The coastal basin will continue to cool a few degrees early this week while mountain and desert regions warm slightly. Patchy coastal fog is possible Monday morning and more likely early Tuesday. Further cooling expected across the entire region starting Tuesday as a low- pressure system begins to move in from the northwest. This will bring widespread light to moderate rain to the region Wednesday into Thursday along with elevated southwest winds for the mountains and deserts as well as thunderstorms for Thursday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
, Today Oct 18 through Tue Oct 20, Pretty big changein temperatures between today and yesterday; 24 hours ago temperatures were 5 to 12 degrees warmer in the coastal and valley areas and 3 to 9 degrees cooler for the mountains and deserts. The difference between this morning's and yesterday's low temperatures is even more significant, with low temperatures across the board 5 to up to 30 degrees cooler this morning compared to yesterday. While offshore winds were present this morning, they were light enough that they had little warming effect, while keeping it dry and clear enough for efficient radiational cooling.
Clear skies will continue today through most of tonight, though there's a chance for at least patchy dense fog to develop early morning on Monday. The cooling trend present today in the coasts and valleys will continue for Monday's high temperatures with another few degrees cooling, while the deserts and mountains warm a couple additional degrees.
There is higher certainty in the return of the marine layer Monday night into Tuesday morning, which will likely start out quite initially low, if not leading to coastal dense fog, before likely rising gradually through Tuesday morning. The cooling trend that started in the coastal basin will continue for Tuesday, as well as spread into the deserts as a low pressure system from the northwest moves towards the area. A small handful of models are indicating that light showers could start late Tuesday, generally just for the inland mountains with increased onshore flow and moisture perhaps leading to weak orographic lift, but the majority keep Tuesday dry. Winds will be average for most areas but start to pick up over the upper ridges into the High Desert late in the afternoon Tuesday.
..Wed Oct 21 through Sunday Oct 25, A broad closed lowcontinues down the west coast Tuesday into Wednesday before stalling just northwest of Pt Conception Wednesday afternoon, then finally sweeping south towards the region late Wednesday. Onshore winds will pick up Wednesday afternoon for the usual mountains, desert slopes/deserts, and marine waters. These winds don't look to be significantly strong, with gusts likely in the 25 to 35 mph range for the most part (to around 20 mph over the coastal waters).
Rain chances increase through the day Wednesday; the current forecast has widespread light to moderate rain moving through the area late Wednesday through Thursday morning. South-southwest flow as the rain band moves in will favor the San Bernardino Mountains, where we're expecting the most overall rainfall. The front looks likely to weaken slightly as it passes through, with lighter rain for south and east San Diego County compared to North County, Orange County, and southwest San Bernardino County. Rain rates will likely be steadily light to locally moderate through this period.
Widespread rain gives way to scattered showers Thursday as the low digs south, finally sweeping right over the area Thursday morning into the afternoon along with the associated vort max. This will bring 1000-700 MB instability of 350-500 J/kg to the area in turn leading to widespread 15 to 25% chance for thunderstorms for the mountains and areas westwards. Thunderstorms will bring the threat for locally heavier rainfall as well as the usual dangerous lighting, gusty winds, etc. so this may be the most potentially impactful period during this event. Snow levels will drop Thursday-Friday, but only to about 8000 feet, so it's unlikely that any area but the highest mountain peaks receive snow.
Showers gradually end sometime between late Thursday and Friday morning. All in all, total rainfall for the region looks to be in the range of 0.7" to about an inch for North County San Diego, Orange County, and the northwest Inland Empire, with 1" to 1.3" likely for the San Bernardino Mountains. South and east county San Diego (including the mountains) as well as the southeast Inland Empire and Riverside Mountains likely to get 0.4" to 0.7", with 0.2" to 0.5" in the High Desert and likely under 0.1" for the low deserts. We've gotten an excellent head start on the Water Year (which started Oct 1st), and this event will keep the trend going.
Models diverge on the location of the low going into next weekend, with some showing the low stalling near AZ over the weekend and others a smooth exit eastwards. In either case, temperatures will start to rebound Friday through Sunday, and winds will likely turn offshore for Saturday-Sunday. The strength of these winds varies quite a bit between ensemble model members, with most of them at this point looking pretty weak, but confidence will increase as the week progresses. Looking out towards the end of the month, some form of troughing is likely on the far horizon. The current Climate Predication Center's outlook for October 26 through November 1 in southern California is for near normal temperatures and leaning above average precipitation (50-60% chance).
181730z. ..VFR conditions expected with mostly SKC.
Coast, There is a 20 percent chance of low clouds and patchy fog developing along San Diego Co coastline, incl vicinity of KSAN and KCRQ, 09-16Z. If clouds do manage to form on land, cigs and vis will be very low, 200-500 ft MSL and 0-3 SM respectively. Any cloud coverage clears 16-18Z Mon.
No hazardous marine conditions are expected through Tuesday. A storm system moves into the region Wednesday through Thursday, bringing possible local gusts around 20 knots and a chance (10-15%) for thunderstorms Thursday.
Elevated surf will continue through mid-next week with multiple south swells moving in through this period. Surf increases slightly today, with surf 3 to 6 feet and local sets to 7 feet along south and southwest-facing beaches through at least Tuesday.
While surf and rip currents will be elevated, daily high tides will not be particularly high, and the threat of coastal flooding is minimal. Further details can be found in the Beach Hazard Statement.
Ca, Beach Hazards Statement through Tuesday morning for Orange County Coastal Areas-San Diego County Coastal Areas.
PZ, None.
