New synopsis to follow around 3 PM.
Mother Nature continues to toy with me, weather forecast-wise. The surface, off-shore flow turned out weaker than the numerical models predicted. The vertical, temperature profile verified about as expected, but an early sea breeze (like yesterday) would cut into the potential warming for the day. I pared back my forecast high for the campus this morning, but it wasn't necessary. Hence, I had to go back to the drawing board and revise the temperature forecast for today/tomorrow. Elsewhere, modest warming also occurred in most areas. Some valley locales reached the mid/upper 70s by early afternoon. The Antelope Valley did warm significantly over yesterday (double digit warming over yesterday thanks to a return of westerly winds pushing out the recent, polar air incursion).
Ignoring my little forecast faux pas, the forecast thinking from yesterday continues. High pressure aloft will last one more day tomorrow before a cold trough moves into the state. Temperatures should be similar to today in most areas (plus to minus two degrees). Noticeable cooling will occur on Saturday when the aforementioned trough moves through the state from the northwest (currently located in the western Gulf of Alaska). Various models show sufficient, cold air instability for some showers in the northern mountains (north facing slopes of the Tehachapis). This could begin early Saturday and last through early Sunday. Snow levels will fall to around 4000 feet by early Sunday, but accumulations should be minor at that elevation (an inch at most though some higher elevation locales could pick up a few inches more).
Some model solutions also show showers occurring in the coastal mountains around San Luis Obispo/Santa Barbara Counties (mainly Saturday morning). There may also be marine layer induced showers around the coastal facing mountains in San Diego County (mainly Saturday). Rainfall from these showers should be well under a quarter inch.
For a time, I wasn't sure whether a defined, marine layer would form with the cold trough. Well, most of the models are predicting a sufficiently, moist layer for widespread, low clouds on Saturday. One model (WRF-ARW) is even predicting patchy, early morning drizzle (solution rejected for now). The low cloud field should become disrupted once a "dry" cold front passes through (near sunset Saturday on the Westside).
Breezy weather should develop in the wake of the trough passage late Saturday. How widespread it gets remains to be seen, but much of the coastal plain should be affected (wind gusts to 20 mph or higher at times). Wind speeds in Santa Ana wind prone areas could see peak gusts over 40 mph late Saturday night or Sunday (not restricted to higher elevations). The wind should diminish some by Monday, but a full fledged, off-shore flow will continue to promote breezy weather in Santa Ana wind prone areas (at least through early Wednesday). The model consensus is leaning against a strong wind event, but peak wind gusts may still reach 45 mph in some spots (especially higher elevations).
The predicted, off-shore flow early next week should produce slightly warmer than normal temperatures by Tuesday (polar air influx may cancel out effects of subsidence warming and produce just seasonable temperatures on Monday). A little more warming may occur on Wednesday (temperatures similar to today), assuming the predicted, off-shore flow doesn't fade till the late afternoon.
Today's model consensus wants to delay a rain threat next week. A threat of showers isn't likely before Thursday now. One model (ECMWF) delays wet weather to the end of the week, and it would be a very minor event. It's unclear if showers will be widespread in areal coverage when the predicted trough reaches southern California (be it Wednesday, Thursday, or Friday). At t his point, rainfall should be well under a quarter inch in most areas. Dry weather should follow for at least a day.
Details of weather remain fuzzy farther out in time, but cooler than normal weather appears likely for the week of Hanukkah/Christmas. A number of model solutions show wet weather for the entire state on a couple of days, but model agreement on which days is poor at this time. One event in southern California might occur on the 22nd (first full day of winter, astronomically speaking). Some model solutions hold onto the prospect of a white Christmas at the local resorts (fresh snowfall even more probable in the Sierras). I'm still reluctant to accept the consensus solution this far in advance despite fair model agreement. I'd like to wait a few more days before getting on the wet weather bandwagon.
UCLA Forecasts and analyses are written by James Murakami
– posted: 2019-12-13