| Day | Date | High/Low | Forecast |
| Thu | 16th | 68/56 | Variable, mostly high clouds but generally sunny day. Some evening high clouds. |
| Fri | 17th | 74/58 | Mostly sunny day with some high clouds. Scattered evening high clouds. |
| Sat | 18th | 80/59 | Variable high clouds but generally sunny day. Partly cloudy evening with high clouds. |
| Sun | 19th | 76/55 | Partly cloudy day with high clouds. Decreasing evening high clouds. |
| Mon | 20th | 73/55 | Mostly sunny day with some high clouds. Chance of late evening low clouds/fog. |
Synopsis
A weak, upper level trough to our southwest brought considerable high clouds to southern California. Marine layer coverage was patchy at best over the coastal waters (aforementioned trough may have helped mix out low cloud field). In addition, an “inside slider” type trough was moving into the Great Basin states. It will make a nearby pass on the Southland (already seeing effects through increased winds in the interior). Late tonight, the nearby passage of the “inside slider” will usher in a period of off-shore flow (tomorrow-Saturday).
Most of the numerical models have backed away from a moderately strong Santa Ana wind event (core of trough should be too far to the northeast at closest approach). Peak mountain, wind gusts may reach 55 mph (beginning overnight hours early tomorrow), but it shouldn’t be widespread in areal extent nor persistent in duration. Some Santa Ana wind prone areas in the lowlands could briefly see wind gusts reach 40 mph, but peak gusts should be mostly under 35 mph. A noticeable decrease in winds should occur Friday afternoon. Weak off-shore flow is predicted for Saturday. A transition to marginal on-shore flow is anticipated for Sunday.
The off-shore flow will promote warmer than normal weather west of the mountains. Saturday should be the warmest of next several days (widespread 80 degree weather probable though isolated readings possible tomorrow). Minor to modest daytime cooling is expected on Sunday (more noticeable by the coast than farther inland. Interior sections should actually warm up modestly as the influx of cool air into the Mohave desert ends.
All the models predict a large, “cut-off” low pressure forming well west of northern California on Monday. Some showers could develop over parts of northern California by the afternoon hours (assumes low pressure doesn’t form too far to the west of the state). The various models haven’t been in great agreement on the future course for the “cut-off” low pressure. There have been forecasts of wet weather reaching the Southland as early as Monday evening or as late as Wednesday afternoon. The latest consensus favors sometime Tuesday, but confidence in timing is still low (numerical models infrequently forecast the motion of “cut-off” lows poorly).
Predicted, available water with the storm is average for this time of year. The region of fair atmospheric instability (to produce thunderstorms) may stay too far north of southern California, but that’s far from certain (at this point). In any case, storm totals (valid period Tuesday-Wednesday) in the lowlands away from the mountains should be under a third inch (potentially up to twice that if thunderstorms actually form). Snow levels should stay above 6500 for most of the storm period (at best, a few inches of “wet” snow at resort level). There are some model solutions showing most showers restricted to around the higher mountains. So, don’t take these storm total forecasts too seriously for the time being.
Although most model solutions show the “cut-off” low weakening and moving east of the state by Thursday afternoon, a few model solutions keep the low pressure staying close enough to support a few mountain showers through Thursday. Some model solutions show a separate “inside slider” moving through the Southland next Saturday (25th), but only wind is predicted for interior sections. No relevant chance of showers appear likely (excepting potential for spotty, marine layer drizzle). Most of the longer range models show only modest warming late next week (includes subsequent weekend). Temperatures could get slightly warmer than normal (similar to recent days).
Next issued forecast/synopsis should be on Monday, 20 April.