| Day | Date | High/Low | Forecast |
| Fri | 15th | 64/48 | Sunny morning; Mostly sunny afternoon with variable clouds. Some evening clouds. Breezy at times in the afternoon through early evening. |
| Sat | 16th | 66/46 | Some clouds possible early morning; Otherwise, sunny day. Clear evening. |
| Sun | 17th | 68/49 | Mostly sunny day with some afternoon high clouds. Some evening high clouds. |
| Mon | 18th | 66/52 | Chance of some morning low clouds; Otherwise, mostly sunny day. Some evening clouds possible. |
| Tue | 19th | 67/53 | Variable high clouds but mostly sunny day. Variable evening high clouds. |
Synopsis
As the computer models predicted earlier this week, a large, upper level trough occupies the West Coast today. The vertical structure of this trough doesn’t favor the maintenance of a frontal cloud band (dissipated on its way southward). So, most of the wet weather associated with this storm comes in the form of cold air instability showers (occurring mostly over and adjacent to the higher mountains). Some convective showers have occurred today in some coastal valley locales (a few producing brief duration heavy showers). However, due to a lack of deep layered, moist air, rainfall totals in southern California should be mostly under a quarter inch (likely well under that value where any showers occur). Snowfall (as low as 5000 feet this afternoon…near 4000 feet late tonight) shouldn’t exceed a couple inches at resort level (convective nature of storm should limit areal coverage). The shower threat should be gone in the Southland by sunrise Saturday (expected by midnight tonight in L.A. County).
While the probability isn’t zero, I decided to omit the slight risk for valley shower clouds from reaching the campus area. Low level, northwesterly winds should continue to cause a “rain shadow” effect (some chance of survival eastern section of the L.A. Basin but only till sunset). I have been a little surprised by the early decrease in wind on the Westside, but that shouldn’t (I think) alter chances for shower clouds to survive the trek from the northern San Fernando Valley down into the coastal plain.
Tomorrow morning may be the coldest morning yet for this new, cold season (thanks to the influx of polar air associated with the upper level trough). Daytime temperatures tomorrow shouldn’t be much different from today’s readings in most locales (maybe slightly higher, depending on how quickly today’s trough exits the state). A little better warming is anticipated for Sunday (west of the mountains) as a brief-lived, off-shore flow occurs. Upper level high pressure is forecast to be relatively weak. So, any warming induced by the off-shore flow should be modest (not expected to warm back to seasonable levels). In addition, poor upper air support should greatly limit the magnitude of wind in Santa Ana wind prone areas (late tomorrow night through Sunday). Peak wind gusts in the higher mountains shouldn’t exceed 45 mph, at best. Most lowland areas that experience Santa Ana winds shouldn’t see wind gusts exceed 25 mph.
The off-shore flow should end Monday afternoon as a new trough quickly passes nearby. This should be a common, “inside slider” trough. Average breezy conditions should occur in the interior before a new round of northerly type winds occur in Santa Ana wind prone areas (late Monday into Tuesday). The trough may promote a few showers in the Tehachapi Mountains (north facing slopes), but the various models don’t favor much in the way of instability showers with this trough (too weak southern end of trough). The off-shore flow should taper off by midweek, but it may not go away completely. The model consensus is for a marginal, weak, off-shore flow to hold on through the end of the work week.
The longer range models disagree on how warm it may get next week. Some model solutions show high pressure aloft getting strong enough to promote widespread 80 degree weather for a day or two (even flirting with 90 degrees in spots). However, the. models also show a stream of high clouds reaching the Southland (riding on the sub-tropical jet stream). This may contribute to more limited warming (coastal plain getting no higher than the upper 70s by midweek). One flaw in my thinking though might be decent vertical air mixing of sub-tropical air down to the ground (from the off-shore flow). In that case, it could still warm into the 80s despite variable high clouds.
Some of the longer range models show a threat of widespread, wet weather in southern California sometime over the subsequent weekend. There could also be a storm threat prior to Thanksgiving Day (Thursday expected to be a dry day though). This scenario includes a storm tapping into an atmospheric river (opportunity for significant precipitation somewhere along the West Coast). Since this is still far out into the future (weather forecast-wise), I’m not taking this potential outcome seriously (yet). At one time, today’s storm was supposed to produce widespread, soaking weather (so much for that forecast).
Next issued forecast/synopsis should be on Monday, 18 November.