Dry and cool today. The marine layer deepens into Monday for more widespread low clouds and fog each night and morning. Weak to locally moderate Santa Ana winds develop Monday and weaken Tuesday with warmer and drier weather. There is a 15-30% chance of light precipitation Wednesday afternoon into Thursday morning, followed by another round of weak to moderate Santa Ana winds Thursday into Friday. Gradual warming Thursday through the weekend with highs near seasonal normals.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Morning Update,
While a coastal eddy didn't not materialize this morning, increased onshore flow from a trough digging through the Great Basin has allowed the marine layer to deepen and spread low clouds/fog into the western inland valleys this morning. Clouds are more patchy and starting to mix out in southern San Diego County but remain more uniform through Orange County and the northwestern Inland Empire, but should eventually clear out by the early afternoon. See the previous discussion below for more details about remainder of the forecast.
From the Previous Discussion,
Increasing onshore flow with the trough passing by to the north will bring breezy west winds to the mountains and deserts, strongest through the passes and on the east facing mountain slopes where gusts 35-45 mph are possible this afternoon and evening. The passing trough will also result in cooler weather with highs near seasonal normals.
The marine layer continues to deepen tonight, potentially spreading into the Inland Empire. Surface high pressure builds over the Great Basin tonight behind the departing trough, driving weak to locally moderate Santa Ana winds by Monday morning. Winds peak during the morning and early afternoon with gusts 30-45 mph below the passes, canyons, and foothills. These winds will bring minor warming west of the mountains where highs will be a couple degrees above normal, with cooling in the mountains and deserts due to cold air advection from the north. Surface pressure gradients and winds weaken on Tuesday, with a return of weak onshore flow by the afternoon for slight cooling west of the mountains and slight warming in the mountains and deserts.
For the midweek system, all of the global ensembles and deterministic models have trended more towards an inside track. By Thursday, only about 36% percent of the members maintain the closed low or deep trough over So Cal on Thursday compared to 75% from the 00Z Saturday runs. The trough still tracks close enough to maintain a 15-30% chance of showers Wednesday afternoon through Thursday morning, though with the trend towards the more inside track, precipitation amounts have decreased. Nbm currently has around a 25% chance of 0.10" or more. Precipitation (assuming we get any) shuts off pretty quickly on Thursday as another round of weak to moderate Santa Ana winds develop behind the trough. Santa Ana winds will likely continue into Friday. For next weekend, ensemble solutions begin to diverge slightly. Around two-thrids of the members maintain a ridge just off the West Coast and dry northwesterly flow over So Cal which would result in warmer weather and perhaps another round of offshore flow, while the remaining members break the ridge down and have more zonal flow for cooler weather and more persistent onshore flow.
301630z. Coasts/Valleys, Low clouds based 800-1500ft MSL have cleared the southern half of San Diego County but remain socked in across Orange County. Low clouds are beginning to filter into the Inland Empire a bit but are unlikely to be able to reach KONT. Clouds will clear back towards the coast 18-20z. Patchy low clouds will redevelop with bases of 1100-1800ft MSL and move ashore again as early as 00-01Z Monday, but will struggle to make it very far inland before they are pushed back towards the coast early Monday morning. Still, some low clouds will likely filter into the Inland Empire some time after 04z Mon. Bases will rise beginning around 11z as the offshore winds kick in, to around 1600-2000ft MSL or higher. Clouds should clear to the coast by 18z Mon.
.Mountains/Deserts, VFR conditions are expected through the TAF period. Strong surface winds gusting 25-35 kt, up to 45 kt through wind-prone passes, at times in the deserts and eastern mountain slopes, generally after 18Z today, prevailing through the evening. Moderate up/downdrafts expected in the vicinity of the mountains. Winds becoming offshore and breezy after 12Z Monday morning for the west mtn foothills locally into eastern valleys, with local areas of LLWS possible for parts of the valleys.
No hazardous marine conditions are expected through Friday.
Ca, None. PZ, None.
