The latest storm zipped through southern California more or less as expected, but measurable rain was more widespread than what most computer models had predicted. Most of L.A. County reported a few hundredths of an inch rain (UCLA recorded 0.03 inch). There were some reports to around a quarter inch rain, but most were north of Point Conception. Snow levels briefly fell down to about 4000 feet though no accumulation occurred south of the Tehachapi Mountains...to my knowledge. Cold air instability showers are possible this afternoon around the higher mountains, but they should be brief in duration (less than 30 minutes in any given location) and generally light in intensity (isolated, brief heavy rain not out of the question though). A few of the showers could "spill" into the adjacent, coastal valleys, but none should reach the coastal plain (shower clouds should dissipate before getting that far away from the mountains...exception would be if one were to form over the Santa Monica range). Any showers should dissipate by evening as upper level low pressure continues to move east and atmospheric instability wanes.
As expected, the storm has brought noticeably cooler weather today (double digit drops from yesterday's readings for well inland areas). Where mostly clear skies and little wind prevails tonight, a chilly overnight can be expected. With the various models predicting a marine layer early tomorrow, low clouds may prevent that much cooling west of the mountains (nothing colder than it got early today). Away from the coast, there should be modest but noticeable daytime warming tomorrow, however. Even better warming is anticipated over the weekend when high pressure aloft is forecast to induce a marginal, off-shore flow pattern. Temperatures should warm well into the 70s in the inland coastal plain, and many valley locales should reach the low/mid-80s. Depending on how much of a marine layer persists (as the NAM model predicts), areas near the immediate coast may not warm beyond seasonable levels. I chose a middle ground for the campus forecast (could warm into the mid/upper 70s if no marine layer prevails over the weekend).
Multiple, weak, upper level low pressure troughs are forecast to pass to our north next week (one actually passing by on Sunday). None of them should affect the Southland that much, but a defined, on-shore flow pattern should occur. Widespread, marine layer clouds should occur in the coastal plain early next week (could also reach valley locales, especially second half of next week). Temperatures should hover near seasonal normals and trend toward slightly cooler than normal weather late next week.
There have been some model solutions showing a "cut-off" low pressure nearing the state from the west late next week (at one time, it was predicted for midweek). Some of the latest runs have delayed the "cut-off" low pressure from reaching the coast till the 29th or 30th. A few of the model solutions also predict sufficient, atmospheric instability for scattered showers accompanying the low pressure (includes the coastal plain). It's far from certain if the wet weather scenario will verify (lack or run to run, model consistency as well as among various models). If the low pressure stays far enough off the coast, high pressure aloft could actually build over the Southland for a time. This scenario would lead to warmer than currently expected weather late next week.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2018-04-19