| Day | Date | High/Low | Forecast |
| Mon | 10th | 71/53 | Mostly sunny day with some afternoon high clouds. Becoming partly cloudy in the evening. |
| Tue | 11th | 60/53 | Mostly cloudy through the evening. Good chance of morning light rain; Chance of afternoon showers. Slight chance of evening showers. |
| Wed | 12th | 57/46 | Mostly cloudy through the evening. Good chance of morning drizzle/light rain; Chance of afternoon showers. Evening rain likely, possibly heavy at times late. |
| Thu | 13th | 55/46 | Rain, heavy at times early morning. Rain tapering to some showers during daylight morning. Chance of brief afternoon showers and possibly breezy at times. Scattered evening clouds. |
| Fri | 14th | 58/49 | Amended…Partly to mostly cloudy day with a chance of showers. Decreasing clouds in the evening. |
Synopsis
A marginal, off-shore flow yesterday brought warmer than normal weather (warmer than I forecast last week). An approaching, “closed” upper level, low pressure is bringing variable clouds today. I had expected the clouds to increase late in the afternoon, but high clouds have increased at a faster pace than I anticipated this morning. In addition, ocean breezes started up more quickly than I expected, which reduced the maximum temperature at UCLA (67 degrees, as of this writing).
All the computer. models have the center of the “closed” low pressure passing south of L.A. County at closest approach (crossing through San Diego County or slightly farther to the south). However, recent model runs have predicted that the low pressure will have a decent sized circulation. The implication is that widespread, mainly light rain (snow levels mostly above 6500 feet for this storm) should include most or all of L.A. County tomorrow morning (first showers probably sometime in the overnight hours tonight). The rain should taper off in L.A. County in the afternoon (slower exit for areas to the south and east of L.A. County…evening hours). A lingering threat of light showers is possible tomorrow evening, but for the most part, a dry evening is expected (expected to remain mostly cloudy though).
Some moderate strength rain may fall in some areas (mainly closer to the upper low pressure center). However, since several hours of steady, light rain is expected, and the low pressure currently has a weak link to an atmospheric river (i.e. rich source of available water vapor), storm totals from this storm may exceed a quarter inch even in lowland L.A. County (higher towards San Diego County). A few to several inches of “wet” snow is probable for the local resorts.
The numerical models continue to forecast an overcast Wednesday despite that the aforementioned, “closed” low pressure should be well to the east of southern California. The reason is that a large, “open wave” trough will be approaching the West Coast that day. Most of the models show marine layer induced drizzle/light rain falling in the morning (potentially all morning, especially around coastal facing foothills and mountains). There may be a temporary break in the wet weather Wednesday afternoon (models aren’t in complete agreement, however). Sometime Wednesday evening, a vigorous cold front should be nearing western Santa Barbara County (front moving from west to east). Even before the front nears, steady precipitation should begin Wednesday evening in L.A. County (early to mid-evening). A period of heavy to intense rain (snow levels falling to 3000 feet Thursday morning) is expected early Thursday (predawn hours, based on current model consensus). Some of the heavy rain may come from thunderstorms (some potential for several weather…strong wind gusts, hail, and even a brief-lived tornado). Once the cold front passes (possibly around sunrise in L.A. County but no later than mid-morning), steady precipitation should taper off to scattered showers for the remainder of the morning. Except for around the mountains, shower chances should diminish greatly Thursday afternoon (wet weather ending by evening).
The second storm currently has a good tap into an atmospheric river (origin near tropical Dateline). Even after the connection gets severed, ample water vapor and good storm dynamics should reach the Southland. Thus, storm totals for the midweek storm could range in the one to two inch range in the lowlands away from the mountains (up to twice that around favored coastal foothills and mountains). 1 – 2 feet of snow should fall at resort level.
Recent model runs now show a chance for an additional trough to bring showers on Friday (highest chance in the afternoon hours). Model consensus isn’t great, but the usually reliable ECMWF model shows widespread, wet weather for a time on Friday. For this forecast, I amended my preliminary forecast to “chance of showers” (previously “slight chance”). However, confidence is the part of the forecast is low at this time. In any case, storm totals should come in under a quarter inch for lowland areas away from the mountains.
Most models favor a dry weekend (cooler than normal but warmer than what occurs during the wet weather). At one time there were some models showing a minor threat of showers late Sunday. However, that threat has been delayed to next Monday. Model consensus is poor on whether widespread, wet weather will occur. Some models show low level winds similar to an “inside slider” type system (more wind than wet weather). Some models also show a closing of the “storm gate” to southern California after the Monday storm (minor storms may still affect northern California next week).
Next issued forecast/synopsis may follow on Friday, 14 March.