NOTE: THE TEMPERATURE/HUMIDITY SENSOR OCCASIONALLY SHOWS SPURIOUS VALUES (DAYTIME ONLY...SO FAR). THE CAUSE IS YET UNKNOWN, BUT RESOLUTION TO THE PROBLEM MAY NOT COME BEFORE THE AUTUMN.
The summer doldrums are here. Low level on-shore flow exists to help moderate temperatures near the Southland coast, but only a very shallow marine layer exists thanks to a strong high pressure over the Southwest (centered drifting around the Four Corners region). That is promoting warmer than normal temperatures across southern California (least warming along the coast, of course). At UCLA, daytime temperatures have hovered around 80 degrees (plus or minus 2 degrees) since the 8th with the exception of last Sunday (84 degrees). Overnight low temperatures have hovered around 65 degrees (plus or minus 2 degrees). I'm not expecting a change for the next week or so. It's just a matter of when warming/cooling cycles occur (something I haven't always succeeded in forecasting correctly). [FYI--Source of recent, campus maximum temperatures from an "in-house only" back-up sensor.]
None of the computer models predict a relevant change to the marine layer for the next several days. Areal coverage of late night/early morning low clouds are likely to continue along the coast, but subtle wind flow changes may alter its coverage from one day to the next. If most model forecasts are right, on-shore flow may strengthen some by the weekend (weakening of the portion of high pressure over the Southland). That may allow for a slight increase in marine layer clouds at that time, but low clouds shouldn't reach any of the coastal valleys, and what low clouds occur should clear away quickly during the daytime, morning hours. By early next week, a potential increase mid/high clouds from the south may help disrupt the coastal, low cloud field.
Temperatures today over most of the coastal plain are a tad lower than yesterday (not sure why). The models show a small decrease in on-shore flow tomorrow. So, it could be a tad warmer tomorrow. A general though minor cooling trend (associated with the weakening high pressure aloft) could begin on Friday or Saturday, depending on which model is right. Still, temperatures headed into early next week don't look to get cooler than normal (seasonable at best). Beyond early next week, the model consensus is poor on a trend (slight majority favor minor warming trend).
The flow of "monsoonal" moisture into southern California has varied in the last week or so (little on some days; fairly abundant for short periods of time too). Early this afternoon some isolated, light showers were occurring over the San Diego County mountains. Isolated thunderstorms were also near the eastern state border west of Bullhead City, Arizona. Most model solutions show a modest increase in moisture and instability for the next couple of days. However, areal coverage of showers/thunderstorms in the Southland mountain/desert region isn't expected to increase substantially (greatest coverage on Thursday). Other than some variable, mid/high clouds, nothing is anticipated west of the mountains through Saturday.
Most of the models predict a weak, upper level low pressure developing west of northern, Baja California early next week. It may help direct variable mid/high clouds into the Southland as early as Sunday. Some model solutions show sufficient, deep layered moisture for an increased chance of mountain/desert showers and isolated thunderstorms. A few solutions even have that occurring west of the mountains (something that's happened a couple of times this month though not all areas experienced the showers). As I've remarked before, a small deviation in predicted wind flow could result in a different outcome. So confidence in some wet weather west of the mountains early next week is quite low for now. However, depending on which model scenario is right, the threat for showers somewhere over the Southland could last till the middle of next week (best chances in the usual mountain/desert region).
Next scheduled synopsis should be on Tuesday, 27 July. Through August, issued forecasts will be made only on Tuesdays unless exceptional weather warrants an additional, weekly forecast (eg. threat of rain for the campus area).
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-07-20