While the 1-2, 2-4 inch snows are beginning to wind down from VW to MA, the attention now turns to California where a set-up known as an ‘Atmospheric River’ begins to setup with the perfect alignment of weather systems. This presents real concern for significant multi-day rainfall and consequencial flooding. A trough developing off the West Coast will have multiple systems dropping into it, while be a subtropical jet extends north to bump up against this trough, the two will become stuck in place for 3-5 days and with a deep plume of tropical moisture, hence the term ‘Atmospheric River’ extending from Northern California all the way back out into the central Pacific, this will result in significant rainfall.

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Furthern north, major wind, rain and mountain snows are expected but as you can see from the latest QPF model (above), the focus of rains take aim at Northern California where the firehose of moisture points to. As much as a foot of rain is expected anywhere from the immediate coast all the way back through the Coastal Ranges, remember that these rains can become enhanced by the orographic influence in which moist air is forced to rise, cool and condense further, ultimately squeezing out more moisture. Some computer models painted a very grim picture with as much as 20″ of rain accummulating. With this setup and persistency in models, I wouldn’t be surprised to see a few 14-18″ totals when all is said and done and unfortunately, flash flooding is highly likely.

Meanwhile, this setup will also support some very mild weather coast to coast. This strong push of ocean air will travel a zonal flow across the country and with the help of downslope warming off the Front Range, an area from eastern Colorado to eventually the Atlantic Seaboard may see upwards of 5 days with highs running 10-20F above normal. Some spots on the Plains will likely return to the 70s, perhaps 80s across Nebraska, Kansas down into Oklahoma and Texas, some 30F above normal.

Here’s the ECMWF upper level chart and 850mb temps for Friday.

While this is welcomed by some, loathed by others as this presents a very mild start to the month of December throughout the Lower. Both the NAO and AO does not support this setup and so, my advise to you would, enjoy it while it lasts!

By days 8-10 on the ECMWF, you can see the cold air regrouping and while the arctic air is never too far north of the border, one would expect this zonal pattern to break and it appears to do so with the trough starting to migrate east. This will allow the arctic air over western Canada to drop south into the interior Pacific Northwest and Northern Rockies. With time, I expect this arctic air to plunge south and east presenting a cold 2nd week of December.

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