| Day | Date | High/Low | Forecast |
| Tue | 27th | 71/58 | Mostly sunny late morning/afternoon. Chance of late evening low clouds. |
| Wed | 28th | 71/58 | Good chance of morning low clouds but mostly sunny by afternoon. Chance of late evening low clouds. |
| Thu | 29th | 74/60 | Chance of early morning low clouds; Otherwise, mostly sunny day with some high clouds possible. Some evening high clouds possible. |
| Fri | 30th | 79/62 | Chance of some early morning low clouds; Otherwise, mostly sunny day with some high clouds. Variable evening mid/high clouds. |
| Sat | 31st | 80/64 | Variable mid/high clouds but generally sunny day. Partly cloudy evening. |
Synopsis
A weak, elongated, upper low pressure trough exists over the state today. Only slight movement to the east is predicted by the numerical models. So, little weather changes are anticipated tomorrow. However, subtle shifts in the low level wind flow may result in a different clearing routine from today (at least, for some locales).
A belt of high pressure aloft is forecast to build into northern California midweek onward. The high pressure should be situated over the interior Southwest by the end of the week. At the same time, a weak, semi-“cut-off” low pressure in the sub-tropics (to our southwest) should move toward central Baja California around Friday or early Saturday. With high pressure building over our area and supportive easterly wind flow on the northern circulation of the “cut-off” low pressure, noticeable warming is expected late this week. The greatest warming (relative to today’s readings) should be for well inland areas (i.e. valleys, mountains, desert). Just how hot it actually gets may depend on the eventual position of the “cut-off” low pressure.
Conditions in the tropics south of Mexico look favorable for a tropical storm to form later this week (name would be Alvin, if sustained winds get to at least 39 mph). A number of model solutions show Alvin will make landfall somewhere in mainland Mexico (north of Puerto Vallarta?). The aforementioned, “cut-off” low pressure may direct some of the tropical storm moisture into the desert Southwest and possibly into coastal southern California as well (valid time sometime Saturday-Sunday window). It’s possible that sub-tropical air may mix down to the ground in the Southland as early as Friday. That would promote very warm and somewhat muggy weather that day (widespread 90 degree weather valleys and possibly some inland, coastal plain locales).
By Saturday, some model solutions show what looks like a “monsoon” moisture incursion into the southern California desert region. Variable mid/high clouds should prevail across the Southland. If the clouds don’t get too thick, Saturday may get a little warmer than Friday in most areas. In the interior, however, a “monsoon” air mass may lower daytime temperatures a bit (offset by muggier feel though). There’s some chance for afternoon instability to produce isolated showers and brief-lived thunderstorms. The chance for instability showers may increase on Sunday if model predictions for the low pressure moving toward Arizona verify. At this time, I don’t see a relevant shower threat in the coastal plain (Saturday or Sunday), but confidence isn’t the greatest. If the “cut-off” low pressure can direct some upper air disturbance our way, then some showers and even thunderstorms would be possible (Saturday night or early Sunday). Any rainfall, however, should be minor for most areas away from the mountains.
By Monday, the “cut-off” low pressure should have moved inland (Arizona), but a cold (for June), upper level trough is forecast to move into the state from the north Pacific. Strong on-shore flow should rapidly whip up a marine layer that day (wind flow pattern will also shunt all “monsoon” moisture east of the state). If some model solutions are right, a couple days of “June Gloom” weather will prevail early next week (includes areas of morning drizzle/light rain). Compared with what’s expected late this week, temperatures will come crashing back to below normal levels (coastal plain may not reach 70 degrees for a couple days).
Improving weather is anticipated later next week, but warmer than normal weather may not occur till the end of next week. Best chances of that would be for well inland areas (some question mark for areas near the coastal…shallow but effective marine layer may limit any warming trend there).
Next issued forecast/synopsis may be on Friday, 30 May.