bursts of heavy rain in one or more squall lines capable of flash flooding, strong to perhaps severe convective or gradient wind gusts, and heavy mountain snow with snow levels falling to around 6 kft. The exact details and timing are uncertain, though a wide array of hazards is becoming increasingly likely Sunday-Monday.

Looking further ahead, the aforementioned cold core will remain anchored across the western CONUS throughout much of the remainder of next week, reinforced by multiple impulses of compact vorticity and accompanying speed maxima pivoting through the western rim of the larger-scale troughing. It appears Southern California will be beneath the trajectory of multiple impulses. Each impulse will have the potential to boost precipitation rates and winds within a rather long duration of on-and-off showers and blustery conditions. Significant dispersion exists amongst models concerning these impulses, though next Wednesday-Thursday is a more-likely period of stronger jet energy passing over the local area, providing more precipitation and gusty winds. Regardless, temperatures through the week in most areas will be in the 50s and lower 60s -- below seasonal normal readings -- while snow levels gradually fall in response to cold advection with each passing impulse. Cold-weather impacts could become of increasing concern through the week.

Regarding precipitation totals through the period, there remains substantial variability from model to model, though most areas will likely experience at least 1 inch of liquid-equivalent of precipitation (perhaps upwards of a few to several inches of precipitation). The progressive nature of each impulse following the Sunday-Monday more-favorable-for-flooding event may tend to mitigate subsequent flooding impacts. However, increasingly saturated ground conditions could offset the short-duration precipitation character to maintain, or even boost, flooding concerns through the week, and uncertainty regarding hydrological impacts grows quickly through the course of next week. And then regarding storm-total snow in the mountains, elevations above 6 kft could receive upwards of multiple feet of snow. As snow levels lower, lighter snowfall accumulations -- though still potentially significant -- could impact elevations as low as 3 or 4 kft including the Grapevine of the Interstate-5 corridor, if enhanced mositure can adequately phase with cooling aloft. This will depend on numerous mesoscale factors for which uncertainty is significant.

Interests across the local area are encouraged to monitor the latest information from the National Weather Service in Los Angeles/Oxnard, California, as forecast details become refined over the coming days.

AVIATION

11/1737Z.

At 1659Z at KLAX, there was a deep moist layer up to around 7000 feet.

Overall, moderate confidence in 18Z TAF Package. There is a 25% chance of LIFR conditions developing at any site from 12/08Z to 12/16Z due to recent rains. Most likely at KPRB (30%).

KLAX...Moderate confidence in TAF. There is a 20 percent chc of 1SM BR OVC004 conds 12/08Z-12/16Z. Good confidence that any east wind component remains below 6 kts through the forecast period.

KBUR...Moderate confidence in TAF. There is a 25 percent chc of 1SM BR OVC004 conds 12/08Z-12/16Z.

MARINE

11/208 PM.

The cold front has moved eastward and is now over land. Small Craft Advisory (SCA) conditions will linger at least into the evening hours especially across the Outer Waters north of Pt Conception. The swing or overnight shift may issue a SCA across the inner waters south of Point Conception for Thursday afternoon and evening timeframe (60% chance).

There is a moderate chance for SCA conditions (winds and/or seas) across the Outer waters Thursday afternoon through Saturday. At times, these hazardous conditions could reach the nearshore waters along the Central coast and western portions of the Santa Barbara Channel.

Another storm system will enter the region Sunday or Monday, bringing hazardous winds and seas along with rain to the entirety of the coastal waters. There is a moderate chance for widespread Gale Force winds early next week.

BEACHES

11/209 PM.

Southwest and west facing shores will be particularly vulnerable to developing the southerly swell and surf through this evening, especially the Santa Barbara South Coast, Ventura County Line, Leo Carrillo, and Zuma Beach.

Swell and surf will likely drop below high surf advisory criteria for Thursday, but high surf is likely to develop again from Friday. It is likely that the swell and surf will remain at or above high surf criteria through the upcoming weekend.

The latest swell models continue to highlight a period of very large waves developing for early next week. There is a high to likely (50-70 percent) chance of widespread high surf between Sunday night and Wednesday as a combination of southwesterly and west-northwesterly swells impact the coasts. Sets above 10 feet will be possible across all coasts, but higher chances for west to northwest facing coasts. There is a 20-40% chance of damaging sets developing between Tuesday and Wednesday, highest for northwest- facing shores along the Central Coast. In addition, there is also some concern for coastal flooding. However, confidence is low with how impactful it will be.

LOX WATCHES/WARNINGS/ADVISORIES

CA...High Surf Advisory in effect until 9 PM PST this evening for zones 87-340-346-354-362-366. (See LAXCFWLOX). PZ...Small Craft Advisory in effect until 3 PM PST this afternoon for zones 645-676. (See LAXMWWLOX). Small Craft Advisory in effect until 9 PM PST this evening for zones 670-673. (See LAXMWWLOX).