Midwest Snowstorm: 19-21 December 2012 (Update)

Over the last couple days model output has honed in on a storm track in the last 24 hours. This allowed the NWS to issue winter storm and blizzard warnings across a large portion of the Central U.S.

The most similar historical events based on last night's North American Model output (20121219/0000F036) continued to show a signal across this area for heavy snow and blizzard conditions based on similar storms over the last 30 years. The top 4 most similar events all were associated with heavy snow in the area.

It is important to remember that similar historical events should not be used deterministically (i.e., this is the most similar analog so this is what is going to happen in the upcoming event) in the forecast process. This is even more important for heavy snow events since the placement of the heavy-snow swath is so dependent on the track of the system. Two historical events can have similar strength systems with similar moisture and temperature to the upcoming forecast but if their tracks are 100 miles different...the snow swaths are different. However, since the analogs are similar to the forecast it's reasonable to assume that the impacts within the area threatened too would be similar. Here are the NCDC Storm Data links with some of these storms.

9 December 2009 and 26 January 1996

However, all of these heavy-snow historical events were associated with life-threatening blizzard conditions (verified through NCDC Offline Storm Data...example shown below from the December 1990 blizzard) and this gives meteorologists confidence.

This confidence is translated in enhanced wording from the NWS to its partners and the public.

...LIFE-THREATENING BLIZZARD CONDITIONS TO DEVELOP OVER MUCH OF THE CENTRAL IOWA TONIGHT AND CONTINUE THROUGH THURSDAY AFTERNOON...  

...LIFE-THREATENING BLIZZARD BEGINS TONIGHT...  

...CONFIDENCE IS HIGH THAT THERE WILL BE A PARALYZING SNOW STORM IN PORTIONS OF WISCONSIN...WITH BLIZZARD OR NEAR-BLIZZARD CONDITIONS LIKELY.

In areas where there is less confidence...the wording is not as strong.

...POTENTIAL BLIZZARD CONDITIONS...

Numerous NWS Forecast Offices across the region have used the Analog Guidance in Area Forecast Discussions and within NWSChat to relay impact information over the last few days.

The analog run that this blog post was written from is available here.


Update 19 December 1825 UTC:

Blizzard warnings are now in effect from Kansas to Wisconsin.