20/530 PM.
A storm will bring rain and mountain snow to the area through Saturday morning, heaviest tonight into Friday morning in Los Angeles County. Gusty east winds are expected Friday in some areas. High pressure will push temperatures back to normal this weekend which will continue through the holiday week.
(Thu-Sun), 20/723 PM.
Showers continue across the area this evening as the main rain band with the system has exited the area. Generally, rain amounts between 0.25 and 0.50 inch have occurred across the coastal and valley areas, so far, with up 0.75 inch in the foothills and mountains. The colder air mass aloft with the storm system is punching into the Central Coast as the system's upper-level center slides through over San Luis Obispo County. Showers will likely become more scattered to numerous overnight tonight and into Friday morning. Jet dynamics, trailing near Point Conception will stretch this broader circulation and allow for a low level circulation to spin up in the southern California bight later tonight. This will likely enhance showers over Los Angeles County overnight tonight and into Friday morning, steepening lapse rates and providing some extra instability.
Some concern exists for a moderate to strong offshore flow event for Friday night and into Saturday morning as the system rotates farther to the south. A 'wet' Santa Ana wind pattern could develop between Friday night and Saturday. The latest NAM-WRF solutions advertise 40-45 knots at 950 mb and 850 mb, which ride high wind warning criteria, while EPS solutions lean more on the side of a moderate advisory for the typical locations, such as KSDB, K3A6, and KCMA. Deterministic NAM-WRF solutions could be the outlier of the solutions, so the best approach is to wait for more data. The next shift will be briefed about the possibility of offshore winds across the region.
***From Previous Discussion***
Very little has changed with the assessment of this storm and the expected impacts aside from the arrival of rain today was a few hours ahead of schedule. The initial round of precipitation has so far lived up to its billing of mostly light rain under a tenth of an inch per hour. This will more or less remain the case through early this evening as it advances through LA County. Meanwhile, along the Central Coast rain will be tapering off this afternoon and evening in most areas except for the far interior sections east of the Santa Lucias where scattered showers may continue into Friday morning. Amounts there have been under a half inch in most locations so far and any additional rain is expected to be on the lighter side.
For the second part of the storm tonight into Friday models have been consistent showing a very favorable severe weather threat across LA County, and in particular the southern part of the county. A strong vorticity lobe is expected to pinwheel around Pt Conception later tonight, resulting in backing winds inland and northwest flow over the Santa Monica Bay resulting in a large scale counter-clockwise rotation centered right over southern LA County. The forecast sounding for KLAX shows the backing winds towards the surface and a hodograph that could certainly support rotation and a small tornado. CAPE values close to 1000J/kg will support moderate updrafts that will be favorable for storm initiation. Overall, the pattern looks consistent with previous severe weather outbreaks, either from strong straight line winds from storms, heavy rain, or from a small tornado. The most favorable area appears to be southeast LA County from Pasadena to LA/Orange County line, but this could shift east or west depending on the ultimate track of the upper low.
Periods of heavy rain are likely at times overnight in LA County and high res models continue to indicate a 10-20% chance of rain rates reaching 1 inch per hour. WPC also has this area in a marginal risk for flooding. This will be a much more scattered heavy rain event than the last event so not all areas will get these rates, but the areas that do will could have significant hydrologic impacts.
The heavy rain potential is expected to decrease Friday morning, however, scattered showers are possible through the day with rain rates generally a quarter inch per hour or less.
Also of interest Friday is the increase in northeast winds creating a semi-wet Santa Ana event across portions of LA and Ventura Counties. A low end wind advisory may be needed tomorrow in the usual Santa Ana wind favored areas.
While the Santa Ana wind will have a detrimental affect on the shower chances across the coast and valley areas, the north facing mountain slopes will benefit from upslope flow that will maintain the shower potential into Saturday. Can't completely rule out a shower at lower elevations but any precip would be isolated and brief.
