Area Forecast Discussion
Issued by NWS Los Angeles, CA

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369 FXUS66 KLOX 230025 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 525 PM PDT Sat Jun 22 2024 .SYNOPSIS...22/222 PM. Excessive Heat Warnings and Heat Advisories are in effect for much of Southern California away from the coast for this weekend, as dangerously hot conditions impact many areas. There will be a gradual cooling trend over many locations during the upcoming week, except for dangerously hot conditions continuing across the Antelope Valley into mid-week. While mostly dry conditions are expected, there is the possibility for a few thunderstorms to form tonight through Monday, as moisture from the remnants of Tropical Cyclone Alberto affects Southern California. && .SHORT TERM (TDY-TUE)...22/222 PM. A large and strong midlevel anticyclone will expand westward across the southern United States this weekend, as diffuse vorticity from the remnants of Tropical Cyclone Alberto circulate around the southern and then western peripheries of the anticyclone. By Tuesday, 500-mb heights are forecast to rise to 594 dam over AZ and NM, with the accompanying ridge axis expanding over the forecast area this weekend into early in the upcoming week. As midlevel heights rise this weekend, temperatures will increase to dangerously hot levels from the Antelope Valley and nearby foothills westward to the Santa Barbara County interior mountains. Excessive Heat Warnings have been posted in these areas, where high temperatures are expected to reach 95-105 degrees (locally to around 108 in the Antelope Valley) producing Major to Extreme HeatRisk. For the Antelope Valley and eastern Antelope Valley Foothills, these conditions are expected to be prolonged into the upcoming week, and the Excessive Heat Warning extends through Thursday. Elsewhere across the region away from the coast, HeatRisk will be relatively lower compared to the Excessive Heat Warning area, though it will still be significant in many areas. In these areas, a Heat Advisory is in effect as potential heat impacts are anticipated, with Moderate to Major HeatRisk. Earlier today, the Heat Advisory was expanded toward (though not including) the coast across Los Angeles, Ventura, Santa Barbara, and San Luis Obispo Counties. Consistent with the latest observational data in these areas, the diurnal erosion of the marine layer will cause its cooling influence to retreat closer to the immediate coast each day this weekend, resulting in a broader area of significant heat impacts. High temperatures in the Heat Advisory area should reach the 90-100 degree range this weekend -- across a large portion of the region away from the coast and outside of Excessive Heat Warning headlines. Overall day-to-day variability in temperatures should be small through early in the upcoming week. However, an expected increase in midlevel moisture from the remnants of Tropical Cyclone Alberto could be accompanied by a couple degrees of cooling day- to-day from Sunday into Monday and Tuesday. As a result, outside of the Antelope Valley and eastern Antelope Valley Foothills where dangerously hot temperatures are expected to continue through Thursday, Excessive Heat Warnings and Heat Advisories are currently set to expire Sunday evening. However, extensions of these headlines in time will be possible in later forecasts (up to 60% probability in some areas). Also, with the increase in moisture tempering nocturnal radiational cooling, overnight low temperatures will remain elevated this weekend -- 70s in many areas -- which will have a compounding effect on heat impacts this weekend. For locations near the coast, outside of ongoing heat headlines, temperatures will be relatively cooler given the influence of the marine layer. Within the shallow marine layer, areas of nighttime and morning dense fog are expected near the immediate coast and nearby coastal valleys through early next week. Gusty south to southwest winds are expected over the Interstate-5 corridor and the western Antelope Valley and nearby foothills each afternoon and evening. While local gusts could reach 35-40 mph, Wind Advisories are not currently anticipated. Present indications are that the aforementioned increase in midlevel moisture accompanying the remnants of Tropical Cyclone Alberto will still have the potential to bring a few thunderstorms across the forecast area. Most locations will remain dry, and chances for thunderstorms are currently very low, less than 20 percent. There is a good chance that no storms form anywhere across the region. However, if enough moisture is able to move into the area this weekend, isolated thunderstorms will have the potential to form. Right now, the most-likely timing of this activity will be tonight, though a couple storms cannot be ruled out as late as Sunday afternoon. If thunderstorms were to form, cloud-to-ground lightning would be the main concern, with highly conditional potential for small hail, gusty winds, and brief heavy downpours. By Sunday night into Monday, the thunderstorm risk will be decreasing, as weak, positive theta-e advection aloft decreases behind the leading edge of the elevated moisture surge. However, a couple thunderstorms could also occur in Santa Barbara and Ventura Counties with the lingering moisture on Monday (15% chance). .LONG TERM (WED-SAT)...22/222 PM. There remains high confidence that the persistent midlevel anticyclone over the southwest states will prolong Major