Slight chance of showers over the San Bernardino Mountains this afternoon. 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 Saturday 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,
Marine layer low clouds are much slower to develop this morning with only patchy coverage near the San Diego County coast. Clouds should slowly fill in over the coastal areas and western valleys by sunrise. Otherwise a band of high clouds is moving northeast over Baja into Imperial County with clear skies elsewhere.
A slow warming trend begins today as a broad upper level high over the southern US begins to expand westward into So Cal. High temperatures today will be near to slightly below normal - generally mid to upper 70s along the coast, mid 80s for the western valleys, low to mid 90s for the inland valleys, upper 70s to mid 80s in the mountains, mid to upper 90s in the high desert, and 105-108 in the low desert. Some residual mid-level moisture remains, but with drying near 850 mb and the subsidence inversion strengthening, any convection over the mountains will be high- based and rather shallow. Can't discount a few showers over the higher peaks of the San Bernardino Mountains, where there is a 15% chance of showers/thunderstorms this afternoon.
The warming trend continues into Thursday when high temperatures climb to 3 to 7 degrees above normal. For Wednesday through Friday, there is around a 15-20% chance of high temperatures of 105F or more over the interior portions of the Inland Empire (Lake Elsinore to Hemet), and a 45-65% chance of high temperatures of 110F or more in the low desert. Widespread moderate HeatRisk is forecast for the valleys, lower elevation mountains, and deserts Wednesday through Friday, with areas of high HeatRisk in the low desert. Low elevation dew points/humidity remain elevated through the week as well, keeping minimum temperatures several degrees above normal. Basically another hot and muggy week is in store. The marine layer will become more shallow with areas of night and morning low clouds and fog becoming confined to the coastal areas and far western valleys.
Thursday through Saturday, the upper level high shifts towards the Four Corners region, allowing for additional monsoonal moisture to track into CA and slow cooling to occur. There remains subtle spread in the ensembles with respect to the position of the high and the orientation of the ridge axis, which will determine how much moisture and instability we see. For now there is at least a slight (15-20%) chance of showers and thunderstorms over the mountains and high desert each afternoon Thursday through Saturday, with lower chances on Sunday as the upper high shifts east and moisture begins to decrease.
161735z. Coast/Valleys, Low clouds will mostly remain off the coast through the early evening hours. Confidence increasing on a later time period for low clouds to impact coastal TAF sites. Low clouds near 1300-1700 ft MSL will move ashore in San Diego County after 04Z, closer to 07Z in Orange County. Cig impacts may be intermittent at KSNA. Low clouds clearing to the coast 15-17Z Mon.
Mountains/Deserts, VFR conditions prevail through the TAF period. SCT-BKN cu AOA 10,000ft MSL building into the afternoon with a slight chance (10-20%) for TSRA over the Riverside and San Bernardino county mountains. Cu dissipates this evening with high clouds moving in from the west. Chance of rainfall nil on Monday.
Gusty northwest winds 20-23 knots near San Clemente Island expected this afternoon and Monday afternoon. Otherwise, no hazardous marine conditions are expected through Thursday.
Ca, None. PZ, None.
