09/206 AM.
Extreme heat and humidity will peak Today and Thursday with a high risk for heat illness. Isolated showers and thunderstorms are possible today as well, mainly over the mountains. Elevated surf and coastal flooding will linger into Thursday, but decrease in severity. Cooler and more normal conditions likely by Saturday.
(tdy-Sat), 09/120 PM.
Overall, 12Z models in very good synoptic agreement through the short term period. At upper levels, high pressure peaks in strength over the area today then gets flattened/weakened Thursday through Saturday as a upper low moves towards the Pacific Northwest. At the surface, onshore pressure gradients will gradually increase with some weak northerly offshore gradients.
Forecast-wise for the short term, main concern is the heat. The combination of weak offshore gradients this morning, strong upper level high and increased atmospheric moisture has allowed for an extremely hot day today with temperatures west of the mountains in the mid 90s to low 100s. With the increased humidity, Heat Risks in the major to extreme categories have been observed today. So, the EXTREME HEAT WARNINGS have verified very well. For tonight, overnight low temperatures will offer little respite from the heat with lows in the 70s to lower 80s. On Thursday, the combination of some weakening of the upper level high pressure and more onshore pressure gradients will generate about 4-10 degrees of cooling west of the mountains with little change across interior sections. Humidity will still remain elevated on Thursday, resulting in continued significant heat issues. At this time, will keep the EXTREME HEAT WARNINGS in effect through Thursday evening to simplify the heat messaging as even with the cooling, many areas would be well within advisory criteria. By Friday and Saturday, a more noticeable cooling trend is expected with increased onshore flow, some return of marine layer stratus and continued H5 height falls.
Secondary concern for today will be the possibility afternoon showers and thunderstorms over the LA and Ventura county mountains. High PWATs and decent instability indices remain in place. So with the plentiful sunshine so far today, expect shower and thunderstorm activity to increase through the afternoon hours. Given the expected slow storm movement and plentiful moisture, there could be some flooding issues across the mountains this afternoon and early evening.
(Sun-Wed), 09/120 PM.
For the extended period, 12Z models continue to be in good synoptic agreement through the period. Essentially, the area will remain "sandwiched" between upper level high pressure over Texas and low pressure over the Pacific Northwest. At the surface, moderate onshore flow to the east will continue with some weak northerly offshore flow at times.
Forecast-wise, the above pattern will lead to a very benign weather pattern across Southwestern California. Skies will remain mostly clear for most areas, but there may be some coastal marine layer clouds during the overnight hours (although the warm coastal waters will help inhibit stratus development). As for temperatures, expect cooler conditions to continue with most areas about 5-10 degrees below seasonal normals. No significant wind issues are expected, although there will be the usual gusty southwesterly winds each afternoon across the mountains and deserts.
09/1742z.
At 1710Z at KLAX, the marine layer depth was 600 feet with a very weak inversion to 2500 feet with a temperature of 32 degrees C.
High confidence in VFR conds for non-coastal TAFs.
Good confidence in VFR TAFs for remainder of sites. KSBP's transition to NW winds may be any time through 22Z.
KLAX, Good confidence in TAF. No significant east wind component expected.
KBUR, High Confidence in VFR TAF. Winds may remain S this afternoon and evening with no NW winds.
09/121 PM.
Moderate confidence in winds and seas increasing to Small Craft Advisory (SCA) levels this afternoon through evening, focused south and west of Point Sal and continuing into early next week. Moderate confidence in winds expanding across all Outer Waters by Thursday, with local gusts to 20-25 knots this afternoon through evening and again continuing into early next week. Winds and at times seas will likely exceed SCA levels into early next week, with a 20% chance for Gale Force winds Thursday/Friday evening, increasing to a 40% chance this weekend.
Across the nearshore waters along the Central Coast, winds may locally reach SCA levels today, with more expansive gusty winds Thursday into early next week, with SCAs likely to be issued.
South of Point Conception, moderate confidence in SCA winds across the western half of the Santa Barbara Channel (locally eastern portions) each afternoon and evening into early next week. Thus, there is a 40-60% chance for SCAs to be issued each day, with highest chances this weekend. Local SCA gusts will be possible each afternoon-evening across the Santa Monica Bay and San Pedro Channel into early next week.
Ca, Extreme Heat Warning in effect until 8 PM PDT Thursday for zones 38-87-88-340>358-362-366>375-548. (See LAXNPWLOX). Beach Hazards Statement in effect through Thursday evening for zones 87-349-350-354-362-366. (See LAXCFWLOX). PZ, Small Craft Advisory in effect from 3 PM to 9 PM PDT Thursday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM Thursday to 3 AM PDT Saturday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Saturday for zones 673-676. (See LAXMWWLOX).
