359 FXUS66 KSGX 110429 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 929 PM PDT Wed Oct 10 2018 .SYNOPSIS... Cool weather will continue as low pressure dominates the West. Scattered light showers are possible tonight through Thursday morning. After some offshore flow and foothill winds Friday morning, moisture from the south will bring a good chance of showers Saturday through Monday with the best chances Saturday afternoon through Sunday morning. Offshore flow continues next week and combines with higher pressure to bring a drying and warming trend beyond Monday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... ...Update... Highs across our service area today ranged from 58F degrees at Big Bear Lake to 89F degrees at Thermal. Some cloudiness developed across the inland valleys and coastal mountain slopes, but it stayed dry. The San Diego/Miramar sounding this evening shows an increase in moisture between 2-5k FT, so a deeper marine layer will be in place tonight. This could lead to sprinkles or patchy light rain west of the mountain crests overnight, with just trace to a hundredth of an inch or so only expected. Lows will be similar to last night, warmest along the coast and in the lower deserts and coolest in the mountains. No changes were made to the forecast this evening. See previous discussion below for further forecast details. ...Previous Discussion (Issued at 300 PM PDT Wed Oct 10 2018)... A really fine day today with scattered fair weather cumulus near the coast and over the valleys. Moderate onshore gradients are noted around 5 mb from the coast to the deserts. The marine layer inversion strength was a bit weaker than this time yesterday hovering around 3.5 degrees Celsius but was relatively deep for this time of the year. A weak shortwave trough and coastal eddy are expected to deepen the marine layer and strengthen the inversion tonight and into Thursday morning, with scattered light rain showers likely west of the mountains and along the coast-facing mountain slopes. Rainfall amounts will mainly be under one-tenth inch through Thursday morning. Cloudiness could persist into Thursday afternoon due to the stronger marine layer. Behind the weak shortwave Thursday, offshore flow will develop Thursday night and continue through Friday morning. Although a short event, a period of gusty offshore winds will likely peak Friday morning with gusts around 40 to 45 mph possible in the favored passes and canyons and in the eastern valleys along the foothills. Tropical Storm Sergio is forecast to make a bee line over the Central Baja Peninsula Thursday through Friday and continue to shear out to the NE over Texas through this weekend. An approaching stronger trough from the northwest will eventually drop an upper low off the coast of SoCal Friday into Friday night which will cutoff from the predominant westerly flow for a time this weekend. This pattern of course is always a forecast challenge...but that said the global models are handling it with some run to run continuity now and in general terms it looks like the upper closed low pressure area will bring the potential for a widespread rain event Saturday and/or Sunday. Linger showers could continue into Monday. The best chance for rain will be Saturday afternoon and Saturday night with the best chance across our southern forecast area, especially closer to the border, as the track of the upper low looks to be over far SoCal and into Northern Mexico. The cutoff low will tap into some of the remnant tropical moisture and there could be some enhanced rainfall across the San Diego County mountain slopes. Further north over OC and the Inland Empire, the chances will be lower but still a decent chance for showers there too, but further from the moisture and forcing of the upper low. At this time the low track could bring enough instability to generate thunderstorms out ahead of it, this best chance for that would be Saturday afternoon and evening. At this time the confidence of thunderstorm chances is very low due to the exact track of the upper low. This will be something we need to get a little better forecaster confidence to add into the forecast. Rainfall amounts with the weekend event could vary from around one tenth to one quarter inch in Orange County and the Inland Empire to one half inch in San Diego County...with up to one inch in the mountains. Much lower confidence but the slow movement of the upper low could bring showers into Monday as well. As the upper low departs to the SE early next week, a period of offshore flow will develop in the wake of the upper low with increasing surface high pressure over the Greek Basin Tuesday and Wednesday. At this time there is still quite a bit of uncertainty but the period Monday night through Wednesday could see a stronger Santa Ana event given the stronger north to south gradient that is being predicted by the medium range models, especially the EC. Will need to keep a close eye on the evolution of this pattern. && .AVIATION... 110330Z...Coasts/Valleys/Coastal Mountain Slopes...Low clouds will overspread the area by 06Z. A very thick marine layer expected by late tonight with bases 1500-2500 ft MSL and top of layer around 5000 ft MSL. Areas of DZ/ISO -SHRA are expected mainly from 10-16Z near the coastal terminals. Expect visibilities to lower to 3-5SM and CIGS 1000-1500 ft MSL near showers. Periods of BKN CIGS expected near inland empire terminals 10-16Z. Expect only slow improvement in CIGS Thursday morning. CIGS should rise to 3000-3500 ft MSL after 16Z and scatter out after 20Z. Desert Mountain Slopes/Deserts...VFR conditions through the period with northwest winds 5-15 knots. && .MARINE... No hazardous marine weather through Friday. A low pressure system will bring a slight chance of showers beginning early Saturday. && .BEACHES... South swells from 200-210 degrees generated by Tropical Storm Sergio will build through Friday. This will bring increasing surf, peaking late Thursday through Friday at around 5-7 ft with higher sets. The swell and surf will decrease late Friday and Saturday. Minor coastal flooding is possible during morning high tides, which peak around 6 ft. There will be strong rip and longshore currents. For more details, see the Beach Hazards Statement and the Surf Zone Forecast. && .FIRE WEATHER... Onshore flow will persist for much of the week maintaining higher humidity and near to slightly below normal temperatures with gusty southwest to west winds in the mountains and deserts each afternoon and evening. Some offshore winds will develop in the foothills Friday morning. A low pressure system will draw tropical moisture into the region Friday afternoon through the weekend with a good chance of showers. Offshore flow will develop again Monday through Wednesday for the potential for stronger offshore winds with much drier air as well. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement through Thursday evening for Orange County Coastal Areas. High Surf Advisory from 8 PM Thursday to 6 PM PDT Friday for Orange County Coastal Areas. Beach Hazards Statement through Friday afternoon for San Diego County Coastal Areas. PZ...NONE. && $$ PUBLIC...Gregoria (Update)/Brotherton (Prev Discussion) AVIATION/MARINE...Moreland