| Day | Date | High/Low | Forecast |
| Mon | 5th | 65/57 | Partial afternoon clearing possible. Mostly cloudy evening. |
| Tue | 6th | 67/57 | Morning low clouds likely but mostly sunny by afternoon. Chance of evening low clouds. |
| Wed | 7th | 69/58 | Chance of morning low clouds; Otherwise, mostly sunny day. Mostly clear evening. |
| Thu | 8th | 73/60 | Chance of some early morning low clouds; Otherwise, mostly sunny day with some high clouds. Some evening high clouds. |
| Fri | 9th | 78/61 | Mostly sunny day with variable high clouds. Scattered evening high clouds. |
Synopsis
An upper level, low pressure center was circulating about Arizona today. It was responsible for some heavy intensity showers/thunderstorms over parts of the high desert a couple days ago. In addition, it helped promote “May Gray” weather west of the mountains. Typically, when a low pressure moves east into Arizona, good clearing occurs over southern California. However, in this case, upper air disturbances rotating around the “parent” low pressure have continued showery weather (mostly eastern Riverside, San Bernardino Counties). Marine layer clouds (over land) have also stayed persistent (clear zones over inner coastal waters though).
The computer models show the aforementioned low pressure moving into New Mexico, but one more disturbance is forecast to pivot through the eastern Mohave Desert late tomorrow into early Wednesday. It’s forecast to be weaker than recent disturbances passing through the Southland, but it should possess enough moisture and atmospheric instability to trigger one more round of showers and brief-lived thunderstorms in the eastern deserts of southern California (Valid time late Tuesday into Wednesday; Arizona later on Wednesday). While I wrote into the forecast for a mostly sunny afternoon, don’t be surprised if tomorrow doesn’t offer much clearing again (not expecting major shift in wind flow pattern just yet).
By Wednesday, high pressure aloft should begin to build into the state. The on-shore flow should weaken steadily for the remainder of the week. With that and high pressure starting subsidence warming from aloft, a noticeable warming trend should ensue (well inland areas at first). Warmer than normal weather is expected for the second half of this week (lasting through the weekend). The key unknown at this point is whether a lingering, shallow marine layer will continue in the coastal plain. If that occurs, the warming trend near the coast should be more muted (getting no higher than the mid-70s in the campus area). For this forecast, I’ve leaned toward a little better warming (following model consensus).
On Friday, a weak, sub-tropical disturbance is forecast to reach the Southland. It should be too weak to present a shower threat, but varying amounts of mid/high clouds should enter the region. There is some chance that the warm, sub-tropical air will mix down to the ground and purge what marine layer may have persisted through that time. If that occurs (??), it would get warmer than currently expected (potentially mid-80s at UCLA; 90s in some valley locales). With less clouds expected on Saturday, it could warm a little further that day (mainly inland areas).
All the longer range models show a new, upper level trough developing along the West Coast next week. Minor cooling may start near the coast on Sunday, but more general cooling should occur as next week progresses. Increasing on-shore flow should promote a widespread return of coastal low clouds. It’s possible that a couple days of “May Gray” weather may return sometime next week. At the least, a return to seasonable temperatures appear likely for most of next week. The new trough is forecast to weaken late next week, but it probably won’t weaken enough to lessen marine layer clouds substantially (i.e. marine layer weather over the subsequent weekend).
Next issued forecast/synopsis may occur on Friday, 9 May.