Dry and cooler this weekend. Patchy fog along the coastal areas and coastal mesas again tonight. Weak to locally moderate Santa Ana winds develop Monday and weaken Tuesday with warmer and drier weather. 20-35% chance of light precipitation Wednesday into Thursday, though confidence remains very low. Another round of offshore winds like follow this system for the late week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Sfc pressure gradients remain weakly offshore this evening, with -0.1 mb SAN-DAG and weak diurnal slope winds. The marine layer is a little deeper than at this time yesterday but low clouds are a little slower to form than yesterday.
From previous discussion, The next shortwave trough is set to move into the Great Basin from the PAC NW tonight into Sunday, bringing stronger onshore winds Sunday afternoon, mainly for the higher elevations in the mountains and adjacent desert slopes. Wind gusts 25-35 mph expected in the high deserts/desert slopes with about 35-45 mph near mountain peaks. The marine layer will likely be less patchy tonight, with cloud cover stretching into portions of the western inland valleys. As cold air spills into the Great Basin behind this upper trough, the offshore pressure gradient is expected to strengthen on Monday, more than what is seen today. This Santa Ana wind event will likely be weak to moderate strength with strongest wind gusts Monday afternoon found in and just below Cajon/Banning Pass and Fremont Canyon up to 45-50 mph, with 25-35 mph gusts spreading into the Inland Empire and inland Valleys of San Diego and Orange Counties. As for temperatures, Sunday will likely be a few degrees cooler than today with the onshore flow, followed by warming and drying on Monday with the offshore flow.
Tuesday into Late Week,
Onshore flow is quickly reestablished Tuesday afternoon, returning temperatures back to near average. Beyond Tuesday, the upper level pattern remains quite uncertain through mid to late week as models continue to disagree on the evolution of the next trough, set to move southward on Wednesday. The GFS and its ensembles have a weaker, shallower, and faster trough, while the ECMWF and its ensembles feature a deeper and slower system. While this disagreement persists, confidence on any precipitation, both timing and amounts, will be low. In a scenario like the GFS's, precipitation amounts will be very low, likely 0.1" or less, with northerly offshore flow building Wednesday into Thursday. If the ECMWF's solution were to verify, precipitation amounts could be closer to 0.25-0.5+" with rain chances stretching into Thursday, delaying northerly offshore flow into late Thursday into Friday. Uncertainty in snow levels exists as well, but will generally be falling to around 6500-6000ft Wed-Thurs. Either way, this storm system isn't expected to be an efficient rain maker, but stay tuned over the next few days to see how this evolves. Still, temperatures are likely to cool below average Wed-Thurs with the trough/cloud cover, though with the potential for offshore flow, its a bit too early to say for sure.
300400z. Coasts/Valleys, Patchy low clouds with bases around 400-700ft MSL have developed offshore this evening, moving ashore after 05Z this evening and into western valleys overnight. Expecting vis to be higher tonight than this morning, but there is the potential for vis to fall to 1/2 SM or lower earlier in the night, especially for higher coastal terrain. Bases will likely rise slightly overnight towards 10-13Z, possibly to 700-1500 ft MSL with a weak coastal eddy. Due to this, there is a 30% chance for CIGs/FG vcnty KONT 13-18Z. Clouds clearing to the coast in most areas by 15-17Z, redevelop at 900-1500 ft MSL and move ashore again after 00Z Mon.
.Mountains/Deserts, VFR conditions 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 16Z through Sunday evening. Mod up/downdrafts expected in the vicinity of the mountains. Winds becoming offshore and breezy early Mon 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 Thursday.
Ca, None. PZ, None.
