| Day | Date | High/Low | Forecast |
| Mon | 19th | 81/64 | Sunny remainder of day. Some evening high clouds possible. |
| Tue | 20th | 77/62 | Chance of early morning low clouds; Otherwise, mostly sunny day with some mid/high clouds. Mostly cloudy evening. |
| Wed | 21st | 69/60 | Rain becoming likely, possibly heavy at times. Intermittent evening rain likely, possibly heavy at times. |
| Thu | 22nd | 66/57 | Showers likely with a slight chance of a thunderstorm. Rain may be briefly heavy. Decreasing chance of showers in evening. |
| Fri | 23rd | 71/56 | Mostly sunny day with some clouds. Clear evening. |
Synopsis
The most recent heat wave is almost over. A large, upper level trough is developing along the West Coast. Low level, on-shore flow toward the low desert has limited today’s warmth (relative to yesterday). A full fledged on-shore flow should develop tomorrow, which, in turn, should (in theory) help form a marine layer (southern waters at first). With all the computer models showing the West Coast trough sticking around till Friday, cooler than normal weather should develop by Wednesday (may return to seasonable, daytime temperatures most areas tomorrow).
Over the last week or so, the various models have disagreed on a common solution regarding the future course of the West Coast trough. Most model solutions now agree that widespread, wet weather should develop in southern California (sans the desert region). It’s not quite unanimous, but most favor steady rain reaching L.A. County Wednesday morning (possibly late morning, with some model forecasts). There continues disagreement on whether widespread, heavy rain will fall in the coastal plain. There will be ample water vapor available, but predicted storm dynamics vary in strength. For this forecast, I’m going with 0.50 – 1 inch storm totals in the lowlands away from the mountains (Wednesday-Thursday period in L.A. County). South facing slopes of mountains potentially could receive twice as much rain (snow levels expected to remain above 7000 feet for most of storm…6000 feet Thursday, but probably no more than a couple inches of “wet” snow).
There will be a chance for brief-lived thunderstorms (best shot Thursday in L.A. County), but even where no thunderstorms occur (presence of lightning defines a thunderstorm), some areas of heavy rain are probable (question mark is “where” specifically). Numerous showers are anticipated on Thursday, but that should be mainly in the AM hours. At least, based on today’s model consensus, clearing should take place by Friday. Dry, sunny weather should return this weekend.
Some model solutions show weak, off-shore flow over the weekend. At this time, no widespread, strong wind in Santa Ana wind prone areas is foreseen. The weekend may get warmer than normal again, but no 90 degree weather is anticipated (for now). Some model solutions show a new storm reaching the state by Halloween (decent, soaking storm). Other model solutions delay onset of widespread, wet weather to the 1st or 2nd of November. It’s a coin toss forecast for now.
Time permitting, a forecast/synopsis may appear this Friday, 23 October.