Cooler conditions today with gradual warming for the middle of the week. Minimal change in temperatures expected for the end of the week into the weekend. A deep marine layer will be maintained into Tuesday morning with night and morning low clouds spreading into the valleys. There is a a slight chance of showers and isolated thunderstorms over the mountains Thursday afternoon, with otherwise dry conditions expected to prevail.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Low clouds are hanging on this morning across the coastal basin with areas of high clouds streaming to the northeast across the area. Cloud coverage could be fairly persistent today for some areas as a closed upper level low move east across the US West Coast. There were a few sprinkles across the coastal basin this morning with continued precipitation to our east in the desert Southwest, but there is less than 15 percent chance of accumulating precipitation through the afternoon. Under the influence of the low, the marine layer will stay 3000 to 3500 ft deep into Tuesday morning with low clouds expected to fill in much of the coastal basin overnight.
Highs today will be a few to around 5 degrees cooler than Sunday and as much as 5 to 10 degrees below average for the valleys. High temperatures on Tuesday will warm a few to around 5 degrees with the greatest warming for the inland valleys. Warming will continue on Wednesday with high temperatures as much as another 5 to 10 degrees warmer for the Inland Empire and inland Orange County. With the warming, high temperatures on Wednesday will be within a few degrees of average for most areas. Another few degrees of warming is expected Thursday and Friday with temperatures along the coast with the highest departure from normal at 7 to 12 degrees. For the latter half of the week, overnight lows will increase to the mid-60s to 70 west of the mountains and in the upper 70s to low 80s in the low desert.
For Thursday, a closed low lingering to our southwest will start to move east. This low will have some upper level moisture entrained in it which may bring some precipitation to the area, most likely for the mountains. The low will bring some increase in instability, so a slight chance of isolated thunderstorms (15-20%) has been introduced to the forecast, mainly confined to the mountains. There is low confidence in any potential impacts, especially in the way of flooding. The spread in NBM guidance between 10th percentile and 90th percentile for 24 hour rainfall is 0 and 0.55 inches respectively.
Tropical Cyclone Polo, off the southwest coast of Mexico, is forecast by the National Hurricane center to become a major hurricane over the new few days with a track eventually and generally toward the west northwest which might put the center a few hundred miles south of the southern tip of Baja early Saturday. There remains a lot of uncertainty in the forecast for late in the weekend into next week but there could eventually be at least some peripheral impacts to portions of Southern California.
211800z, Coast/Valleys, SCT-BKN low clouds with bases 2000-3500 feet MSL still cover much of the coastal basin but CIGs are mostly above 3000 ft MSL. Low clouds with similar bases will begin to increase in coverage around 23Z, covering the coastal basin by 12Z Tue. Most CIGs are likely to be above 2000 ft MSL.
Elsewhere, SCT clouds AOA 15000 feet MSL with unrestricted VIS through tonight.
No hazardous marine conditions are expected through Friday.
Ca, None. PZ, None.
