Weather Synopsis – January 17, 2025

James Murakami

DayDateHigh/LowForecast
Fri17th60/50Mostly sunny remainder of day. Chance of evening low clouds.
Sat18th61/50Chance of morning low clouds; Otherwise, mostly sunny day. Chance of some evening low clouds.
Sun19th61/51Chance of early morning low clouds; Otherwise, mostly sunny day. Chance of evening low clouds.
Mon20th65/53Chance of some early morning low clouds; Otherwise, mostly sunny day. Clear evening and possibly breezy at times.
Tue21st70/56Mostly sunny day and possibly breezy at times. Some evening high clouds and possibly breezy at times.

Synopsis

The overall, large scale wind flow pattern over the West hasn’t changed drastically. An upper level ridge (high pressure) continues over much of the northeastern Pacific, and an upper level trough (low pressure) is over central Canada. However, there has been just enough of an eastward shift in the pattern to relieve southern California of persistent, off-shore flow (same shift to the Canadian trough will contribute to the cold snap in the eastern U.S. over the weekend. The return to a full fledged, on-shore flow resulted in widespread, marine layer, low clouds this morning (not a solid, widespread blanket of low clouds though). I’m unsure if a widespread blanket of low clouds will make an appearance tonight and intermittently through the MLK, Jr. weekend. The air mass above the marine layer is still quite dry. So, with any appreciable vertical mixing, much of the low clouds could dissipate in places (not necessarily all over land).

With or without marine layer clouds, today’s noticeable drop in daytime temperatures west of the mountains should continue through the weekend (maybe slightly warmer than today for well inland areas, if on-shore flow weakens as the computer models show). Even a diluted marine layer and on-shore flow should help maintain the cooler than normal weather. Overnight low temperatures, however, should be less cold than the last few days (increase in water vapor, which is a green house gas).

The vast majority of model solutions show a return to off-shore flow sometime early next week (some model solutions show a marginal, off-shore flow on Sunday but only briefly). The models continue to disagree on how strong Santa Ana winds may get, not to mention when peak winds may occur. Most Santa Ana wind prone areas should have breezy episodes starting around late Monday. The first maximum in winds might occur late Monday night into Tuesday morning before a relative lull takes hold (i.e. less wind but not calm in wind prone areas). A second period of heightened winds may occur late Wednesday into Thursday morning. Peak wind speeds potentially may exceed 65 mph in the higher mountains (45 mph lowland locales), but again, model solutions vary (some scenarios favor a “weak Santa Ana wind”…peak gusts mostly under 45 mph). More solutions favor the first increase in wind being the stronger event. There are currently no forecast indications of a widespread, significant wind event (as happened on the 7th).

In my last synopsis, I mentioned that completely dry Januarys are quite rare. At UCLA, only 1976 was totally rainless (January records going back to 1933). In 1972 only a trace of rain fell on the campus that January. Well, the longer range model forecasts, while far from unanimous, do indicate a chance at widespread, wet weather sometime during or after the subsequent weekend (25th or later). Instead of an “inside slder” trough, a number of model solutions predict an “outside slider” (not an official term…my usage) reaching southern California. This would be a cold storm (snow levels possibly getting as low as 2500 feet). Being a cold system, rainfall in the lowlands will be limited (likely less than a half inch away from the mountains). However, that doesn’t mean heavy showers won’t be possible (cold air instability may promote isolated, short duration thunderstorms). The wet period may not last beyond a two day period (some models favoring 28th-29th). I should state again that the wet weather is far from certain (confident in cooler than normal weather though). Even if the wet weather materializes, there are a number of model solutions showing only a minor storm (rainfall totals mostly a tenth inch or less).

Next issued forecast/synopsis should be on Tuesday, 21 January.