February 1-2, 2011 Blizzard Summary
Blizzard Overview - Jim AllsoppNorthern Illinois and northwest Indiana were walloped by one of the most powerful winter storms in history on February 1 and 2, 2011. Snowfall totaled 20.2 inches officially at Chicago’s O’Hare Airport, making it the third largest snowstorm on record. The total at Rockford was 14.3 inches, which is fourth on the all time list. The snowstorm was accompanied by fierce winds, gusting to 50 to 60 mph. The intense winds and heavy snow reduced visibility to near zero at times and produced widespread snow drifts of 2 to 5 feet, and a few drifts of 10 feet or more. The storm was powerful enough to generate vigorous updrafts, resulting in lightning, thunder, and small hail.
Snowfall
Lake effect snow began over parts of northeast Illinois during the morning of February 1. Meanwhile heavy snow from the storm was spreading into central Illinois from the south during the late morning. By evening rush hour, snow had overspread much of northern Illinois and northwest Indiana. Snow diminished over most areas during the late night and early morning hours of February 2, but a band of lake effect snow continued over the Chicago metro area into mid morning, and swung across northwest Indiana during the early to mid afternoon. Snowfall totals were generally 6 to 12 inches south of a line from Gibson City to Rensselaer, but one to two feet over most of northern Illinois and northwest Indiana. The blowing and drifting made snowfall measurement very challenging, even for the most experienced weather observers. In addition, the convective nature of the snow resulted in some variability of the snowfall intensity. Lake effect and lake enhancement generally made snowfall totals highest in Lake, McHenry, Cook, and DuPage Counties in Illinois, as well as Lake and Porter Counties in northwest Indiana.
map of snowfall over northern Illinois and northwest Indiana
Here is a list of snowfall totals through 4 pm February 2. Most observers report 24 hour snowfall at around 700 AM. Some spotters and observers provide additional reports through the day. The snowfall listed here is the total through the end of the event. Additional totals will be added to this news story Thursday morning.
ALSIP 5NNE (COOK)............................22.5
ELK GROVE VILLAGE (COOK).....................22.0
WESTCHESTER (COOK)...........................22.0
DOWNERS GROVE (DUPAGE).......................21.1
CHICAGO MIDWAY 3SW (COOK)....................20.9
CHICAGO OHARE (COOK).........................20.2
NORTH AURORA 2NE (KANE)......................19.0
DEKALB (DEKALB)..............................18.7
OAK BROOK (DUPAGE)...........................18.3
WORTH (COOK).................................18.1
ROCKFORD 4NW (WINNEBAGO).....................17.2
PEOTONE (WILL)...............................16.1
POLO (OGLE)..................................14.1
Wind
Winds gusted to 45 mph to over 60 mph during the evening of February 1. The strongest winds were along the Lake Michigan shore. Here is a list of peak wind gusts.
CHICAGO LAKEFRONT 70
BURNS HARBOR 67
WAUKEGAN HARBOR 63
PONTIAC 61
CHICAGO OHARE 61
AURORA 59
ROMEOVILLE 59
CHICAGO MIDWAY 58
WEST CHICAGO 54
LASALLE/PERU 53
WAUKEGAN 53
CALUMET 53
JOLIET 52
DEKALB 52
ROCHELLE 49
KANKAKEE 49
WHEELING 49
ROCKFORD 48
LANSING 47
MICHIGAN CITY 46
MIDEWIN 44
STERLING 43
VALPARAISO 43
MORRIS 43
Records
Snowfall records for Chicago date back to 1886. O’Hare Airport is the official observing site for Chicago. The 20.2 inches that fell with this storm is the third largest snowfall in Chicago history. It was the biggest snowstorm ever in the month of February. Here is a list of the top ten snows in Chicago;
1. 23.0 inches Jan 26-27, 1967
2. 21.6 inches Jan 1-3, 1999
3. 20.2 inches Feb 1-2, 2011
4. 19.2 inches Mar 25-26, 1930
5. 18.8 inches Jan 13-14, 1979
6. 16.2 inches Mar 7-8, 1931
7. 15.0 inches Dec 17-20, 1929
8. 14.9 inches Jan 30, 1939
9. 14.9 inches Jan 6-7, 1918
10. 14.3 inches Mar 25-26, 1970
Snowfall records for Rockford go back to 1906. The Chicago-Rockford International Airport is the official observing site for Rockford. The 14.3 inches that fell at Rockford was the fourth largest snowstorm in history, and the biggest ever in the month of February. Here is a list of top 10 snowstorms for Rockford;
1. 16.3 inches on January 6-7, 1918
2. 16.0 inches on March 30–31, 1926
3. 15.0 inches on March 21–22, 1932
4. 14.3 inches on February 1-2, 2011
5. 13.8 inches on March 1–2, 1948
6. 12.9 inches on December 11–13, 1909
7. 12.5 inches on February 10–11, 1944
8. 12.3 inches on January 11–14, 1979
9. 12.0 inches on January 17–19, 1943
10. 11.5 inches on January 14–15, 1943
Meteorology - Stephen Rodriguez
This very complex and strong system which brought blizzard conditions across a large portion of the nation’s midsection on Tuesday, February 1, 2011, began to first show up in computer models by the middle part of last week. Although at that time, model guidance was widely variable in terms of placement and timing of this system. It wasn’t until this past weekend when guidance began to converge on a solution - one which would bring heavy snow, strong winds, and crippling conditions to northern Illinois and northwest Indiana. This would be in contrast to what has been observed so far this winter. Although the area has observed accumulating snowfall this winter, what had not been observed in some time is a storm moving in from the southwest - an orientation that is capable of ingesting copious amounts of moisture into the storm and provide widespread heavy amounts of snowfall.
