The 1991 Halloween blizzard was a powerful storm that caused a period of heavy snowfall and ice accumulation, which affected parts of the Upper Midwest of the United States, from October 31 to November 4, 1991. Over the last week of October 1991, a large storm system over the Atlantic Ocean (1991 Perfect Storm) blocked most of the weather patterns over the eastern half of the United States, and in turn, moisture from the Gulf of Mexico was funneled straight northward over the affected region. By the time the precipitation stopped falling, many cities in the eastern half of Minnesota and northwestern Wisconsin had witnessed record early-season snowfall accumulations, while parts of southern Minnesota and northern Iowa were crippled by a large ice storm. Arctic air that was pulled southward behind the storm had combined with the heavy snow pack to produce many record low temperatures. Between the blizzard and the ice storm, 22 people were killed and over 100 were injured.
Meteorological synopsis
The 1991 Halloween Blizzard developed from a strong arctic cold front that pushed south and east through the central United States several days prior. On October 28, temperatures to the east of the cold front were above normal. High temperatures reached into the 70s from the middle Mississippi River Valley south into northern Texas, and into the 80s across much of central and southern Texas. Meanwhile, high temperatures remained below Fahrenheit across most of Montana and Wyoming.
The contrast between the two air masses was stark, and by the morning of October 29 the cold front was about halfway through Texas. At 6 am CST, the temperature in the western Texas city of Amarillo had dropped to with a strong northerly breeze. Farther east in Texas, the temperature was 64 in Dallas – a 42-degree temperature difference over about . In the northern United States, morning lows were much colder. Temperatures were in the single digits across Montana and Wyoming and in the teens in North Dakota and South Dakota.
By October 30, the cold front had pushed east to the Texas shoreline with the Gulf of Mexico, and stalled in that location. As an upper-level shortwave trough approached the Southern Plains, it aided the development of an area of surface low pressure along the sharp temperature gradient near the Texas Gulf Coast. The development of low-pressure systems along coastal fronts in this fashion is relatively common in the cool season along the Texas Gulf Coast and along the Atlantic Seaboard near the Gulf Stream current.
From October 30 – 31, this low-pressure system slowly became better organized over Texas, before it ejected north over the Mississippi River Valley. This meridional trajectory of a low pressure track (almost due north from the western Gulf) is climatologically favorable to produce very heavy snowfall in the winter months because it allows copious amounts of moisture to flow north where it can interact with colder air. Cooler readings lingered at the very end of October across the Upper Midwest, and a re-enforcing pool of Arctic air was just beginning to push southeast through the western Canadian Provinces.
On November 1, the low-pressure system moved north from western Illinois into the Upper Peninsula of Michigan and the minimum pressure fell about 24 millibars in 24 hours, indicative of rapid deepening and strengthening of the cyclone. The low pressure eventually became occluded, weakened, and then continued to dissipate as it pushed east across northern Ontario in subsequent days.
Precipitation
The precipitation from this system began falling as snow in Iowa late on October 30, and by the morning of October 31, it had changed over to ice and spread northward across southeast Minnesota. During Halloween day, the precipitation spread to the north, where it fell mostly as rapidly accumulating snow. The snow continued to develop northward, eventually spreading into the Minnesota Arrowhead. Eventually all of the precipitation changed over to snow, and in some areas it would continue falling until November 4.
Snow
The heavy snow began falling across east central Minnesota and northwest Wisconsin during the late afternoon hours of October 31. By midnight, the Twin Cities had already recorded 8.2 in (20.8 cm) of snow. This not only set a record for the largest amount of snow on that date, but also for the most snow ever recorded in the Twin Cities during the month of October. As the storm system pushed its way northward, more communities in eastern Minnesota and northwestern Wisconsin began to experience similar conditions; and thundersnow occurred as far north as Duluth.