Several Air Masses can affect this area but most behave or have been modified to behave with Continental characteristics

Summer Air Mass Associations

·         Maritime Tropic(mT)- strengthened by the sub-tropic lows

·         Continental Polar(cP)- weaker than in winter but still causes frontal wedging in summer months

·         Maritime Polar(mP)- from the west coast, but orographic lift over the rocky mountains causes most moisture to be lost before it reaches North Dakota

Winter Air Mass Associations

·         Continental Polar- as the polar highs strengthen in winter this air mass dominates

·         Continental Arctic(cA)- this can bring subzero temperatures

·         Maritime Tropic- as fronts collide cyclones can be created bring warm fronts to the north

Pidwirny, M. (2006). Original Publication "Air Masses and Frontal Transitional Zones". Fundamentals of Physical Geography, 2nd Edition. Accessed via online post 10/01/12 http://www.physicalgeography.net/fundamentals/7r.html

Frontal air mass collisions regularly take place south of North Dakota as continental polar highs and maritime tropical lows seasonally weaken and strengthen. The high latitude of Dickinson, ND makes the continental polar air mass the most influential air mass association and as a result this area usually experiences extended periods of cold fronts.  This causes the typical cold and dry winters; however severe blizzards occur periodically as low pressure systems build to the south near Colorado or to the north near Alberta, Canada (Black, 38pg).  These mid-latitude cyclones can bring occluded warm fronts creating short durations of high precipitation. 

1Example of mid-latitude cyclone developing south of North Dakota that will eventually bring a warm front, Map Courtesy of meted.ucar.edu, Accessed 11/10/12

As the sub-tropic lows of the Gulf of Mexico strengthen in summer, warm maritime tropical fronts with moist air reach far enough north to cause the summers of Dickinson to be reported as semi-humid (currentresults.com).  While the sub-tropic highs have strengthened, the oscillating rossby waves periodically bring the continental polar air mass further south.  This wedges the warmer air upward and causes brief but severe thunderstorms with heavy precipitation.

2 A 1998 winter mid-latitude cyclone caused severe storms across the United States, this diagram shows the initiation of the cyclone.  Notice how Dickinson, indicated with the yellow circle, sits in the cold front.  From http://www.cmmap.org/scienceEd/summercourse/summerCourse11/docs/KateThursdayPM.pdf Accessed 11/14/12

This diagram shows the progression of the cyclone, and shows how the warm tropical air can be skewed to the north.  Dickinson still sits in the cold front but moist tropical air is being brought into this cold area, carrying heavy amounts of precipitation. http://www.cmmap.org/scienceEd/summercourse/summerCourse11/docs/KateThursdayPM.pdf Accessed 11/14/12

                                                                                                                             

This is a broader example of a high pressure cold front sweeping across the western United States, eventually occluding with a warm front.  Dickinson is again sitting in the cold front of this occlusion.  www.weatherchannel.com   

References

·         Black, R. E. 1971. A Synoptic Climatology of Blizzards on the North-

     Central Plains of the United States. NOAA Tech. Memo. NWS CR-39.

     38 p.