Several inches of snow in the mountains are expected with this system with the snow level around 6000 feet. Could see 8-12 inches of snow by Saturday in the LA County mountains above 8000 feet with the beneficial upslope flow Friday and Saturday.
Sunday is expected to be dry but on the cooler side.
(Mon-Thu), 20/226 PM.
While temperatures might cool a little on Sunday as the east winds turn off and return to a more normal onshore pattern, temperatures will remain around normal through next week (including Thanksgiving). The marine layer with low clouds and fog should also make a return, but might be limited by Wednesday as north winds start to form. A rain-free period through at least Black Friday looks almost certain. There are growing signals however for another storm sometime in the Saturday through Tuesday time range (Nov 29-Dec 2).
21/0130z.
At 2352Z at KLAX, there was no marine inversion. There was a deep moist layer up to 6000 feet.
A storm system will continue to bring widespread rain to the area tonight through Friday with periods of MVFR conditions, at times reducing to RA with possible IFR. The highest chances of heavy rain and IFR conds will be over LA County tonight through around 18Z Fri. Increasing east winds could cause LLWS 5-15 knots at most airports, especially Ventura and LA Counties. 10-20% chance of thunderstorms through 22/00Z. Low to moderate confidence in timing (+/- 3 hours).
KLAX, Low to moderate confidence in the 00Z TAF. Periods of RA and IFR conds likely through 05Z, but with a 30% chance of persisting through 12Z. 20% chance of thunderstorms, with +RA possible along with erratic, stronger wind gusts. Winds will remain easterly through the period, peaking in strength between 08Z to 15Z.
KBUR, Moderate confidence in the 00Z TAF. Periods of RA and IFR conds likely through 05Z, but with a 30% chance of persisting through 12Z. 20% chance of thunderstorms, with +RA possible along with erratic, stronger wind gusts.
20/714 PM.
Low confidence in forecast, especially regarding winds. It is best to alter any plans and remain in safe harbor until conditions improve this weekend.
A storm system will continue to move south before stalling inside the Southern California Bight, bringing periods of gusty winds, a large west- northwest swell (especially for the Outer Waters), rain, and possible thunderstorms into the weekend. The highest thunderstorm chances are from tonight into Friday morning. Any thunderstorm can produce erratic wind shifts, heavy rain, lightning, and even waterspouts. There is a 10% chance of brief Gale Force winds south of Point Conception through late tonight.
A combination of Small Craft Advisory (SCA) winds and seas will be common across the coastal waters through much of the weekend. Seas will increase to 10-12 feet across the Outer Waters, 10 feet for the nearshore Central Coast Waters, and 4 to 6 feet inside the Southern California Bight by late tonight. Wind directions will vary considerably through the course of the storm which will will result in rough, confused seas, especially south of Point Conception.
NE winds will become common tonight through Saturday morning as the core of the storm system remains nearly stationary and spins inside the Southern California Bight. Winds will be strongest from Ventura to Santa Monica (20-30 knots), but NE winds gusting up to 25 knots will occur nearshore from Palos Verdes south through Orange County. NE winds may reach the Channel Islands, but much lower chances for Catalina Island.
Ca, Flood Watch in effect until 7 AM PST Friday for zones 88-362-366>373-379-380-548. (See LAXFFALOX). High Surf Advisory in effect until 3 AM PST Monday for zones 340-346. (See LAXCFWLOX). High Surf Advisory in effect from 3 AM Friday to 3 AM PST Monday for zone 354. (See LAXCFWLOX). Winter Weather Advisory remains in effect until 10 AM PST Friday for zone 377. (See LAXWSWLOX). Winter Weather Advisory remains in effect until 4 PM PST Friday for zones 379-380. (See LAXWSWLOX). PZ, Small Craft Advisory in effect until 3 AM PST Friday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PST Saturday for zones 650-655-670-673-676. (See LAXMWWLOX).
