| Day | Date | High/Low | Forecast |
| Mon | 28th | 62/52 | Mostly sunny after low clouds clear late morning. Mostly clear evening. |
| Tue | 29th | 67/55 | Chance of some morning low clouds; Otherwise, mostly sunny day. Chance of late evening low clouds. |
| Wed | 30th | 65/56 | Chance of morning low clouds; Otherwise, mostly sunny day with some high clouds. Chance of evening low clouds. |
| Thu | 1st May | 64/56 | Good chance of morning low clouds but mostly sunny afternoon. Chance of evening low clouds. |
| Fri | 2nd | 64/56 | Good chance of morning low clouds but generally sunny afternoon. Partly to mostly cloudy evening. |
Synopsis
The storm that passed through southern California held together better than I expected. Most areas received under a quarter inch rain (UCLA 0.21″), but there were some locales (not all by the mountains) that received from a half to two-thirds of an inch (Northridge 0.61″). Another surprise (to me) was how much clouds persisted yesterday (even isolated sprinkles/light showers). The axis of the storm had passed to the east, but low level winds didn’t respond in kind (an example where the upper level circulation didn’t “communicate” with its lower level reflection).
Even today, low clouds were widespread for most of the morning (most of the inner coastal waters did clear out quickly though). At least this afternoon is mostly sunny for most locales (exceptions though in Orange/San Diego Counties and a portion of the Inland Empire). With high pressure aloft building into the state tonight/tomorrow, there should be much less low clouds tonight into tomorrow morning (in theory). Some predicted northerly winds should dilute or even purge the current marine layer, but I left a chance for some morning clouds tomorrow morning (in case the northerly winds are lacking).
The predicted high pressure isn’t expected to be particularly strong nor well developed. In fact, it should loosen its grip on the Southland tomorrow afternoon. Modest daytime warming is anticipated tomorrow (especially well inland locales), but it doesn’t look like warmer than normal will occur (at best, some inland areas may reach seasonal normals). On Wednesday, a new, albeit weak, upper level low pressure is expected to drop south into the state. This should help to increase the on-shore flow again. Temperatures that day should also dip slightly in most areas (compared to what occurs tomorrow). A little further cooling is forecast for Thursday when the aforementioned low pressure should be somewhere over the Southland. This low pressure should be moving east of southern California on Friday. That should allow for some slight warming for well inland areas (little or no change from Thursday expected near the coast).
What effects the coming low pressure will have on the local marine layer is a little uncertain. The various models do predict a minor deepening of the marine layer (leading to more extensive, areal coverage), but the day to day “behavior” of the low cloud field is infrequently difficult to predict accurately (complexities of marine layer physics). I’m not expecting a persistent overcast into the afternoon hours for most areas, but I wouldn’t be surprised if such a thing happens on Thursday. In addition, if the marine layer deepens enough, there could be spotty, early morning mist/drizzle (mainly up against coastal facing foothills/mountains). The chief threat would be Thursday morning.
If the current model consensus holds true, the forecast, upper level low pressure could even promote isolated, afternoon showers in the higher mountains as well as parts of the desert region. Some models show this happening Wednesday afternoon (Tehachapis), but the greatest threat would be Thursday afternoon (possibly Friday afternoon too if the low pressure moves east more slowly than most models predict). Showers, if they occur, should be minor. For now, no thunderstorms are forecast (predicted low pressure should be too weak to produce enough deep layered, atmospheric instability).
Of more weather interest is the forecast of another, cold storm (for early May) this weekend. A cold front is forecast to pass through southern California Saturday afternoon. It would be followed by a period of cold air instability showers Saturday night into Sunday morning (timetable subject to change, depending on how the various models handle the projected low pressure in future runs). Unlike the past weekend storm, the next one may not have a low level circulation passing directly through the Southland (model consensus poor on this detail…for now). If so, low level winds may not favor widespread precipitation (restricting wet weather more around the mountains). The predicted dynamics and available, water vapor look good for widespread precipitation, however. Since last weekend’s storm was initially not forecast to produce widespread, wet weather, the models could change their tune as the event nears. We’ll see.
Next issued forecast/synopsis may be on Friday, 2 May.