to Extreme HeatRisk across the Antelope Valley and eastern Antelope Valley Foothills through the middle of the upcoming week. The Excessive Heat Warning is currently in effect for these areas through Thursday. This will occur as 500-mb heights peak around 596 dam over the southern Rockies by the middle of the upcoming week. Meanwhile, strong heating across the Intermountain West, combined with an approaching midlevel trough over the Pacific Northwest, will drive strengthening onshore flow and perhaps an increase in marine stratus near the coast. In addition to bringing gusty southwest winds over interior sections, this will cool off the coast and coastal valleys by a few degrees into the middle of the upcoming week, with highs in the 70s and 80s. This will confine significant heat impacts to interior sections progressively farther away from the coast from day to day. However, aside from Excessive Heat Warnings extending through Thursday in the Antelope Valley and the eastern Antelope Valley Foothills, additional heat headlines will be possible across interior mountains and valleys following the expiration of heat headlines this weekend (60% chance). By the end of the upcoming week and into next weekend, medium-range model guidance indicates lowering 500-mb heights corresponding to a cooling trend for interior areas, as well. High temperatures for next Friday and Saturday may not rise above 100 degrees over the local area. Also, for middle to latter parts of the upcoming week, offshore surface ridging beneath subsidence upstream of the aforementioned midlevel trough, may favor increasing Sundowner Winds for southern Santa Barbara County. However, the potential for wind headlines is currently unlikely (less than 20% chance). Present indications are for dry conditions to prevail through middle and latter parts of the upcoming week and into next weekend. However, the strong heating across the Intermountain West ahead of the aforementioned trough will foster an additional northward influx of monsoonal moisture across the Desert Southwest. Richer tropospheric moisture is expected to reside east of the forecast area for the upcoming week. However, given anticipated daily increases in convection over the higher terrain of Arizona, there is a remote potential for residual convective debris to be advected over the local area for the middle of next week. Very low predictability in convection-augmented vorticity fields and moisture enhancements will still be accompanied by a non-zero risk for showers and thunderstorms across the forecast area, as minor midlevel impulses and moisture enhancements circulate around the larger-scale anticyclone. However, confidence in such activity is too limited for mention in the forecast at this time. && .AVIATION...
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23/0024Z. At 0005Z at KLAX, the marine layer was 800 feet deep. The top of the inversion was at 1800 ft with a temperature of 27 deg C. Very good confidence in valley and desert TAFS. Moderate confidence in all coastal TAFS due to uncertainty in timing/dissipation of cigs and flight categories. Based on latest satellite trends, have moved up timing of cigs for KOXR, KCMA, and KSBA (where there is increased confidence of cigs). With shallow and strong inversion in place, increased confidence of cigs lowering to LIFR for most coastal areas, with a 20 percent chance of VLIFR (except 60 percent for KSMX). There is a 10-20 percent chance of SHRA or TSRA starting in LA county late tonight, overspreading the forecast area on Sunday. KLAX...Low confidence in TAF. Increased chances of IFR/LIFR cigs at KLAX later tonight, but still considerable uncertainty in timing of cig arrival/dissipation (+/- 3 hours from TAF timing). No significant east wind expected. There is a 10-20 percent chance of SHRA or TSRA late tonight through at least Sunday afternoon. KBUR...High confidence in TAF. There is a 10-20 percent chance of SHRA or TSRA late tonight through at least Sunday afternoon.
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&& .MARINE...22/1253 PM. For the Outer Waters, high confidence in current forecast. Small Craft Advisory (SCA) winds are not expected through at least early next week, though there is a 40% chance for SCA winds to return by Tuesday or Wednesday. For the Inner Waters north of Point Sal, high confidence in current forecast. SCA conditions are not expected into next week. For the inner waters south of Point Conception, high confidence in current forecast. SCA winds are not expected through at least early next week. In addition, there is a slight chance of thunderstorms over the waters late tonight through at least Sunday afternoon, as remnant moisture aloft from Tropical Cyclone Alberto advances into the region. Areas of dense fog will likely continue at times through early next week. Please see the Marine Weather Statement (LAXMWSLOX) for information of the thunderstorm potential, as well as the potential for dense fog. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Heat Advisory remains in effect until 8 PM PDT Sunday for zones 38-88-342>345-352-356>358-368>375-380-548. (See LAXNPWLOX). Excessive Heat Warning remains in effect until 8 PM PDT Sunday for zones 353-376>379-381. (See LAXNPWLOX). Excessive Heat Warning remains in effect until 8 PM PDT Thursday for zones 382-383. (See LAXNPWLOX). PZ...NONE. && $$ PUBLIC...Cohen AVIATION...Gomberg/Munroe MARINE...Munroe/Rorke SYNOPSIS...Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox