| Day | Date | High/Low | Forecast |
| Mon | 11th | 72/57 | Hazy sunshine remainder of day. Good chance of evening low clouds. |
| Tue | 12th | 68/57 | Morning low clouds likely but hazy sunshine by afternoon. Evening low clouds. |
| Wed | 13th | 67/56 | Morning low clouds likely but generally sunny afternoon. Evening low clouds likely. |
| Thu | 14th | 69/57 | Morning low clouds likely; Clearing late morning Chance of evening low clouds. |
| Fri | 15th | 70/57 | Chance of morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds. |
Synopsis
High pressure aloft covers much of the West today. Much warmer than normal weather prevails for areas that aren’t affected by the marine layer. Interior sections (i.e. deserts) are sweltering in triple digit heat (over 110 degrees in parts of the low desert). However, it’s come to that time of year when the magnitude of warmth isn’t just dictated by distance from the ocean. Early this afternoon, a shallow but effective marine layer has limited the beaches around Malibu to the low 60s. Just a few miles inland in the Santa Monica Mountains (elevation around 1500 feet above sea level), it’s about 25 degrees warmer. Had the marine layer been at least highly diluted, the temperature contrast would have been much less.
Once again, I over forecast temperatures for a predicted warming trend this past weekend. The computer models forecast much less marine layer near the coast (UCLA included) than what actually transpired. Even most coastal valleys stayed a little cooler than what the models predicted. Today is the warmest day of this heat spell, but for some areas near the coast, it’s barely warmer than normal for this time of year. Dominant high pressure aloft has limits in warming when on-shore flow can continue to support a shallow but defined marine layer.
For this forecast, I chose to keep a dominant marine layer going for this week. Most models predict strengthening on-shore flow through Wednesday (minor drop off later but not significant). They also predict high pressure aloft getting displaced east by an approaching, weak, upper trough from the south. High pressure aloft is forecast to rebound into the Southwest in the second half of the week, but it shouldn’t get as strong as today’s high pressure.
All this should mean that the marine layer should have a big influence on temperatures west of the mountains (less so in the interior). That, of course, will depend on on-shore flow strength and marine layer depth (very shallow depth means less influence for well inland areas). One thing that bothers me for this forecast is that satellite imagery shows a band of clearing off the coast that extends west of Baja California. Strengthening on-shore flow should produce modest but noticeable cooling for well inland areas (with or without marine layer clouds). However, how much daytime cooling occurs tomorrow and Wednesday might also depend on whether the marine layer gets disrupted as it seems over the coastal water this afternoon.
The apparent dilution of the coastal low field in place could be temporary (just a few hours?). Every year, I wrestle with how much influence the marine layer will have on a given day (numerical models aren’t always helpful). I’ve leaned toward typical marine layer (‘Tis the season for May Gray weather). So are the models this time around. Still, my confidence in today’s campus forecast isn’t the highest.
Other than minor ups and downs in temperature, fairly static weather is anticipated for the coastal plain this week. (nothing unusual by this time year). This will probably extend into next week. No major trough or ridge is currently forecast to influence Southland weather.
Next issued forecast/synopsis should be on Friday, 15 May.