09/751 PM.
Extreme heat and humidity will continue through Thursday with a high risk for heat illness. Cooler and more normal conditions are likely by Saturday, with a likely return of the marine layer.
(Wed-Sat), 09/751 PM.
***UPDATE***
It was a record-breaking day today, with five of our 14 climate stations breaking their calendar day records, and two tying. That is impressive on its own merit, but adding the fact that the heat index readings were 5-10 degrees above the air temperature and that the nights were downright steamy, and this was truly an extremely hot day. Temperatures are on track to lower a few degress tomorrow as the surface pressure gradients trend onshore, but any relief will largely be unnoticeable as the humidity remains elevated and the nights warm. The Extreme Heat Warning will remain in play as a result. Friday and Saturday are still on track for big drops as the onshore flow really ratchets up and the high retreats to the east. Extensions to the heat hazards are unlikely.
We had a couple of mountain showers and thunderstorms today, but it was less than what could have occurred with the very high convective parameters (like off the charts K-index readings near 47). Thankfully there were not any upper level features to initiate convection, and that proved to be the limiting factor. The convective parameters are down for tomorrow, but still elevated enough to keep an eye out for a possible mountain shower or thunderstorm, mainly over the San Gabriels.
The only other thing to write about are likely advisory-level Sundowners that should form tomorrow night, which looks poised to last through the weekend.
***From Previous Discussion***
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.
09/2344z.
At 23Z at KLAX, there was a surface-based inversion, with an inversion top at 1500 feet and 33 degrees C.
High confidence in VFR conditions at all sites through at least Thursday with typical winds.
KLAX, High confidence in VFR conditions through at least Thursday, with any east winds staying under 8 knots.
KBUR, High confidence in VFR conditions through at least Thursday.
09/746 PM.
Moderate confidence in winds and seas increasing to Small Craft Advisory (SCA) levels this evening, focused south and west of Point Sal. Moderate confidence in winds expanding across all the outer waters Thursday and 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 during the evenings on Thursday and Friday, increasing to a 40% chance this weekend.
Across the nearshore waters along the Central Coast, winds will increase to SCA levels Thursday and continue into early next week, especially during the afternoon and evening hours.
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.
09/719 PM.
Moderate coastal flooding will continue through Thursday, mainly around evening high tides for south facing shores. Even though the south swell from post tropical cyclone Marie is subsiding, the tides remain very high. The actual observed tides have been running about 6 to 8 inches above predictions, with the evening tides near or exceeding 7 feet.
While impacts will not be as extreme as the past few days, possible impacts include continued beach erosion, flooded sandy beach areas, bike paths, and parking lots, as well as low-lying roads. Some areas that may continue to see impacts are: the Alamitos Peninsula in Long Beach, Avalon on Catalina Island, and Oxnard to Malibu (including the Pacific Coast Highway near Point Mugu).
Beach Hazards Statements (BHS, CFWLOX) have been issued for the beaches of Los Angeles, Ventura, and southern Santa Barbara Counties due to the very high evening tides through Thursday.
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).
