Well, in the last synopsis, we were in the midst of a storm. Up to midday, it was a significant storm in L.A. County, but it wasn't phenomenal. That changed later in the afternoon for some parts of the County (prolonged downpours along with the wind). In the end, widespread storm totals in the two- three inch range did occur (not five inches in the coastal plain as one model had predicted at one time, however). The one surprise for me was that the snow level dropped sooner than I expected (snowfall at the resorts though were in line with expectations).
An atmospheric river continued over the Southland this morning. However, without any relevant storm dynamics over the region (actually high pressure aloft encroaching from the south), only light rain fell from the narrow band of clouds (other showers occurring west of L.A. County too). Rainfall today has been under a tenth inch (0.04 inch at UCLA). While the threat of showers should continue for the remainder of the afternoon, any additional rainfall should be insignificant (trace amounts to maybe a couple hundredths of an inch rain).
An upper air disturbance, embedded within the large scale, low pressure trough over the West Coast, will pass by tomorrow afternoon. It may help trigger some showers over the north facing Tehachapi Mountains, but only some increase in clouds are expected for most of the Southland tomorrow afternoon. Once the disturbance passes, increasing north winds should develop. A breezy evening is expected in Santa Ana wind prone areas. Peak wind gusts in the higher mountains may exceed 65 mph for a time (lowland areas prone to Santa Ana winds might see gusts reaching 40 mph). The wind should diminish Thursday morning, but a complete cessation of these winds may not occur till Friday.
The various computer models don't forecast a full-fledged, off-shore flow developing. It comes close Friday morning, but it lacks depth and longevity (in the forecast). Thus, the predicted north winds should simply import polar air without warming up as much as with a typical, Santa Ana wind event (nights will definitely have a winter chill, especially wind sheltered areas). Daytime temperatures, at best, may reach seasonable levels by Friday. Highs tomorrow/Thursday may actually be slightly below today's levels (which were noticeably higher in most areas than yesterday...reached the low 70s in San Diego today).
Most of the longer range models show a new storm reaching southern California on Sunday. There is relatively high confidence in widespread, wet weather on Sunday and Monday. Some model solutions show a chance of showers starting as early as Saturday evening, but that is probably a bit too fast (L.A. County). There is, however, uncertainty regarding greater details about this predicted storm. Some models predict a connection to an atmospheric river. There are model solutions depicting a fairly strong storm reaching the Southland although not quite as fierce as last Friday's storm. Other model solutions show a rapidly weakening storm, which produces modest rains (under a half inch). At this point, I'd lean toward an decent but average soaker (widespread one inch rainfall in the lowlands away from the mountains; Snow levels remaining relatively high during the height of the storm).
A second, colder storm would follow rapidly on Monday (snow levels falling to 5000 feet or slightly lower). Showery weather would prevail that day. Additional, lowland rainfall would be in the quarter to half inch range (possibly higher where any thunderstorms develop). That storm would exit the Southland on Tuesday (most showers that day would be near/over the mountains). Dry and sunny weather would begin the new month.
Today's model consensus isn't ready to close the "storm gate" to the state next week. There are model solutions depicting a strong high pressure forming over the state late next week (would bring warmer than normal weather). However, many still show the storm track running through the state. The path, however, wouldn't permit any connection to any atmospheric river. These would be the more typical, cold, Gulf of Alaska storms (passing squarely through northern California but the Southland gets the weaker, southern portions of the storms). It's possible that the model consensus will change in the coming days (including details about the storm predicted for Oscar Sunday). So, stay tuned.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2017-02-21