It doesn’t get much better than this plot below for our Catalina weather 10-13 days ahead, which is always pretty fuzzy-looking as a rule. “Better” means for rain chances here, which is not everyone’s “better.”
Here’s the excitement:
Note blank area, that is, an area free of lines area centered on the gambling and other mischief-permitting State of Nevada1.
Note how the red lines dip down into Mexico, whilst the blue lines bulge northward into Canada along the West Coast. 
This error-filled plot2 tells us that it is almost certain that a trough will be in the lower middle latitudes where we are on January 22nd or so. Just about guaranteed.
In the meantime, those blue lines indicated that a ridge of high pressure is going to divert northern storms into Canada and southeast Alaska. Sometimes we refer to situations like this as “split flow”; the southern portions of storms in the Pacific move ESE toward southern California and the Southwest, while the northern portions split off to the NE, as is happening now. Weak upper level disturbances pass overhead, the next one tomorrow, and with it, a little more rain, the models say.
U of AZ mod has rain moving in toward dawn tomorrow with totals here amounting to about like that last rain, 0.10 to 0. 25 inches. Given model vagaries, probably the lower and upper limits here are likely to be 0.05 (worst case scenario) and 0.50 inches (best case scenario), so a best guess would be the middle of those, 0.275 inches, not too much different from the AZ mod. This is the sports-like part of weather forecasting. What’s your estimate, fantasy or otherwise?
BTW, there were quite a few stations reporting over an inch of rain in Santa Barbara and Ventura Counties during the past 24 h, and so while weak, this system is pretty juicy, lots of liquid water as measured by dewpoints which should rise into the upper 40s to low 50s here during the next day. Also, there have been some embedded weak Cumulonimbus clouds and that’s a possibility here tomorrow, too, as the rainband goes by. You’ll be able to tell that by strong shafting below the clouds. As always, hoping we here in Catalina get shafted tomorrow.
But the ones these days are weak, while the split ahead in 10-13 days is likely to contain much stronger disturbances, well, at least ONE before it gives out.
Yesterday’s clouds
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1What a great and honest state motto that would be! “Nevada: That US State where gambling and other social mischief is OK with us!”
2Don’t forget that due to growth in computer capabilities, we can now have many model runs from the same data and be done with them in a timely way. These “spaghetti”, “ensemble” or better yet, “Lorenz” plots are computer model runs with deliberate (!) slight errors introduced to see how the model forecasts of high and low pressure centers changes, given a few slight errors. This is because there are ALWAYS errors in the data anyway, there are always error bars on measurements, etc. By doing this, only the strongest signals in the forecasts remain, indicated by grouping of lines these two colors of lines, red and bluish. So, the forecast of the jet stream coming out of Asia is very, very reliable. Things go to HELL, downstream (toward the east), but some likely patterns can still be seen, such as the one over the Southwest US where a trough/low is almost certain in our area then. Will it bring us rain in Catalina? Hell, I don’t know because if the trough is a little too far to the east of us, we might only get cooler. However, since Cloud Maven person has a postive rain bias, he will say, “Absolutely. There will be rain in the Catalina area on January 22nd or so”–the actual timing might be off by a day or so.
