| Day | Date | High/Low | Forecast |
| Mon | 19th | 74/56 | Sunny day. Clear evening. |
| Tue | 20th | 78/59 | Sunny day. Clear evening. |
| Wed | 21st | 78/59 | Mostly sunny day with some high clouds. Mostly clear evening. |
| Thu | 22nd | 76/58 | Chance of some early morning low clouds; Otherwise, mostly sunny day with some high clouds. Mostly clear evening. |
| Fri | 23rd | 73/56 | Chance of early morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
Synopsis
After several, upper air troughs promoted cooler than normal weather, building high pressure aloft has brought noticeable warming today to most areas in southern California. This high pressure should strengthen some more over the next couple of days. So, the warming trend should continue through midweek. The next cooling cycle in the second half of the week should be modest as a predicted trough for late in the week should be weak.
There has been some disagreement amongst the computer models about how close the center of high pressure aloft will get to the Southland. The latest model runs have backed down a little on how hot it will ultimately get. Widespread 90 degree weather in the valleys is almost a certainty (already a smattering in the western Inland Empire today). Widespread 80 degree weather in the inland, coastal plain is also expected. However, closer to the coast, there is some doubt about whether 80 degree weather will occur. Weak on-shore flow might stay just strong enough to prevent that. I leaned today’s campus forecast that way but confidence isn’t high. In the valleys, isolated triple digit heat is probable around midweek, but it still may not get as hot as it did with the the most recent heat spell.
All the models predict a weak, Pacific trough passing through the state late this week. The timing of the trough passage is still a bit uncertain (as early as late Friday or delayed to Saturday morning). A return of widespread, marine layer clouds in the coastal plain is likely. It could happen as early as Thursday morning, but Friday is probably the more likely date. There is a chance for most valley locales to see early morning low clouds Saturday morning, but those clouds should clear away fairly quickly during the day. The same should apply to most of the coastal plain. Some uncertainty is whether low clouds will hang in stubbornly along some beaches (Friday). None of the models (today) suggest persistent, “May Gray” weather this week (including chances for marine layer drizzle). This should extend to the Memorial Day weekend.
Weak high pressure aloft should build briefly late Saturday into Sunday, but a separate trough should pass through the state early next week. So, any warm up on Sunday should be minor (no more than a few degrees above normal for well inland locales). Some form of troughing over or near the state is expected for all of next week. This should promote an on-shore flow pattern of varying strength (one day to the next). Marine layer clouds should be a daily, weather staple in the coastal plain (possibly some valley locales too). Temperatures next week should hover near seasonal normals (a little above normal some days; a little below normal other days). Potentially, this sort of weather may continue into the subsequent weekend. Some model solutions show a chance for building high pressure at the end of the week, but it’s far from certain.
Next issued forecast/synopsis may be on Friday, 23 May.