327 FXUS66 KSGX 081627 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 827 AM PST Sun Dec 8 2024 .SYNOPSIS... Low clouds and patchy dense fog are possible along portions of the immediate coast early this morning. Cooler conditions are expected today and Monday. Areas of low clouds may linger along the coast into early Monday. Moderate to locally strong Santa Ana winds in the mountains and valleys Monday through Wednesday, strongest Tuesday, with critical fire weather conditions expected. Slight cooling with less wind and a deeper marine layer will return Thursday through Saturday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... .Morning Update... The marine layer this morning was just deep enough for areas of dense fog to form along the coast, including at the San Diego Airport. Clouds may locally stay near the beaches today, but much of SoCal should see mostly clear skies today. The marine layer will deepen slightly into tonight, so low clouds will go further inland covering the coast and far western valleys into Monday morning, where foggy conditions will be limited to higher terrain away from the coast. The Santa Ana wind event that is poised to occur across the region from Monday through Wednesday remains on track. Wind speeds have trended slightly upward as the pressure gradient is expected to be a bit higher (10-12mb gradient from San Diego to Las Vegas), so will look into fine tuning the forecast this afternoon. The timing of the strongest winds continues to be placed on Tuesday with lowest relative humidity expected on Tuesday and Wednesday. Please use this time to prepare for fire weather hazards and windy conditions across the mountains and western valleys. .Previous Discussion (323 AM Sunday)... An upper level trough of low pressure will dig south into the Intermountain West Monday. A corresponding surface high will build over the Great Basin, generating an offshore pressure gradient for Southern California. The offshore gradient will generate gusty Santa Ana winds for the mountains and adjacent valleys in Southern California Monday through Wednesday. Gusty north to northeast winds will begin to increase Monday for the San Bernardino and Santa Ana mountain foothills; below the Cajon Pass into the Inland Empire; and into portions of inland Orange County Monday. As the upper level low continues to dig south, winds will become easterly Tuesday with stronger winds expected in the San Diego and Riverside mountain foothills into adjacent valleys, and below the San Gorgonio pass near Banning. Strongest winds are expected on Tuesday. These winds will also usher in very dry desert air. Gusty winds combined with very low relative humidity and dry fuels will create critical fire weather conditions. More information on wind speeds and relative humidity trends can be found in the Fire Weather discussion below. Winds will weaken through the day Wednesday with no offshore wind concerns currently forecast for the end of the week. For Thursday, most of the mean ensemble solutions are showing some degree of troughing near the US West Coast, although there is uncertainty in the placement and strength of the trough. Even with the uncertainty in the details of the trough, the ridge should be tampered enough to allow for cooling and a deeper marine layer. For Friday and Saturday, most mean ensemble solutions keep troughing near the US West Coast indicating the gradual cooling and deep marine layer will maintain until at least Saturday. Although chances remain less than 10 percent, light precipitation is still not out of the question by next weekend depending on how far south the trough digs. && .AVIATION... 081630Z....Coasts/Valleys...Low clouds (based around 100-300ft MSL) and areas of fog (with VIS 0-3 SM) remain largely offshore this morning, with the only patch currently onshore over North Island and near/just west of KSAN. Clouds and fog could linger along the coast from La Jolla southward through Imperial Beach, but otherwise, low clouds should remain offshore through the afternoon, leading to fair skies with SCT-BKN high clouds. Much better confidence for more widespread, though could be somewhat patchy, low clouds this evening. They are expected to move into coastal sites as early as 01-03z Monday, spreading 10-15 mi inland by 12z. Bases will likely be slightly higher, around 500-800ft MSL, but areas of VIS 2-5 SM possible along the coasts and coastal mesas, with VIS 0-3 SM into the western valleys and where clouds intersect higher terrain. Any clouds/fog will quickly clear out by 17-18z Monday. .Mountains/Deserts...Aside from SCT-BKN high clouds at or above 20,000ft MSL, mostly clear skies and VFR prevail through Monday morning. && .MARINE... Areas of low clouds and fog with visibilities around 1-2 NM continue to linger in the southern and eastern waters this morning, but should decrease in coverage by this afternoon. Another round of low clouds/fog is possible tonight into Monday, but should be less dense over the waters, with better visibility and higher clouds bases. Increasing offshore winds on Tuesday with gusts up to 20-25kts, primarily for the northern San Diego County coast. && .FIRE WEATHER... Moderate to locally strong east to northeast Santa Ana winds are expected Monday through Wednesday. Winds will be predominately from the north to northeast Monday before becoming more easterly Monday night into Tuesday. Winds will peak in strength late Monday night through Tuesday, and gradually fade through Wednesday. Strongest winds will occur on mountain foothills, through passes, and into adjacent inland valleys. Gusts 50 to 65 mph expected through favored passes and on mountain foothills, a few isolated gusts to 70 mph are possible Tuesday during the peak of the event in typically wind prone locations. Gusts of 35 to 45 mph are expected in parts of the inland valleys and inland Orange County. Relative humidity Monday afternoon is expected to fall to 15 to 25 percent for inland areas north of San Diego County. By Tuesday afternoon, relative humidity values will fall to 5 to 10 percent for inland areas and 15 to 25 percent at the coast. Overnight humidity recovery will be poor, only increasing to 15 to 30 percent Tuesday night into Wednesday. Low humidity, strong winds, and dry fuels will lead to critical fire weather conditions from Monday through Wednesday morning, highest risk on Tuesday. Wednesday and Thursday will see lighter winds, but humidity will still lower near 5-20% each afternoon for inland areas. Humidity recovery is expected to improve by Thursday night. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Red Flag Warning from 10 AM Monday to 4 AM PST Wednesday for Riverside County Mountains-Including The San Jacinto Ranger District Of The San Bernardino National Forest-San Bernardino County Mountains-Including The Mountain Top And Front Country Ranger Districts Of The San Bernardino National Forest-San Bernardino and Riverside County Valleys -The Inland Empire-San Diego County Inland Valleys-San Diego County Mountains-Including The Palomar And Descanso Ranger Districts of the Cleveland National Forest-San Gorgonio Pass Near Banning-Santa Ana Mountains-Including The Trabuco Ranger District of the Cleveland National Forest. PZ...None. && $$ UPDATE/PUBLIC...APR/CO AVIATION/MARINE...Munyan