FXUS66 KLOX 011156 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 456 AM PDT Sun Sep 1 2024 updated aviation discussion .SYNOPSIS...01/415 AM. A warming trend will occur for the rest of this weekend and into early parts of the upcoming week, before significant warming and dangerous heat materialize for mid to late week. While the significant heat will start as early as Tuesday for the Western San Fernando Valley, the most dangerous heat will be on Thursday and Friday. Areas of low clouds and patchy night through morning dense fog will affect areas near the coast through Monday, persisting along the Central Coast throughout the week, with clear skies elsewhere for the remainder of the week. Precipitation is not expected. A cooling trend is expected next weekend. && .SHORT TERM (TDY-TUE)...01/415 AM. A compact upper low and surrounding trough are forecast to track from the waters off the northern California and Oregon coasts to the northern Rockies through Tuesday. This track will be to the north of an upper anticyclone re-focusing from the central Rockies and Great Basin to the Desert Southwest. Subsidence-induced surface ridging over the east Pacific waters upstream of the upper low will maintain offshore pressure gradients for SBA-SMX through early in the upcoming week. Modest Sundowners are expected to bring generally sub-advisory-level winds to southwest Santa Barbara County. However, wind-prone areas in the Santa Ynez range near Gaviota and Refugio could experience occasional 45-mph wind gusts during the evening hours through Monday, and there is a 20% chance for short-duration Wind Advisories to become necessary. Meanwhile, the east Pacific surface ridge will continue to favor a modest north-northwesterly coastal jet off the Central Coast into the upcoming week. Speed shear east of this jet across the Southern California Bight will sustain a broad and diffuse cyclonic eddy over the coastal waters south of Point Conception, with multiple circulation centers. On the east flank of this broader circulation, return flow will maintain the inland presence of the marine layer near the coast of Los Angeles and Ventura Counties, where night through morning low clouds are expected through Monday, dissipating by each afternoon. However, for the Central Coast, more prominent onshore flow along the edge of the coastal jet will foster a stronger influx of the marine layer. Also, patchy night through morning dense fog is expected in the marine layer, and there is a 20% chance for localized short-duration Dense Fog Advisories to become necessary. The eastward progression of the aforementioned upper low and upstream subsidence will correspond to an offshore trend in pressure gradients early in the upcoming week. The marine layer edge will correspondingly retreat closer to the coast, especially south of Point Conception, which will lessen the coverage of clouds and fog. By Tuesday, any night through morning low clouds and fog are expected to be confined to the Central Coast vicinity, with clear skies prevailing elsewhere. Given the offshore trend in pressure gradients and retreating marine-layer edge, temperatures will warm by 1-3 degrees from day to day from the beaches to the coastal valleys. High temperatures by Tuesday will generally range from the middle and upper 70s at the beaches, to the 80s and 90s and locally near 100 degrees over the coastal valleys, with highs upwards of 95 to 103 degrees over the interior valleys. Cooler conditions will persist near the Central Coast where onshore flow will be stronger. There is a notable exception regarding these Tuesday temperatures, however, and that is across the Western San Fernando Valley. Present indications are that the greatest day-to-day warming will be over the Western San Fernando Valley. This is a mesoscale zone where encircling hills and mountains will locally favor greater downslope flow associated with the offshore-trending pressure gradients, and related adiabatic warming will locally accentuate heating rates. By Tuesday, high temperatures around Woodland Hills are projected to increase to around 107 degrees, bringing Major HeatRisk to the Western San Fernando Valley as early as Tuesday. This will mark a localized early onset of the significant heat, which will affect a much broader portion of Southern California later in the week. Given the strong synoptic and mesoscale signals favoring the early onset of significant heat across the Western San Fernando Valley on Tuesday, combined with high confidence that some of the most significant heat impacts occur in this zone later in the week, an Excessive Heat Watch has been issued for the Western San Fernando Valley starting on Tuesday. There is also a 30% chance for heat headlines to become necessary as early as Tuesday over nearby locations from the Santa Clarita Valley through the Eastern San Fernando Valley through Calabasas and Agoura Hills, as well as the Santa Monicas -- especially if temperatures were to trend warmer in subsequent forecasts over these areas. Dry conditions are expected to prevail through the remainder of this weekend and into early parts of the upcoming week. Diurnally- enhanced orographic circulations into modest midlevel moisture could yield a few high-based cumulus build-ups during the afternoon hours each day around the San Gabriel Mountains. However, with precipitable water over the higher terrain remaining below 1 inch, precipitation is not expected. .LONG TERM (WED-SAT)...01/415 AM. By middle and latter parts of the upcoming week, the continued re-focusing of the aforementioned upper anticyclone across the Desert Southwest will bring rising midlevel heights over the region. Medium-range model guidance suggests 500-mb heights rising upwards of 595-597 dam over the area by Thursday. This will occur after the previously mentioned marine-layer edge retreats closer to the immediate coast while pressure gradients continue