February 2011 Blizzard: One Year Later

It's one year later but still fresh in everyone's mind.  The third largest snowfall in Rockford's history hit on the eve of January 31st and lasted through February 2nd.  Hard to believe one year has passed and how things have been so different this year than they were last winter. 

The snow that fell came in two parts; an upper level disturbance out ahead of the main low January 31st and then the main storm itself.  The upper level disturbance didn't produce much snow...only 0.8" fell.  What was most memorable for everyone was the 10.9" that fell on the first of February followed by the 3.4" that fell on the second.  The first signs of this possible winter storm started to develop in our computer weather models about a week before it actually hit.  With each run that came out, morning and evening, most models were holding on to the idea that there would be a winter storm and it would likely hit somewhere in the Midwest and Great Lakes.  It was just a matter of where the exact track would go that we wouldn't know until a few days before.  The storm brought extremely strong winds, heavy snowfall: rates of 2" to 3" per hour for some time, and crippling conditions across northern Illinois.

 A Blizzard Watch had been issued a few days in advance of this storm because the National Weather Service felt very confident that this system would have major impacts on the region.  When the storm moved closer Blizzard Warnings were issued on the first of February.

Early on February 1st the main low started to move out of the south Plains.  As it did, there were a few lake effect snow showers that had set up across northeastern Illinois.  Those lake effect snow showers even extended as far west as Rockford, which was quite impressive.  It wasn't until mid-afternoon that we started to feel the full effect of this system; winds started to increase and the snow grew heavier.  Around 2pm & 3pm drifting snow was already becoming a problem in open locations and visibility was going down.  I can still remember thinking as I was traveling to a hotel where I would try to catch a little sleep: "This is going to be a bad one".

Further south across Missouri and central Illinois blizzard like conditions continued to pour into the National Weather Service offices as the snow fell.  Winds were increasing and would soon gust as high as 50-55mph across northern Illinois later that evening.  A large area of moderate to heavy snow was falling just north of the strengthening low pressure system as it moved through southern Illinois.  What made this system so dynamic and intense was the fact it had a lot of upper level support with it.  The upper level trough began to move in and actually took on a negative tilt.  A negatively tilted upper level trough means the system, as a whole, is strengthen.  The surface low is being enhanced by the low pressure system several thousand feet above.  As long as the air continues to rise in the storm the storm will continue to strengthen.  A positively tilted trough isn't as strong.  At the same time this was occurring there were intense pressure falls out ahead of the surface low; another indication the storm would continue to intensify.  Smaller scale features, known as mesoscale features, also aided in the intense snow that set up across northern Illinois.

Later that evening on the 1st and into the wee hours of the 2nd is when the most intense snow fell.  Snow plows were called off due to safety concerns...highways were closed due to the intense nature of the blowing and drifting.  Visibility was at zero and we were experiencing true blizzard conditions.  During the overnight, the low continued to move east but as it did moisture continued to get wrapped into the storm producing lake effect snow across northeast Illinois.

Some other features that made this storm so impressive were the lightning strikes that were observed along with the thundersnow.  I'm sure many have seen The Weather Channel's Jim Cantore and his reaction to the thunder that occurred while he was live in Chicago.  To get thunderstorms in the spring and summer you need three components: lifting/forcing, moisture and instability.  You can also get these components in winter weather too.  It's just not as frequent and when it does occur you know you're dealing with an impressive storm.  With such strong features in the upper levels of the atmosphere the forcing, or lift, was not lacking.  This storm was also able to keep it's moisture supply due to the warm, moist conveyor belt pulling in moisture from the gulf.  Instability was also in full supply across northern Illinois.  The wind was also a major factor with this storm.  Strong winds not only at the surface but aloft aided in the blinding blizzard conditions.  Peak wind gusts at the Chicago/Rockford International Airport reached 48 mph.  In Rochelle winds peaked at 49 mph, DeKalb 52 mph and at the Chicago Lakefront a peak wind gust of 70 mph was recorded!

All in all, a total of 15.1" fell at the Rockford airport making this snowstorm the third largest in Rockford's history.  Significant blowing and drifting occurred with visibility near zero through the most intense part of the storm.

Here is a break down of how much snow fell  in six hour periods from the 31st through the 2nd from the Chicago National Weather Service.

Rockford, IL
Date
Time
Snowfall Amount (inches)


1/31
12PM-6PM
0.3

6PM-12AM
0.5


2/1
12AM-6AM
0.1


6AM-12PM
0.2


12PM-6PM
1.9


6PM-12AM
8.7

2/2
12AM-6AM
3.0

6AM-12PM
0.4


Rockford Statistics
January 31st-February 2nd Three-Day Storm Total Snowfall: 15.1" (Third Highest on Record)
Snowfall Total from the Afternoon of February 1st through the Morning of February 2nd: 14.0"
Calendar Day Total for February 1st: 10.9"