Marine Weather and Tide Forecast for Simi Valley, CA

.SHORT TERM (WED-SAT)...11/238 PM.

Early-afternoon satellite loops indicate that the forecast area remains along the poleward side of an extensive channel of moisture streaming east-northeastward from well offshore Baja California to the Central Rockies. The interaction of a reinforcing upper cyclonic circulation passing southward to the west of 30N 130W is facilitating the development of an inflection point in the moisture channel -- within a couple hundred miles south of the Channel Islands. This is causing the back edge of midlevel moisture to stall over the eastern/southern forecast area, despite the maintenance of the initial upper low presently located near the Central California coast.

As a result, it appears that -- while large-scale ascent over the local area is considerably weaker today than yesterday -- modest ascent peripheral to the reinforcing cyclone persists over the region (albeit decreasing). And with lingering midlevel moisture, slight chances for showers continue through late this afternoon across most areas south of Pt. Conception. This shallow convective activity will form and re-generate amidst modest diurnal heating and in the vicinity of sea-breeze boundaries and mountain-valley circulations, and then dissipate quickly by the evening in response to nocturnal cooling. To the north, weakening upper- level ascent around the initial upper low will still maintain low precipitation chances (generally less than 30%) north of Pt.
Conception through late this afternoon. Any additional rainfall amounts should be a tenth inch or less area-wide. Also, while the back-edge of richer moisture aloft is stalling over the area, most of this moisture is concentrated high in the atmosphere, as the midlevel dry slot from the initial upper low previously overspread the region. Thus, a general clearing trend will continue to be noted through the rest of today, with increasing sunshine especially for locations west/north of L.A. County.
However, a steady stream of high clouds will persist over the L.A.
area -- through tonight and all the way through Thursday night -- owing to the steady-state character of the moisture channel, anchored by the circulation around the reinforcing upper perturbation.

In response to these developments, precipitation development is mostly not expected starting later this evening. While the initial upper low will eventually move southward along the California Coast Thursday into early Friday, its diffusion and prior influx of the dry slot will be unfavorable for precipitation, though a couple mountain showers cannot be entirely ruled out around the peak of the diurnal heating cycle on Thursday, especially over the San Gabriels, however this activity should remain largely inconsequential. Thereafter, a low- amplitude, progressive, and shortwave ridge will cross the region through Saturday, resulting in no precipitation area-wide.

Also, VAD Wind Profile data suggest gradual weakening of the low- level jet that was earlier reinforced in conjunction with the developing moisture-channel inflection. Surface observations across the San Gabriels indicate wind gusts continue reaching around 30-40 mph in isolated locations. However, with the reinforcing upper low gaining further displacement away from the area along its southward track well offshore, the low-level mass response crossing the mountains and related surface winds are subsiding (albeit slowly owing to the complex interaction between the two upper lows), and the earlier Wind Advisory was allowed to expire at 1PM PST this afternoon. Nevertheless, locally gusty, though sub-advisory-level, southerly winds will be likely through this evening over the high terrain. Subsequently, a surface ridge will be slow to build north of the area causing local pressure gradients to turn modestly offshore and favor both weak Santa Ana winds and weak NW/N winds for Thursday into Friday. However, overall, winds will be notably weaker than they were during the past 24 hours.

Temperatures through Thursday will remain cool (near to below normal) in most areas with highs in the 50s and 60s. Temperatures in the coastal valleys and L.A. Basin will rise into the upper 60s and the lower 70s for Friday into Saturday, as midlevel heights rise. However, the strength of warming on Saturday will be tempered by the influx of mid and high clouds ahead of the next system. A few areas of patchy fog near the coasts and in the southern Salinas Valley cannot be ruled out each morning and night, especially when the ridge builds over the area Friday into Saturday, though confidence in fog development is too limited for inclusion in the forecast -- given the progressive pattern aloft and uncertainty in the re-establishment of the marine layer.

.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
12/0342Z.

At 2335Z at KLAX, there was a deep moist layer up to around 4800 feet.

Overall, moderate confidence in 00Z TAF Package. There is a 25% chance of cigs at any site from 12/08Z to 12/16Z due to recent rains. Most likely chances for cigs at KPRB.

KLAX...Moderate confidence in TAF. There is a 20-30 percent chc of cigs from 12/08Z-12/16Z. Good confidence that any east wind component remains below 6 kts through the forecast period.

KBUR...Moderate confidence in TAF with VFR conditions expected.

MARINE
11/738 PM.

Northwest winds will increase to 20-25 knots across the waters southwest to southeast of Point Concpetion (including nearshore)
later Thurs afternoon through the evening. The waters southwest to west of Point Concpetion will feature NW to NE winds remaining at Small Craft Advisory levels through Sat morning. A 40% chance of SCA level winds exists for the inner waters south of Point Conception Fri afternoon and evening. Additionally, the waters northwest of Point Conception will see increasing NW to NE winds and seas late Thurs night, and a SCA will likely be needed (60% chance).

While winds are expected to remain below SCA levels for the nearshore waters along the Central Coast, there is a 30% chance for NW to NE gusts reaching 25 knots late Thurs through Sat morning.

A series of storm systems will begin to enter the region Sun, bringing hazardous winds and seas along with rain and a low chance of thunderstorms to the entirety of the coastal waters through at least the middle of next week. There is a moderate chance for widespread Gale Force winds early next week. Confidence is growing in dangerous conditions for all boaters, thus it is encouraged to think about altering plans during this timeframe.

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 from 3 PM Thursday to midnight PST Thursday night for zones 650-655. (See LAXMWWLOX).
Small Craft Advisory in effect from 3 PM Thursday to 9 AM PST Saturday for zones 673-676. (See LAXMWWLOX).