We still have another couple of days of dry but cloudy conditions across the region before the ridge of high pressure breaks down and allows a wet and aggressive storm to come through over the weekend. Temperatures will warm to the mid-50s Wednesday and will climb into the low 60s by Friday, when the first wave of precipitation will likely start. The Friday event is shaping up to be a moderate strength Atmospheric River event, with high snow levels (8,000’+), and most of the computer models seem to be in agreement with that picture. That would result in significant rain, especially in the mountains below the passes.
The weekend gets a little less certain, but colder air does move into the region, dropping snow levels to below Lake Tahoe Saturday and possibly all the way to the valley floor by Sunday.
So what makes an Atmospheric River (AR) so wet? There are two main factors. First, the moisture source is usually straight out of the tropics, which of course is a very wet air mass. But even more important is the configuration of the precipitation plume. I’ll expound on this tomorrow.