FXUS66 KLOX 311138 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 438 AM PDT Wed Jul 31 2024 .SYNOPSIS...31/338 AM. High pressure aloft to the east of the region will build west into the Four Corners for late week. A warming trend will continue into early next week. Temperatures will be well above-normal across the interior by Friday and Saturday. There is moderate-to- high chance of dangerously hot conditions developing, especially over the weekend. Any low clouds and fog will remain confined to the coastal areas and lower valleys. Monsoonal moisture may bring a few showers and thunderstorms to the mountains and Antelope Valley late in the week into the weekend, and it is possible for some showers to drift west of the mountains at times. && .SHORT TERM (TDY-FRI)...31/419 AM. The latest water vapor imagery shows an upper-level ridge of high pressure centered over north Texas. An upstream ridge is building into the Pacific Northwest as an upper-level trough that brought cooling earlier week exits into the northern Plains. Low clouds continue to well-entrenched along the Central Coast but are more broken up south of Point Conception and into the southern California bight. The high pressure system to the east will build into Four Corners Region through Friday. A warming trend, already developing, will turn hotter over the weekend. Temperatures will likely be well above normal over the weekend, but questions remains how hot the air mass will get. The latest deterministic solutions and a bulk of the forecast ensemble members seems to suggest monsoonal moisture getting piped into the area as the upper-level high builds west. The latest deterministic solutions suggest an easterly wave developing and pushing a more moist air mass into the region between tonight and into Thursday. By Thursday afternoon, EPS ensemble members suggest precipitable water value mans approaching 1.50 inch at KLAX and 1.25 inch at KPMD, or values closer to 125 to 150 percent of normal for this time of year. EPS and GEFS ensemble members have a decent amount of model perturbations with precipitation at KPMD, KGXA, and KSDB with measurable precipitation between Thursday night and Friday. The forecast places more confidence in the moisture parameters versus precipitation expected in generating the forecast as one would be at the mercy of convective parameterizations working as advertised. As a result, the PoP forecast leans more heavily into the ensemble based precipitable water value means increasing and interacting with the complex terrain of southern California. How hot the air mass gets may depend how much moisture makes its way north over the next several days starting tonight. The easterly wave could end up producing a cloud deck that cuts into temperatures with a mostly cloudy day on Thursday and Friday. The forecast leans more heavily towards the hottest days being over the weekend or into early next week. Some coordination took place with surrounding offices about an excessive heat watch for the interior portions of the area, but there is still a good deal of uncertainty with the monsoonal moisture spilling northwest into the region over the coming days. For the current time, the forecast holds without headlines, but there is some concern that any breaks in the clouds in the short term toward Thursday and Friday could lead to warmer temperatures than forecast. With the high pressure system building west over the coming days, the marine layer depth should thin additionally as the subsiding air with the high pressure system presses down on top of the marine layer depth. A thin marine layer depth already this morning, producing patchy dense fog in some areas, could thin more for the latter half of the week. Dense fog issues will continue to plague the area at least through tonight. After tonight, the monsoonal mositure could start to play tricks on the low cloud field. With the increase in monsoonal moisture, the clouds could mess with the radiative processes in the marine layer stratus deck and play tricks on the marine layer and either break it up or scour it completely. There is a chance that low clouds and fog could become non-existent with the easterly wave spreading across the area between late tonight and Friday. .LONG TERM (SAT-TUE)...31/436 AM. The upper-level ridge of high pressure builds into Utah over the weekend and could temporarily shut off the monsoonal moisture and heat up the air mass. There is a moderate-to-high chance that significantly hotter temperatures will develop over the weekend, possibly producing dangerously hot conditions. Future shifts may need to consider heat headlines as confidence is shifting for the hottest weather for the weekend and into early next week. The threat of monsoonal moisture will linger into the period as the ridge position will be consistent with the pattern lingering. EPS and GEFS ensembles seem to hint at the moisture remaining into early next week as preciptable water means remain elevated, but the main question that remains is with the amount of moisture and if it will produce any afternoon and evening showers and thunderstorms over the mountains and desert. The forecast remains consistent with PoPs for now and keeps a slight chance in the forecast each afternoon. && .AVIATION...31/1138Z. At 1046Z at KLAX, the marine layer was around 800 feet deep. The top of the inversion was at 2600 ft with a temp of 24 deg C. High confidence in CAVU TAFs for KPRB, KBUR, KVNY, KPMD, and KWJF. Low to moderate confidence in all coastal sites, with IFR to LIFR conds forecasted, but categories may be off by at least one category thru the late morning. Flight cat changes may be off +/- 4 hours from current forecast. There is a 30% chance of VLIFR conditions at KOXR, KCMA, KSMX, and KSBP thru 18Z. KLAX...Low confidence in TAF. Flight cats may bounce between IFR- VFR conds due to stratus hugging the coast through 18Z, when VFR conds are expected through 04Z. There is a 30% chance cigs initially arrive below 005 by 04Z. Flight cat changes may be off by +/- 4 hours. No significant east wind component is expected. KBUR...High confidence in CAVU TAF. && .MARINE...31/258 AM. For the Outer Waters, high confidence in current forecast. There is a 70-80% chance of Small Craft Advisory (SCA) level winds through late tonight. There will likely be a lull in the winds Thursday morning, but there is a 60-70% chance of SCA level winds returning Thursday afternoon and persisting much of the time into Sunday, with Friday and Saturday looking to be the strongest. For the Inner Waters north of Point Sal, moderate confidence in current forecast. There is a 50-60% chance of low end SCA winds to pick back up in the afternoon through the evening hours today, therefore a SCA was issued through 9 PM. There is a 20-30% chance of SCA winds in the afternoon to evening hours Thursday and Friday, then a 40-50% chance of SCA winds in the afternoon and evening hours Saturday and Sunday. For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. There is a 20-30% chance of SCA level winds Wednesday through Friday in the afternoon and evening hours, and a 40-50% chance of SCA level winds Saturday and Sunday in the afternoon and evening hours across the Western Portion of the Santa Barbara Channel. Otherwise, at this time, SCA level wind and seas are not expected thru Sunday. Dense fog, with visibilities less than 1 NM, will impact most of the coastal waters thru 12 PM today. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...Small Craft Advisory in effect from 3 PM this afternoon to 9 PM PDT this evening for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Thursday for zones 670-673-676. (See LAXMWWLOX). && $$ PUBLIC...Hall AVIATION...Lewis MARINE...Lewis/Smith SYNOPSIS...Hall weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox