In another example of Mother Nature playing tricks with weather forecasters, a small area of showers and a few thunderstorms occurred this morning. None of showers were of consequence (just sprinkles...one foot rain...drops spaced about one feet apart), and none of the lightning was cloud to ground, to my knowledge. It seemed to start over northern San Diego County during the 8 AM hour. It passed through southeast L.A. County in the 9 AM hour (I was an eyewitness) As of this writing, it was dissipating over the San Joaquin Valley. There was atmospheric instability above the 10000 feet elevation mark, but deep layered moisture was far from impressive. Also, the lower atmosphere was very dry (discounting very shallow marine layer in the coastal plain). The latter explains why most rain drops evaporated before reaching the ground (only largest drops survived trek). However, none of the numerical models resolved an apparently, small disturbance (to help trigger the showers/thunderstorms).
Atmospheric instability from an influx of "monsoonal" moisture also encompasses the southern mountain/eastern desert region of southern California. Radar imagery shows some showers occurring in the eastern deserts early this afternoon. Some of those clouds may briefly turn into thunderstorms. Isolated thunderstorms could also form over some of the southern mountains before the afternoon ends. However, there are no indications (precursor clouds) for further, "surprise" showers west of the mountains (upper level winds not conducive for any mountain/desert showers to drift west of the mountains). None of the showers in the interior are expected to be significant. A deep layer of dry air resides in the lower atmosphere (just like west of the mountains). All the various models maintain a flow of "monsoon" moisture well aloft, but they do decrease the amount over successive days. Also, atmospheric instability is forecast to decrease after today. So, the chance of Southland mountain/desert showers and/or thunderstorms should wane after today (threat should be altogether gone by Sunday).
It's been quite hot in the interior as was predicted by the models (hottest desert locales over 120 degrees). Many coastal valley locales also have experienced triple digit heat for the last few days. However, for the coastal plain, it has remained cooler than what I expected (warm but not excessive). There has been sufficient on-shore flow to limit the warming trend there. With high pressure aloft over the Southwest (now centered over Arizona), warmer than normal is expected to continue through at least Saturday. Any cooling trend over the next few days should be a slow, gradual one (least effective in the interior). A return to seasonable temperature may not come for most areas till early next week.
A nagging question mark for me though is whether the marine layer could become better established in the coming days. Thanks partly to a large, coastal eddy southwest of the coastal islands, low clouds extended well inland in San Diego County this morning (low clouds restricted mainly to the coast elsewhere). The various models show a steady increase in on-shore flow after tomorrow. The various models aren't predicting a relevant, marine layer deepening phase till Monday, but models infrequently handle this occurrence poorly. The dominance of high pressure aloft in the Southwest should prevent any real deepening of the marine layer through at least Saturday. However, a strong enough on-shore flow could bring about better cooling in the coastal plain than I currently show. It's possible that UCLA daytime temperatures could wind up several degrees lower than I show today.
All the models show the center of high pressure aloft getting displaced back into Mexico early next week. Weak, upper level low pressure is forecast to move over northern California around the middle of next week. This predicted shift should promote more seasonable temperature even in the interior (hot but better than currently). There is some chance for a day or two of widespread, "June Gloom" weather next week, but that's far from certain (aforementioned low pressure may not get close enough for such weather). Some model solutions show a new though modest, warming cycle late next week. Other model solution delay that warming for the final, few days of June.
Next scheduled synopsis should be on Tuesday, 22 June.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2021-06-17