Slightly cooler conditions expected into Monday. Patchy dense fog is possible over the coastal waters and along the immediate coast tonight into Monday, with the potential for patchy dense fog to develop Monday night into Tuesday. Warmer conditions return for the middle to end of next week, with periods of Santa Ana winds. Cooler conditions and onshore flow return for the end of the week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Update: The forecast continues to remain on track, with a weak trough passing to the north and then a rex blocking pattern setting up thereafter going into the middle of the week, with an upper level ridge to the north and an upper level low to the south. The pressure gradient will cause mild Santa Ana conditions to return to the area and lead to a gradual warmup through Wednesday. After that, there will be a cooldown from Thursday through next weekend. There may also be a slight chance of rain as the trough begins to transition over the western states and possibly allows an area of low pressure to drop down over SoCal, although at this time there is too much disagreement with the deterministic models to have high confidence of this, although ensembles do reflect that we will be returning to a more active weather pattern going into the latter part of the weekend and into early next week.
Previous discussion submitted at 1131 AM:
Weak onshore flow returns for today and Monday as a weak trough moves across the West Coast. This will allow the marine layer to rebuild after a period of prevailing offshore flow. Patchy dense fog is expected to develop tonight over the coastal waters and along the immediate coast. Areas of visibility of 1 mile or less may impact travel at times. Patchy dense fog may redevelop for Monday night into Tuesday for similar areas and with similar impacts.
Highs today are expected to be a few degrees cooler than yesterday along the coast and western valleys, with cooling spreading further inland for Monday. Even with widespread cooler conditions on Monday, highs will remain 5 to 10 degrees above average, locally up to 15 degrees above average in the mountains. By Tuesday, an upper level high pressure will settle over the West Coast, kicking off another period of warming. Wednesday is currently forecast to be the warmest day of the week. West of the mountains widespread highs in the upper 70s to upper 80s, locally low 90s are expected. The mountains and High Desert will see highs in the upper 60s to low 70s, with highs in the low desert in the 80s. The majority of global ensembles show the upper level high pressure shifting east on Thursday, which will allow for a few degrees of cooling across the area but maintain above average temperatures.
With the upper level ridge in place over the Western United States, a surface high is expected to form over the Great Basin for the middle to end of the week. This will bring another period of Santa Ana winds. The CW3E West-WRF ensemble guidance maintains a 60 to 80 percent chance of a weak to moderate wind event with a 30 percent chance of a moderate wind event for Wednesday and Thursday. Current guidance indicates locations more prone to easterly/northeasterly winds will be more impacted than areas more prone to northerly winds.
Uncertainty grows for Friday onward as ensemble solutions begin to diverge on the evolution of the upper level pattern, particularly with the progression of an upper low moving into the West Coast. A cooling trend for Friday and Saturday looks likely, but how much cooling we'll see will depend on how the low tracks. A few members of each of the global ensembles are producing light precipitation with this low over the weekend, but not enough to introduce a slight chance into the forecast. The alternative to any precipitation would be another round of offshore flow and warming conditions by Sunday.
020400, Coastal locations beginning along south San Diego will see low clouds and patchy fog moving in after 09Z with a 45% chance of vis less than 2 SM. Areas further north along the coast are expected to see patchy fog after 13Z but with only a 10% chance of vis less than 5 SM. Low clouds and fog will clear around 15-17Z. Everywhere else, VFR conditions expected through the TAF period.
Patchy fog likely to develop over the coastal waters overnight through late Monday morning, with areas of visibility under 1 SM possible. Otherwise, no hazardous marine conditions are expected through Thursday.
Ca, Coastal Flood Advisory from 4 AM Monday to 10 AM PST Tuesday for San Diego County Coastal Areas.
High Surf Advisory from 4 AM Monday to 10 AM PST Tuesday for San Diego County Coastal Areas.
Beach Hazards Statement from 4 AM PST Monday through Monday evening for Orange County Coastal Areas.
PZ, Coastal Flood Advisory from 4 AM Monday to 10 AM PST Tuesday for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm.
High Surf Advisory from 4 AM Monday to 10 AM PST Tuesday for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm.
