Area Forecast Discussions are the real forecasts, written 3 to 4 times daily by NWS meteorologists reading the models and interpreting what they mean for your area. They offer far deeper insight than any weather app, but they're written in dense meteorological shorthand. Plaincast uses Anthropic's Claude to summarize them into plain English, side by side with the annotated original.
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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.
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 precipitable water 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.
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.
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