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Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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689 FXUS66 KLOX 301643 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 943 AM PDT Sun Jun 30 2024 .SYNOPSIS...30/217 AM. A significant heatwave will impact a large part of the area for the upcoming week. Temperatures will become dangerously hot across interior areas, where daily highs up to 100 to 105 degrees will be possible, with highs upwards of 105 to 115 possible over interior valleys and foothills including the Antelope Valley. Late in the upcoming week very warm to hot conditions could extend closer to the coast. Also, areas of night and morning dense fog will be possible near the coast today and early next week. && .SHORT TERM (TDY-TUE)...30/905 AM. ***UPDATE*** Surface observations indicate that fog is dissipating across the region. Therefore, the Dense Fog Advisory has expired. However, patchy fog will continue through the late morning near the coast. The forecast has been updated to reflect the expiration of the Dense Fog Advisory. Otherwise, no changes have been made to the ongoing forecast. ***From Previous Discussion*** AMDAR soundings out of KLAX show the marine layer at 900 ft deep. There is only light onshore flow and no eddy so there is not that much inland penetration and the vlys will remain clear. The marine layer is shallow enough that there will be pockets of dense fog. The capping inversion in rather strong (rising 10 degrees C in 600 ft) and this along with a strong 8 mb onshore push to the east this afternoon will likely keep some beaches cloudy all day. The marine layer has shrunk 600 ft since ydy and this means no cool air for the vlys. This and 2 dam of hgt rises will bring 3 to 6 degrees of warming to most areas except, perhaps, the beaches. This will bring most max temps away from the coast to 4 to 8 degrees above normal. A weak trof will move over the state on Monday. The slight lift from this trof will deepen the marine layer and allow for some of the low clouds to reach the vlys. The deeper marine layer will also eliminate the threat of dense fog. Sightly lower hgts and the deeper marine layer will combine to 2 to 5 degrees of cooling to most of the area. A dangerous and long duration heat wave will begin on Tuesday as very warm east Pac upper high noses into the state. Hgts will rise to 592 dam. There will still be onshore flow at the sfc but it will be weaker than on Monday. The marine layer will again be squashed down to under a 1000 ft keeping it out of the vlys. Most areas away from the beaches will see 2 to 5 degrees of warming. An exception being western SBA county where much weaker onshore flow will allow for about 6 degrees of warming. All of the vlys will record highs in the 90s while the far interior, lower mtn elevations and the Antelope Vly will all see triple digit heat (The AV the warmest with max temps 104 to 107 degrees) An Excessive Heat Watch for interior areas for the significant and long duration heatwave begins on Tuesday Please see the product LAXNPWLOX for details. Hot temperatures and low humidities will combine to bring increased fire danger to the interior (please see Fire Weather Planning Forecast discussion for more details). In addition, gusty west to northwest winds are expected each afternoon over most of the interior areas. The strongest gusts (35-45 mph) will occur over the I-5 corridor and the western Antelope Valley/foothills. These gusts will come in just under wind advisories criteria. .LONG TERM (WED-SAT)...30/414 AM. Both the GFS and the EC along with their respective ensembles are in very good agreement and there is high confidence that dangerous heat wave will grow and peak during the extended period. The very warm east Pac high will push further into the state and hgts will peak at 597 dam which is in the 97th percentile of hgts for July. While the anticyclonic flow around the upper high will bring some offshore winds at higher levels moderate to strong onshore flow will continue at the sfc. This will spare the coasts from the most serious of the heat. The marine layer will likely be squashed to under 600 ft by the hgts and this will allow the coastal sections furthest from the beaches to warm significantly. Look for 3 to 6 degrees of warming Wed, 2 to 4 degrees Thu and then another 3 to 6 degrees on Friday which will be the peak of the heat before 2 to 4 degrees of cooling occurs Sat. On Friday Look for many readings from 100 to 106 degrees in the vlys. The lower elevations of the mtns and the far interior will see max temps from 110 to 115 degrees. These max temps are 12 to 18 degrees above normal. Very warm overnight low temperatures (esp in the warm sector of the inversion 800 to 1200 ft elevation) will worsen the effects of extreme heat. Some locations in this heat bad will not see temps fall below 80 degrees. The Excessive Heat Watch continues for a large portion of the region away from the coast through late Friday and may need to be extended into the weekend. Even though they are not included in the watch the interior coastal sections may well need Heat Advisories issued on Thursday and Friday as the marine layer shrinks until it is too shallow to provide much relief to the portion of the coast not in the near shore area. Night through morning low clouds will continue through the period driven ashore by moderate to strong onshore flow. The marine layer will be so shallow and capped by such a strong inversion that some beaches may stay cloudy through the period or at least the bulk of it. Afternoon convection looks unlikely but there is a non zero but less than 10 percent chc that some monsoon moisture could creep in and trigger a round of showers/tstms. && .AVIATION...30/1643Z. At 1615Z at KLAX, the marine layer was 600 feet deep. The top of the inversion was at 3800 feet with a temperature of 25 Celsius. High confidence in 18Z TAFs for KPRB, KBUR, KVNY, KWJF and KPMD as CAVU conditions are expected thru the period. For coastal sites, low to moderate confidence in 18Z TAFs. Timing of return of LIFR/VLIFR conditions could be +/- 4 hours of current forecasts. There is a 10-20% chance that coastal sites could remain VFR thru the period. KLAX...Overall, moderate confidence in 18Z TAF. There is a 30-40% chance that IFR CIG/VSBY restrictions could return as early as 22Z. Timing of flight category changes this evening and overnight could be +/- 4 hours of current forecasts. No significant easterly wind component is expected. KBUR...High confidence in 18Z TAF as CAVU conditions are anticipated thru the period. && .MARINE...30/737 AM. For the Outer Waters, moderate to high confidence in current forecast. Today 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 this afternoon and tonight and a GALE WARNING remains in effect. For Tuesday through Thursday, 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 Thursday, high confidence in winds and seas remaining below SCA levels. For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. For the western half of the Santa Barbara Channel, there is a 40-50% chance of SCA level winds this afternoon/evening and a 20-30% chance of SCA level winds Monday afternoon/evening. Otherwise, high confidence in winds and seas remaining below SCA levels through Thursday across the Inner Waters south of Point Conception. Dense fog (with visibilities one nautical mile or less) will threaten all waters through at least tonight. There is a 40 percent chance that the fog will clear later tonight over Santa Barbara and San Luis Obispo Counties as the northwest winds increase. Any fog that forms for the remainder of the week will likely be dense, but expecting the coverage to shrink a little each day as associated low cloud coverage gradually diminishes. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Excessive Heat Warning 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 Tuesday morning through late Friday night for zones 38-88-343>345-353-376>379-381>383. (See LAXNPWLOX). Excessive Heat Watch in effect from Wednesday morning through late Sunday night for zones 342-348-351-352-356>358-369>375-380-548. (See LAXNPWLOX). PZ...Small Craft Advisory in effect from 2 PM this afternoon to 6 AM PDT Monday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 2 PM PDT this afternoon for zones 670-673. (See LAXMWWLOX). Gale Warning in effect from 2 PM this afternoon to 6 AM PDT Monday for zones 670-673. (See LAXMWWLOX). Small Craft Advisory in effect from 2 PM this afternoon to 3 AM PDT Tuesday for zone 676. (See LAXMWWLOX). && $$ PUBLIC...Rorke/Cohen AVIATION...RAT MARINE...RAT SYNOPSIS...Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox