| Day | Date | High/Low | Forecast |
| Mon | 20th | 70/54 | Mostly sunny day with some high clouds. Chance of evening low clouds by late evening. |
| Tue | 21st | 67/53 | Mostly cloudy day and possibly breezy at times in the afternoon. Good chance of light rain in the afternoon. Decreasing clouds in the evening. |
| Wed | 22nd | 68/54 | Mostly sunny day with scattered high clouds. Scattered evening high clouds. |
| Thu | 23rd | 71/55 | Mostly sunny day with mainly, scattered high clouds. Chance of evening low clouds. |
| Fri | 24th | 70/56 | Chance of morning low clouds; Otherwise, generally sunny day. Chance of evening low clouds. |
Synopsis
A large, upper level, “cut-off” low pressure was centered several hundred miles west of Fort Bragg, California. A frontal band out ahead of the “cut-off” low was producing wet weather across much of coastal northern California. This low pressure will promote inclement weather up north through early Wednesday (potentially, the last good opportunity to add relevant,widespread snowfall in the Sierras this season). Down in southern California, most areas aren’t expected pick up more than a quarter inch rain (San Luis Obispo County should receive a quarter to three quarters inch though). By the time the frontal band reaches L.A. County, lowland areas away from the mountains may not get more than a tenth inch (wouldn’t be surprised if southeast L.A. County barely records measurable rainfall…rapid weakening of storm front predicted by the computer models). Southland snow levels shouldn’t fall below 6500 feet (above 7000 feet for most of the event), and accumulation isn’t expected to exceed a couple inches (slushy snow at resort level)
The model consensus favor light showers developing in L.A. County around 1 PM, give or take an hour. Steady, light rain may fall in some areas of the County, but it could be intermittent in other areas, especially the southern most part of L.A. County. In any case, the wet weather should come to an end by late afternoon (4 PM, give or take an hour…timing based on what most models predicting…admittedly, risky to accept at face value when dealing with “cut-off” lows).
A return to dry, mostly sunny weather is anticipated on Wednesday. Weak high pressure is forecast to build into the Southland on Thursday. The nearby pass of the weak, “inside slider” type trough may even promote a brief, marginal, off-shore flow Thursday morning. Only modest, daytime warming is forecast that day (at or slightly above normal temperatures). Minor cooling and possibly the return of coastal low clouds are forecast for Friday. This happens as a weak, upper level trough develops off the Southland coast. The trough should stick around for much of the weekend. So, weekend temperatures should be slightly below normal (with or without the appearance of marine layer clouds west of the mountains).
There is uncertainty regarding some model scenarios that include cold air instability showers over parts of the Southland (main day of concern Saturday). Most shower activity, IF it occurs, should be around the higher mountains. Unlike tomorrow’s system, isolated, brief-lived thunderstorms can’t be ruled out (valid time in the mid/late afternoon hours). Available water vapor would be limited. So, weekend storm totals should be under a tenth inch most areas that actually see rain. Up to a third inch rain is potentially possible with thunderstorms. Sunday should be less prone to showers as the models predict the upper level trough weakening and moving well inland.
A separate, upper level trough is forecast to visit the state early next week. This trough should be too weak to produce wet weather (excepting marine layer induced drizzle in spots). However, cooler than normal weather is likely. Most models show weak high pressure developing in the second half of next week, but it may not last. Some models show multiple, upper troughs passing through the state later (as early as the subsequent weekend). So, any warming cycles should be brief in duration for the remainder of the month (getting only slightly warmer than normal, at best). There are also some solutions showing minor wet weather toward the end of the month.
Next issued forecast/synopsis should be on Thursday, 23 April.