Sunday, November 16, 2014

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How a Storm Named Nuri Froze the U.S.

This past week has been highlighted by extremely cold temperatures for the majority of the eastern two-thirds of the country and the Pacific NW. Cold air outbreaks east of the Rockies are typical in November, but this year’s is particularly unique.  Record low highs and lows have been shattered from Montana to Texas and the cold continues to hang around with more records likely to be broken in the Eastern Gulf States midweek.  

So what makes this year’s cold air outbreak so unique and long lived?  Well in this case it all stemmed from super typhoon Nuri.  Super typhoon Nuri developed east of the Philippines and quickly exploded to a category 5 storm with a max wind speed of 180 mph.  At one point Nuri was forecasted to exceed the strength of Haiyan, the storm that devastated the Philippines last year. Instead, Nuri quickly weakened as it tracked northward and lost its tropical cyclone designation east of Tokyo on November 7th.  

The cyclone continued northward and began to phase with the jet stream... Sound familiar?  Superstorm Sandy should come to mind.  With the support of the jet stream, the cyclone underwent “explosive cyclogenesis,” or deepened rapidly, becoming one of the strongest storms ever recorded in the Bering Sea.  The storm track and evolution of Nuri can be seen in Figure 1.

Figure 1: Nuri’s storm track, storm type and recorded wind speeds.

Although Nuri and the ensuing extratropical cyclone was primarily a storm for the fishes, the amount of warm moist air that the storm transported into the Bering Sea was exceptional.  Figure 2 shows the 500 mb (the height in the atmosphere that controls the majority of the weather over the mid-latitudes) state of the atmosphere over the Northern Hemisphere.  The blue box shows the same area for each 24 hour time step. 

Figure 2: 500 mb state of the northern hemisphere

 In panel a. the extratropical cyclone can be seen entering the Bering Sea at the western edge of the box as the circles.  The purple shading below the cyclone represents the wind which is acting to transport warm air (green shading) from the Central Pacific into and over Alaska. At time step b., the cyclone is beginning to weaken (fewer circles) and so is the northward transport of warm air (less purple shading).  Even with the weakening storm and decreasing transport, a huge increase in green shading can be observed over Northwest Canada.  

The warm air continues eastward and expands all the way to the western shores of the Hudson Bay, as can be seen in panel d. This huge influx of warm air displaces the notoriously cold air of the Arctic Circle and drives it southward on a crash course with the Central U.S.   The cold air can be seen overtaking the Northern Great Plains in panel d. as the dark blue shading.  Figure 3 shows the departure from normal temperatures observed over the U.S at the same time as panel d.

Figure 3. Note: the labeled contours represent the true departure of high temperature from normal as the color fill and corresponding color bar does not get negative enough. Source: CPC 

Some areas experienced high temperatures as much as 38 degrees below normal as can be seen in Figure 3. That is extremely rare but not unheard of. Since records began in the mid to late 1800s there have only been two other cold air outbreaks of this magnitude in November, which occurred in 1880 and 1976. If extreme cold air outbreaks in November are any indication of Sierra snowfall, we are in for a dismal snow year.  The winters of 1881 and 1977 were the two lowest on record…

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