FXUS66 KLOX 140514 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 1014 PM PDT Sun Oct 13 2024 .SYNOPSIS...13/616 PM. Little change in weather expected through Tuesday, except a cooling trend and expanding low clouds over some coastal areas each day. An upper level low will move over the western United States Wednesday into Thursday, bringing widespread cooling and possible drizzle. North to northeast winds will bring warming and drying to the area Thursday into the weekend. && .SHORT TERM (SUN-WED)...13/913 PM. The latest satellite imagery shows an upper-level ridge of high pressure remaining in place over the region. Some building of the ridge aloft will take place through Monday as the feature amplifies ahead of a broad upper-level trough over the eastern Pacific Ocean. Despite the ridge in place, temperatures in the lowest levels of the atmosphere will cool through the period due an upper-level trough to the northeast over Utah enhancing onshore flow. This moderately strong onshore flow regime will remain in place over the coming days and likely continue a cooling trend into midweek. Night through morning low clouds and fog will continue to be a staple of the forecast through next several nights and mornings. The marine layer depth will become slightly shallower over the next couple of days due some slight weakening of the onshore flow, but overall not much change is expected. A marginally strong northerly pressure gradient over the region last night into today has weakened some this evening. The latest forecast guidance suggest winds being a touch weaker than previously surmised, but enhanced breezy northerly winds will continue through the Interstate 5 Corridor and across southern Santa Barbara County tonight. In the latest forecast update, a few tweaks were made to low clouds and fog for tonight and into Columbus Day, but overall the forecast package was in reasonably good shape. ***From Previous Discussion*** On Wednesday, a combination of a slight lowering of 500 mb heights, strong onshore flow, and a fairly large coastal eddy, will result in a deepening marine layer bringing the potential for patchy drizzle for areas south of Point Conception. There is a slight chance of (very light) rain across the northwestern half of the Central Coast and San Luis Obispo County as a result of a dying front moving into that area. Any precip accumulation for this area is expected to be under a 0.01 inch, so not very impactful and just barely worth mentioning. High temperatures on Wednesday will cool a few degrees across the entire forecast area but keep in that 75 to 85 range for the interior mountains and Valleys, and 65 to 75 range for the coasts and coastal valleys. .LONG TERM (THU-SUN)...13/239 PM. Upper level heights will continue to decrease on Thursday, resulting in a few more degrees of cooling across the entire forecast area. High temps Thursday will likely fall in the 70 to 80 range across the interior valleys and mountains, with the coastal areas falling to 60 to 70 degrees - making the forecast area around 5-10 degrees below normal for this time of year. Gusty northwest to north wind gusts of 30 to 50 mph will be likely over the wind prone mountain and valley areas into Thursday night. Starting Thursday evening, there is a lot more uncertainty than usual in the extended forecast, mainly due to disagreement between the deterministic models and the different ensembles, resulting in a wide range of outcomes for later this week. A deep low pressure system that is currently just south of Kodiak Island, Alaska will start to move southeastward, resulting in a deep trough moving across the entire western half the U.S. through the latter half of the week. Where exactly and how fast this low pressure system (and resulting trough) will move is the question as it will alter the direction and strength of winds Friday through Sunday. Unfortunately, the answer to this question is what the models are struggling to agree on. The latest deterministic ECMWF run and ensemble members are starting to align with the GFS and its respective ensemble members, showing the low pressure system moving southward over the meeting point of Nevada, California, and Arizona by Friday afternoon. If this happens, winds will increase on Friday from the north, and shift north to northeast overnight Friday, bringing a weak to moderate Santa Ana wind event with a warming and drying trend. Wind gusts could be in the 40 to 60+ mph range, with healthy upper level support and offshore surface pressure gradients. An additional component that the most recent GFS and ECWMF deterministic runs are hinting at is some wrap- around moisture and a little bit of instability, which would could bring some showers to the northern slopes of the interior mountains - but again, extremely high uncertainty in this happening. If we take a