to trend offshore. This will cause significant warming area-wide, with the most dangerously hot conditions expected Thursday and Friday. One exception will be near the Central Coast, where more prominent onshore flow along the edge of the coastal jet will continue to maintain stronger presence of the marine layer. By Wednesday, high temperatures are expected to reach 100-105 degrees in many interior sections, and the 90s to around 103 degrees across most coastal valleys, with 70s and 80s near the coast. Another few degrees of warming will occur going into Thursday and Friday, bringing widespread Moderate to Major HeatRisk -- even to locations nearing the coast, aside from the Central Coast. Present indications are that isolated locations could experience Extreme HeatRisk on Thursday and Friday across Los Angeles and eastern Santa Barbara Counties away from the immediate coast. Warm overnight low temperatures in the lower to middle 70s will bring little relief from the heat in many locations. The most prolonged heatwave and most significant heat impacts are expected to focus through late week in the Western San Fernando Valley due to the aforementioned mesoscale effects. This is where the Excessive Heat Watch, which starts on Tuesday, continues through Friday. High temperatures Thursday and Friday will likely exceed 110 degrees around Woodland Hills, and there is a 20% chance for these temperatures to reach Top-10 readings at Pierce College -- in excess of 115 degrees. Needless to say, this has the potential to be a very dangerous heatwave for the Western San Fernando Valley. Elsewhere across much of the region, there is a greater than 95% chance for heat headlines to be issued for Wednesday and/or Thursday through Friday in subsequent forecasts. This includes a greater than 95% chance for Excessive Heat headlines to become necessary in some areas. By next weekend, model guidance suggests midlevel heights lowering over the region as an upper trough crosses the Pacific Coast. While model solutions vary considerably regarding the depth, amplitude, and phasing of this disturbance, there is at least medium confidence that the significant heat will have concluded prior to next weekend. Aside from persistent marine stratus and night through morning fog along the Central Coast where onshore flow will be strongest, most areas will experience clear skies for much of the upcoming week. Precipitation is not forecast to occur through next weekend, as the upper pattern does not favor any appreciable influx of monsoonal moisture. && .AVIATION...01/1155Z. At 0915Z at KLAX, the marine inversion was 700 feet deep. The top of the inversion was at 3000 feet with a temp of 29 deg C. High confidence in 12Z TAFs for KPRB, KBUR, KVNY, KPMD, and KWJF. Moderate confidence in 12Z TAFs for KLGB, KLAX, KSMO, KCMA, KOXR and KSBA. Arrival of cigs this morning may be off 1-2 hrs from current fcst. Cigs may arrive as MVFR and bounce between IFR- MVFR once they move in this morning. However, there is also a 40% chance that no low clouds move into these airfields this morning. There is a 50% chance at best for IFR cigs to move into KLAX and KLGB by 10Z or 11Z tonight. Moderate confidence in 12Z TAFs for KSBP and KSMX. LIFR/IFR conds are expected until later this morning, altho conds may remain IFR. IFR/LIFR cigs/vsbys are expected at KSMX aft about 03Z. Arrival of the cigs may be 2-3 hours too early from current fcst. KLAX...Moderate confidence in 12Z TAF. IFR cigs are expected about 14Z-17Z, but there is a 40% chance that no low clouds develop. IFR/MVFR cigs are forecast to move back into the airfield by 10Z tonight and persist until about 17Z Mon, but timing could be off +/- 2 hours. No significant east wind component is expected. KBUR...High confidence in 12Z TAF. && .MARINE...01/159 AM. For the outer waters, there is moderate to hi confidence in the current forecast. Small Craft Advisory (SCA) winds can be expected for much of these waters through tonight, and continue for the southern two outer waters zones (PZZ673/676) for Monday and Monday night. Conds should then be below SCA levels Tuesday through Thursday altho there is still a 20%-30% chance of occasional SCA gusts especially afternoons and evenings for the southern outer waters. For Thursday night, there is a 40%-50% chance of SCA level wind gusts for the central and southern outer waters. For the inner waters north of Point Sal, generally hi confidence in the current forecast. Conds are expected to be below SCA levels thru Thursday night. However, there is still a 20%-30% chance of SCA wind gusts at times afternoons and evenings through Monday. For the Santa Barbara Channel, moderate confidence in the current forecast. There is a 50% chance of SCA level wind gusts late today and this evening, otherwise conds are expected to be below SCA levels thru Thursday. However, there is still a 20%-30% chance of SCA wind gusts at times afternoons and evenings Monday through Wednesday for the western portions. There is also a 40%-50% chance of SCA level wind gusts for western portions Thursday evening. For the southern inner waters, there is moderate to hi confidence in the current forecast. Winds and seas are likely to remain below SCA levels through Tuesday. There is a 20%-30% chance of SCA wind gusts afternoons/evenings mainly western portions Tuesday night through Thursday, increasing to a 40%-50% chance Thursday evening. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Excessive Heat Watch in effect from Tuesday morning through Friday evening for zone 372. (See LAXNPWLOX). PZ...Small Craft Advisory in effect from 5 PM this afternoon to 3 AM PDT Monday for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Monday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Tuesday for zones 673-676. (See LAXMWWLOX). && $$ PUBLIC...Cohen AVIATION...Sirard MARINE...Sirard SYNOPSIS...Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox