A warming trend will continue through through Monday, with a shallower marine layer. Night and morning low clouds will be restricted to the coastal areas and adjacent valleys through this period. Temperatures will peak on Monday around 10 to 20 degrees above normal for inland areas, with areas of moderate to high heat risk. A cooling trend with a deepening marine layer will return for the middle and end of next week as an area of low pressure approaches the West Coast.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
This morning, The sounding from yesterday afternoon and recent pilot reports indicate the marine layer is about 1500-1700 ft deep. At this hour, low clouds cover the coastal areas extending almost 15 miles inland. Sfc pressure gradients remain onshore and high-resolution models show the marine layer low clouds reaching into the northwest portions of the Inland Empire with significantly less coverage than the last 2 nights. Low clouds are expected to clear to the coast by late this morning.
The extensive high pressure ridge continues to strengthen over the west coast and today is likely to be generally warmer than yesterday. High temperatures today will be within a degree or two of normal near the coast while in the inland valleys, mountains and deserts temperatures will be 5-15 degrees above normal. Monday will likely be the warmest day of the week as the high pressure ridge reaches its peak intensity. High temperatures on Monday will be up to 9 degrees above average in the coastal areas while in the inland valleys, mountains and deserts they will be 10-20 degrees above average. Overnight low temperatures will also be above normal and the low deserts will likely experience high HeatRisk today and Monday while the Inland Empire will likely experience moderate HeatRisk only on Monday. The marine layer will be at its shallowest tonight into Monday morning, restricting the low clouds to within a few miles of the coast.
Models remain in good agreement through Tuesday with respect to the large-scale pattern. Solutions begin to diverge significantly after Tuesday introducing more uncertainty into the forecast. We can say with reasonable confidence that a cooling trend will begin on Tuesday as the upper ridge begins to weaken and shift east in response to a low pressure system moving in from the northwest. This low pressure will move inland on Thursday with the closed upper low over central/northern CA (depending on the model solution). Thursday will likely be the coolest day with daytime high temperatures mostly within 3 degrees of seasonal averages except in the low deserts where temperatures will likely be about 6 degrees above seasonal averages. Temperatures then trend upward next weekend as the low pressure system moves east and a high- zonal pattern develops aloft. The marine layer will deepen, allowing low clouds to spread farther inland during the nights and mornings, moderating temperatures west of the mtns. Conditions are likely to remain dry through the forecast period.
100930z, Coast/Valleys, Low clouds based 1000-1500ft MSL are covering up to the inland valleys, beginning to reach into the western Inland Empire after 11z. 30% chance for IFR cigs to reach KONT by 12z; 50% chance by 14z. VIS generally over 5SM for coastal areas up to the valleys around I-15. VIS 0-4SM (BR/HZ) and patchy fog for elevated inland valleys, including western parts of the Inland Empire through 16z. Bases lift after 15z and scatter out 16-18z. Low clouds with bases 700-1200 ft MSL develop at the coast 01z Mon and move inland through 06z. VIS reductions and fog to occur similarly in eastern interior valleys after 05z Mon with a higher chance for vis impacts to reach down into lower valleys around the I-15 corridor.
Mountains/Deserts, SKC and VFR through early Monday. Gusts up to 30 kts along desert slopes (40 kts Banning Pass) Sunday evening.
No hazardous marine conditions are expected through Thursday.
Ca, Heat Advisory from 10 AM to 8 PM PDT Monday for San Bernardino and Riverside County Valleys-The Inland Empire.
Extreme Heat Warning from 10 AM this morning to 8 PM PDT Monday for Coachella Valley-San Diego County Deserts-San Gorgonio Pass near Banning.
PZ, None.
