Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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296 FXUS66 KLOX 300657 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1157 PM PDT Sun Sep 29 2024 .SYNOPSIS...29/923 PM. A warming trend will occur through at least Wednesday as a ridge of high pressure builds over the region. Dangerous hot temperatures could develop in some valley and mountains locales on Tuesday and Wednesday when the hottest weather is forecast. A slight cool down is expected for late this week and into the weekend, but temperatures will remain above normal away from the coast. Except for some coastal low clouds and fog at times during the night and morning hours, skies will continue to be mostly clear for much of the forecast period. && .SHORT TERM (SUN-WED)...29/956 PM. The latest fog product shows low clouds and fog less entrenched than the past several nights. The low cloud field is likely starting to feel some of the effects of the trough being nudged to the south and west by the upper-level ridge building in. The latest AMDAR soundings and earlier balloon soundings from KVBG indicate a shallow marine layer depth in place. Any low clouds and fog that develop tonight could turn dense. The latest high- resolution indicate a moderate to high chance of dense fog overnight tonight and into Monday morning, but the main question is if the modeled extent of the low cloud field will match up with reality. An upper-ridge of high pressure over the eastern Pacific Ocean will merge with another ridge over the Sonoran Desert, then the larger high pressure system force the trough vacate the region by Tuesday. A warming trend will take shape through Monday, then turn significantly warmer, especially away from the coast for Tuesday and Wednesday. The latest deterministic solutions indicate onshore flow weakening and turning offshore for through the first half of the week, but a bias correction of the modeled pressure gradients would suggest weak onshore flow remaining in place on Tuesday. It will be a tough forecast at the coast for middle portion of the week depending upon what unfolds as there is some offshore low-level component. Higher confidence exists in the hottest temperatures developing inland and well away from the coast. Dangerously hot conditions could develop between Tuesday and Wednesday with the highest likelihood across the valleys, foothills, and mountains. A few tweaks were made to temperatures for Tuesday and Wednesday, mainly to warm overnight low temperatures slightly across the region. The latest update has been issued to add some tweaks to low clouds and fog and tweak temperatures over the next several days. ***From Previous Discussion*** A weak H5 upper level low (584 dam) was centered over nrn L.A. County early this afternoon. This upper level low is forecast to drift S of the area tonight thru Mon then largely dissipate Mon night. Upper level ridging is expected to build into central CA Mon thru Tue then expand some into srn CA for Wed. The forecast area will be on the southern periphery of this upper level ridging on Tue with H5 heights increasing to around 591 dam then H5 heights should lower slightly to 588-589 dam on Wed. A broad easterly flow aloft can be expected over the region Tue thru Wed. There will be increasing offshore pressure gradients tonight thru Mon with little change thru Tue. A slight onshore trend is expected Tue night and Wed, but pressure gradients will remain rather weak. In addition, significant warming is expected in the boundary layer up to at least 950 mb thru Mon, also helping to shrink the marine inversion to about 600 ft or less. There will likely be a surface-based inversion along the Central Coast by Mon morning. Low clouds and fog are expected again tonight into Mon morning but there will be much less coverage, with just the immediate Central Coast and some coastal areas S of Point Conception being affected. The night and morning low clouds and fog should then be confined to the immediate L.A. County coast Mon night through Wed morning. Any low clouds will continue to be accompanied by patchy dense fog thanks to the shallow marine inversion. Otherwise, mostly clear skies will cover the forecast area tonight through Wed, except some hi level clouds may move into the area from the E by Tue. Breezy onshore flow is expected afternoon and evening hours, with breezy offshore flow night and morning hours Mon and Tue especially for the foothills including the Santa Lucia Mtns in SLO County. There will also be gusty NW to N sundowner winds in SW SBA County this evening, with local gusts close to Advisory levels possible. The offshore pressure gradients will help to bring significant warming to the forecast area for Mon thru Wed. By Tue afternoon temps will be 10-20 deg above seasonal norms with little change expected Wed. This combined with overnight lows quite a bit above normal will bring heat concerns to the area especially Tue and Wed. As a result, Heat Advisories and Excessive Heat Watches will be issued with the afternoon zone package for the vlys, mtns and Antelope Vly foothills. The hottest temps Tue and Wed should be in the 95-105 degree range for much of this area, altho isolated max temps could be a degree or two warmer. .LONG TERM (THU-SUN)...29/221 PM. The EC and GFS mean ensembles were is somewhat good agreement for the extended period, with the deterministic showing more differences especially later in the fcst period. Overall, the mean ensembles indicate that upper level ridging will prevail over the forecast