Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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514 FXUS66 KLOX 010324 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 824 PM PDT Sun Jun 30 2024 .SYNOPSIS...30/219 PM. A significant heatwave will impact a large part of the area this week and continue into next weekend. Temperatures will become dangerously hot across much of the area. High temperatures by mid to late week are expected to reach 95 to 105 degrees in many areas away from the coast, with highs upwards of 105 to 115 over interior valleys and foothills including the Antelope Valley. Also, by late week, very warm to hot conditions could extend closer to the coast. Patchy night and morning dense fog will be possible near the coast early this week. && .SHORT TERM (SUN-WED)...30/823 PM. ***UPDATE*** There was a very weak upper level trough over the area early this evening with H5 heights around 590-591 dam. The high H5 heights was helping to promote a strong and shallow marine inversion this evening. The depth of the inversion was around 800 ft at both VBG and LAX. A warm layer was noted in the soundings at around 2400-2600 feet or so with temps in the low 80s. Skies were mostly clear across the region early this evening, except for some low clouds and fog mainly along the Malibu coast and portions of the L.A. County beaches. Thru the overnight hours, low clouds are expected to expand over the SoCal Bight as an eddy develops, with low clouds pushing into much of the coast from L.A. county this evening to the VTU and S SBA County coast later tonight. Low clouds are also expected to quickly develop along the Central Coast overnight and move inland to the Santa Ynez Vly late tonight. Patchy dense fog will likely accompany the low clouds overnight, but there is also a chance the dense fog could have a larger aerial extent which would lead to the issuance of a Dense Fog Advisory. Otherwise and elsewhere, mostly clear skies can be expected thru the night. As a result of the aforementioned warm layer in the soundings, low temperatures tonight in the foothills and lower mtns should be quite mild. Temps are expected to fall only into the lower 70s for much of this area altho a smattering of mid 70s would not be out of the question. ***From Previous Discussion*** Midlevel heights will continue to gradually rise early this week, as an upper-level ridge builds over Southern California. Then by Wednesday, a strong upper-level anticyclone over the east Pacific will begin expanding eastward over California, bringing more significant midlevel height rises. In response to these developments, the warming trend will continue early this week, and will become more significant on Wednesday. Tuesday high temperatures are expected to have warmed to 95-105 degrees in most areas, 105-110 over the Antelope Valley, and the middle 70s to the middle 80s near the coast. Temperatures are expected to increase further, by another few to several degrees, for Wednesday. Much of the earlier Excessive Heat Watch has been upgraded to an Excessive Heat Warning for interior areas starting on Tuesday, with the onset of the significant heatwave. Thereafter, with the continued warming trend into Wednesday, confidence continues to increase that significant heat impacts will extend farther toward the coast as the week progresses. Correspondingly, by Wednesday across portions of the coastal ranges, foothills, and valleys of Southern California closer to (but not including) the coast, high temperatures are forecast to become dangerously hot -- with highs up to 95 to 105 possible, and upwards of 105 to 110 possible over valleys well away from the coast. An Excessive Heat Watch has been issued starting on Wednesday for these areas -- from the San Fernando Valley through the Santa Monicas, Calabasas, and Agoura Hills, to the central Ventura County valleys and through the Santa Ynez Range and Santa Lucia Range. Local surface pressure gradients will be neutral to moderately onshore through the middle part of this week. This overall pattern will maintain night and morning low clouds and fog in the marine layer near the coast and over coastal valleys, scattering out and clearing during the afternoon. With the continued rising heights over the region, patchy dense fog will remain possible in the shallow marine layer over coastal areas and some coastal valleys. Otherwise, dry conditions are expected into middle parts of this week. Occasionally gusty southwest to northwest winds are expected each afternoon over interior areas. The strongest winds, up to 25-35 mph, are expected over the Interstate-5 corridor as well as the Antelope Valley and nearby foothills. However, the issuance of Wind Advisories is not expected. .LONG TERM (THU-SUN)...30/219 PM. Dangerously hot conditions are expected to continue over the region through the remainder of this week and into next weekend, as the strong upper-level anticyclone continues expanding eastward over California. Medium-range model guidance continues to suggest 500-mb heights rising to around 597-600 dam to the north of the area by late this week and into next weekend. This should correspond to daily high temperatures of 95-105 degrees in many areas away from the coast, with highs upwards of 105-115 over interior valleys and foothills including the Antelope Valley. Warm overnight low temperatures will worsen the effects of extreme heat, with lows remaining above 80 degrees at some locations over interior valleys and nearby foothills. Excessive Heat Warnings and Watches continue through late this week, next weekend, and into early Monday morning, July 8. HeatRisk is forecast to become Major to Extreme in most areas away from the coast during latter parts of the week and into next weekend. In addition, very warm to hot conditions will have the potential to expand and bring significant heat impacts toward the coast, especially for Thursday and Friday. This would occur as the marine layer becomes increasingly shallow and retreats toward the coast provided the rising heights aloft, which could also eventually limit the chances for night and morning dense fog. Heat headlines may need to be expanded to include a larger portion of Southern California in later forecasts. This is especially the case for southern Santa Barbara County, where local SBA-SMX pressure gradients are forecast to become offshore around 1.5-2 mb for Thursday and Friday -- behind a weak wave tracking over the northern Rockies late this week. The mass response to the offshore pressure gradient -- including enhanced Sundowner winds -- will have the potential to transport the extreme heat over interior areas toward the Santa Barbara County coast, where some model projections are indicating 950-mb