Late Monday night into early Tuesday morning, an upper level system and associated surface low began to eject out of the southern Plains. As this occurred, several different types of winter precipitation begin to overspread portions of the mid Mississippi valley. Despite some lake effect showers which developed early in the day Tuesday, the main area of accumulating snow did not reach the northern portions of Illinois and Indiana until after noon. At this time, the area of low pressure had reached southern Missouri and southern Illinois while strengthening. A large area of moderate to heavy snow just north of the strong low pressure system overspread much of the area. It’s during this time when this system furthered intensified as an upper level trough began to take on more of a negative tilt with strong pressure falls and rises being observed at the surface. Several mesocale, or small scale, factors were occurring which helped bring more widespread intense snowfall to the area between the hours of 6PM Tuesday and 12AM Wednesday. With strong low pressure moving through east central Illinois, the deformation axis or snow band pivoted northwest over northern Illinois. This deformation axis provided snowfall rates of 1 to 2 inches per hour across the area for several hours before it began to shift out of the region. Mid and upper level forcing was greatest early in the evening as the upper level trough progressed northeast across the region. Then low and mid level forcing continued later in the evening as strong mid level frontogenitical forcing was observed for several hours.
By late Tuesday and early Wednesday morning, the system continued to quickly exit off of to the east. As this occurred, wrap around moisture shifted east across northern Illinois and northwest Indiana, helping to maintain a continuous light to moderate snowfall. It was also during this time that upper level flow began to shift such that this system snow transitioned more to lake effect snowfall. This next radar image indicates this with a band of lake effect snow originating from northern Lake Michigan southwest into southern Wisconsin and northeast Illinois. Also on this image are surface observations along Lake Michigan. There are two things to note with these observations. First, was the strong wind speeds, with gusts up to 50 MPH being observed. The second is with the orientation of these observations. Instability over the lake as well as a long fetch are essential for lake effect snow development. Another thing that helps more intense bands of snow to develop is surface convergence. Notice that several areas along the western shores of Lake Michigan have wind barbs which come together over eastern Wisconsin and Illinois. This is an indication that strong surface convergence is occurring. This strong convergence aided with this last area of more intense snow to fall over northeast Illinois through the middle part of the day Wednesday, and then eventually into northwest Indiana late in the day on Wednesday as this all shifted to the east.
Lightning
Although lightning is not something you think of when discussing winter weather, it was observed all across the region on Tuesday, and really increased in frequency late Tuesday Night. The image below show lightning strikes given by the Lightning Detection System setup across the US.
So what caused the lightning across the region Tuesday night? For the development of spring time thunderstorms, several components are needed: lift or forcing, moisture, and instability. These components can also be discussed with winter time lightning. With a strong upper level trough and mid level frontogenesis, forcing was definitely not lacking with this event. This system was also able to pull in a good amount of moisture as it evolved over the central part of the country, providing the second component. The only component left to discuss, which would continue to provide good vertical motion for charge separation, or for the potential for lightning, is instability. The image below is a cross section of the atmosphere at around 6pm Tuesday night for areas in Madison, Wisconsin south to Champaign, Illinois with Ohare airport centered on the screen. The purpose of this image is to best describe the instability present over northern Illinois and northwest Indiana. The solid streamlines are essentially the vertical motion provided by system scale forcing as well as mid level frontogenitical forcing. The image in the background is our instability for this event. The blue colors, right above the best vertical motion, are the areas of this best instability. This instability helps any vertical motion rise faster and easier, which then helps with snowfall production and charge separation, or lightning. With summer time thunderstorms, what can happen when all of this is present is for hail to reach the ground. Now, of course, we are talking about the winter time so you would think that there would not be a need for a mention of hail. But there were several observations across the area of hail that did fall along with the snow.
Winds
Not only did this strong area of low pressure bring very heavy snowfall across the region, but it also provided a setup for very strong winds which aided in the blinding blizzard conditions. A strengthening and unstable upper level trough provided an attendant strong surface low to deepen as it tracked northeast across portions of the Midwest. With this deepening low moving in from the south and a strong ridge of high pressure to the north, a strong pressure gradient setup across the Midwest late in the day on Tuesday. These lines of pressure, or isobars, were tightly packed over much of the region, in particular northern Illinois and northwest Indiana. These tightly packed isobars associated with this strong area of low pressure were one indication for the potential for very strong winds during this event.
There were several other aspects to this system which also helped with very high wind speeds across the area on Tuesday. One of which had to do with the mid and upper level portions of this system. This dynamic system brought with it very strong flow at all levels of the atmosphere, which included levels just off of the surface. The image below is of a forecast sounding which depicts moisture and thermals profiles as well as winds for all levels of the atmosphere. This image is of a forecast sounding for a point located near O’Hare Airport. The first part of this image to focus on is on the right, where wind barbs give the wind speed and direction at different level of the atmosphere. The wind barbs are indicating that winds of around 50 to 60 MPH are located only a couple of thousand feet off of the ground. With a well mixed lower part of the atmosphere, as can be seen with the thermal and moisture profile, these wind speeds can easily be transferred down to the surface, which is exactly what occurred late Tuesday night. Areas along the shore of Lake Michigan also saw stronger winds due to the fact that these winds were out of the northeast, which gave an unimpeded flow right off of the lake.
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