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…
The descriptions on your blog are amazing. Sabrina
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