731 FXUS66 KLOX 260629 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1129 PM PDT Tue Sep 25 2018 .SYNOPSIS...25/844 PM. Fair skies, except for an overnight coastal marine layer that may spread to some valleys into next week. A high will move in midweek for a warming trend into Thursday. Then a low should arrive by Friday for a cooling trend into next week, with temperatures below normal and more clouds. Patchy overnight drizzle may occur early next week south of Lompoc. && .SHORT TERM (TUE-FRI)...25/823 PM. Satellite imagery showing low clouds and fog making a return to coastal areas this evening across Los Angeles, Ventura, and Santa Barbara counties. An upper level ridge of high pressure will continue to build over the region during the next 24 hours resulting in a shrinkage of the marine layer depth. Earlier this morning, the marine layer depth was around 2500 feet across the LA basin, with current ACARS data now showing a depth around 1600 feet. By Wednesday morning, the marine layer depth is expected to shrink to around 1200 feet. As a result, expecting low clouds and fog to mainly be confined to coastal areas tonight into Wednesday morning, extending locally into some of the lower valleys. Onshore pressure gradients are trending weaker this evening as compered to yesterday evening, and this weakening of onshore flow will continue through Wednesday morning. In fact, some light offshore winds are expected to develop overnight into Wednesday across the mountains and deserts, extending into some of the interior valleys. Offshore wind speeds will mainly be in the 10 to 20 mph category, but could see isolated gusts up to 30 mph in favored mountain locations on Wednesday. This offshore component combined with the upper level ridge of high pressure will result in a faster burnoff of the low clouds and bring substantial warming across interior sections. In fact, many valley, lower mountain, and desert locations will climb well into the 90s on Wednesday, with warmest locations potentially climbing to around 100 degrees on Thursday. These very warm temperatures combined with low humidities. breezy offshore conditions, and very dry fuels will lead to elevated fire weather concerns across interior sections on Wednesday, continuing into Thursday. *** From previous discussion *** Cooling trend to begin Friday as onshore flow will definitely be stronger. Models in good agreement showing the upper low at the base of the Rex block near 33n/140w finally shifting slowly northeast to near 38n/130w Friday afternoon, or about 500 miles west of the Bay Area. Assuming this happens we should see surface pressures lowering inland allowing that onshore flow to increase. There also is some cooling aloft that will increase the marine layer depth. Interestingly models show some drying in the boundary layer Thu into Fri south of Pt Conception, suggesting a substantial decrease in stratus coverage both days. Not much confidence in this either and will continue with a solid marine layer north to south for now. .LONG TERM (SAT-TUE)...25/159 PM. The Friday upper low continues its northeast movement into nrn CA and OR with the trough extending south through srn CA. A cooling trend with a deepening marine layer will continue through the weekend with temps 3-6 degrees below normal on average. May even be some spotty drizzle at times across coast/valleys south of Pt Conception. Confidence in the forecast drops off considerably starting next Monday as a complex upper level pattern sets up along the west coast. Models are really struggling handling a number of significant factors including a tropical system off the coast of Mexico (Rosa), several other tropical systems across the Pacific, and a pretty deep upper level trough approaching the west coast. Solutions have varied quite a bit run to run and there's a very large spread in the ensembles. At this point it appears that the influence of Rosa will be minimal locally as models have it tracking too far south and east, though timing and strength are very different between the models. The best chance for any precipitation here will probably be from the upper trough approaching from the west but there are a number of complicating factors with that too, including how much moisture will be available, timing, and strength. And model solutions will likely change several times between now and next week. But for now feel like it's worth introducing small precip chances into the forecast for next Monday and Tuesday, again mainly from the upper trough but also the very small probability that moisture from Rosa will drift into our area. Either, neither, or both are possible scenarios next week and it may take a few more days at least before confidence in one solution improves. Until then enjoy the model roller coaster. && .AVIATION...26/0627Z. At 0545Z at KLAX, the marine layer was near 1500 feet deep. The top of the inversion was around 3700 feet with a temperature of 25 degrees Celsius. Good confidence in cstl/vly TAFs through 14Z then moderate confidence 14Z-20Z due to uncertainty in clearing times which could be off by as much as 2 hours. Good confidence again after 20Z. KLAX...Moderate confidence in clearing time with near equal chcs between 18Z and 20Z. Good confidence in VFR conds aft 17Z. Good confidence in no east wind component greater than 5 kt. KBUR...Moderate confidence in clearing time with near equal chcs between 15Z and 17Z. Good confidence in VFR conds aft 17Z. && .MARINE...25/740 PM. For the outer waters, moderate to high confidence in wind and sea conditions remaining below Small Craft Advisory levels (SCA) through Friday. There is a 30 percent chance of Small Craft Advisory winds for the outer waters near Point Conception Wednesday evening, and again Friday afternoon and evening. No SCA issues expected for the inner waters through at least Friday, and likely through the weekend. Areas of dense fog with visibility one nautical mile or less will likely affect portions of the coastal waters N of Point Conception, including the nearshore waters north of Point Sal, late tonight through Wednesday morning. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...NONE. && .HAZARD POTENTIAL OUTLOOK (FRI-TUE). No significant hazards expected. && $$ PUBLIC...Gomberg/MW AVIATION...Rorke MARINE...Smith SYNOPSIS...STu weather.gov/losangeles