Weak on-shore flow prevails today as a result of a weak, upper level trough of low pressure along the state coast (trough also responsible for the variable high clouds). Daytime temperatures today are the same or slightly cooler than yesterday in most areas. In a few areas near the coast, however, it's ended up a little warmer than yesterday. That includes the campus, where today's high looks to be three degrees higher than yesterday...four degrees more than my forecast shows for today). Other than better vertical air motion today compared with the previous couple of days, I can't explain why it's warmer in some spots.
Today's slightly cooler than normal weather will lead to warmer than normal weather this weekend (Monday as well). All the computer models predict the current low pressure getting displaced to the south as high pressure builds into the state. A weak off-shore flow pattern will take hold. Significant wind gusts (20 mph or higher) could begin this evening in some areas of northern Ventura/L.A. Counties. Winds associated with the pattern won't be especially strong (as Santa Ana wind events go). However, at higher elevations, peak wind gusts may reach 50 mph on Saturday (slowly decreasing winds late Sunday morning). Some lowland areas prone to Santa Ana winds may see wind gusts reach 35 mph although this shouldn't be widespread nor persistent. The off-shore flow should end by Monday evening.
The off-shore flow will also induce another cycle of much warmer than normal weather, not to mention very low humidity Widespread 80 degree weather for inland areas should occur tomorrow. This will likely include the coastal zone on Sunday and perhaps Monday. Readings into the low/mid-90s are a distinct possibility on Sunday/Monday (latter day should be limited to valley locales). By Tuesday, however, temperatures should drop noticeably as a cold, upper low pressure approaches (back to seasonal normals or even cooler in some areas).
Well, there is good model agreement on the first, widespread wet weather event in southern California since 28 September. This storm will be understandably much cooler than the one in September (coastal plain should struggle to reach the low 60s for a day; Snow levels down to 6000 feet). This storm is predicted to become a semi-"cut-off" low pressure around the middle of the week. Thus, forecast details regarding this storm remain on the fuzzy side ("cut-off" lows are a weather person's woe). Depending on which model solution is right, a number of scenarios are possible.
Complicating matters is the potential entrainment of moisture from tropical storm Raymond (currently about 550 miles south of the southern tip of Baja California). Our current, weak trough is forecast to become a "cut-off" low pressure itself (predicted to spin a few hundred miles west of central Baja California). Raymond will die off as tropical storm by Monday, but some of its high, water vapor content air may get caught up in the circulation of the Baja low pressure. That in turn may provide a temporary source of moisture for the cold low pressure that eventually parks itself west of southern California. Of course, a number of things must happen at the right time in the right place for significant moisture import into southern California.
IF the aforementioned scenario occurs (a minority model solution at present), significant rainfall could occur over portions of southern California (particularly the Inland Empire and San Diego County). An inch or more of rain is possible at lower elevations (snow limited to very high elevations much of the time). One monkey wrench into this wet scenario is if the cold low pressure winds up farther off the coast than many model solutions show. In that case, most of the heavy rain would fall over the ocean. There are a number of variations that could verify sometime late Tuesday through Friday.
For now, I'm expecting a good chance of showers on Wednesday. Rain in the lowlands wouldn't occur for lengthy periods except around the mountains. Most of the rain should be light in intensity and storm totals for most areas should be under a quarter inch. More showers are probable on Thursday, but most of these showers should be to the east and south of L.A. County by Thursday afternoon. Some lingering showers could occur Friday morning, but for the most part, the storm should be done by then.
A number of model solutions are trending back toward a dominant high pressure along the West Coast over the subsequent weekend and beyond. If this prevails, we'll be back to dry and warmer than normal weather at times (valid for Thanksgiving week) The model scenario that keeps the "storm gate" open to the state would become less likely. However, I suspect what the forecast, "cut-off" low pressure does next week will have be big bearing on the weather pattern to follow for the rest of the month. In short, stay tuned...
UCLA Forecasts and analyses are written by James Murakami
– posted: 2019-11-15