| Day | Date | High/Low | Forecast |
| Mon | 8th | 79/63 | Sunny day. Evening low clouds developing. |
| Tue | 9th | 75/63 | Morning low clouds likely but sunny afternoon. Evening low clouds likely. |
| Wed | 10th | 72/62 | Morning low clouds likely; Generally sunny afternoon. Evening low clouds likely. |
| Thu | 11th | 72/62 | Morning low clouds likely; Generally sunny afternoon. Evening low clouds likely. |
| Fri | 12th | 72/62 | Morning low clouds likely but sunny afternoon. Chance of evening low clouds. |
Synopsis
On-shore flow is picking up in strength this afternoon. This results from a developing, upper level low pressure along the West Coast (high pressure in the Southwest finally giving way the low pressure). Temperatures today are running below yesterday’s levels in most areas of southern California (at or slightly above normal levels for this time of year). As the aforementioned trough strengthens over the state, cooler than normal will be the rule for rest of the work week (may feel a bit chilly to some after all the recent warmth).
While I’m confident in the forecast of cooler than normal weather, I’m less so in the predicted “behavior” of the expected, marine layer return (starting tonight). The local, low cloud field was tattered in appearance (from satellite imagery) this morning (actually little of it south of Point Conception). All the computer models predict widespread low clouds (coastal plain and possibly some coastal valley locales) tomorrow. Areal coverage is expected to include all areas west of the mountains by Wednesday. With such a deep marine layer, there would usually be a chance for spotty, early morning mist/drizzle (especially in the presence of a coastal eddy circulation). However, extensive low clouds aren’t common to September (October too). It’s possible that the low cloud cover in the next few days will be no worse than partly cloudy. Of course, with the complexities of marine layer physics, I wouldn’t be surprised if a spate of “June Gloom” like weather were to occur (i.e. persistent low cloud overcast for much of the period).
The upper level trough currently has a weak and narrow cold front accompanying it (west of northern California this afternoon). Some minor showers should affect parts of northern California tomorrow (mostly close to the coast). Cold air instability showers (cold for September) over the northern Sierras are expected on Wednesday/Thursday (mainly afternoon hours). Unfortunately, relevant showers aren’t expected in the area of the brushfires in the central Sierras. In fact, increasing winds may fan the flames more. Down in southern California, any marine layer induced drizzle shouldn’t produce relevant rainfall either. Strong on-shore flow for next few days, however, should induce breezy weather in the interior.
The West Coast trough is predicted to weaken and move inland on Friday. Some warming is expected that day, but that should apply mainly to well inland areas (staying cooler than normal that day, however). More general warming is expected on Saturday when weak high pressure aloft passes overhead (no higher than seasonable levels). Another trough (weaker than the one expected this week) should induce some cooling again for Sunday/Monday. Most models show high pressure aloft building into the state thereafter (possibly lasting into the subsequent weekend. Most models favor warmer than normal weather, but it’s not clear if it will get much warmer than normal.
Also uncertain is whether southern California will experience any more “monsoon” moisture incursions this season. Typically, such a weather pattern is rare by mid-September. However, some model solutions show just that sometime between the 17th and 21st. One scenario even includes guiding moisture from a decaying tropical cyclone into the region. Much can occur between now and the predicted period. A small error in predicted wind flow can make the difference between a wet and dry forecast. It’s a wait and see situation for now.
Next issued forecast should be on Monday, 15 September.