FXUS66 KSGX 191616 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 916 AM PDT Thu Sep 19 2024 .SYNOPSIS... A low pressure system moving across southern California will deepen the marine layer and maintain well-below average high temperatures through Friday with chances for showers for late tonight through Friday afternoon along with a slight chance of thunderstorms for Friday afternoon. Then dry and much warmer for the weekend into next week with high temperatures warming to a few degrees above average on Sunday and 4 to 8 degrees above average for inland areas for Monday through Wednesday. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... .Morning Update... Low clouds are slowly clearing along the foothills of the peninsular ranges; the valleys and coastal area will be clearing later this morning. Visible satellite shows the closed low nearing Point Conception as dry weather will continue for much of today. Latest hi-res guidance from the WRF and HRRR are showing light drizzle and showers, notably over San Diego County, overnight into Friday morning. As the low pushes inland, wrap around moisture will occur as the coolest air of the system pushes over the region on Friday as well. Scattered showers will occur across the area with a slight chance of thunderstorms for places primarily north of SD County. Hourly rain rates will be near one third of an inch in heavier showers across the mountains, less so across lower elevations. Cooler weather will continue through Friday as well. Moderate to high probabilities exist for temperatures below 80 degrees for most areas, except the lower deserts which will see high temperatures near the 90 degree mark. Confidence is high that the area of low pressure will push eastward into Arizona by the weekend, providing drier and sunnier conditions with temperatures rising near average. .Previous Discussion .SHORT TERM (Today through Saturday)... The center of a closed low pressure system is moving southward off the coast of central California early this morning. The center is expected to move to near Point Conception for late this afternoon and across southern California during the day on Friday, then weakening as it moves into Arizona Friday night and into Colorado on Saturday. The marine layer has deepened to around 4000 feet. However, cloud bases have also risen with the depth of the cloud layer around 1000 to 1500 feet with some holes in the coverage. High temperatures will be a few degrees cooler today for the mountains, inland valleys, and high desert with high temperatures for today for the mountains and inland valleys around 15 degrees below average. Chances for light precipitation today will be mostly 10 percent or less and greatest over the coastal waters this morning. The greater chances for showers are for late tonight through Friday afternoon. The HREF shows the potential for island band showers to develop and move into San Diego County from the coast to the mountains for late tonight into Friday morning. During the late morning and afternoon on Friday, moisture in the cyclonic circulation around the upper low could bring showers from the northeast along with a slight chance of thunderstorms for the mountains, deserts, and inland valleys of southwestern San Bernardino and western Riverside Counties. NBM has increased chances for measurable rainfall at Big Bear to 57 percent on Friday with a 19 percent chance for one-quarter inch or more of rainfall. Slightly cooler high temperatures are expected for inland areas on Friday, then a few degrees warmer on Saturday near the coast to as much as 10 to 15 degrees warmer for the mountains. Even with the warming, Saturday high temperatures will still be a few degrees below average to as much as 5 to 10 degrees below average for the valleys. && .LONG TERM (Sunday through Wednesday)... For Sunday and Monday, low pressure will weaken with generally northwest flow aloft across the area. By Tuesday and Wednesday, a blocking pattern may develop across the western states. Dry weather is expected for Sunday through Wednesday. High temperatures are expected to warm several degrees for Sunday and Monday with Sunday high temperatures a few to around 5 degrees above average for inland areas and around 4 to 8 degrees above average for inland areas for Monday through Wednesday. With the warming, high temperatures for the Inland Empire on Sunday would be mostly in the upper 80s to mid 90s and mostly in the 90s for Monday through Wednesday. NBM shows greater spread in the high temperature guidance for the middle and latter part of next week with the greatest spread for the coast and valleys. && .AVIATION... 191500Z...Coast to mountain slopes...Low clouds fill the entire coastal basin this morning, with varying bases 3000-4000 feet MSL, and tops to 4200-4600 feet MSL. Scatter out for many areas should occur 16-19Z, but BKN clouds will likely linger or regenerate along the coast, but based 3500-4000 feet MSL. Low clouds to eventually fill the entire coastal basin again 01-09Z into Friday with similar bases/tops. Patchy -RA or -DZ most likely along the coast and up against the coastal mountain slopes after 06Z Friday, which would reduce vis. Mountain ridges to deserts... SCT high clouds today and tonight. && .MARINE... No hazardous marine conditions are expected through Monday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ PUBLIC...APR/17 AVIATION/MARINE...CSP