temperatures reaching upwards of around 40C. This could bring Major to Extreme HeatRisk toward or even right up to the immediate coast of southern Santa Barbara County (60% chance). Present indications are that precipitation will be unlikely (below 20% chance) through this week and into next weekend. However, given how hot and strongly mixed the boundary layer will be over interior sections during latter parts of the week and into next weekend, any low-predictability monsoonal moisture influx south of the midlevel ridge axis could introduce chances for showers or thunderstorms over higher terrain. Chances are too low for mention in the forecast at this time, though this scenario will continue to be monitored. The combination of the significant heatwave, with dry boundary layer conditions, and diurnally enhanced southwest to northwest winds over interior areas, will increase the fire-weather risk this week and into next weekend. This will also be of particular concern in southern Santa Barbara County for Thursday into Friday, when elevated to critical fire-weather conditions will be possible in conjunction with the Sundowner winds. For additional information, please reference the Fire Weather discussion below. && .AVIATION...30/2328Z. At 23Z at KLAX, the marine layer was 800 feet deep. The top of the inversion was at 2000 feet with a temperature of 29 Celsius. High confidence in VFR conditions for for KPRB, KBUR, KVNY, KWJF and KPMD through at least Monday. For KOXR KLAX KSMO, there is a 50-70% chance of 2-4 hours of IFR ceilings through 04Z. Low confidence on timing of any breaks or ceilings. Moderate confidence in all airports that get ceilings seeing LIFR conditions at times, low confidence on coverage and timing. VLIFR dense FG possible anywhere, with higher chances than last night. KLAX...There is a 60% chance of 2-4 hours of IFR ceilings through 04Z, low confidence on exact timing. There will likely be a 3-6 hour break before the ceilings come back for good, moderate confidence in LIFR conditions most common by as early as 06Z and as late as 10Z. High confidence in any easterly wind component staying well below 06 knots. KBUR...High confidence in VFR conditions through at least Monday with seasonal winds. && .MARINE...30/812 PM. For the Outer Waters, moderate to high confidence in current forecast. Through Monday night, high confidence in Small Craft Advisory (SCA) level winds from San Nicolas Island northward. For the waters north of Point Conception (PZZ670/673), there is a 60-70% chance of Gale force winds tonight and 30-40% chance of Gale force winds Monday afternoon and Monday night. A GALE WARNING will remain in effect through tonight for PZZ670/673. For Tuesday through Friday, high confidence in winds and seas remaining below SCA levels. For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. Today through Monday, there is a 50-70% chance of SCA level winds in the afternoon and evening hours. For Tuesday through Friday, high confidence in winds and seas remaining below SCA levels. For the Inner Waters south of Point Conception, moderate confidence in winds staying under Small Craft Advisory (SCA) levels as the winds are already lowering at the mid-channel buoy. So the previous SCA was let go. There is a 20% chance of SCA level winds Monday afternoon/evening but likely will stay very localized. Otherwise, high confidence in winds and seas remaining below SCA levels through Friday across the Inner Waters south of Point Conception. Patchy dense fog (with visibilities one nautical mile or less) will continue to impact the coastal waters through the week. The areal coverage of the dense fog will lessen from day to day. && .FIRE WEATHER...30/429 PM. Elevated fire weather conditions will continue across the interior valleys, mountains, and deserts through Monday due to hot temperatures, low humidities, and locally gusty onshore winds. Beginning on Tuesday, and continuing into next weekend, high pressure will build into the region from the Eastern Pacific. This will likely bring a significant and long duration heat wave away from the coast from Tuesday through next weekend. High temperatures between 100 and 110 degrees will be common across the interior during this period, with temperatures potentially rising to between 110 and 115 degrees across the Antelope Valley and San Luis Obispo County interior valleys during the peak of the heat Thursday through Saturday. Overnight lows will also remain quite warm, adding to the potential for dangerous heat mid to late week. Widespread humidity values between 7 and 15 percent will likely occur across the interior during this period, along with poor humidity recoveries, especially in the mountains, foothills, and Antelope Valley. Typical gusty onshore winds are expected across interior sections in the afternoon and evening hours, with gusts of 20 to 35 mph common. There is also the potential for gusty sundowner/I5 corridor winds late Thursday into Friday night which could bring hot and dry conditions into portions of southern Santa Barbara county. Given the very hot temperatures, low relative humidity, and locally gusty winds, an extended period of elevated to brief critical fire weather conditions are likely from Tuesday through Sunday for valley, foothill, mountain, and desert locations. Elevated to critical fire weather conditions may expand into southern Santa Barbara county if the sundowner winds develop late Thursday into Friday night. The hot and unstable conditions will also increase the threat of large vertical plume growth for any fires that develop across the interior. The very hot and dry conditions will also cause a significant drop in both live and dead fuel moisture levels this week. These drying fuels combined with the expected fire weather conditions and increased outdoor activities associated with the 4th of July will cause the large fire threat to become high for areas away from the coast Tuesday through next weekend. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Excessive Heat Warning remains in effect from 11 AM Tuesday to 5 AM PDT Monday for zones 38-343>345-353-376>379-381>383. (See LAXNPWLOX). Excessive Heat Watch remains in effect from Wednesday morning through late Sunday night for zones 88-342-348-351-352-356>358-369>375-380. (See LAXNPWLOX). PZ...Small Craft Advisory in effect until 6 AM PDT Monday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PDT Monday for zone 650. (See LAXMWWLOX). Gale Warning in effect until 6 AM PDT Monday for zones 670-673. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Tuesday for zone 676. (See LAXMWWLOX). && $$ PUBLIC...Cohen/Sirard AVIATION...Kittell MARINE...RAT/RK FIRE...Gomberg SYNOPSIS...Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox