Upper level high pressure will slowly build in from the east this week, bringing gradual warming, a shallower marine layer, and dry weather. Shower and thunderstorm chances return for the mountains Thursday through Sunday with subtle cooling as the upper high shifts east.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
, UPDATED AVIATION DISCUSSION,
Water vapor imagery shows a broad upper level high across the southern US centered over Texas. This high slowly expands westward through Wednesday before shifting back eastward and amplifying on Thursday. This will produce a gradual warming trend through the middle of the week along with a shallower marine layer. Areas of night and morning low clouds and fog will generally be confined to the coastal areas and far western valleys Monday night through much of the week. With the ridge axis directly overhead, the subsidence inversion and mid-level drying will prevent any deep convection from developing. There could be a few shallow cumulus clouds over the mountains each afternoon through Wednesday, but no precipitation is forecast.
In terms of temperatures, highs today will be near to a couple degrees above normal - upper 70s for the coastal areas, mid 80s to low 90s for the western valleys, mid to upper 90s in the inland valleys, mid 70s to upper 80s in the mountains, mid 90s to low 100s in the high desert, and 105-110 in the low desert. Temperatures increase to around 3 to 7 degrees above normal by Thursday, which is still forecast to be the hottest day of the week for the mountains westward. Heat in the low deserts will likely peak earlier in the week as a return of monsoonal moisture on Thursday may limit heating there. By Wednesday, there is a 90-100% chance of at least moderate HeatRisk over the inland valleys, lower elevation mountains, and deserts. Chances of major HeatRisk are around 10-30% over the far inland valleys, around 50% for the lower elevation pars of the high desert, and 55-85% for the low desert. While high temperatures won't be significantly above normal, continued humidity and above normal overnight temperatures will contribute to the higher HeatRisk. These probabilities increase slightly on Thursday. Only a couple degrees of cooling is expected each day Friday through the weekend as the upper high shifts back eastward.
As the high becomes centered closer to the Four Corners region late in the week, south to southeasterly flow aloft will help bring in another round of monsoonal moisture. Moisture begins to increase by Thursday, but with the ridge still overhead instability is lacking, so precipitation chances over the mountains is only 10-15%. Chances increase to around 15-20% each afternoon for the mountains and parts of the deserts Friday through Monday.
171800z. Coast, Intermittent low clouds along the beaches of SD county will begin to clear after 19-20Z, keeping SCT-FEW conditions at KSAN. Patchy low clouds to gradually increase overnight into Tuesday. Highest chance of cigs around 1000 feet MSL in San Diego County 12-16Z Tuesday, more isolated cloud coverage near KSNA. Clouds clear to the coast after 15Z Tue.
Otherwise, FEW-SCT high clouds AOA 15000 feet MSL increasing this evening, lingering into Tuesday. SCT-BKN clouds 10000 feet MSL over mountains 19-00Z.
Occasional northwest wind gusts will exceed 20 knots in the outer waters near San Clemente Island this afternoon and evening. Otherwise, no hazardous marine conditions are expected through Friday.
Ca, None. PZ, None.
