08/103 AM.
A warming and drying trend will develop through the weekend as offshore flow establishes beneath building high pressure aloft. Temperatures above seasonal levels are likely Saturday through Monday with 80s and 90s being common across most coastal and valley locations. A storm system will likely move over the region the latter half of next week and could bring moderate to heavy rainfall.
(Sat-Tue), 08/735 PM.
***UPDATE***
Strong onshore trends in the N/S direction (-5 ydy to only -1.6 mb today) led to a few surprises. Notably 8 to 10 degrees of cooling across The Long Beach Area and into the San Gabriel Vly, the Ventura cst and the SBA south coast. Most of the rest of the area saw some warming with mid 80s to lower 90s common in most vly locations (just not the San Gabriel). The onshore trends are also generating a big surge in marine layer stratus that currently covers most of the coasts south of Pt Conception with a tendril extending around the Pt and into western the SBA county coast. The low clouds are also making a run for the San Gabriel vly. The marine layer is under a 1000 ft deep and it is likely that there will be some patchy dense fog as well.
High pressure will build into NV overnight and there will increase the offshore flow esp from the east. With no really upper level support this offshore push will not generate advisory levels but there will be 25 to 30 mph gusts in the usual Santa Clarita Vly to Camarillo corridor. These winds will chase the low clouds away from the Malibu to Ventura County area, but low clouds may linger across the LA South coast and San Gabriel Vly.
The offshore flow will bring a big warm up to most of the area, esp the areas that were cool today. There is a 25 percent chc that the Long Beach area will only warm 8 to 12 degrees not 12 to 18.
Updated the forecast to bring more clouds and dense fog to the overnight period and portions of the morning period.
***From Previous Discussion***
Overall, 12Z models in very good synoptic agreement through the short term period. At upper levels, ridge builds over the area, peaking in strength on Monday, before weakening on Tuesday. Near the surface, weak to moderate offshore flow will prevail through Monday with weak diurnal flow on Tuesday.
Forecast-wise, main focus in the short term will be offshore winds and temperatures. With respect to winds, LAX-DAG gradient looks to peak around -4.0 to -4.5 mb in the morning hours through Monday. However, upper level wind and thermal support is not too impressive. So, will anticipate gusty northeast winds in the usual Santa Ana wind-prone spots of Ventura/LA counties as well as the Santa Lucia Range. Looking at high resolution ensembles, the chances for widespread advisory-level winds are about 30-40%. So at this time, will not issue any wind products with the expectation of only localized advisory-level winds through Monday.
As for temperatures, the combination of the offshore surface gradients and upper level ridge will bring quite the warming trend to the area through Monday. At this time, based on model data and TEMP STUDY data, high temperatures on Sunday and Monday look to be around 10-15 degrees above normal for most areas. Some locations will approach record highs, but do not anticipate any record breaking readings. Overnight lows will also be warm, especially across the foothills and valleys. At this time, the chances for any heat-related products looks to be about 20-30%. So, will not issue anything at this time, but if things look to be a bit warmer during the day and/or night, then this will need to be reconsidered.
As for clouds, the marine layer stratus looks to "hug" the immediate this evening and into early Sunday morning before the developing offshore flow finally obliterates it. So, there will be some dense fog this evening and into early Sunday morning for the immediate coastal areas south of Point Conception. Otherwise, skies will remain clear trough Monday. By Tuesday morning, there will be the chance of some stratus/fog returning to coastal areas.
(Wed-Sat), 08/103 PM.
Overall for the extended, 12Z models continue to be on the same synoptic page. There are some differences in the details (timing, amounts, etc.), but all models indicate a wet and unsettled period of weather.
On Wednesday, increasing clouds and cooler temperatures can be expected a storm system approaches the West Coast. Some light warm frontal precipitation will be possible Wednesday evening, ahead of the storm.
For Thursday and Friday, both the deterministic GFS/ECMWF and their respective ensembles indicate a storm will roll across the area and will bring significant measurable rainfall to the area. At this time, the ECWMF and its ensembles are slower with the progression of the system and generate more significant rainfall the GFS gang. So with the model differences, confidence in the forecast details remains on the low to moderate side. Current best forecast is for a widespread 1-3 inch rain event with some chance of a lesser event or even a heavier event. Additionally, based on thicknesses, snow level look like they could drop to around 6000 feet and the resorts could see some decent snow accumulations. However, much like with rain totals, confidence in the exact snow levels remain on the low to moderate side.
09/0538z.
At 0415Z at KLAX, the marine layer was 1100 feet deep. The top of the inversion was 2500 feet with a temperature of 24 C.
High confidence in TAFs KPMD and KWJF.
Moderate confidence in TAFs for KPRB, KSBP, and KSMX. There is a 20-30% chance for VLIFR to LIFR conds at KSBP from 03Z-18Z, and KPRB from 12Z-17Z. There is a 30% chance that no flight restrictions develop for KSMX.
Low confidence in TAFs for coasts and valleys of LA and Ventura Counties. There 15% chance for no clearing at KOXR, KCMA, KSBA, KSMO, and KLAX. Clearing and arrival times may be off +/- 3 hours. While cigs are present at KSMO, KOXR, KCMA, KBUR and KVNY, there is a 40% chance vsbys of 1/4SM and cigs VV001.
KLAX, Low confidence in TAF. 20% chance for vsbys less than 1/2SM and cigs less than VV002, between 10Z and 17Z. There is a 20% chance of no daytime clearing on Sunday. No significant east wind component expected.
KBUR, Low confidence in TAF. 40% chance for VV001-VV002 cigs and vsbys less than 1/2SM from 07Z-17Z, with the greatest chance before 13Z.
08/827 PM.
Seas have dropped just below Small Craft Advisory (SCA) levels across all the outer waters. 10-15 knot E to NE winds will be common early Sunday morning through Monday afternoon nearshore from Ventura to Point Dume. Otherwise, relatively mild conditions will continue into at least Tuesday.
Moderate to high confidence that a storm system will approach the Coastal Waters as early as Wednesday night, bringing rain and at least Small Craft Advisory level winds and large steep and choppy seas to the region, including nearshore. There is a potential for Gale Force Winds. Confidence is low in timing and magnitude of impacts.
Ca, NONE. PZ, NONE.
