High pressure aloft is weakening and shifting east and onshore flow has returned. Temperatures will moderate near the coast as marine air spreads inland but record-setting heat will continue for many inland areas today, including the low deserts. The cooling trend will continue through Monday, but with daytime temperatures remaining above seasonal averages. There will be stronger and gusty west winds each afternoon and evening in the mountains and deserts today and Monday. Gusts of 35 to 45 mph are likely in the wind-prone areas. Dry and a little warmer for Tuesday through Saturday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Mostly sunny with some high clouds this afternoon. Temperatures are generally a few degrees lower than at this time yesterday but some locations in the coastal areas are as much as 7-12 degrees cooler. This is a result of the return of onshore flow and maritime air. The onshore flow is already strong enough that wind-favored locations in the mountains and deserts are reporting westerly winds gusting 20-35 mph.
From previous discussion, The axis of the upper level ridge has shifted to the east as a low pressure system off the coast of northern CA moves toward the coast. A weakening cold front is associated with this system and is approaching Point Conception from the west. The band of frontal clouds will likely move into SoCal over the next 24 hours but is unlikely to bring any precipitation. This low pressure system will move eastward into the Great Basin by Tuesday. It will bring more significant cooling for Monday when daytime high temperatures will be in the 60s and 70s west of the mtns, in the 70s in the high deserts, the 50s in the mtns and in the upper 80s to about 90 in the low deserts. These values are still near seasonal averages in the coastal areas but above seasonal averages elsewhere. The onshore flow will continue to strengthen, peaking on Monday when it will produce southwest to west winds gusting 35-45 mph in the mountains and deserts before weakening on Tuesday. Patchy marine layer low clouds and fog will likely return to the coastal areas late tonight then again Monday and Tuesday nights.
Temperatures will rebound somewhat for Tuesday and Wednesday as the low pressure trough moves into the Rockies and a weak transient ridge of high pressure moves over the region. Daytime high temps will again be 5 to 11 degrees above seasonal averages with mostly sunny skies after the low clouds and fog dissipate.
Numerical models remain in good agreement into Thursday before solutions diverge as we move into next weekend. Another low pressure system will move inland to the north on Thursday bringing slightly lower temperatures for Thu and Fri along with another round of strong onshore flow on Thursday.
Forecast details become more uncertain after Thursday as the deterministic models show a closed upper low forming near SoCal by Saturday. These models disagree significantly on the location of the closed upper low. While a slim majority of ensemble members across model platforms agree (in general) with this solution, a significant minority of ensemble members produce a solution where the upper trough remains an open wave and continues to progress eastward into the southern Rockies. The first solution has the potential to produce precipitation for SoCal while the second solution keeps us dry. For now, the forecast remain dry but stay tuned.
011800z. Coast, VFR conditions and mostly clear skies prevail for the first half of the period with marine layer clouds increasing after 06z Monday. These low clouds are expected to stretch 10-15 inland, based around 400-800 ft MSL with VIS at sea level generally 3-5 SM in BR, lower VIS for coastal mesas. Scattering expected by 18z Monday.
Elsewhere, VFR with SKC-FEW250. Winds will begin to increase after 20z in the mountains and deserts with gusts up to 20-35 knots with locally higher gusts in the mountain passes.
Northwest winds increase over the outer waters this afternoon creating hazardous conditions for small craft. Peak gusts of 20 to 25 kt are expected, with seas of 4 to 7 ft. Strongest winds and highest seas will be in the outer waters near San Clemente island. Winds and seas will decrease Monday night into early Tuesday.
Ca, None. PZ, Small Craft Advisory from 4 PM this afternoon to 9 PM PST Monday for Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.
