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Steering the Storms – Split and Unified Jet Streams

December 3rd, 2016

Much of our winter snowfall occurs with weather systems originating over the Pacific Ocean.  The path and intensity of these systems is determined by the upper level winds steering the storms.   Two primary configurations are seen with split or unified jet streams, and both of these primary setups have many possible variations.

Big Snowmakers

The strongest and most moisture-laden systems for the north-central states move into the southwest US before swinging northeastward.  This can occur most readily when there is a single unified jet stream producing a deep low pressure trough.   In this situation, moist air from the Gulf of Mexico can be in close proximity to true Arctic air creating very turbulent weather and rapid changes from spring-like to winter weather.   We had a taste of this type of pattern with the powerful storm system on November 27th and 28th.  One thing missing was abundant Arctic air (as the true Arctic branch of the jet stream was way up over northern Canada – see purple arrows) which allowed the low pressure center to track from Nebraska into the eastern Dakotas.  As a result, Minnesota and Iowa saw mainly rain with a smaller area of heavy snow centered over North Dakota.

Upper-level winds 11-28-2016

This is the upper-level wind configuration from the morning of November 28th — when low pressure was over the eastern Dakotas — and the resulting weather. (Map from Unisys Weather.)




 Less Snowy Patterns

Alternative patterns tend to occur often during El Nino winters, featuring a split jet stream.  The two branches of the jet stream are not necessarily weak but they remain “out of phase”.  In many cases, a southern branch of the upper winds flowing across the southern US will keep storm systems near the Gulf Coast.  This is why El Nino winters are often wetter and cooler/cloudier in the south.  At the same time, a northern branch of the jet stream will flow across southern Canada and keep Arctic air locked up farther north.  I have seen the northern Plains left literally high and dry for weeks on end during the winter months with this type of pattern.   A subtle variant of this split pattern is expected to keep the risk of heavy snow away from Minnesota during the coming week.  We will not be spared from bitter cold, however, as the northern branch of the jet stream shifts to flow more from northwest to southeast (see map below).  The high pressure system seen over northwest Canada has literally crossed over the Arctic Circle and is a chunk of the extreme cold which was over northern Russia during November.

Weather pattern for December 8th, 2016.

The pattern shown here (as forecast for Thursday, December 8th) will bring much of the northern and central United States the coldest temperatures since February. (Map from Unisys Weather.)



Coming Soon to Your Backyard

Keep informed of the snowfall forecast in the days leading up to the cold outbreak.  It is HARD for temperatures to stay below normal for any length of time in December, January, or February unless we also have a normal (white) groundcover of snow.   The very limited snow cover in Minnesota as of December 3rd (see map below from the National Operational Hydrologic Remote Sensing Center) could help limit the intensity of the cold air over the coming week.  Little to no new snowfall this week would allow lakes to freeze solidly and quickly.  Occasionally, a bitterly cold outbreak occurring with little insulating snow cover will produce greater risk of frozen underground pipes, though this would be out of character with often-snowy La Nina events.  I expect this winter pattern to bring more abundant snowfall to Minnesota and Iowa within seven to ten days!

Snow cover as of December 3, 2016.

A weekend weather system will boost the very limited snow cover in Wisconsin and far eastern parts of Minnesota and Iowa.







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