774 FXUS66 KLOX 212128 AFDLOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 228 PM PDT Fri Jul 21 2017 .SYNOPSIS... Onshore flow will continue night through morning low clouds and fog will continue through the period. High pressure will dominate through Saturday, then a weak upper trough will bring some modest cooling. Monsoonal moisture could bring an unstable weather pattern to mountains and desert early next week. && .SHORT TERM...(TDY-MON) Strong high pressure aloft centered over the Plains States will continue to assert an influence on the weather pattern across the region through Saturday. Despite model solutions indicating a slight increase of onshore flow tonight and into Saturday, 500 mb heights increase by a decameter and 950 mb temperatures warm some. The air mass will warm into Saturday. The marine layer depth should thin to around 900 feet or so. Southerly flow aloft will develop on Sunday as a shortwave trough of low pressure near 34N and 137W deepens as it approaches the North Coast of California. Model solutions strengthen onshore pressure gradients, which would suggest a cooling trend; however, a battle will be staged between a southwest flow aloft/onshore flow regime and a monsoonal flow pattern. Confidence is growing for a monsoonal flow pattern to win out. The forecast remains consistent for the current time, but significant changes could be made for early next week. .LONG TERM...(TUE-FRI) Model solutions are continuing to develop a monsoonal flow regime for the region between Sunday night and Monday. NAM-WRF solutions continue to be aggressive relative to GFS solutions in the possibility of shower and thunderstorm activity spilling over into the coastal and valley areas. If this trend continues, future shifts will need to increase PoPs and add mentions of shower activity for Monday and Monday night for the coast and valleys. 850-700 mb mixing ratios increase in excess 10 g/kg across the area and NAM-WRF precipitable water values increase above 2 inches across portions of the area. A warm, humid and sticky weather pattern looks to be developing for early next week. PoPs have been increased across the area. Though the forecast averages out to a slight chance of showers and thunderstorms for the Mountains and Desert, the Eastern San Gabriel Mountains and the eastern portion of the Antelope Valley (Pearblossom, Littlerock, and Lake Los Angeles) have a better chance of showers and thunderstorms. Stay tuned as this will be a fluid situation for early next week. Southwest flow aloft should start to reestablish on Tuesday. Slight chance PoPs have been left in the forecast for now as isolated showers or thunderstorms cannot be ruled out. Certainly, drier southwest flow aloft could prevail and convective showers could not develop. Stronger onshore flow should develop for mid-to-late next week keeping near-to-slightly above normal temperatures in place across the region. A persistent marine layer will likely allow for night through morning low clouds and fog for the coast and possibly the lower valley areas. && .AVIATION...21/1545Z. At 1545Z, the marine inversion at KLAX was based near 1700 feet. The top of the inversion was 3700 feet with a temperature of 22 degrees Celsius. Overall, moderate confidence in 18Z TAF package. High confidence in return of MVFR/IFR conditions to coastal sites tonight, but only moderate confidence in timing. KLAX...moderate confidence in 18Z TAF. Timing of return of stratus/fog could be +/- 2 hours of current 08Z forecast. KBUR...high confidence in 18Z TAF. && .MARINE...21/100 PM. For the Outer Waters, good confidence in current forecast. Small Craft Advisory (SCA) level winds will continue through Saturday morning then will slowly diminish by Saturday evening. So the SCA may need to be cancelled early. There will be a 30% chance of some gusts reaching Gale force levels this afternoon and evening. For Sunday through Wednesday, winds and seas are expected to remain below SCA levels. For the Inner Waters, good confidence in current forecast. For the waters north of Point Sal, SCA levels are likely this afternoon and evening then winds and seas will remain below SCA levels Saturday through Tuesday. For the waters south of Point Conception, winds and seas will remain below SCA levels through Wednesday. && .BEACHES...21/1245 PM. A potential exists for significant surf to impact Southern California beaches late next week and into the following week. Model solutions have been consistent with developing a tropical cyclone off the Mexican Coast and pushing it into an area consistent with higher surf for the region. Model solutions are consistent in developing swells between 25 and 35 feet off the Baja California Coast. Early guidance suggests a moderately long period swell impacting the Southern California Coastline as early as next Friday, building into next weekend. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement in effect through Sunday evening for zones 40-41-87. (See LAXCFWLOX). PZ...Small Craft Advisory in effect until 9 PM PDT this evening for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 9 PM PDT Saturday for zones 670-676. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Sunday for zone 673. (See LAXMWWLOX). && .HAZARD POTENTIAL OUTLOOK...(MON-FRI) A monsoonal flow pattern developing on Sunday will transport subtropical moisture northwest into Southern California. Shower and thunderstorm chances are in the forecast for Monday and Tuesday, but there is a possibility that shower activity could spill over into the coastal and valley areas. && $$ PUBLIC...Hall AVIATION...Thompson MARINE...Thompson BEACHES...Hall SYNOPSIS...B/Hall weather.gov/losangeles