February 1-7, 2019 Atmospheric River Event

by John Farley

During the first week of February, 2019, much of the United States was impacted by an atmospheric river (hereinafter, AR) event. In these events, a very active jet stream brings storms and moisture across the Pacific Ocean into the U.S. west, with long bands of precipitation crossing the ocean into the U.S. West. This results in persistent and heavy rain in coastal and valley areas and snow in mountain areas. It is particularly associated with heavy rain in coastal California and heavy snow in the Sierra Nevada. A Web page from NOAA further explaining AR events can be found here. This particular AR event was a bit different from many in that it did not follow the common "pineapple express" pattern. In other words, the jet stream and moisture flow did not come all the way across the Pacific from the west or west southwest from Hawaii to the West Coast, as is the case with many AR events. Rather, the jet stream went very far north, near Alaska, then dove southeast just off the Pacific coast, then made an abrupt turn to the east-northeast over California and tracked to the upper Great Lakes and east to northern New England.

This track led to heavy snow - in some cases VERY heavy - across the Sierra Nevada and much of the Rockies, as is typical with AR events. But it also had impacts somewhat different from what you might expect with most AR events, discussed further below. There were two main low pressure systems associated with this AR event, which followed similar but not identical tracks. The first system was a relatively warm system and, once inland, followed a slightly more northerly track than the second one. As a result, this system produced rapid warmups in areas of the Midwest that had experienced record cold a few days earlier. In parts of Iowa, Illinois, and Wisconsin that had experienced temperatures of -30 and lower (with wind chills in the -50 to -60 degree range), temperatures rose by 60-70 degrees (and even more in some locations), and rain fell in places that had been far below zero a few days earlier. Although it did produce heavy mountain snows, this first system produced rain at fairly high altitudes. For example, while this first wave of this AR event did produce more than a foot of snow at Wolf Creek Ski Area in Colorado, nearby Pagosa Springs got mostly a mixture of rain and sleet, even at elevations as high as 7500 feet.

The trailing system was slightly colder and tracked slightly farther south. It was, for example, cold enough to produce a rather unusual snow event in Seattle, where schools were closed on February 4, 5, and part of the 6th. In Salt Lake City, the snow was heavy enough to close schools for the first time in 20 years. Snow and graupel (snow pellets) fell in Las Vegas, and there were even a few snowflakes in San Francisco. But the big impact of the storm was in the mountains, particularly the Sierra Nevada, where even ski areas were forced to close because of the severe conditions. Between the two main waves of energy and some snow that fell in between, California's Mammoth Mountain got 7 1/2 to 11 FEET of snow. Squaw Valley was not far behind with 5 to 7 feet, and all roads in Yosemite National Park were closed for a time due to concerns about falling trees. As shown on the map below, very heavy snow of a foot and a half or more also fell in numerous locations up and down the chain of the Rocky Mountains:

In southwest Colorado, the second wave of the AR event brought two more feet of snow to the Wolf Creek Ski Area, bringing their 4-day storm total to more than 3 feet. Not far west of there, the Purgatory ski area got more than two feet. High-valley locations like Durango and Pagosa Springs that got rain and sleet with the first wave of the AR event got snow this time, with up to 6 inches in the Durango area and around 7 inches in Pagosa Springs. A period of steady snow fell in these areas, followed by temporary clearing and then intermittent brief but intense convective snow squalls. This picture shows one of these squalls in Pagosa Springs on February 6:

The next picture, taken a few minutes later, shows the accumulation of the snow:

This radar image, from later that evening, illustrates the AR character of the event - note that precipiation is scattered all the way from Nevada, Utah and Idaho to the East Coast:

In this image, the first low pressure system is in northern New England, with the second one over Colorado. But note that precipitation is not just falling near the storm centers, but over a widespread area including some areas in between that are not near either of the storm centers. This illustrates the AR character of this event. The fact that it was fairly stormy and unsettlled between the two storm centers can also be seen in this picture that I took while skiing at Wolf Creek on February 4:

At this time, the cloud base was intermittently rising and falling above and below the 11,700 foot level where this picture was taken. This low cloud base accounts for the rime on the trees. Just over a foot of snow had fallen on the 2nd and 3rd, and the area was now between the two storm centers associated with this AR event. However, low clouds, strong wind, and intermittent snow, graupel, and drizzle continued to fall, with a half-inch or so of snow falling during the day on the 4th and 3 more inches that night. By the evening of the 5th, the second main wave of snow had arrived and that brought nearly two more feet, yielding a 4-day storm total of 37 inches at Wolf Creek. As this second wave of the storm continued east, it also produced substantial snowfall of 6 to more than 12 inches over portions of eastern Montana, the Dakotas, Minnesota, Wisconsin, and Upper Michigan, as shown on the first map above. But note that even this colder storm produced snow mainly north of a line from the Quad Cities to Chicago to Detroit to Buffalo to north of Boston. However, although the snow line was fairly far north, the storm did produced ice-storm conditions from central Missouri northward into central and eastern Iowa, northern Illinois, southern Wisconsin, and parts of Michigan. Many of these areas got ordinary rain with the first low-pressure system associated with this AR event, but by the time the second system arrived, the surface air was just enough colder to get freezing rain. Farther south, heavier rain and thunderstorms were common, especially across the Mid-South and the Southeast Coast. In addition, there were a fair number of severe weather reports, especially on February 6 and 7, when there were nearly 100 severe-weather reports over the two days. This even included a handful of tornado reports, in AR and TN on the 6th and in IN and OH on the 7th. The heavy, coast-to-coast precipitation that occurred over the 7-day duration of this AR event can be seen on this cumulative-precipitation map:

There are areas on both coasts that received more than 5 inches of precipitation over the seven days. Many areas in between received 2 to 5 inches. Snow-water equivalent amounts of over 1 inch are common in areas where the P-type was snow, and in some places in the Sierras, snow moisture exceeded ten inches.

While this AR event was subsiding from west to east by February 7, the jet stream pattern that produced it looks to remain in place for a while longer. Yet another storm is expected by the weekend, with a slightly more northerly track once inland, but also with the potential for yet another significant snowfall in the Seattle area, perhaps the largest in years. And this pattern will likely continue to produce below-normal temperatures in the west and far north, but above-normal temperatures in the east and deep south.

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