Potential Atmospheric River Event Late This Week

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January was a horrible month for precipitation across California – specifically in northern/north-central California, which was between the cutoff lows that trickled into southern California and clipping cold fronts in far northern/northwestern California. That said, precipitation across much California was for the most part 0 to 25% of normal; yet strangely the desert(s) of interior southern California did actually pick up normal to even slightly above normal precipitation. This was due to the cutoff lows that sat around in the southwestern U.S., spiraling moisture around and over the southern California deserts more than anywhere else for the most part.

30 day ending feb 1

30 day precipitation from normal in percent. Image via California Climate Data Archive (CalClim).

It appears that this pattern consisting of a dry northern California and a (previously) unsettled southern California will flip through the week this week, with the current pesky ridge shifting southeast into the desert southwest, and by late this week, be far enough south to allow a large upper low and long fetch of westerlies to surge out of the central Pacific, toward the west coast, and essentially sit off the western U.S. coast & in the Gulf of Alaska from Friday into the weekend, feeding the mid/upper-level westerly jet right into northern California and the rest of the Pacific Northwest.


This belt of westerlies and associated disturbances (upper lows, surface lows, vort maxes, etc.) will latch onto a long fetch of subtropical moisture originating from near and east Hawaii, with precipitable water values associated with this long fetch of moisture in the 1.5 – 2″ that become aimed directly at northern California for what at the moment is an unknown period of time. The GFS is extraordinarily wet, keeping this feed of moisture and jet energy focused on northern California from as early as Thursday, through Monday, dumping a massive amount of rainfall across northern California no matter how you look at it, drought or not. The ECMWF is much, much dryer – indicating rain from late Thursday/early Friday through early Saturday, with a break Saturday into most of Sunday, and another moderate-strength wave later Sunday into early Monday. While the ECMWF still dumps a good bit of rain, but it’s only half of what the GFS has. The cause of these major differences lies in the placement of the lows. The GFS keeps two (deep) parent lows in the eastern Pacific, around 45N and 150W, while the ECMWF brings the low(s) eastward into British Columbia quickly, which sends the moisture plume out and away with the cold front instead of stalling it like the GFS does, as it keeps the lows quasi-stationary and aims this atmospheric river at northern California for days instead of a day.

About 65% of the 12z GFS ensemble members indicate potential for a widespread 5″+ rainfall event from San Francisco northward, with every single other ensemble dumping 3″+, so there’s a crazy amount of confidence in a significant amount of rainfall occurring Friday through Monday. The GEM, UKMET, and US Navy (NAVGEM) models tend to lean more toward the ECMWF’s solution… thus, it appears the GFS and a good chunk of its ensembles could be off here, or at least in the small boat of outlier solutions dumping insane amounts of precipitation over the weekend into early next week. We’ll have see in the coming days how models trend.

Forecast 10 day precipitation totals via the GFS ensembles. Each different image is a different ensemble member, and thus each has different output variations. Image via WeatherBell.

Forecast 10 day precipitation totals via the GFS ensembles. Each different image is a different ensemble member, and thus each has different output variations. Image via WeatherBell.

I’ll hold off on precise or even rough rainfall amounts just yet given the crazy differences between the two major models here, but will post an update either Tuesday or Wednesday with a first-call map depicting some forecast amounts, and more information overall. Consider this post more of a wake-up call for the blog, and expect more to come through the week.

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