Through Wednesday, a weak low pressure system moving into southern California will deepen the marine layer to around 3000 feet with night and morning coastal low clouds spreading farther into the valleys and with slower afternoon clearing for San Diego County coastal areas. It will be slightly warmer for Thursday and Friday with the marine layer decreasing in depth, then slightly cooler for the weekend.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
A weak upper low just offshore is contributing to strengthening onshore flow and cooler temperatures today. South to southeast winds will turn out of the west this evening with gusts 25-45 mph for the deserts and desert slopes of the mountains both today and Wednesday afternoon/evening. On the west side of the mountains, visible satellite indicates that marine layer clouds have held strong over San Diego and Orange Counties through this morning. Additionally, temperatures through the coastal basin are 15 to 25 degrees cooler than yesterday at this time. All signs indicate that our first true "May Gray" day is upon us with coastal areas expected to stay cloudy through the afternoon. Low clouds will fill most if not all of the coastal basin tonight with areas of drizzle expected, especially near the coastline in the early morning. This will locally create minor visibility reductions. As the low pressure moves overhead on Wednesday, a weakened inversion and weaker onshore flow should allow for a mostly sunny afternoon.
Despite the cooling trend, temperatures will still reach the low 100s today across the low deserts. Even Wednesday, which is expected to be the coolest day, will be around 5-8 degrees above normal in the deserts with temperatures nearing 100 again in the Coachella Valley. Meanwhile, temperatures will remain near to slightly below average in the coastal basin through Wednesday. Starting Thursday, a weak upper ridge will attempt to nudge into the region. This will bump temperatures up by a few degrees for all areas and decrease the depth of the marine layer for both Thursday and Friday. Thus, temperatures in the low deserts are expected to reach back into the low 100s Thursday through Saturday.
Model guidance begins to diverge slightly over the weekend, but the general consensus is that an upper low pressure system will pass to the north sometime late Saturday into early Sunday. This will herald increased onshore flow and a cooling trend resulting in near average temperatures for all areas by Sunday. A minority of model solutions produce a few hundredths of an inch of QPF for coastal areas Sunday into early Monday. However, this likely reflects the potential for marine layer drizzle and nothing more. Beyond this weekend, guidance leans toward a warming trend and slightly above normal temperatures through mid next week.
130430z, Coast/Valleys, Patchy low clouds with bases mostly 2000- 3000 ft MSL have partially cleared out of SD County. Cloud coverage will gradually fill back into SD county and the Inland Empire. Moderate confidence of full cig coverage by 09z, high confidence of full coverage by 12z. Cloud bases drop slightly throughout the night to 1500-2500 ft MSL as cloud tops increase mostly above 4000 ft MSL, yielding to coastal -DZ most likely 10-15z Wed. Patchy vis reductions 2-6SM for elevated inland valleys (above 1000 ft MSL) and 0-3SM throughout foothills. Bases lift quickly above 2000 ft most likely by 15z. Scatter out 17-19z inland with patchy BKN clouds lingering through the afternoon within 20 miles of the coast. Most bases rise to above 3000 ft by 18z where clouds remain. Clouds with similar bases (1500-2500 ft MSL) start to develop again after 00z Thu, primarily over SD County, expanding from south to north. About a 65% cig coverage over SD County by 04z, reaching to the foothills.
Mountains/Deserts, SCT high clouds and VFR conditions into Wed. West winds with gusts 25-35 kt (locally to 40 kts) weaken slightly Wednesday morning, then strengthen again after 20z Wed afternoon.
No hazardous marine conditions are expected through Sunday.
Ca, None. PZ, None.
