Warm with weak offshore flow into Monday. Patchy dense fog is expected over the coastal areas late tonight into Monday morning. Gradual cooling Tuesday and Wednesday. Increasing winds and widespread precipitation Thursday and Friday, potentially continuing into Saturday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Evening update, It was a hazy afternoon and evening for the coastal areas, but visibility improved once the Sun set. Marine layer low clouds have already spread into the western valleys of San Diego County, but are struggling to for over northern Orange County. Offshore surface pressure gradients are weakening, and with that winds along the coastal slopes and below the passes are gusting to 20-35 mph. Hi-res guidance shows these winds remaining confined to the foothills, so they may be not have much impact on clearing out the low clouds earlier. There is potential for visibility to drop again tonight with dense fog developing, especially on the coastal mesas.
Previous discussion, High pressure aloft will continue to strengthen through Monday. The high pressure combined with weak to moderate offshore flow at the surface will result in above average temperatures across the area. Highs Monday will be as much as 12 to 17 degrees above seasonal averages. Areas of minor to moderate HeatRisk are expected, with the highest HeatRisk expected in the western valleys. Northeast to east winds are expected to have a peak Monday morning before slowly diminishing into Monday evening. Winds will be most widespread on the coastal slopes of the mountains and through passes and canyons. Peak gusts of 30 to 40 mph are expected in the favored locations.
Cooler conditions are expected by Tuesday, although highs will remain above average, as the area of high pressure weakens ahead of an incoming low pressure system. As onshore flow continues to increase for mid-week the marine layer will begin to deepen, diminishing chances for coastal dense fog. Cooler conditions will also spread inland due to the increase in onshore flow. By Wednesday, high temperatures are expected to return to near normal west of the mountains, but remain about 10 degrees above average in the deserts. Further cooling is expected for the end of the week with widespread below average temperatures although there remains about 10 degrees of spread between high temperature guidance for inland areas from Thursday onwards.
There remains some spread in model solutions from Thursday onward, but better agreement between the majority of solutions is starting to emerge, especially when it comes to timing. Precipitation is looking likely to start later in the day Thursday, with the highest chances of precipitation late Thursday into Friday, chances look to diminish late Saturday. The NBM shows a 45 to 65 percent chance for a 3-day total of one inch or more of precipitation from the mountains to the coast, with the mountain slopes most likely to reach that threshold. Nbm chances for the deserts exceeding 0.25 inch of rain over 3-days is about 30 percent for the low desert and 60 percent for the High Desert. An increase in winds can also be expected over the mountains and into the deserts. Gusts to at least 40 to 50 mph are looking likely over the mountains and into the deserts, with timing of the strongest winds still uncertain. Finally, it does look like this system will be cold enough to produce some mountain snow. There remains considerable uncertainty in the snow levels, but right now it looks like the snow level during the highest chance of precipitation is around 6500 feet, with some ensemble solutions showing snow levels as low as 5500 feet. By next Sunday, most ensemble solutions have the low pressure system to our east, but about 40 percent of solutions still have the low in a position that would result in wetter conditions for Southern California.
100415, Coasts/Valleys, Low clouds based 400-800 ft MSL currently over San Diego County coast and western valleys and moving northwards into Orange County. Widespread VIS reductions along the coast 2-6 SM in BR, with dense fog at higher coastal/west valley terrain. CIGs and VIS likely to gradually lower overnight, to 200- 600 ft MSL and 1/4-3 SM. Meanwhile, weak easterly winds likely to result in intermittent clearing, possibly as far as the coasts in areas, especially after 06Z. FG retreats to the coast more fully 13- 16Z. Widespread VFR conditions Monday afternoon with low clouds mostly clearing from nearshore waters.
Inland Valleys/Mountains/Deserts, Clear skies and VFR conditions through the period. Localized east to northeast winds through passes and coastal slopes through Monday. Intermittent gusts 25-35 mph in these areas. Winds peaking early Mon morning before gradually decreasing over the day.
Areas of fog redevelop over coastal waters overnight, intermittently reducing visibility to less than 1 nautical mile. Fog will clear out by late Monday morning. Otherwise, gusty southwest winds pick up late this week ahead of a weather system. Low confidence on timing and peak intensity at this time, though sustained winds 20 to 30 kts and periods of rain are likely.
Ca, None. PZ, None.
