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Duluth, MN

* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database

Description of Duluth

Duluth is located at the narrow western tip of Lake Superior and sits along a ridge that extends along the north shore of the lake. Elevation increases abruptly from the lakeshore to a few miles inland, roughly 600 to a little over 800 feet. Duluth's terrain and proximity to Lake Superior cause large differences in snowfall within the city. Snow is enhanced when winds blowing into the hill force air to rise, cool, and release additional moisture. Snow can also be enhanced when winds blow onshore from the lake and the air over the lake is cold and unstable. The eastern half of Duluth is more susceptible to lake enhanced snow since a wider range of wind direction can produce a long fetch over the lake.

Seasonal snowfall varies greatly within the city and it's surrounding communities. The mean (average) seasonal snowfall at the National Weather Service Forecast Office in Duluth near the Duluth International Airport is 90.2 inches based on records for the 30-year period of 1991 to 2020. The previous 30-year average for the period of 1981 to 2020 was 86.1 inches. The National Centers for Environmental Information updates climatological averages every 10 years by dropping the oldest 10 years and adding the most recent 10 years. Please note that the office was moved a very short distance in 1996 from the airport to its current location along Miller Trunk Highway so some of the measurements included in the average are from the airport location. Snowfall averages which include periods of record longer than 30 years, but do not include measurements taken prior to the office being moved from downtown Duluth to the airport in 1950, are also near or above 80 inches. The National Weather Service location gets more snow than most of the city of Duluth. The office is at a higher elevation and is farther away from the warming influence of Lake Superior but is still close enough to get the added moisture. For more information on Duluth's weather station history, go to the National Weather Service's office history page.

After 19 or more years of making personal observations, examining snowfall reports given to the National Weather Service, and talking to people who live in the area, seasonal snowfall appears to be 20 to 30 inches greater on top of the hill than at the bottom near the lakefront. The lower elevations of the far western communities such as Gary-New Duluth get around 60 inches while the higher areas several miles inland get from 85 inches to around 90 inches.

Duluth snowstorm tracks

Storms that produce snow in Duluth can come from almost any direction. This author likes to classify them into five general categories based on the track of the primary surface low pressure system. Storms can be hybrids of the categories listed. Storms of different categories can merge to produce a stronger storm.
1. Clippers
2. Pacific Northwest Storms
3. Southwest Storms
4. Lake-Effect Storms
5. Back-Door Storms

Clippers


The presence of an upper-level high pressure ridge over the west coast of the Canadian province of British Columbia forces storms north into southeast Alaska, the Yukon Territory, and northern British Columbia. The remains of the storms then dive to the southeast through the Canadian provinces of Alberta or Saskatchewan and then into the northern United States where they 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 or Iowa, before turning east. Clippers are usually fast-moving weather systems that lack access to moisture even if they are fairly strong.

Pacific Northwest storms


Pacific Northwest storms are those that enter Washington State or Oregon and sometimes southern British Columbia and northern California. They move east across the northern half of the United States and typically produce 3 to 5 inches in Duluth as they pass through. The storms sometimes amplify and slow down, allowing them to ingest more moisture, then produce snowfalls of 6 inches or more.

Southwest storms


Strong southwest storms produce the majority of larger snowstorms in Duluth of 8 inches or more. They originate in the southern Rockies, the southern half of the Plains, or the middle to lower Mississippi Valley. Many southwest storms are hybrids where an upper trough in a southern branch of the jet stream phases in (that is combines) with an upper trough in a northern branch to create a larger, stronger storm with access to both cold air from the north and moisture from the Gulf of Mexico to the south.

