Duluth MN



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Duluth Storm Tracks

Storms that produce snow in Duluth can come from almost any direction but this author likes to classify them in four general categories. Some storms can be hybrids.
1. Clippers
2. Pacific Northwest Systems
3. Southwest Storms
4. Lake-Effect Storms

Clippers


An upper level high pressure ridge along the west coast of British Columbia, Canada forces storms north into southeast Alaska, the Yukon Territory, and northern British Columbia. The remains of the storms then dive south on the east side of the ridge through typically Alberta or Saskatchewan, into the northern United States, then turn east across the Great Lakes. Sometimes turn east sooner and stay in southern Canada. Sometimes they drop much further south into the Middle Mississippi Valley states, such as Missouri, before turning east. Even North Carolina where the author of this website grew up can get snow from clippers. Bet you lifetime northerners never figure that! Clippers are usually fast moving, small in size, and lacking moisture. They do not have to be weak.

Pacific Northwest Systems


These Pacific storms enter Washington State, Oregon, or southern British Columbia and move east across the northern half of the United States. The storms sometimes dip a little southeast then turn to the northeast, looking like southwest storms. These bread and butter systems can produce 3 to 4 inches a shot. If they slow down, amplify, and ingest more moisture or the surface low passes south of Duluth in such a way as to produce an east wind of Lake Superior, you can get amounts of 6 inches or more.

Southwest Storms


Southwest storms are well know for being responsible for producing most, but not all, major snowstorms of 8 inches or more. They originate in the southern Rockies, southern half of the plains, or the middle and lower Mississippi Valley. Many southwest storms are hybrids. Split upper level jet stream flow patterns, such as frequently occurs during El Nino affected winters, often result in these hybrids. An upper level low pressure trough in the southern branch of the jet stream combines with a Pacific Northwest system in the northern stream to form a larger stronger storm system. The proximity of the southern trough to the Gulf of Mexico allows it to ingest very high amounts of moisture. Even when widespread thunderstorms break out in the Southern Plains and Lower Mississippi Valley, enough moisture can still get far enough northward to support snowfall totals of 8 inches or more.

Lake-Effect Storms
Lake-effect storms are not really a storm track but the snow does come from an east to northeast direction since Lake Superior is obviously east of Duluth. This classification includes storms where the dominant generation of snow if off the lake rather than a synoptic (large scale) storm system. Most of these storms do however get help from weak storms such as clippers that produce snow to help moisten the air over the lake. Lake-effect snow of which a storm system's contribution is dominant or at least significant is usually called lake enhancement. The upward vertical motion associated with a clipper also helps destabilize the air. An exception would be lift is occurring due to warm advection but enough warm air moves over the lake to decrease the lapse rate and stabilize the air. A good example of lake-effect that occurred in association with a clipper-like system is the December 4th, 2007 lake effect storm.

More information
Sometimes Pacific Northwest storms or even clippers produce major snowstorms. In these cases the storms slow down and the surface low dips farther south as the upper trough amplifies. Amplification could be due to the blocking of an upper level ridge to the east. Perhaps a clipper and Pacific Northwest system are merging (phasing in) to produce a larger, more amplified and slower moving trough. The slower movement and farther south position of the surface low will allow the storm to draw more moisture from the Gulf of Mexico. Strong frontogenetical forcing accompanies some of the storms and produces very narrow bands of high snow accumulations, such as the Pacific Northwest type system on February 24th, 2006.


Terrain and Lake Influences

Duluth's location at the western tip of Lake Superior makes the city a very unique place to forecast. The lake itself, the hill which the city is built along, and high elevations along both sides of the lake cause extensive variation in snowfall both seasonally and for any given storm. Differences in seasonal snowfall vary in excess of 20 inches from the bottom of the hill to the top of the hill along and inland of the ridge crest. For a more detailed discussion of lake and terrain influences go to the Duluth Local Influences page.


