Yesterday, it didn't warm up as much as expected. The marine layer became quite shallow, and areal coverage was limited, but apparently, vertical air motions weren't enough for noticeable warming in most areas west of the mountains. Today, in spite of some marine layer clouds in some areas (early today)., it finally warmed noticeably. Increasing "monsoonal" moisture appears to have promoted greater, vertical air motions than yesterday. Of course, at the time I issued the preliminary forecast this morning, that wasn't the case (this morning's computer model runs weren't inclined toward additional warming over yesterday).
Upper air temperatures aren't forecast to change from today's readings, but surface on-shore flow is expected to weaken further (compared with this morning). This should mean that the warming trend should continue , but near the coast (campus included), a combination of factors (cloud cover, sea breeze strength, magnitude of vertical air motions) may not allow for much additional warming over today (UCLA 84 degrees, as of this writing). I postcasted today's portion of the forecast, but I left the rest of the forecast unchanged. I may need to increase temperatures for Tuesday/Wednesday, however, depending on what the model output shows tomorrow.
I'm more confident of further warming for well inland areas, but there is some uncertainty concerning the current, "monsoon" moisture incursion. If the variable cloud cover turns out more persistent in some places, daytime heating may get restricted somewhat. Subsidence warming is less efficient in humid air compared with a drier air mass. In any case, Wednesday is expected to be the warmest day this week in the coastal plain (most coastal valleys too). Considering how warm it has gotten today, there is potential for widespread 90 degree weather in the inland coastal plain for the next couple of days (as high as mid-90s). Triple digit heat in the valleys are almost certain, but how widespread it gets is uncertain.
Isolated thunderstorms developed over the southern mountains late this morning and afternoon. Additional storms may occur in the eastern desert. Some model forecasts show isolated thunderstorms for the Tehachapi Mountains, but so far, only cloud build-ups have occurred (atmospheric instability marginal). There is an upper air disturbance over northern Baja California, which is forecast to move northward tonight. While not explicitly forecast by the numerical models, "enhanced" showers and isolated thunderstorms could occur tonight in portions of southern California (continuing through the overnight hours). This should occur mostly to the east of L.A. County, but I wouldn't rule out sprinkle showers occurring over the eastern half of L.A. County tonight.
As is often the case, details of the predicted, "monsoon" flow vary with each model. So, just where shower/thunderstorm activity will be greatest each of the next few days is uncertain. However, for the most part, it should be restricted to the southern mountains (San Bernardino range southward) and the deserts to the east (mainly afternoon hours). There is a chance for flash flooding with the thunderstorms (high water vapor content). Today's model consensus has the areal coverage decreasing after Wednesday (decreasing "monsoon" moisture into the Southland). If the models are right, it should become too dry for thunderstorm formation by Friday.
The upper level, high pressure that is responsible for the hotter, more humid weather will drift westward from it current location around the Four Corners area. Its center may reach the Mohave desert by the weekend. This should increase temperatures in the interior, especially if "monsoon" moisture decreases significantly. Warmer weather should also prevail for well inland areas west of the mountains this weekend. However, low level, on-shore flow is predicted to strengthen, which may cancel some of the warming effects by the proximity to the high pressure center. The various models predict slightly cooler weather in the coastal plain for late this week (leveling off for the weekend). However, even with the stronger, on-shore flow, temperatures this weekend will still be warmer than normal. Any marine layer will be quite shallow, which will limit any relevant cooling to the coast.
Only minor weather changes are forecast for next week. Depending on which model solution is right, a new, "monsoon" incursion may start around the end of the month (continuing first days of August). Only minor ups and downs are expected in temperature trends for next week (no return to seasonable temperatures foreseen for most areas...maybe near immediate coast only).
UCLA Forecasts and analyses are written by James Murakami
– posted: 2019-07-22