Warmer conditions expected through Monday with periods of weak offshore flow. Locally dense fog in the coastal areas this morning is possible again tonight into Monday morning. Cooler conditions expected by the middle to end of next week. A windy and wet pattern is expected to develop for the end of the week into next weekend.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
This morning, the marine layer is a little shallower than yesterday morning, with low clouds and fog confined to the coastal areas. Areas of dense fog have developed in the San Diego County coastal areas in spite of strengthening offshore flow while the low clouds and fog have begun to clear out of portions of Orange County. Sfc pressure gradients are trending stronger offshore with -8.4 mb SAN-TPH, and the favored locations in the mountains and coastal foothills are currently reporting wind gusts from 15 to 32 mph. There is about a 40 percent chance that the offshore winds and the associated dry air will cause the fog in San Diego County to lift before 9 am.
By Monday, as the pattern amplifies, the upper level high pressure will be centered approximately over Southern California with a closed upper low forming about 1500 miles west southwest of San Diego. The building high pressure aloft and offshore flow will continue the warming trend through Monday which will be warmest day, with daytime high temperatures reaching the low 90s in the inland valleys. These temperatures are about 15 degrees above seasonal averages. The offshore flow will continue to strengthen, with northeast to east winds peaking this afternoon/evening and again Monday afternoon/evening. Strongest winds will likely produce gusts of 30-45 mph in the wind-prone locations of the mtns and coastal foothills.
For Tuesday, the high begins to weaken and the flow at the sfc begins to turn onshore. This will bring some cooling to the coastal areas and western portions of the inland valleys but daytime high temperatures in the IE and parts of the San Diego County valleys will still reach the upper 80s. The marine layer will begin to deepen and low clouds will increase in coverage and begin to spread inland beyond the immediate coastal areas early Tue morning.
For Wed into next weekend, A vigorous low pressure trough from the Gulf of Alaska will absorb the low to our west into the mean flow, merging into an extensive low pressure system off the coast. As this system moves inland, it will bring much cooler conditions, stronger onshore flow, increasing clouds and the potential for widespread rain. By next Sunday, the potential for precip will diminish as the system moves east into the Rockies and a high pressure ridge begins to move in from the west.
Confidence in forecast details is still low due to the spread in model solutions but it looks like Friday will be the coolest, windiest day, with temperatures 5 to 15 degrees below seasonal averages in the coastal areas and inland valleys. In those areas, daytime high temps will be generally in the 60s. The stronger onshore flow could produce westerly winds gusting to 40-45 mph in the favored mtn passes and adjacent desert areas. The beginning and ending times, and the precip amounts are still very uncertain but about 40% of ensemble members across model platforms show some precip by Thursday afternoon. By Friday afternoon, about 60% of ensemble members show some precip. The NBM shows a 45-65 percent chance for a 3-day total of one inch or more of precip from the mtns to the coast, with the mtn slopes most likely to reach that threshold. Snow levels could get down to about 6500 ft Friday night so the resorts in the San Bernardino mtns could get some fresh snow.
091000z, Coasts, Areas of FG throughout coastal SD county to the coastal mesas up to 400 ft MSL creating widespread VIS reductions, 1/8-1/2SM. In Orange county and western portions of the Inland Empire, low clouds are based higher, around 500-900 ft MSL, with VIS 2-5SM due to BR/HZ. Intermittent offshore flow will clear out low clouds and FG in places.
Scattering to the coastline 14-17Z this morning. Patchy low clouds/FG with similar bases to form along immediate coast after about 02Z Mon, pushing up to the coastal mesas and reducing VIS 0- 2SM in even more patchy fashion than this morning.
Valleys/Mountains/Deserts, Clear skies and VFR conditions through the period. Localized east to northeast winds through passes and coastal slopes today, peaking tonight. Intermittent gusts 25-35 mph in these areas.
Patchy fog along coastal waters reducing visibility to less than 1 nautical mile through this morning. Otherwise, gusty southwest winds pick up late this week ahead of a weather system. Low confidence on timing and intensity of wind and precipitation at this time, though sustained winds in excess of 20 kts and periods of rain by late Thursday are possible.
Ca, Dense Fog Advisory until 9 AM PST this morning for San Diego County Coastal Areas.
PZ, None.
