Area Forecast Discussion
Issued by NWS San Diego, CA

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000 FXUS66 KSGX 172044 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 144 PM PDT Tue Oct 17 2017 .SYNOPSIS... Light offshore winds and high pressure aloft will result in another very warm day today, with scattered high clouds. An upper level disturbance may bring some sprinkles or light showers to the mountains and high deserts late tonight into early Wednesday, with a small chance of thunderstorms. A deepening trough over the West will bring a cooling trend Wednesday through Friday with the marine layer returning and deepening. Expect night and morning clouds over the coast and portions of the valleys. On Saturday the flow turns offshore as high pressure builds. That will bring Santa Ana conditions, clear skies and hot weather, especially Sunday and Monday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Infrared satellite imagery this afternoon shows cold cloud tops (peaking around -55 deg C) and thunderstorms associated with a short-wave trough offshore of Point Conception, while scattered mid and high level clouds can be seen stream into Southern California from the southwest. The 12Z Miramar sounding indicated a dry layer from the surface up through 700 mb, a moist layer from about 660-580 mb, and then dry air above that. Light offshore flow continues across the region, as the San Diego to Tonopah offshore pressure gradient is at 5.2 mb and the San Diego to Las Vegas gradient is at 2.1 mb. On water vapor satellite, an upper level ridge can be seen over much of the Southwest. This ridge and the light offshore flow is resulting in another warm day across Southern California today, with 1 pm temperatures in the 90s in the inland valleys and 80s at the coast. The ridge will start to get flattened and pushed south tonight as a low pressure system from Alaska moves south off the coast of British Columbia. This will result in onshore flow returning and likely a return of the marine layer at the coast. There could be dense fog along the coast late tonight into Wednesday morning, but confidence is low at the moment as the HRRR and other hi-res models show almost no fog developing. On Wednesday, the aforementioned short-wave trough gets picked up by the west flow aloft as the large scale trough off of British Columbia deepens further. The short-wave trough will move across Point Conception late tonight and then across northern San Bernardino county Wednesday morning into Wednesday afternoon. The bulk of the moisture and lift will be close to the short-wave while the best elevated instability will be south into Riverside and San Diego counties. With those parameters not really lining up, confidence in any widespread elevated convection occurring is low at the moment, with best chances for some isolated thunderstorms being in San Bernardino county and the best chances for any measurable rainfall being in the mountains where the rain will not have to go through a deep dry layer. There`s a chance that nothing more than sprinkles/virga will occur as hi-res model composite reflectivity forecasts show not much of anything developing and the 18Z NAM12 is further north with the short-wave. A cooling trend will begin on Wednesday and continue through Friday as the large-scale trough deepens further and moves inland over the Pacific Northwest. The coolest day will be Friday when highs fall to 5-10 degrees below normal. Onshore flow will strengthen as a result, with strong and gusty west winds in the mountains and deserts (gusts 30-40 mph, local 50 mph). Finally, the marine layer will deepen each day, eventually reaching around 3500-4000 ft msl on Friday. Expect night and morning marine layer stratus clouds over the coast and portions of the valleys. There may be just enough moisture and lift Friday morning for some drizzle or isolate light showers over and west of the coastal slopes of the mountains. Surface high pressure building over the Great basin will turn the flow offshore starting Saturday which, in addition to a building upper level ridge, will create significant drying and warming. It will continue to get hotter Sunday and Monday as the ridge builds further and becomes anomalously strong, with the ECMWF showing 500 mb heights of around 594 dm on Sunday and 596 dm on Monday. Santa Ana winds will continue (right now looking to be around moderate strength) as well, so expect it to get pretty hot even all the way to the coast. && .AVIATION... 172015Z...SCT clouds at/above 15000 ft MSL will prevail through Wed with unrestricted vis. There is a small chance of thunderstorms late night and Wed, mainly over the mountains of San Bernardino and Riverside Counties, likely with bases above 10000 ft MSL. Stratus/fog is unlikely in coastal areas tonight or early Wed. Stratus/fog will likely return late Wed afternoon or evening. && .MARINE... No hazardous marine weather is expected through Thursday. Winds will increase to 20-25 kt in the outer coastal waters late Thursday night and Friday, decreasing by Saturday. && .BEACHES... A south southwest swell from 200 degrees will peak today-Wednesday at 3-4 ft/16-18 seconds. This will produce 3-6 ft surf with sets to 7 ft over Orange County and south in San Diego County to Solana Beach. Highest surf will occur along southwest facing beaches. Strong rip and longshore currents will accompany the elevated surf. Swell and surf will lower Thursday. A Beach Hazard Statement is in effect for elevated surf and strong rip currents through Wednesday. && .FIRE WEATHER... Very warm and dry weather (humidity once again down to 5-10 percent) will continue today, but with light winds. An upper level disturbance may bring some sprinkles/light showers to the mountains and high deserts late tonight into tomorrow morning, with a small chance for dry lightning strikes. Increasingly cool and more humid Wednesday through Friday as a trough moves in over the West. Gusty westerly onshore winds in mountains and deserts Thursday and Friday, with gusts 30-40 mph and local 50 mph. Another round of moderate Santa Ana winds likely Saturday through at least Monday, with day-time humidity falling down to at least 10 percent Sunday and Monday (with poor overnight recoveries) along with hot conditions and gusty northeast winds. This would likely result in critical fire weather conditions through and below the mountain passes and into the coastal slopes and foothills Sunday through at least Monday. && .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 Wednesday evening for Orange County Coastal Areas-San Diego County Coastal Areas. PZ...NONE. && $$ PUBLIC/FIRE WEATHER...Harrison AVIATION/MARINE...Maxwell

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