By:
Bob Henson
, 11:14PM,GMT on March 11,2015
Precipitation came in as the 27th lowest on record, with the 48-state average of 6.12” falling 0.67” below the 20th-century norm. Although seven of the last 10 winters have been drier than average nationwide, there is no discernable trend in winter precipitation for the 48 states as a whole (see Figure 2).
Figure 1. Winter temperature rankings (December-February) for the period 1895-96 through 2014-15. Image credit: NOAA National Climatic Data Center
Figure 2. Winter precipitation rankings (December-February) for the period 1895-96 through 2014-15. Image credit: NOAA National Climatic Data Center.
In many ways, this was a tale of two winters, both in time and space. December was extremely warm nationwide, second only to 1939, while January and especially February featured a split between record or near-record warmth across much of the West and intensifying cold over most of the East. More than half of the 48 states saw either a top-ten warmest or top-ten coldest February (see Figure 3). Five states had their warmest February on record. Interestingly, although every state from Ohio, Pennsylvania, and New York through New England had its second-coldest February, no state set a monthly record, with 1934 holding on to top billing (except in Ohio, where the coldest February remains 1978). Nevertheless, as we reported in a post last week, a number of locations had their coldest single month or their coldest February on record, while some Western cities had their warmest winter month on record.
Figure 3. Temperature rankings for February 2015. A value of 1 indicates the coldest February in the 121-year record, whereas 121 indicates the warmest. Image credit: NOAA National Climatic Data Center
Overall, last month’s Northeast chill was remarkable given that February has been warming at a rapid clip over that region: more than 4°F since 1900. The West has only been warming at about half that rate in the long term during February, but the heat was on last month in many areas, especially California, where February was 1.3°F warmer than the previous record-holder (1962).
In spite of the relentless snowfall that buried New England in February—especially eastern Massachusetts, where Boston and Worcester had their snowiest month on record—the Northeast as a whole had its 12th driest February on record. The main reason was the absence of rain. Typically, heavy rains can drench the Northeast even in midwinter, whereas little rain occurred there with the persistently cold pattern this past month. Two snowy places that did manage to set records for their highest February precipitation were Boulder, CO (3.69”, including 54.8” of snow) and Riverton, WY (1.28”, including 17.8” of snow). Across most of the West, precipitation ended up well below the winter norm, and the amount of moisture locked up in snowpack by early March was far below average from California through Washington and southwest British Columbia all the way to southern Alaska.
For much more detail, see the NOAA summaries for December, January, and February (which includes winter 2014-15), and our WU posts for December and January.
How well did the seasonal forecasts do?
For those courageous forecasters who attempt to predict how each winter will fare, the last few years have been challenging indeed. This time, the impacts of what’s been a marginal El Niño ended up weaker and patchier than expected. The west-to-east temperature contrast noted above overwhelmed the more prototypical El Niño winter pattern of milder-than-average weather across the northern tier of states and cooler-than-average conditions over the Sunbelt.
Figure 4. A comparision of seasonal temperatures forecasts for winter 2014-15 issued by NOAA (left) and WSI (center) with actual temperatures (right). Image credit: Todd Crawford, WSI, with data from NOAA.
Several private forecasting firms went for more widespread cold across the East than was projected by NOAA, including WSI. According to chief meteorologist Dr. Todd Crawford, WSI’s forecasters believed that the weak El Niño would push the influence of the tropical Pacific farther east than it was in the winter of 2013-14. “This resulted in an eastward shift of the poleward-and-downstream ridging relative to last year, which meant that the largest negative temperature anomalies ended up farther east than last year,” Crawford told me.
Another closely watched factor is the extent of snow cover across Siberia in the autumn. Studies led by Judah Cohen (Atmospheric and Environmental Research) suggest that greater October snow extent may trigger surface high pressure and lead to stratospheric disruptions that produce spells of winter cold and snow over eastern North America and Eurasia, linked to negative phases of the Arctic Oscillation (AO). Last October, snow cover in Siberia was close to a record-high extent in October, implying an enhanced risk of midwinter cold and snow in the U.S. Northeast. Although a sudden stratospheric warming (SSW) did occur, and AER’s U.S. temperature forecast verified well, the AO ended up positive rather than negative during most of the winter, especially in February. In an email, Cohen explained: “I do believe that the tropospheric pattern from late January though February was reflective of the stratospheric circulation forced by the SSW in early January. We are trying to understand why the AO stayed positive, and I think that we have some good leads.”
Jan Null (Golden Gate Weather Services) takes a closer look at how NOAA’s temperature and precipitation forecasts verified for winter 2014-15. Another useful tool is the interactive verification website provided by NOAA’s Climate Prediction Center. It’s worth noting that CPC’s seasonal forecasts are probabilistic: the areas marked “A” are not blanket forecasts of above-average conditions, but rather denote a shifting of the odds toward above-average and away from below-average, with the odds of near-normal conditions fixed at 33.3% except where huge departures are expected. More explanation can be found at a CPC tools page.
Bob Henson
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