Note: No forecasts will be issued on Tuesday/Wednesday/Friday, 25-26,28 July.
The latest "monsoon" moisture incursion is promoting considerable mid/high clouds over parts of southern California today. Some of those clouds are producing sprinkles and light rain. Over parts of the eastern desert and southern mountains (San Diego County), there are also some thunderstorms (heavy rain there). This activity is forecast to continue well into the evening hours in some areas (showers over parts of L.A. County will probably diminish though). While considerable moisture is forecast tomorrow, a subtle shift in predicted winds aloft should limit the chance for showers and isolated thunderstorms to the mountain/desert region (as is more typical with "monsoon" incursions into the Southland). If today's computer model consensus is right, the threat of showers/thunderstorms will end after tomorrow (at least for a couple days).
Where mostly cloudy skies prevail today, it's a little cooler than it got yesterday in the coastal plain. It's noticeable more cooler in some areas farther inland, especially where it has rained. However, the increased muggy atmosphere has somewhat offset the lower temperature readings. Only a small decrease in water vapor content is forecast tomorrow. So, it should be another muggy day for most areas. More drying (water content-wise) is forecast by Wednesday, but most of the models aren't predicting a complete elimination of the current, "monsoon" air mass. So, it's likely to stay a bit more humid that what we may be accustomed to.
Sunny weather should return to the L.A. Basin by Wednesday (may actually be sunnier tomorrow than it is today). What I'm unsure of is how well developed will be the coastal low cloud field after tomorrow. It's currently in a disrupted state (no uniform coverage of low clouds thanks to the "monsoon" incursion). The low cloud field should "heal" itself Tuesday night./Wednesday morning, but whether a widespread overcast prevails is still uncertain (better chances Orange/San DIego Counties than other areas). By the end of the week, however, strengthening high pressure aloft should limit low cloud coverage (just to the coast).
A new "monsoon" incursion is also forecast by most models this weekend into early next week. The model don't agree on how much of an incursion will occur. Some model solutions show one similar to the current situation. Other model solutions show only a minor moisture incursion or limit deep layered moisture/instability to the southeast corner of the state. At this point, I'm leaning toward a chance for mainly afternoon thunderstorms for the southern mountains/eastern deserts this weekend. Except for a few model scenarios, areas west of the mountains should remain rain free (muggier again, however).
The few wetter scenarios involve the future course of hurricane Hilary (a far more formidable storm than was tropical storm Don was in the Caribbean last week). At the least, Hilary should promote much higher than normal surf at south facing beaches (probably by the weekend). Some of its moisture could get caught in the wind flow that promotes the weekend "monsoon" incursion. One scenario actually brings the remnants of Hilary all the way to the Southland (making landfall in Santa Barbara County), but this solution has been rejected (model keeps the storm unrealistically strong as it leaves warm waters...needed to maintain tropical cyclones). While some substantial moisture from the storm could reach the Southland early next week, odds are that won't happen (based on the majority of model solutions).
Many of the longer range models continue to forecast a significant heat wave next week. There is potential for widespread 90 degree weather in the coastal plain (campus included). This would last for several days. If the predicted center of high pressure stays far enough to the east, the heat in the coastal plain might not get severe, but at this point, I'd be prepared.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2017-07-24