Possibly the Thundersnow Capitol of the Upper Midwest

Researchers and the University of Saint Louis conducted extensive research in thundersnow climatology and forecasting. One piece of resulting literature, "A Climatology of Thundersnow Events over the Contiguous United States" (Market, Halcomb, and Ebert, 2002), presents a map of the United States showing number of thundersnow occurrences in 30 years ending in 1990. Duluth leads the pack for locations in the Middle and Upper Mississippi Valley. High frequencies also extend south into west central and southeast Wisconsin.

Now obviously many years have passed since 1990 so maximum areas of thundersnow activity may have shifted. The data on the map also rely on official 3-hourly observations and likely missed a few events on shorter time scales and between the observation locations.

In spite of the data collection challenges, Duluth still seems to justify the results. For example, Thundersnow dumped 6 inches in two hours on the evening of January 1st, 2005 at the National Weather Service office. The March 1st and 2nd blizzard, 2007 produced five hours of thundersnow with snowfall rates easily 1 to 2 inches per hour. The rates were more impressive than they seem considering the snow was dense enough to feel like walking on white sand. The April 10th to 12th, 2008 blizzard produced nearly six hours of thundersnow including cloud-to-ground lightning observed by this author. Cloud-to-ground lightning also occurred a little less than a week earlier on April 5, 2008 with mixed precipitation changing to snow to produce a quick inch of soggy accumulation.


El Niño and La Niña influences on Duluth seasonal snowfall

There is some degree of correlation between La Niña and snowfall for Duluth but it is more a matter of distribution during the season. The following image is from the National Weather Service's Climate Prediction Center (CPC).



The top image shows mean seasonal snowfall for neutral El Niño-Southern Oscillation (ENSO) seasons including weak El Niño and weak La Niña episodes. The lower-left image shows departures from neutral seasons for El Niño-influenced seasons. The lower-right image shows departures from neutral for La Niña-influenced seasons. Please note this not the same as comparing to mean seasonal snowfall for all seasons El Niño, La Niña, and neutral combined. There appears to be a little more correlation with La Niña to getting more snow than with El Niño to getting either more or less snow.

Important note here. The data does not include April. Duluth easily gets snow in April, sometimes major storms. Some examples include 12.2 inches at NWS Duluth on April 3 to 4, 2007, 9.9 inches at NWS Duluth on April 10 to 12, 2008, and 9.4 April 3 to 4, 2014. Then there is the great snow blitz of April, 2013 when 50.8 inches fell in 19 days. The positive anomaly for snow during La Niña is strongest in late March and in April based upon examination of data kept by NWS Duluth through 2006, the year that this author left work at that office.

Since that time, three additional La Niña episodes occurred and a 4th is in progress for Winter 2016-1017 at the time of this writing. Relative to the snowfall mean for ALL seasons ( ), Duluth was a few inches above the mean for one year, a few inches below for another, and well below the mean for the third.

The extent of influence of any climate pattern depends on it's strength, persistence, and interaction with other patterns. A weak to low end moderate La Niña developed for Winter 2011-2012. Snowfall for that season was only around 50 inches, give or take a few inches based on location in the local area. A dominant positive phase of the Arctic Oscillation (especially November through mid January) appears to have combined with La Niña to keep the jet stream flow too far north. A more detailed discussion of what happened and why it happened is being written.


Recent Major Snowstorms Near or Exceeding 20 inches

23.3 inches on December 2 to 5, 2013 (near 72 hour event)

Blizzard 24.5 inches on December 24 to 26, 2009

Blizzard 19.6 inches with 5 hours of thundersnow and wind gusts over 60 mph near lake shore on March 1 to 2, 2007

27.1 inches on January 25 to 27, 2004

 

Climate Information Links

Climate Data for Minnesota - Minnesota State Climatology Office
Local Climatological Data - National Weather Service, Duluth, MN


References

Market, P. S., C. E. Halcomb, and R. L. Ebert, 2002: A Climatology of Thundersnow Events over the Contiguous United States. Wea. Forecasting, 17, 12901295.