05/534 PM.
A cooling trend will begin across Southwest California on Monday with temperatures back to near normal by mid week. Rain is likely to affect the area sometime between Thursday and Saturday.
(Sun-Wed), 05/728 PM.
***UPDATE***
For the overnight period, a weak and dry disturbance will move overhead, spreading high clouds across much of the area. This disturbance, along with the high cloud cover, could help to disrupt the marine layer enough to prevent low clouds from forming. Otherwise, there is a chance of low clouds and patchy dense fog along the near-coastal areas of the LA Coast late tonight into Monday morning. A better chance of low clouds and patchy dense fog will affect portions of the Central Coast tonight.
A cooling trend is expected across the area Monday through mid week. Looking at Monday, significant cooling is expected west of the mountains, especially for the coastal plains. The Central Coast will likely see highs drop by 10 to 15 degrees, with 5 to 10 degrees of cooling for the coast and valleys of LA and Ventura Counties.
Gusty onshore winds are affecting interior areas tonight, with speeds generally 15 to 25 mph, locally stronger near Lake Palmdale. Northerly gradients will begin to increase on Monday, with potential for Advisory level winds along the western Santa Barbara South Coast and through the Interstate 5 Corridor, especially Tuesday and Wednesday. At the same time, gusty onshore winds will continue to affect some of the interior passes and canyons into the Antelope Valley.
***From Previous Discussion***
Forecast-wise for the short term, main concern will be the winds. With the increasing northerly offshore gradients, northerly winds will become more widespread from day-to-day across the usual locations (such as the Santa Ynez Range and I-5 Corridor). For tonight, expect any northerly winds to remain below advisory levels. However by Monday night, and continuing through Wednesday, the chances for advisory-level winds increase. Based on HREF data, there is about a 60-80% chance of advisory-level northerly winds in the previously mentioned areas. So, future shifts will need to seriously consider this possibility. As for the onshore winds, the moderate onshore east-west gradients will generate the typical gusty southwesterly winds across interior sections. At this time, any advisory-level winds are expected to remain localized, mainly confined to the desert foothills.
As for temperatures, will expect several degrees of cooling for most areas on Monday. For Tuesday and Wednesday, there looks to a bit of a warming trend for the area (as the upper level ridge peaks in strength). No matter the day-to-day changes, most areas will remain around 6-12 degrees above normal through Wednesday.
(Thu-Sun), 05/143 PM.
For the extended, 12Z models continue to exhibit decent synoptic agreement, indicating a wet and unsettled pattern as an upper low moves across the region. Given the nature of upper lows in Spring, confidence in the details of the storm (timing, precipitation amounts, etc.), but there is high confidence in measurable rainfall for the area.
The upper low, and its associated rainfall, will begin to impact the area Thursday night/Friday with the rain chances continuing through Saturday. On Sunday, the upper low will be south and east of the area, but there still could be a chance of some additional wrap-around showers (mainly across LA county). Based on WPC QPF guidance, best forecast for now remains rainfall totals between 0.25 and 0.75 inches with local amounts around 1.00 inch. This matches up well with GFS/ECMWF ensembles indicating a 50-70% chance of totals between 0.50 and 1.00 inches. For amounts over 1.00 inch, ensembles indicate a 10-20% chance of that occurrence.
Along with the rainfall, there will definitely be a convective element with this system. With any upper low in April, there is always a concern about thunderstorms. Looking at ensemble MUCAPE, there is a noticeable increase Friday through Saturday. Additionally, 500 MB temperatures drop to around -23 degrees Celsius. So, that is enough to warrant a 10-20% chance of thunderstorms for all areas in the Friday through Saturday time frame. Of course, the exact track of the upper low will determine which areas will be most at risk for thunderstorms, but cannot get that detailed this far out. If thunderstorms do develop, there will be the potential for heavy showers, strong outflow winds and hail.
06/0033z.
At 0003Z at KLAX, there was no marine layer. There was an inversion with a top near 1600 ft with a temperature of 23 C.
High confidence in all TAFs for the valley and desert sites and low confidence in coastal TAFs except moderate confidence in KSBA where no CIGs are expected. Otherwise, lowered CIGs/VSBY are possible for all coastal sites, but low confidence in flight categories and timing. IFR conds are forecast but any site could be off by a category. The timing of low clouds arrival may differ by +/- 3 hrs from TAF times.
KLAX, High confidence in the 00Z TAF through 06Z. Low confidence after 06Z. Arrival and clearing times of CIGs may be off +/- 2 hours. CIG heights could range from: 004-015. No significant east wind component is expected.
KBUR, High confidence in VFR conds for the 00Z TAF.
05/715 PM.
Small Craft Advisory (SCA) level winds will develop south of Point Conception early Monday morning and will become widespread by Monday afternoon to include all Outer Waters, nearshore along the Central Coast, and the Santa Barbara Channel.
SCA winds will continue across all Outer Waters through Wednesday. Winds will diminish later in the week with moderate chances for SCA winds lingering south of Point Conception on Thursday and Friday. The Inner Waters will reach SCA levels at times mainly during the afternoon and evening hours - lower chances along the Central Coast later in the week. There is a 30% chance of GALE force winds Wednesday night across the Santa Barbara Channel and near the Channel Islands.
A storm system is expected to impact the coastal waters sometime Thursday into the weekend bringing rainfall and the potential for thunderstorms. Confidence is low on exact timing and details for now.
Ca, NONE. PZ, Small Craft Advisory in effect from 3 PM Monday to 3 AM PDT Tuesday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM Monday to 3 AM PDT Wednesday for zones 650-670. (See LAXMWWLOX). Small Craft Advisory in effect from 9 AM Monday to 3 AM PDT Wednesday for zones 673-676. (See LAXMWWLOX).
