An off-shore flow developed yesterday, but it peaked early today (in terms of wind speeds in Santa Ana wind prone areas). There will be one more day of off-shore flow tomorrow (first half of the day), but it's expected to be fairly weak wind-wise (no upper air support for significant wind predicted by the computer models). On-shore flow should begin tomorrow afternoon, but it shouldn't pick up much strength till Sunday (increasing wind in the interior expected that day).
Daytime temperatures today have been a little lower than expected in most areas. At UCLA, it looks like it won't get warmer than 73 degrees (steady ocean breezes currently blowing). Tomorrow may still reach forecast temperatures despite a decaying off-shore flow (predicted deep layered, easterly winds could promote better warming than today). At the least, it should get as warm as today (weaker off-shore flow offset by predicted warmer temperatures aloft along with subsidence from the easterly wind flow). Noticeable daytime cooling will occur on Sunday with the return of a full fledged, on-shore flow. Temperatures should return to seasonable levels in most areas that day(slightly warmer than normal in in the interior).
A cold, upper low pressure trough that develops along the West Coast on Sunday will send a cold front into the Southland early on Presidents Day. The predicted, wind flow pattern should promote a deep marine layer Sunday night into early Monday morning. The passing cold front could trigger marine layer showers though most of that should be around foothills/mountains. Also, most of the model solutions show that occurring south and east of L.A. County. Lacking a good moisture source (other than the local marine layer), rainfall (where it occurs) should be no more than a tenth inch (maybe a little higher in the mountains). I wrote for a "slight chance" of showers for early Monday, but based on today's model consensus, that would occur before sunrise (should wet weather actually occur).
For L.A. County, most of Monday should be a sunny and breezy, cool day (threat of showers should mostly end by the afternoon to the south and east of L.A. County). Post-frontal winds may reach 30 mph in the coastal plain though 20 mph gusts should be more common (afternoon hours). The influx of polar air should keep daytime temperatures below normal despite expected sunshine. If today's ECMWF model is right, it won't even reach 60 degrees west of the mountains that day (current forecast based on less cold air reaching the Southland)
Off-shore flow should develop on Tuesday and continue into Wednesday. Breezy weather in Santa Ana wind prone areas should occur (peak gusts may reach 45 mph in the higher mountains for a time). This Santa Ana wind event will be a "cold" type in that subsidence warming won't be able to overcome the influx of polar air into the Southwest. Daytime temperatures would, at best, reach seasonable levels. Nights would be seasonable chilly (30s to low 40s in most wind sheltered locales west of the mountains).
Temperatures should moderate some later next week, but the model consensus maintains a cool, northwesterly wind flow pattern on the West Coast. Thus, it's nearly a guarantee that cooler than normal weather will prevail through the end of next week (quite possibly for the rest of the month). What is more uncertain is whether widespread wet weather will develop in the state.
The predicted trough of low pressure for Sunday into Monday should promote cold air instability showers in the state. Except for marine layer induced showers, wet weather should be confined to near/over the mountains. At least a couple more "short wave" troughs are forecast in the latter half of next week and beyond. The predicted wind flow pattern will preclude any of the storms from tapping into a moisture rich, atmospheric river. So, storm totals from any given storm shouldn't exceed about half an inch rain (several inches of snow above 4000 feet though an inch or so possible as low as 2000 feet)). Some model solution had shown a better over water trajectory for a projected, Southland storm over the subsequent weekend. However, the model consensus has put that off a few days (almost never promising when the models want to keep delaying a predicted event). For now, I'm just taking a "wait and see" approach.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2018-02-16