Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
090 FXUS66 KSGX 160429 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 930 PM PDT Sun Sep 15 2024 .SYNOPSIS... Light rain will occur over and west of the mountains late tonight and Monday, mainly in the morning. It will continue to be cooler Monday with strong gusty winds in parts of the mountains and deserts. Brief warming will occur Tuesday before cooler windy weather returns by Thursday, along with a chance of light rain. Warmer weather will return Friday through next weekend as ridging builds in from the west. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Update... Marine layer clouds are already widespread and extend up to over 5000 feet in parts of the mountains, especially in San Bernardino County. The 00Z NKX sounding already had 0.82" for precipitable water, and this was almost entirely from the marine layer. Mid-level water vapor satellite imagery shows the low a bit west of San Francisco, and that low will gradually move into the Great Basin. Height gradients will gradually increase overnight with tight gradients aloft through Monday night, though the trough axis will not actually move through until very early Tuesday morning. Moisture will be sufficient for light rain, and orographic precip will occur along the SW mountain slopes, especially in San Bernardino County where 850 mb winds will be 15-25 knots and nearly perpendicular to the mountain crests in a nearly saturated atmosphere. While MOS is running quite low for precip chances due to limited statistical data for stratiform rain in the dry season, ensemble precipitation chances are quite high, with most solutions from GEFS, ENS and HREF showing measurable precipitation from the coast to the mountains Monday morning, albeit mostly 0.10" or less, except on orographic slopes where 0.25" to locally over 0.50" is expected. Therefore, I increased the PoPs to "likely" category, generally 60-70% over most of the area. With that trough axis not moving through until early Tuesday morning and a continued deep marine layer, there could be a bit more rain, albeit light, as late as a little after sunrise Tuesday. Temperatures will be well below normal with upper 60s-lower 70s west of the mountains and only in the 80s in the lower deserts Monday. In many areas these will be the lowest highs since early June, with the deserts having possibly the lowest highs since May. Monday night could have lows near freezing at Big Bear, though snow chances are minimal as the freezing/sub-freezing air will be above the moisture. Thursday still has a chance of rain, and GEFS/ENS are showing a lot of ensemble members with at least light precipitation. Instability could be a little greater as the closed upper low could be more over SoCal versus central California, so the rain could be more showery then. From previous discussion... An upper level low pressure system digging south into California from the Pacific Northwest will increase onshore pressure gradient for Monday. The increased onshore flow will deepen the marine layer to 5500-6000 ft and will produce areas of light showers west of the mountains and on the coastal mountain slopes. For the coastal areas and valleys, rainfall totals will generally be 0.05 to 0.15 inches. High-res guidance is still suggesting higher amounts on the south facing slopes of the San Bernardino Mountains. The HREF has a 50-70% chance of total rainfall accumulations greater than 0.50 inch for portions of the coastal slopes of the San Bernardino mountains, and a 20% chance of greater than 1 inch. Much of the shower activity will occur Monday morning. More isolated shower activity is possible into Monday afternoon along the coastal slopes of the mountains. An embedded shortwave will move across the area Monday night into early Tuesday. That may spark another round of isolated showers Monday night into early Tuesday, most likely in San Diego County. Southwest to west winds will begin to increase this afternoon and evening for the desert mountain slopes, deserts, and passes. Peak wind gusts for the desert slopes of the mountains and into the deserts will be 35 to 45 mph, with a few local gusts to 50 mph possible through the San Gorgonio Pass. Winds are expected to strengthen for Monday afternoon and evening. The strongest winds are expected through passes and into the deserts. There is a 60 to 70% chance of gusts greater than 55 mph below the San Gorgonio Pass and along I-8 near the Imperial County border. Forecast peak gusts are be 45 to 55 mph, with a few local gusts to 60 mph possible. Deep boundary layer moisture will still be in place for the middle of the week, but with a weakened/non-existent marine inversion cloud coverage may be patchy. Brief warming will occur Tuesday with a minor cooling on Wednesday as another trough digs south along the California coast. There remains spread amongst the ensembles in exact timing of the trough passage. This introduces some uncertainty in the forecast, particularly surrounding high temperatures on Wednesday and Thursday as well as timing of any precipitation and gusty winds. Current forecast has minor change in temperature from Wednesday to Thursday and best chance of precipitation Wednesday night into early Thursday for areas in and west of the mountains. The forecast also has the strongest winds Wednesday night into Thursday for desert mountain slopes, deserts, and through passes. As the trough moves east ridging will build in from the west for Friday into the weekend for warmer and fair weather. There is the potential for weak offshore flow sometime next weekend as an upper low/inside slider moves into the Great Basin, but confidence is low. If any offshore flow develops, it would contribute to additional warming west of the mountains. && .AVIATION... 160320Z...Coasts/Valleys...Hazy conditions continue in the inland valleys due to smoke from the Line and Bridge Fires in the San Bernardino Mountains. Low clouds with bases varying 2000-3000ft MSL and tops to 4000ft MSL continue to increase in coverage at this hour, covering all of San Diego and Orange counties west of the mtns. By 15Z expect all areas, from the coast to the coastal mtn slopes, to be covered by low clouds. CIG impacts are likely at all TAF sites except KPSP and KTRM. Light rain/drizzle could develop after 09z, with very low accumulation expected. Similar to previous nights, smoke from the Line Fire may be trapped by the marine later and result in VIS reductions down to 3SM for KSBD with the only other VIS reductions seen where marine clouds intersect with terrain. Clearing on Monday will likely be similar to what was observed today. Mountains/Deserts...Smoke from the Line and Bridge fires will produce localized VIS restrictions VCNTY San Bernardino County Mountains, and the Airport fire will produce similar conditions in the Santa Ana Mountains through Monday. West-northwest winds strengthen this evening, with gusts up to 40kts possible through the mountain passes, continuing into Monday. Light rain will be possibly tonight into Monday for the coastal slopes of the mountains but little impacts expected at TAF sites KONT and KSBD. && .MARINE... Expect light precipitation late tonight through Monday morning, with less than 0.10" expected. Winds will strengthen from the west to northwest in the outer coastal waters Monday and Monday night, with gusts up to 20kts likely. Otherwise, no hazardous marine conditions are expected through Friday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Wind Advisory through 9 PM PDT Monday for Apple and Lucerne Valleys. Wind Advisory from 11 AM Monday to midnight PDT Monday night for San Diego County Deserts. Wind Advisory through midnight PDT Monday night for San Gorgonio Pass Near Banning. PZ...None. && $$ PUBLIC...Maxwell (Update)/CO (Previous Discussion) AVIATION/MARINE...PG