look at the past runs of the ECWMF and the majority of the EPS projections, the low pressure system and trough will sweep eastward over Colorado and confine the winds to the interior mountain areas with speeds peaking in the 20 to 30 mph range. Temperatures and the impact to the marine layer will be dependent on which scenario comes into fruition. High pressure should build aloft once the low moves through by the weekend, and temperatures will rise. If the strong offshore flow scenario wins out however, coastal and adjacent valley areas would see highs in the 90s being fairly common by next weekend. If the weaker wind scenario plays out, weekend temperatures will struggle to get out of the 80s. To wrap it all up and really drive home the point, there is a lot of uncertainty in the long term. The majority of the most recent model runs are trending toward gusty north to northeast winds Friday through the weekend, resulting in a weak to moderate Santa Ana event bringing a warming and drying trend across the forecast area. However, since this is the first run that models are starting to agree, there is still a considerable amount of uncertainty, and would like to see more consistency across each model and each run. && .AVIATION...14/0511Z. At 2243Z at KLAX, the marine layer was around 1300 ft deep, with an inversion up to 3900 ft with a temperature of 18 C. High confidence in TAFs for KPMD and KWJF. Low confidence in TAFs for KPRB, KBUR, and KVNY due to uncertainty in arrival time and flight cats. Arrival time could be off +/- 2 hours and flight cats will likely bounce between VLIFR to LIFR once cigs arrive. Moderate confidence in KLAX, KSMO, and KLGB. There is a 20% chance conds remain MVFR-VFR thru the period. There is also a 20-30% chance cigs don't clear in the morning/afternoon, with best chances at KLAX. Low to moderate confidence in remaining coastal TAFs. Lowest confidence at KSMX and KSBP due to uncertainty in minimum flight cat. LIFR-IFR conds are most likely, but there is a chance for VLIFR conds at KSMX (30%) and KSBP (20%) thru the morning hours. There is a chance that conds remain IFR or greater thru the morning hours at KSBA (30%), KOXR (30%), and KCMA (40%). KLAX...Moderate confidence in the 06Z TAF. There is a 20% chance cigs remain OVC010 or higher thru the morning. There is a 30% chance cigs don't clear at all this afternoon. There is a 20% chance of a 6 KT east wind component between 08Z-20Z Mon, otherwise no significant east wind component expected. KBUR...Low confidence in the 06Z TAF. Arrival of cigs tonight may be off +/- 2 hours, and vsbys of 1/4SM-1/2SM and cigs VV001-VV002 will be possible from 08Z-17Z. && .MARINE...13/841 PM. For the waters beyond 10 NM from the Central Coast to San Nicolas Island (Outer Waters), moderate confidence in the forecast. Winds will likely remain at Small Craft Advisory (SCA) levels tonight through late Monday night, with a lull possible Monday morning. Tuesday afternoon/evening there is a 30% chance of SCA winds, followed by a 60% chance Wednesday. By late Wednesday night, there will be a 50% chance of GALES lasting through Friday (Otherwise, SCA winds are likely Wednesday night through Friday). Seas of 10 to 15 feet are expected Thursday and Friday. Nearshore along the Central Coast, SCA conditions are likely through late tonight, mainly for seas as the wind gusts are subsiding below 20 knots. SCA conditions are possible (50%) again Monday late afternoon through the evening hours, less likely during the same time period Tuesday (30%). Wednesday afternoon through Friday, SCA winds are likely (50-60%) with a chance of Gales (40%) Thursday afternoon and night. Large seas of 9 to 14 feet are forecast for Thursday through Friday. For the Santa Barbara Channel, SCA level winds are possible (40% chance) across the western and southern portions of the Channel during the afternoon through evening hours Monday and Wednesday. SCA winds will be likely (50-70% chance) Thursday and Friday, with a 20% chance of GALES. Seas are likely to peak around 8 to 12 feet across the channel Thursday and Friday. For the nearshore waters off the Los Angeles and Orange County coasts, high confidence in winds and seas staying under SCA levels through Wednesday. Thursday and Friday there is a 50% chance of NW to W SCA winds and seas. SCA level winds from the NE due to Santa Anas are possible (30% chance) Saturday, but confidence in timing and maximum wind speed is low. Seas of 6 to 10 feet will be possible Thursday night into Friday. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...Small Craft Advisory in effect until 3 AM PDT Monday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Tuesday for zones 670-673-676. (See LAXMWWLOX). && $$ PUBLIC...Hall/Lund AVIATION...Lewis MARINE...Smith SYNOPSIS...RK/Smith weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox