Altadena, CA Weather

Synopsis

26/811 PM.

Gradual cooling will continue through the week, with increasing cloud coverage and a high chance of rain sometime from Friday- Monday, with highest chances Saturday (Halloween) through Sunday. Additionally, coastal flooding issues will continue due to very high tides.

Short term (mon-thu)

26/809 PM.

Very quiet weather will continue over the area through Thursday with minimal weather impacts expected aside from the continual coastal flooding threat due to very high tides.

The upper pattern will continue feature weak troughing with gradually decreasing 500 mb heights over the next few days. Weak onshore flow will likely continue each day as well. Temperatures will cool each day, bringing the region to generally below seasonal normals as far as max temps go by Thursday.

Increasing cloud coverage will become common through the week as well, with skies becoming partly to mostly cloudy thanks to the influx of mid and high level clouds. Mid and high clouds will become most extensive Wednesday and Thursday. Some coastal areas may see a return of marine layer clouds over the next couple of nights, but confidence is fairly low, especially with increasing higher clouds. However, Wednesday appears to have the better chance of more widespread low clouds, with low clouds potentially extending into some coastal valley areas.

Long term (fri-mon)

26/230 AM.

On Thursday, slightly cyclonic SW flow develops over Srn CA as another upper low spins out of British Columbia. It will be a mostly cloudy day due to a combination of low mid and high clouds. Due to the extensive cloud cover max temps will flatten out and almost all areas save for the mid and upper mtn elevations will see max temps in the upper 60s or lower 70s.

On Friday the flow turns more to the SW and the cyclonic turning sharpens. The day will most likely resemble Thursday will cloudy skies and cooler than normal temps (4 to 8 locally 12 degrees under normal). But there will be a 10 to 20 percent chc of brief and light shower or two.

All eyes on the next storm system, which unfortunately, remains a low confidence forecast. One problem comes from timing and placement of a moisture plume moving in from the SW and the other is the evolution of the upper low over WA and OR. Until the mdls settle down and begin to converge on a solution the forecast will have to be broad brushed. There is very high confidence that it will rain during the weekend, but low confidence on onset timing and peak timing. Rainfall amounts are also a low confidence prediction. The current estimate calls for most areas south of Pt Conception to recieve about an inch with higher amounts in the foothills and coastal slopes. Most mdls keep most of the plume to the south of Pt Conception so areas north of there will probably see half that amount. There is a 20 to 30 percent chc that rainfall totals will be double that amount. Stay tuned for the latest details on this developing storm system.

Snow levels will generally be high, although light snow may occur at high elevations on the backside of the storm.

Max temps will only be in the mid to upper 60s through the weekend.

An active weather pattern may continue behind next weekend, with a 50-60 percent chance of above normal precipitation into the following week.

Aviation

26/2336Z.

At 2256Z at KLAX, the marine layer was 1100 feet deep with an inversion topped at 1800 feet with a temperature of 20 degrees C. High confidence in VFR TAFs at KBUR, KVNY, KPMD, & KWJF.

Moderate confidence in VFR TAFs at KOXR, KCMA, & KSBA. There is a chance for IFR cigs 12Z-18Z at KSBA (20%) & KOXR (30%).

Low confidence in TAFs for KPRB, KSBP, KSMX, KSMO, KLAX, & KLGB. 30-50% chance VFR conds prevail. If cigs develop, 30-40% chance for 1/4SM-1/2SM vsbys and VV001-002 conds at KPRB, KSBP, & KSMX.

KLAX: Low confidence in TAF from 10Z-18Z, high confidence otherwise. 40% chance VFR conds prevail. Cigs may arrive as low as BKN007 from 10Z-15Z. Any east wind component is expected to remain below 7 kt.

KBUR: High confidence in VFR TAF.

Marine

26/1026 AM.

For the Outer Waters, moderate to high confidence in current forecast. This afternoon through Wednesday, high confidence in winds increasing to SCA levels. On Thursday and Friday, there is a 50-60% chance of SCA level winds continuing.

For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. For this afternoon and evening, there is a 60-80% chance of SCA level winds. For Tuesday through Friday, there is a 30-50% chance of SCA level winds each afternoon and evening.

For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. Through Friday, high confidence in winds and seas remaining below SCA levels for most of the southern Inner Waters. The only exception will be the western half of the Santa Barbara Channel with a 50-60% chance of SCA level winds in the late afternoon and evening hours Monday and Tuesday, then again on Friday.

Dense fog is likely to form over portions of the coastal waters over the next 48 hours, but low confidence on exactly when and where.

A significant storm system will impact the area sometime in the late Friday through early next week timeframe. There is a wide range of outcomes, but there is at least a low threat of a 12-24 hour period of strong winds and step seas.

Watches, warnings and advisories

CA: Coastal Flood Advisory in effect until 1 PM PDT Thursday for zones 87-340-346-349-350-354-362-366. (See LAXCFWLOX). PZ: Small Craft Advisory in effect until 9 PM PDT this evening for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Tuesday for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Wednesday for zones 670-673-676. (See LAXMWWLOX).