10/322 AM.
Today will be very warm and dry, especially across the Los Angeles and Ventura County valleys. A cooling trend will start Tuesday, ahead of the incoming storm system. Rain may start as early as Wednesday night, however the heaviest rainfall is likely to occur Thursday morning into Thursday night.
(tdy-Wed), 10/259 AM.
The high pressure that has been over the region these past few days will have one final day in the area before weakening and moving eastward on Tuesday. As the high pressure moves out of the area Tuesday into Wednesday, the offshore flow will weaken and switch to onshore by Wednesday. Then, a trough will start to push into the west coast starting late Wednesday night.
Low clouds and dense fog are currently stretching along the coasts and pushing a little bit inland this morning, so dense fog advisories are currently in effect until 9am. Could see the Central Coast advisories being dropped early due to visibilities already lifting, likely due to the influx of higher level clouds disrupting the marine layer. With a similar marine layer pattern, high 500 mb heights, and similar offshore flow pattern, not expecting high temps today to be too different compared to yesterday. Meaning, highs will be in the 80s to 90s for a large portion of the region, except in the 60s to 70s closer to the coasts. These temperatures still remain about 10-20 degrees above normal for this time of year and some valley locations may come within a few degrees of daily record high temperatures again. As for winds, the weak Santa Ana winds will continue, but with lack of upper level support, expecting winds to remain under advisory levels.
On Tuesday, a significant cooling trend will start up as the onshore gradients start to increase and the 500mb heights start to drop. Most areas will see a 5-10 degree drop in temperatures both Tuesday, and again Wednesday. The marine layer will also deepen as a result of the onshore trends and lowering heights, and low clouds will likely push back into the valleys by Wednesday morning.
(Thu-Sun), 10/321 AM.
While still outside the high resolution model window, the EC and GFS as well as their ensembles are somewhat coming into agreement as well as the very end of the NAM window for the upcoming storm. Timing wise, Thursday looks to be the main period for rain, though there's about a 12 hour window where the models disagree on start times, with the NAM on the earlier side, bringing rain across the Central Coast as early as Wednesday evening, and the GFS and EC leaning towards the late Wednesday night to early Thursday morning time period. Regardless of the start time, the models do suggest that the front moving through will bring a heavy, but relatively short period of rain, falling within about 5-10 hours.
Like many of the systems we get in SoCal, orographics will play a role with south-southwest facing upslope areas getting twice to three times the amount of rain at coasts. With the GFS and its ensemble members showing rain totals slightly lower than the EC and its ensemble members (which seems to be more often than not), still leaning toward the previous forecast of rain totals of 1-2 inches across coasts and valleys and 2-4 inches across foothills and mountains. As for rain rates, between a quarter and half inch per hour rates are possible, with as much as 0.75 per hour in the upslope areas. However, there remains a 30 percent chance for higher totals than what is forecast. Especially as there is a possibilty for convection as this system is cold, models are ever so slightly hinting at upper level divergence, and the upper level jet overhead. If convection does come to be a factor, rain totals and rates could be locally higher wherever an individual cell may form.
Snow levels will likely remain at or above 8000' for most of the storm, then drop to around 6000 feet as the last of the precip is moving through later Thursday night into Friday morning. At this point it does not appear to be cold enough for snow on the Grapevine, but some light accumulations (2-4") are possible down to around 6000'.
Dry and cool weather expected next weekend with highs mostly in the upper 60s to low 70s.
Additionally, looking out ahead to early next week, another storm could be on the tails of the one at the end of this week. However, early model projections at this time, do show the storm would be lighter.
10/1120z.
At 0840Z at KLAX, the marine layer was 400 feet deep. The top of the inversion was near 1900 feet with a temperature of 27 degrees Celsius.
High confidence in VFR TAFs for KPRB, KBUR, KVNY, KPMD, and KWJF. There is a 10% chance for VLIFR-LIFR conds at KPRB, KBUR, and KVNY from 14Z-17Z. 30% chance for cigs to develop after 11/10Z.
Low confidence in TAFs for KSBP and KSMX due to low clouds remaining on the periphery of each terminal. If cigs are able to come in, vsbys of 1/2SM or less and cigs VV002 or lower are likely. 40% chance for VFR conds to prevail until a return of low clouds tonight (+/- 4 hours for arrival). There is a 30% chance for no low clouds later tonight.
Moderate confidence in coastal TAFs. VLIFR to LIFR conds will continue through the morning, and clearing times may be off +/- 2 hours. MVFR vsbys may linger through 22Z. Low confidence in return of low clouds tonight. 20-40% chance for no low clouds late tonight, highest at KCMA. If cigs develop, 25% chance for 1/2SM vsbys and cigs VV002.
KLAX, Moderate confidence in TAF. Clearing between 15Z-19Z. 3-5SM vsbys may linger through 23Z. Low confidence in arrival of OVC002-007 cigs tonight. 30% chance for no low clouds. If cigs develop, 25% chance for 1/2SM vsbys and cigs VV002. No significant east wind component expected.
KBUR, High confidence in TAF. 10% chance for VV001-VV002 cigs and vsbys less than 1/2SM thru 17Z. 30% chance for cigs OVC002-007 to develop after 11/10Z.
10/300 AM.
High confidence in winds and seas remaining relatively calm through Wednesday. Then southerly winds ahead of the incoming storm system will pick up late Wednesday, likely reaching 20-30 knots across a majority of the waters. After the front passes Thursday, gusty NW winds of 20-30 knots will pick up and continue across the entirety of the coastal waters through late Friday, potentially into Saturday morning. Gale Force gusts up to 40 knots are likely (60% chance) during this timeframe, especially south of Point Conception including the Santa Barbara Channel and nearshore waters adjacent to Los Angeles and Orange counties.
Along with winds, the aforementioned storm system will bring large, rough seas to the Coastal Waters Thursday afternoon through late Friday night, including 8-10 foot seas for the inner waters south of Point Conception. There is also a chance for thunderstorms with frequent lightning and potential waterspouts with the frontal passage. Thunderstorms can also create sudden, intense wind shifts.
Ca, Dense Fog Advisory in effect until 9 AM PST this morning for zones 340-346-349-350-354-362-366. (See LAXNPWLOX). PZ, NONE.
