Off-shore flow continues today, but it's weaker than yesterday. Much higher than normal temperatures characterize this off-shore flow more than wind (weekend winds never reached levels predicted by some computer models). Yesterday, some areas recorded triple digit highs for the day (Orange/ San DIego Counties and Inland Empire). The high for the nation yesterday was 102 degrees at Miramar Naval AIr Station in San Diego County. Despite temperatures getting into the mid/upper 90s in many areas ( including near the immediate coast), no temperature records for the date were set (97 degrees at UCLA yesterday, but the record for the date was 102 degrees in 1961).
This morning, I thought it could get slightly warmer today than it got yesterday. A weaker off-shore flow would promote an earlier ocean breeze, but temperatures aloft were as high as yesterday, and it was running a little warmer than yesterday at mid-morning. As it turned out, a pseudo-sea breeze (ocean breeze but too dry to be considered a true sea breeze) began earlier than yesterday. The campus appears to have peaked at 95 degrees (11:27 AM) as temperatures have mostly hovered in the upper 80s early this afternoon. Areas farther inland, however, should reach the same levels, if not slightly higher than what occurred yesterday (ocean breezes too weak to have appreciable effect farther inland).
The off-shore flow pattern will continue to decay steadily through Wednesday (should be completely gone by the afternoon). Tomorrow's weather should still be quite warm, but more cloudiness is expected. Currently, a flow of moisture from the tropics is occurred west of Baja California (marks the western edge of the upper high pressure responsible for our hot weather). Disturbances embedded in the flow are promoting scattered showers and thunderstorms. Today's models show a chance for showers and isolated thunderstorms reaching western Santa Barbara County (and points north) tomorrow. The moisture is forecast to migrate eastward during the day. While the models show the threat of showers only in the southern Sierras and perhaps northern Tehachapi Mountains, the predicted high water vapor content of the air mass does make be nervous about tomorrow's weather. I've kept it out of the "official" forecast, but I wouldn't be surprised if some sprinkles/light showers developed late tomorrow afternoon or evening in L.A. County (not just around the mountains). At the least, it could become mostly cloudy sometime in the afternoon.
A fair amount of moisture and atmospheric instability is predicted for Wednesday in the southern mountains. If that holds up, a chance for isolated showers and thunderstorms could be possible (mainly San Diego County). Any shower clouds would drift into the deserts as upper levels winds are forecast to be from the west. Elsewhere in the Southland, there should less mid/high clouds compared with what's expected tomorrow. Over the coastal waters, low clouds should begin to form, and by Thursday morning, widespread low clouds should return to the coast (possibly encompassing much of the coastal plain). The marine layer should reach peak depth Friday morning when low clouds should reach most valleys (not necessarily as an overcast, however).
A decent strength storm should bring widespread precipitation to the northern third of Calfornia late Thursday into early Friday morning. At one point, some of the models showed moisture associated with a "cut-off" low pressure (located 1500 miles west of Los Angeles this morning) getting caught up in the circulation of the northern California storm. More recent model runs have pretty much abandoned that scenario (NAVGEM holds onto it but barely) In fact, the only affect on the Southland should be the increasing on-shore flow. Once a "dry" cold front passes Friday afternoon, the marine layer should get disrupted again (low cloud purged or at least greatly reduced in areal coverage).
Temperatures late this week should fall to seasonable levels or even slightly lower by Friday. However, yet another off-shore flow is predicted to develop this weekend. All the computer models predict a another heat spell comparable to the current one. Widespread 90 degree weather would occur on Sunday/Monday next week (possibly Tuesday too). Some model solutions maintain much warmer than normal weather for the entire week (just modestly cooler in the second half of the week). Other model solutions, however, show a more robust cooling trend late next week. So, there is considerable uncertainty for the latter half of next week.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2017-10-16