| Day | Date | High/Low | Forecast |
| Mon | 14th | 70/61 | Morning low clouds clearing to a sunny afternoon. Evening low clouds returning. |
| Tue | 15th | 71/61 | Morning low clouds but mostly sunny afternoon. Evening low clouds likely. |
| Wed | 16th | 72/61 | Morning low clouds likely but sunny afternoon. Chance of evening low clouds. |
| Thu | 17th | 73/63 | Chance of morning low clouds; Otherwise, mostly sunny day with variable mid/high clouds. Partly cloudy evening. |
| Fri | 18th | 75/63 | Partly cloudy day with mainly mid/high clouds. Decreasing mid/high clouds in the evening but chance of late evening low clouds. |
Synopsis
Mild weather (for mid-summer) continues across most of southern California (coastal plain in particular). Although high pressure aloft has been centered near or over us for a couple days, a healthy, low level on-shore flow has prevailed (not a common outcome when high pressure of current magnitude is overhead). That, in turn, has favored widespread, marine layer clouds in the evening/morning hours (some beaches only seeing partial afternoon clearing on some days). All the latest, computer model forecasts show only minor day to day weather changes through mid-week.
The various models show high pressure aloft weakening over the next few days (cooling trend for the interior, which has been running on the toasty side recently). A weak, upper level trough is expected to develop over the Pacific Northwest states, and an extension of this trough may reach well into California (favors cooler weather). The model consensus favors a minor increase in on-shore flow for a couple days, but it’s actually unclear (to me) how much weather influence this trough will have on the marine layer. Subtle, low level wind shifts may play a bigger role in the day to day “behavior” of the marine layer (i.e. areal coverage and persistence through the day). Still, at least in terms of temperatures, I’m not expecting more than a couple degrees change from one day to the next (through Wednesday).
The models predict a weak, upper level low pressure forming over central Baja California tomorrow. The models show this low pressure moving northward into Arizona by late Friday. The models have disagreed on how close the low pressure will get to southern California as it heads for Arizona. It is relevant because this low pressure is forecast to bring up “monsoonal” moisture into the Southwest. A low pressure position into the state (even if for a brief time) could help trigger scattered showers and isolated thunderstorm (as early as Thursday afternoon). The chances for showers west of the mountains looks remote at this time (decided not to include any shower threat in the campus forecast), but variable mid/high clouds are a definite possibility. Some of that sub-tropical air may mix down to the ground sometime in the Thursday-Friday window. In that event, it should feel a little on the muggy side (even without showers…showers making it muggier though). At this point, no significant mountain/desert showers and isolated thunderstorms (mainly afternoon hours) are foreseen late this week (unsure if significant showers will reach the Grand Canyon area…at this time).
By Saturday, any “monsoon” incursion should be diminishing in the Southland. Should the local, low cloud field get disrupted by the incursion, it should “repair” itself over the weekend. However, most of the models show a weaker on-shore flow than it is currently. So, there is a chance that marine layer, areal coverage will be less this weekend than it has been running lately. Temperatures may correspondingly increase some (valleys in particular). Still, the coastal plain should be within a degree or two of seasonal normals.
High pressure aloft is expected to dominate the weather in the Southwest next week, but at least early in the week, its weather influence on the coastal plain should be limited. At most, limited low cloud coverage and slightly warmer than normal weather may prevail (least warming at the beaches, as commonly is the case). Later next week, the high pressure may exert more influence in warming the entire Southland. However, at this time, the model consensus doesn’t favor any excessive heat west of the mountains. There is also some chance for a “monsoon” moisture incursion late next week, but model agreement is just fair, at best. Just a small error in predicted winds may keep deep layered moisture/instability to the south and east of the state.
Next issued forecast/synopsis should be on Monday, 21 July.