Note: A forecast will not be issued on Tuesday, 11 December (possibly Wednesday as well).
In the lowest, few thousand feet of the atmosphere, a weak, off-shore flow continues over southern California. However, higher aloft, a sub-tropical, jet stream is directing considerable mid/high clouds through the Southland (reason for mostly cloudy skies since yesterday). An upper level trough of low pressure over northern California will move east and shunt the mid/high clouds out of the Southland tonight. So, mostly clear skies should return for tomorrow.
The southern end of the aforementioned trough is promoting some convective clouds over and to the southwest of Santa Barbara County. Radar imagery does show some faint returns suggesting sprinkles/light showers in that area. However, the base of the clouds are quite high (about 15000 feet), and there's dry air beneath it (partly from the off-shore flow). Hence, it's unlikely that any rain drops are reaching the ground from those clouds. The southern portion of the trough is forecast to develop a secondary, low pressure center early tomorrow, but by that time, all the upper level moisture will be gone (nothing left for the low pressure to trigger showers). There is a small chance that the north facing slopes of the Techachapi Mountains may see isolated showers (low clouds in the southern San Joaquin Valley ). Otherwise, the passing trough tomorrow should be a non-event for most areas (i.e. no wet weather).
As mentioned before, low level, off-shore flow continues, and it's forecast to continue for most of the week. Today's computer models forecast occasional bursts of stronger wind in Ventura/L.A. Counties (mostly higher elevations of Santa Ana wind prone areas). This should last through early Thursday. Peak mountain gusts shouldn't get much above 40 mph, and few lowland locales are expected to see any wind gusts exceeding 25 mph. A predicted, trough passing to our north may promote a marginal, on-shore flow at the end of the week, but it's far from certain (at this time).
Daytime temperatures today are slightly to modestly lower in most areas thanks to the cloud cover and approaching trough (would be several degrees lower in the coastal plain though if on-shore flow were present). Slightly cooler weather may occur for the next couple of days, but a rebound in temperatures similar to yesterday is expected on Thursday (moderate strong high pressure aloft and off-shore flow promoting greater, subsidence warming). If a marginal, on-shore flow does develop, temperatures at the end of the week should fall back to seasonable or slightly below normal levels.
Most of this weekend should be dry and mostly sunny. Temperatures should be at or slightly below normal levels. Sunday evening or Monday, however, some of the longer range models show a chance of wet weather reaching southern California. It isn't a majority model solution at present, but enough model solutions show some wet weather that it merits monitoring (at least scattered, light showers possible). Even if widespread, wet weather develops, the predicted storm should be a "run of the mill" storm (average for this time of year).
Dry and warmer weather would prevail by the middle of next week. The longer range models are in surprisingly good agreement on a warm, off-shore flow (no widespread, significant wind). For a couple of days, there is potential for temperatures to flirt with the 80 degree mark (isolated readings probable if the predicted high pressure verifies). Cooler weather would occur at the end of next week (astronomical arrival of winter on the 21st). A few model solutions show much colder than normal as Christmas Day nears (even a chance for cold air instability showers). However, a fair number of model solutions show renewed, off-shore flow. That would favor warmer than normal weather. It's still too early to say with any confidence which scenario may verify.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2018-12-10