Wet May 2019 at UCLA
Note: No forecast will be issued on Wednesday (22 May).
The storm that came into southern California over the weekend proved wetter than expected in many areas. As some computer models predicted, some light showers started Saturday evening in L.A. County, albeit a little faster than those models predicted. Although most showers ended yesterday afternoon in L.A. County (away from the mountains), some showers developed last night on the Westside (some other areas too though nothing measurable). Additional showers occurred early today from the western Inland Empire southward. That resulted in much more impressive rainfall than expected there (half to one inch rainfall noted in a number of those areas). Rainfall did exceed a half inch in some parts of the coastal plain west of L.A. County, but that had been anticipated. The biggest surprise (to me) was how cold the storm ended up being (resulted in several inches of "wet" snow at resort level).
Yesterday's storm is now wreaking weather havoc on parts of the southern Rockies and Great Plains today. For our area, weak high pressure should promote a largely dry day tomorrow. However, mostly cloudy conditions may result for a part of the day in some areas west of the mountains. There could even be some marine layer induced, early morning showers down in San Diego County. A new, upper air trough of low pressure is forecast to swing into the Southland from the north on Wednesday. That should lead to more showers for the Inland Empire and points south into San Diego County (similar to early today). Predicted, northwesterly winds should promote a rain shadow effect in L.A. County, but showers could occur early in the morning over the north facing mountains in the Techachapis.
Some of the various models do predict enough atmospheric instability for mountain showers elsewhere in southern California in the coming days. However, confidence is low in where these showers may occur. Some of the model solutions also include a chance for widespread showers west of the mountains on Thursday. Since a small error in predicted, wind flow could result in a mostly dry weather day (excepting around the mountains), I only show a "slight chance" of showers for Thursday. Depending on subsequent model runs, I may upgrade the threat or yank away the rain forecast altogether.
The upper level low pressure responsible for the rain threat will weaken on Friday. Chances for mountain showers should decrease, but the cooler than normal weather that starts out this week should continue that day. In fact, with most model solutions showing upper low pressure remaining somewhere over the western states this Memorial Day weekend, any warming trend should be modest at best (maybe back to seasonable levels for Memorial Day).
Some of the longer range models show an upper air trough bringing widespread showers on Sunday. One model solution even shows a storm wetter than yesterday's storm. At this point, model solutions are rather varied (some include just partly cloudy weather). The usually reliable ECMWF has been predicting wet weather, but it hasn't been consistent with its forecasts lately (at one point, it didn't favor yesterday's wet weather). So, caution is advised in accepting any given forecast for the holiday period. Hopefully, clarity will develop in the next couple of days.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2019-05-20