Weather and Climate Playmakers (Blog 2)

Duluth's Weather and Climate Playmakers: Air Masses, Fronts, and Unique Factors

• Air Masses

The air mass which has the greatest effect on the weather and climate of Duluth, MN is cool, dry Contintental Polar air that is present year-round.  The much colder and very dry Continental Arctic (cA) air mass often pushes southward into Duluth during the winter, however, the mass quickly receeds due to summertime insolation of the northern latitudes.

Duluth lies well within the exent of the cP air mass which brings cool air that mollifies
the summer heat; a buldging of the the cA in the winter months brings bitter cold.

Occasionally, summer heatwaves can occur when warm Marine Tropical (mT) and Continetal Tropical (cT) airmasses push northward into the region.  A late season push of mT air has the potential for early snowstorms as was seen in 1991 (See figure below).

The Halloween Blizzard of 1991.  A warm, moist mT front swept north from the
Gulf of Mexico that pounded Duluth with 36.9" of snow.

• Fronts and Mid-Latitude Cyclones

Although mid-latitude cyclones often mature further south, Duluth is still affected by these cyclones.  The diagram below shows Duluth in the center of the the occluded low pressure, just at the tail of the cold occlusion.

This low pressure "land-bomb" late October, 2010 was touted as stronger
than the historic storm that took the Edmund Fitzgerald.

Mid-latitude cyclone with occlusion and low pressure system near Duluth.

The "Current Fronts" map shown below indicates a cold front moving towards Duluth. With no warm front visible in its path, it is unlikelythat an occluded front will form and spawn a mid-latitude cyclone. This will likely push through bringing cooler temperatures in the next few days.

Weather Underground Front Map shows advancing cold front.

• Unique Factors

In addition to the topographic and other factors introduced in the previous blog, there are several others worth mentioning: 

 -Ice Storms:  Related to the topography, the lake provides frequent, near-freezing but moist easterly winds that when forced to rise over Duluth's topographic barrier drop below freezing.  Advection of cold air from the lake bolsters the process and the city freezes over.

Iced branches near Lake Superior's north shore

-Alberta Clippers: Fast-moving low pressure systems that form in the Canadian Rockies then cut across the upper Midwest.  The clippers bring bitter cold to Duluth and drive engine block heater sales.

"Exact Storm Track Uncertain;" clippers are small and fast which complicates forecasting

-Colder by the Lake:  As stated in the previous blog, Lake Superior helps keep Duluth cool in the summer, but when the head of the lake freezes in winter cold air sinks to lower elveation and pools at lake level.

During the winter, locations on Duluth's western ridge are often several
 degress warmer than those along the shore where cold air is occluded from interaction with the
warmer water by a layer of ice and snow.

-Thundersnow Captial of Upper Midwest?: Cloud to ground lightning alongside snowfall can be seen in Duluth on rare but instances; on Jan. 1, 2005 the national Weather Service recorded 6" of snow in two hours.  Over data ranging from 1960-90, the Duluth region led the Middle and Upper Mississippi Valley in frequency of thundersnow storms.



Sources:

"Air Masses."  http://www.srh.noaa.gov/jetstream/synoptic/airmass.htm.

"An Immaculate Mid-Latitude Cyclone and its Decay." http://www.washingtonpost.com/blogs/capital-weather-gang/post/an-immaculate-mid-latitude-cyclone-and-its-decay/2011/09/27/gIQAEaTt1K_blog.html.

"Duluth (Orographic City) Local Influences." http://snincr.net/duluth_local_effects/duluth_local_effects.php

"Duluth Storm Tracks." http://snincr.net/winter_profiles/DuluthMN.html.
"General Climate Description of Minnesota." http://climate.umn.edu/doc/online_resources.htm

"20th Anniversary of the Halloween Blizzard."  http://climate.umn.edu/doc/journal/halloween_blizzard_20th.htm.

 "Weather and Climate - Cyclone Image." http://www.mrsciguy.com/weather.html.