Note: No forecast will be issued on Monday, 21 August.
Upper level low pressure continues today over the western states. However, it's weakening, and high pressure will begin to build into the state. The marine layer depth over the L.A. Basin early today was nearly unchanged from yesterday, but the low clouds were thinner than yesterday (no reports of overnight drizzle). So, along with the weaker on-shore flow, clearing took place more completely in most areas (exceptions in eastern Santa Barbara/western most Ventura Counties and a portion of the Palo Verdes peninsula, as of this writing). On-shore flow is expected to weaken further over the next couple of days. So, marine layer clouds should be less extensive/persistent tomorrow and Friday (probably about the same Saturday morning too). Low clouds may reach some valley locales early tomorrow, but it may not necessarily occur as a solid overcast (may apply to parts of the coastal plain too). Low clouds should be confined to the coastal plain on Friday/Saturday mornings, but I'm unsure whether a widespread overcast will occur.
The uncertainty about the "behavior" of marine layer clouds isn't anything new (often a challenge to get it right). However, an added question mark is what effect a predicted, vestige, upper low pressure will have on the low cloud field. High pressure aloft will cover most of the southwestern states by late tomorrow, but over the Southland, this "leftover" low pressure could help support the low cloud field more than I currently anticipate (marine layer might stay deeper than usual). The weak low pressure should also limit subsidence warming from the high pressure in the region. Well inland temperatures, at most, will get slightly above normal on Friday/Saturday. The coastal plain and perhaps, some valley locales, shouldn't exceed seasonal normals (leaning toward it staying a tad below normal).
There are some model solutions that show the low pressure producing some atmospheric instability (Friday and Saturday...possibly into early next week). That would lead to at least some afternoon cloud build-ups over the mountains (especially Tehachapis). However, except for the NAVGEM model, none show sufficient instability for developing for showers/isolated thunderstorms. If a long fetch of southerly winds aloft develop early next week (related to a new, upper low pressure forming off the state coast), there is some chance of a modest, "monsoon" moisture incursion (adding to the instability in the southern mountains/eastern desert). Today's model consensus keeps this moisture to the east and south, however.
With a new, upper low pressure forming off the state coast early next week, another cooling cycle should occur (beginning on Sunday). This low pressure will be weaker than the one that promoted such "cool" weather yesterday (valley temperatures were upper 70s to mid-80s only). So, temperatures early next week should not be as low as yesterday. In fact, if the upper low pressure develops too far off the coast, any cooling trend should be minor (not even felt in the interior). At this point, a "middle of the road" approach to the forecast appears appropriate. Any new, warming trend next week should hold off till late in the week.
A number of longer range models show warmer than normal over the subsequent weekend and beyond (for a few days at least). It's unclear how warm it may get (solutions range from slightly warmer than normal to sizzling weather). Once again, much seems to hinge on the predicted motions of tropical cyclones that may occur in the eastern Pacific and Caribbean Sea. At one point, heat wave weather was predicted for much of this week. Needless to say, I have no confidence in the model trends beyond early next week.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2017-08-16