Southwest storm blocking patterns
Concerning snow producing storms that approach from the southwest, a northern branch of the jet stream can dip far enough south to block a southern storm system from moving far enough north to spread snow into the Duluth, MN and Superior, WI area. One particularly annoying blocking pattern is when a trough embedded in the northern jet sits over Hudson Bay with northwest wind flow aimed into the Western Great Lakes on the west side of the trough. In this case, the snow-producing southern storm system is deflected or sheared off to the east. Sometimes if the southern storm is moving slow enough, the northern jet stream trough will pass to the north and only temporarily suppress snow to the south. The southern storm and the snow it produces will then have a chance to slip northeast behind the exiting northern jet stream trough. Also, if another trough in the northern jet is approaching from the west, that trough can draw the southern storm to the northeast.

The April 13 to 16, 2018 snowstorm that hit Minnesota and Wisconsin hard with widespread 12 to 24 inches of snowfall accumulation is a case where the scenario just described played out. The onset of continuous accumulating snow in Duluth, MN, and Superior, WI was initially held to the south by a northern jet stream upper-level trough north of the Great Lakes. As the trough moved to the east, the next trough in the northern jet sank southeast into the West Coast of the United States. That next trough appeared to help push the southern storm to the northeast in the break between the two northern jet stream troughs. A summary of the April 13 to 16, 2018 snowstorm includes more details.

Lake-effect storms
Lake-effect storms are not really a storm track. A dominantly lake-effect storm is one where the main production of snow is from the lake rather than a larger scale (synoptic scale) storm system which could also be producing snow at the same time. Snow from synoptic-scale storms can help moisten the air over the lake so that less dry air has to be overcome before clouds and snow can form. Snow from a synoptic-scale storm can also seed clouds that already exist over the lake. When a synoptic-scale storm is the primary snow producer, snow from the lake is usually referred to as lake enhancement rather than lake-effect.

Back-Door Storms


This term is borrowed from the concept of the back-door cold front. A back-door cold front is one that comes from the east or northeast. Back door storms are those that would approach from any east component direction, but usually from the southeast. Back-door storms tend to be hybrids of southwest storms. They have a strong left turn that causes their track to bend west of due north as they move into the western part of the Great Lakes region. A snowstorm that dumped heavy snow on Duluth the day before Christmas in December 2007 is an example.

More information
Sometimes Pacific Northwest storms and even clippers produce major snowstorms. In these cases, the upper trough embedded in the jet stream amplifies and slows down. The surface low pressure center intensifies or may redevelop farther south then intensify as it moves northeast. Amplification of the trough could result from being blocked by a strong high pressure ridge to the east. The slower movement allows the storm more time to ingest moisture and increases the duration of the snowfall.

Possibly the thundersnow capital 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 the number of thundersnow occurrences in 30 years ending in 1990. The map shows Duluth with one of the highest occurrences of thundersnow. The area of high frequency covers the Texas Panhandle, eastern Colorado, western Kansas, and western Nebraska. The area then extends northeast into Wisconsin and northeast Minnesota. Many years have passed since 1990 so maximum areas of observed thundersnow may have shifted.

In spite of any data collection challenges and elapsed time since observations used for the research were taken, personal observations of thundersnow in Duluth seem to justify the results. For example, Thundersnow dumped 5 inches of snow in two hours on the evening of January 1, 2005. The National Weather Service Forecast Office in Duluth measured 6 inches in the same two hour period. The March 1 to March 2 blizzard in 2007 produced five hours of occasional thunder with snowfall rates 1 to 2 inches per hour. The rates were more impressive than they seem considering the snow was so dense it was like walking on sand. The April 10 to April 12 blizzard in 2008 produced nearly six hours of thundersnow including cloud-to-ground lightning observed by this author. Cloud-to-ground lightning was also observed a little less than a week earlier on April 5, 2008. The lightning occurred as mixed precipitation changed to snow to produce a quick inch of soggy accumulation.

