With the coldest nights of winter in our rearview mirrors and brief cameo of springlike temperatures today across the inland northwest with highs in the 60s all the way up into Kalispell Montana, it’s time to reflect on winter. My previous commentary noted the relatively boring temperatures regionally this winter and in particularly the lack of any brutally cold air Arctic mass diving south toward the inland northwest. While cold air outbreaks did impact much of Alaska and Europe this winter, a noticeable lack of the real bone-chilling cold has been alluded to across much of the contiguous US in various media reports.
Lets take a peak at some numbers to quantify just how un-cold it really was. Per USDA cold hardiness maps released earlier this year, I examined the coldest minimum temperature recorded each winter (herein defined as Nov-Feb). From the United States Historical Climate Network (1218 stations in the lower 48) we can easily track the coldest night per year when the records are complete enough for the past 100+ years. Per the last 62 winter (1950-2011) the median coldest night per winter provides us with what we might generally expect (values in degrees F). The numbers for this winter (Nov 2011-Feb 2012) across the lower 48 paint a similar picture spatially; however, the values are dramatically different if you examine the maps in a bit more detail. Note that missing data for 2011-2012 is due to stations not reporting at least 100 days during the four-month period.
A difference of the two maps provides that quantitative support for media hype. Simply put, as the coldest night this winter in the lower 48 weren’t down-coat worthy, or at least not as much as we’re accustom to. Across pretty much all of the inland northwest the minima extrema was about 10 F warmer than usual, with similar type numbers for all but California, the desert SW and Florida, which by the way, force you to check your down-coat at the door and do not generally observe as much variance in the coldest minimum temperatures of the year as locales in the interior.
It is challenging to lump all of the stations across the lower-48 into a single metric due to differences in how climate manifests itself spatially and irregularly spacing of observations. To work around some of those problems, I normalized data across the USHCN network [assuming something about a statistical distribution] and show in the plot below the mean standardized anomalies for the past 100 years (black line shows an 11-year moving average of the annualized data). Due to the way stations tend to be concentrated across eastern 2/3rds of the country, this doesn’t truly give us the most accurate picture, but in some ways is more closely tied to population centers. However, at least through this vantage, the winter of 2011/12 will be noted by a very benign version of Jack Frost in the US and an off-year for sales of down coats and other winter “accessories”.
Update: I ranked the coldest winter temperature observed in 2011/2012 for each station in the USHCN network that had at least 50 years worth of records. Below I show the percentile ranking for this winter. The larger circles denote a station sets an all-time “warmest-cold”. About 13% of the stations, concentrated primarily from the Virginia’s westward toward the midwest saw their warmest coldest winter on record.