A two week vacation by Ol' Man Winter came to an end last Friday (at least in southern California). Winter is expected to hold sway through at least early next week, if today's computer model consensus is right.
The second in a series of storms is currently passing through the Southland today. In L.A. County, residual, instability showers persist though mainly over/adjacent to the mountains. I hadn't expected any showers would survive the trek from the north into the L.A. Basin. Northwesterly wind flow typically dissipates such showers, but one shower cloud managed to drop a brief sprinkle on UCLA around 2:15 PM. I haven't amended the campus forecast, but I wouldn't rule out one or more sprinkle showers before sunset (atmosphere should get steadily more stable thereafter). For the most part though, storm totals from earlier today shouldn't vary much (UCLA recorded 0.12 inch). Like the first storm on Saturday, rainfall varied due to the showery nature of the storms. One to two tenths of an inch were common in the L.A. County, but some totals were in the three to four tenths range (away from mountain locales). As with both storms, storms totals were noticeably higher to the east and south of L.A. County (half to one inch totals). Snowfall at the local resorts was plentiful (around a foot for each storm).
The wind and residual showers should fade out tonight in most areas (some showers might linger down in San Diego County through much of the evening). We should be between storms tomorrow. If it clears out and calms down as expected, it should be quite chilly early tomorrow. Despite expected, ample sunshine, afternoon temperatures tomorrow should be only a few degrees higher than today's readings. Increasing clouds are anticipated tomorrow night. By Wednesday, a good part of northern California should be wet. There is a question mark about how far south the wet weather will get on Wednesday. The various models predict a deep, moist, marine layer. Storm dynamics aren't predicted to get this far south (i.e. L.A. County), but any weak forcing from an upper air disturbance may permit marine layer showers. I placed a chance for occasional light showers in the campus forecast, but most activity in L.A. County should be around the south facing mountains (rainfall well under a quarter inch). Chances for wet weather should be higher to the west of L.A. County (steady, occasionally heavy rains in San Luis Obispo).
The various models pivot the orientation of the storm front early Thursday. The northern portion takes on a more north-south pitch (earlier southwest to northeast). The southern portion edges eastward a little. The expected change should help translate the frontal rains eastward during the day, albeit slowly. However, predicted, model winds (easterly) may actually make it less conducive for marine layer showers on Thursday. I wrote for a continued chance of showers for Thursday, but that may need tweaking tomorrow (cloudy though dry weather possible). Also, if the front progresses more slowly than expected, steady rain in L.A. County might not come till the first hours on Friday.
Unless the model change their tune (???), a period of widespread, heavy rain is likely in the lowlands of L.A. County Friday morning. Model forecasts have varied less since daily, aircraft reconnaissance into the storm began a couple days ago (part of research on atmospheric rivers in the eastern Pacific). Of course, just how much rain may fall in L.A. County remains to be seen, but 1.5 to 2.5 inch totals (away from mountains) appear reasonable (tap into the atmospheric river good but not spectacular). Compared with the first two storms, the third storm will be warm in comparison. Still, snow levels should run around 5500 feet for a good portion of the event. Potentially, a couple feet of "sticky" snow may occur at the local resorts.
Depending on which model is right, showers should end rather quickly by Friday evening. A fourth, yet much weaker storm will affect northern California over the weekend. Other than San Luis Obispo/western Santa Barbara Counties, no wet weather is expected this weekend this far south. Some model forecasts show a fifth storm passing through the state early next week. Some model solutions show a minor rain event sometime Tuesday or Wednesday of next week in southern California (wetter farther north). After that, the models are divided. Some leave the storm gate open, but others close it as high pressure takes hold (back to warmer than normal weather for the rest of next week). Take your choice.
Next synopsis should be issued on Tuesday, 26 January.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-01-25