| Day | Date | High/Low | Forecast |
| Mon | 13th | 68/55 | Possible partial afternoon clearing; Otherwise, mostly cloudy through the evening. Chance of rain developing late evening. Possibly breezy at times. |
| Tue | 14th | 63/53 | Overnight rain, heavy at times with gusty wind overnight; Slight chance of a brief thunderstorm. Rain tapering to a chance of a few showers by afternoon. Evening clearing. |
| Wed | 15th | 64/52 | Sunny day. Clear evening. |
| Thu | 16th | 71/54 | Sunny day. Clear evening. |
| Fri | 17th | 75/56 | Sunny day. Clear evening. |
Synopsis
Typically, November heralds the beginning of the rainy season in southern California. However, there have been a number of Octobers with widespread, significant (half inch or more rain). At UCLA, more than an inch of rain fell on today’s date (back in 2007). The campus record for the most rain on an October date goes to the 14th (3.03 in 2009). That record will probably hold for at least another year, but the first storm of this rainy season with widespread, significant precipitation is expected tonight into tomorrow (some details vary but all computer model forecasts agree).
A “closed”, upper level low pressure was centered off of the north Oregon coast this morning. The low pressure center should get as far south as about the Monterey area early tomorrow before it starts to track inland (into Nevada early Wednesday). There have already been showers falling as far south as north Santa Barbara County (interior facing mountains). A cold front accompanying the upper low pressure will sweep through the Southland on Tuesday, and it is this front that should provide most areas with a period of gusty winds, heavy rain, and brief-lived but potent thunderstorms (not all areas though). If some model solutions are right, the wind flow pattern may support short-lived, weak tornadoes early tomorrow.
As is often the case, model forecasts vary in small details of expected weather. Based on the model consensus, the heaviest, consistent rain in the L.A. Basin should fall sometime between 2 – 8 AM (about a three hour window in any given location). Once the cold front passes, there should be some clearing (as early as late morning on the Westside). Isolated, lingering showers are possible in L.A. County, but most of that should be near/over the mountains. General clearing should occur by sunset (from west to east).
Last week, I expected lowland, maximum rainfall (away from the mountains) in L.A. County would be 0.75 inch. Well since that time, the model consensus increased that total to about 1.75 inches (low end about half inch). Coastal facing mountain slopes experiencing thunderstorms potentially could receive 2 – 4 inches of rain. Since the cold core of the storm should stay well to the north, snow levels during the height of the storm should remain above 7000 feet. Up to a foot of “wet” snow could fall at those higher elevations though. Snow levels could fall to near 6000 feet late tomorrow, but no widespread accumulation is expected.
Except for some possible, lingering, light showers in San Diego County early Wednesday, most of the Southland should be mostly clear that day. A weak, off-shore flow is predicted by most models Wednesday night into Thursday, but significant wind (25 mph or higher) should be restricted to higher elevations in Santa Ana wind prone areas. The off-shore flow should become marginal thereafter (just enough to suppress the daily sea breeze for a time lasting into Saturday). By late in this period, well inland areas may get slightly warmer than normal (around seasonable levels elsewhere west of the mountains).
Model agreement isn’t great at this time, but a new, upper level trough is forecast to past through the state late Sunday into Monday. Cooler weather should develop, but how much is still uncertain (probably no worse than slightly cooler than normal). Some models show an “inside slider” type storm capable of instability showers (marine layer induced but mostly to the south and east of L.A. County). No relevant rainfall is expected though if such weather happens. The remainder of next week should be sunny and warmer (potential for weak, off-shore flow by midweek…warmer than normal possible late next week).
Next issued forecast/synopsis should be on Monday, 20 October.