The following list shows all personal observations of thundersnow in the period from Winter 2001-2002 to Winter 2019-2020. Notice that the vast majority of the events are late in the season, March to early May.
March 13, 2004   (snow and snow pellet mix)
January 1, 2005   (5 inches in two hours)
March 1, 2007
April 5, 2008   (cloud-to-ground lightning observed)
April 11, 2008   (cloud-to-ground lightning observed)
November 13, 2010
April 15, 2011
April 11, 2013
April 14, 2013
February 20, 2014
November 18, 2016   (sleet, then snow-sleet mix, then snow)
May 8, 2019
March 28, 2020

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 may be 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. 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. Yellow/orange to red indicate negative departures. Blue to violet indicate positive departures. 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.

An important note here. The data does not include April. Duluth easily gets snow in April, sometimes major storms. Some examples from the National Weather Service in Duluth include 12.2 inches on April 3 to 4, 2007, 9.9 inches on April 10 to 12, 2008, 9.4 inches on April 3 to 4, 2014, and 13.4 inches on April 15 to 16, 2018. 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 some data kept by the National Weather Service in Duluth at least as of 2006, the year that this author left work at that office. Since that time, additional La Niña episodes have occurred.

The extent of influence of any climate pattern depends on its strength, persistence, and interaction with other patterns. A high end 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 the middle of January) resulted in a strong jet and associated storm track staying too far north. The Climate Patterns discussion addresses the Arctic Oscillation's effect on the jet stream in more detail.

Recent major snowstorms near or exceeding 20 inches based on measurements at the National Weather Service in Duluth

21.7 inches including blizzard conditions on November 30 to December 1, 2019

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

24.5 inches including blizzard conditions on December 24 to 26, 2009

19.6 inches including blizzard conditions and thundersnow on March 1 to 2, 2007

27.1 inches on January 25 to 27, 2004

Personal measurements for the same storms in the same order are 20.0, 28.0, 22.0, 19.0, and 21.8.

Snow amount frequencies

The seasonal frequency of storm snowfall totals of 8 or more inches, 10 or more inches, and 12 or more inches are calculated for the winter seasons of 2001-2002 through 2021-2022. The first set of numbers is the total number of occurrences for each category over the twenty season period. The second set of numbers is the frequency of each category. Data is given for two locations. On location is the National Weather Service at Duluth (DLH). The other location is specified as DAK for personal measurements taken near the ridge crest of Central Duluth.

A lot of major snowstorms are prolonged events where breaks in snowfall or changes back and forth between precipitation types can occur within the context of the same storm system. Sometimes several storms combine to produce an extended continuous snowfall. Add in some lake-effect/lake enhanced snow and the scenario is even more complicated. The National Weather Service for each storm usually issues text products listing snow totals reported from various sources. The statements help determine which DAK measurements to include for each storm.

Important to note is that the snow amounts used for the calculations were not rounded to the nearest inch. Call it a personal preference. The number 9.6 is not 10. The number 10.0 is 10.

Total number of times each snow amount threshold was reached or exceeded (Winter 2001-2002 through Winter 2021-2022)
DLH    8 inches or more - 46, 10 inches or more - 29, 12 inches or more - 16
DAK    8 inches or more - 39, 10 inches or more - 27, 12 inches or more - 15

Seasonal frequency of each threshold being reached or exceeded (Winter 2001-2002 through Winter 2021-2022)
DLH    8 inches or more - 2.19, 10 inches or more - 1.38, 12 inches or more - 0.76
DAK    8 inches or more - 1.86, 10 inches or more - 1.28, 12 inches or more - 0.71

Climate information references

Weather and Science Facts - Current Results Publishing Ltd.
Climate Data for Minnesota - Minnesota State Climatology Office
Local Climatological Data - National Weather Service Forecast Office, Duluth, MN
Snow Reports - National Weather Service Forecast Office, Duluth, MN

Research article reference for thundersnow

Market, P. S., C. E. Halcomb, and R. L. Ebert, 2002: A Climatology of Thundersnow Events over the Contiguous United States. Wea. Forecasting, 17, 1290-1295.

Reference for El Niño-Southern Oscillation

Climate Prediction Center (CPC) of the National Weather Service