2008/2009 Exclusive Winter Outlook

We’ve pored over the data, run tests, used many beakers and test tubes… okay, maybe not quite that extreme.

We won't soon forget last winter when more than 70 inches of snow fell in the Rock River Valley. There will not be a repeat of that this season. However, we are expecting slightly above average precipitation. As far as temperatures go, it appears the cold weather from November will be carried on into December and possibly January. Because we'll be starting the season in a deficit (temperature-wise), we feel compelled to continue with a slightly below-average temperature pattern through winter 2008/2009.

Summary:
Last winter, we were in a strong La Nina pattern. That has since subsided this fall into a near-neutral pattern, perhaps a weak La Nina. Generally, La Nina leads to temperatures that are cooler than average in the Midwest. Other large-scale signals indicate the potential for a similar situation in which cold air is able to build up in Alaska/northern Canada, then spill southward into the central United States.

The National Weather Service office in Detroit came up with a number of analogue years; that is, years in the past that were similar to what occurred this past year. They used years in which a La Nina was ongoing from winter through the summer, with a return to neutral conditions by fall. We used that technique in our Winter Outlook by looking back at what conditions Rockford experienced in the seven analogue years. While there was certainly variability in temperature and snowfall, we were able to identify a couple of general trends.

Temperature:
We can already get a glimpse into a good part of December using our ensemble computer models. What they suggest is a continuation of the cold. Aside from the first week of November, a significant long-term pattern has developed bringing us repeated bouts of cold weather. This trend is also very similar to one of the aforementioned trends from the analogue years. We found that December typically ended up with much below average temperatures – sometimes to the tune of 5°F below average. It also appeared that a significant cold wave would affect the Midwest after the middle of the month, sending Rockford’s temperatures below zero for several days. While we aren’t giving specifics within this Winter Outlook, it is hard to ignore such suggestions.

In any event, some moderation in temperature is expected into January (partially based on data from analogue years). Given that we are in a weak La Nina, or perhaps non-existent La Nina by February or March, we are given fewer signals of any deviation above or below. As a result, we are going to trend to near normal temperatures in January. For February, we are not seeing any notable indication for either above or below average temperatures, but data from analogue years points to the possibility for slightly warmer than average conditions. Even if February does end up warmer than average, we do not feel it will outdo the very cold December and near-average January.

In a nutshell, our Winter Outlook calls for below-average temperatures on the whole, particularly during the month of December.

Precipitation:
As with temperatures, a lack of a strong La Nina or El Nino doesn’t lend itself to making a precipitation outlook easy. Looking back at our analogue years, snowfall amounts had a high level of variability. A few years had vastly higher than average snowfall totals, while others had scarce amounts of snow. We are calling for average to slightly above average snowfall this winter, partly due to the fact that colder than average temperatures will favor snow.

Since cold air intrusions will be frequent for the first part of winter, we expect Alberta Clipper systems to be quite common. While these usually are starved for moisture and therefore produce relatively low snowfall totals, their frequency may mean more small snows more often. Recall only to last year when we had so many Alberta Clippers; the minor amounts of each one added up to a sizable portion of the 72.9 inch total for the season.

There is some indication that the dominant "Texas Hook" storm track may be a bit farther northwest this year than last year, putting the heavier snow bands northwest of Rockford. That doesn’t mean we won’t have any significant snowstorms; they just may become rarer into February and March of 2009.

Eric Sorensen
Adam Painter
Justin Gehrts


For additional reading:
NWS Detroit Winter Outlook - http://www.crh.noaa.gov/images/dtx/climate/winterout08.pdf [Note: keep in mind that the specifics pertain to Lower Michigan, not necessarily northern Illinois/southern Wisconsin]

WREX Winter Outlook Notes - http://www.justingehrts.com/files/winter_outlook_notes.pdf