area Thu thru Sun, altho the EC continues to be much more pronounced with the ridging. Pressure gradients should turn more onshore for Thu with slightly cooler temps expected altho remaining quite a bit above normal. It may be warm enough to extend Heat Advisories for some inland areas. For Fri thru Sun, temps will again cool slightly but continue to be 5-15 deg above normal away from the immediate coast. However, the EC ensembles, with significant upper level ridging next weekend along with persistent offshore pressure gradients, indicates temps could be much warmer than is currently forecast. The marine inversion should remain rather shallow thru the extended period. As far as any night and morning low clouds and fog, they should be minimal Thu and Fri, then possibly affect mainly portions of the Central Coast and VTU/L.A. County coasts Sat and Sun. Otherwise, mostly clear skies will continue except for the chance of some high clouds moving in at times. && .AVIATION...30/0656Z. At 0505Z at KLAX, the marine layer was 1100 feet deep. The top of the inversion was at 2000 feet with a temperature near of 25 C. High confidence in desert TAFs and KPRB. Moderate confidence in TAFs for KBUR and KVNY with a 25 percent chc of LIFR conds 11Z-15Z. Moderate confidence in coastal TAFs. Flight cat changes could be off by +/- 90 minutes. Cigs could be off by +/- 200 ft. Low confidence in timing of return of low clouds Monday evening. KLAX...Moderate confidence in TAF. There is a 25 percent chc that vis will remain AOB 3SM. Good confidence in VFR conds arriving between 17Z and 19Z with a better chc of it occuring between 17Z and 18Z. Low confidence in low cloud arrival timing Monday evening. KBUR...Moderate confidence in TAF through 15Z then high confidence. There is a 25 percent chc of 2SM BR OVC003 conds 11Z-15Z. && .MARINE...29/932 PM. In the Outer Waters (Central Coast to San Nicolas Island), current Small Craft Advisory (SCA) looks on track with SCA level winds lasting into early tomorrow morning. Then, moderate confidence in sub advisory winds Monday through Wednesday morning. There is a 10% chance for SCA level winds for the waters north of Point Sal Monday night, and a 20-30% chance Tuesday night into Wednesday focused on the waters south of Point Conception. There is a better chance (40%) for SCA level winds during the afternoon thru late evening hours Wed and Thurs, and another chance Friday afternoon thru evening for the entire Outer Waters. For the inner waters north of Pt Sal, high confidence in SCA winds ending tonight. There is a 20-30% chance of SCA level winds Wed/Thur afternoon and evening, and possibly also Friday. For the Santa Barbara Channel, and inner waters off the LA and OC coasts, conditions are expected to be below SCA levels through Tuesday morning. There is a 20-50% chance of SCA level winds in the Santa Barbara Channel and southern inner waters Tuesday afternoon and evening. Wednesday may also be windy for the waters south of Point Mugu, but will more than likely remain below SCA levels. && .FIRE WEATHER...29/1000 PM. Warm and dry conditions are expected to continue away from the coast through Wednesday. High temperatures between 90 and 100 degrees will be common through Monday, then increasing to 93 to 106 degrees on Tuesday and Wednesday. The warmer temperatures are expected to spread into many of the valleys and coastal foothills Monday through Wednesday as a weak offshore wind influence develops. Dry air with humidity levels between 5 and 20 percent over the interior valleys, mountains, and deserts earlier today will expand into the coastal valleys and foothills through Wednesday. By Monday and Tuesday, the winds will become more northeast to southeast over the interior, likely expanding into other areas such as the Santa Lucias, Santa Susana mountains, and portions of the LA/Ventura county valleys. On Monday, southeast winds gusting 20 to 30 mph will be common across the San Gabriels and Ventura county mountains, with isolated gusts as high as 35 mph across ridgetops. Mixing heights are expected to peak between 8,000 and 13,000 feet this afternoon across the interior, then increase to between 10,000 and 17,000 feet Monday through Wednesday due to increasing temperatures and instability. As a result, there will be an increased risk of large vertical plume growth of new fires across the interior Monday through Wednesday. mainly in the mountains and foothills. The combination of hot temperatures, very low humidities, locally gusty winds and potential for large vertical plume growth will result in elevated to brief critical fire weather conditions today in the mountains, expanding into the Santa Lucias, Santa Susana mountains, and portions of the valleys Monday into Wednesday. A slight cooling trend could develop Thursday into Friday, then a possible offshore wind influence may reinforce very warm and dry conditions next weekend. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Heat Advisory in effect from 11 AM Tuesday to 8 PM PDT Wednesday for zones 38-342>345-348-351-356>358-369>371-374>377-380-382-548. (See LAXNPWLOX). Excessive Heat Watch in effect from Tuesday morning through Wednesday evening for zones 88-352-353-372-373-378-379-381. (See LAXNPWLOX). PZ...Small Craft Advisory in effect until midnight PDT tonight for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PDT Monday for zones 670-673-676. (See LAXMWWLOX). && $$ PUBLIC...Hall/Sirard AVIATION...Rorke MARINE...Phillips FIRE...Gomberg/Hall SYNOPSIS...Hall weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox