The North Pacific–Driven Severe Midwest Winter of 2013/14

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The North Paciﬁc–Driven Severe Midwest Winter of 2013/14

ALAN MARINARO Meteorology Program, Department of Geography, Northern Illinois University, DeKalb, Illinois

STEVE HILBERG Midwestern Regional Climate Center, Climate and Atmospheric Science Division, Illinois State Water Survey, Champaign, Illinois

DAVID CHANGNON Meteorology Program, Department of Geography, Northern Illinois University, DeKalb, Illinois

JAMES R. ANGEL State Climatologist Ofﬁce for Illinois, Climate and Atmospheric Science Division, Illinois State Water Survey, Champaign, Illinois

(Manuscript received 24 March 2015, in ﬁnal form 28 July 2015)

ABSTRACT

The severe 2013/14 winter (December–March) in the Midwest was dominated by a persistent atmospheric circulation pattern anchored to a North Pacific Ocean that was much warmer than average. Strong teleconnection magnitudes of the eastern Pacific oscillation (2EPO), tropical Northern Hemisphere pattern (1TNH), and second-lowest Hudson Bay 500-hPa geopotential height field are indicators that led to severe winter weather across the eastern United States. Unlike in previous cold and snowy midwestern winters, Atlantic Ocean blocking played little to no role in the winter of 2013/14. The primary atmospheric feature of the 2013/14 winter was the 500-hPa high pressure anchored over the North Pacific in response to the extremely warm sea surface temperature anomalies observed of 13.7 standard deviations. Only one other severe midwestern winter (1983/84) since 1950 featured a similar Pacific blocking. The accumulated winter season severity index, which is a metric that combines daily snowfall, snow depth, and temperature data for the winter season, was used to compare the 2013/14 winter with past winters since 1950. Detroit, Michigan, and Duluth, Minnesota, experienced their worst winter of the 55-yr period. Seven climate divisions in northern Illinois, eastern Iowa, and parts of Wisconsin experienced record-cold mean temperatures. These climate conditions were associated with a number of impacts, including a disruption to the U.S. economy, the second-highest ice coverage of the Great Lakes since 1973, and a flight-cancellation rate that had not been seen in 25 years.

1. Introduction Quayle 1980) such as severe cold-air outbreaks, record numbers of snowstorms, and a wide range of impacts on The 2013/14 winter in the Midwest, which was char- society (Changnon and Changnon 1978a,b; Changnon acterized by frequent snowstorms and outbreaks of 1979; Changnon et al. 1980). Documented impacts sug- arctic air, brought back memories to long-term residents gest that the 2013/14 winter was severe. Economists re- of winters experienced in the late 1970s. The three ported that the unusually cold and snowy weather consecutive winters from 1976/77 through 1978/79 fea- conditions experienced in early 2014 negatively im- tured record-breaking weather conditions (Diaz and pacted the economy by ‘‘disrupting production, construction, and shipments, and deterring home and auto sales’’ (Sharf 2014). Similar to the situation during Corresponding author address: Alan Marinaro, Dept. of Geography, Northern Illinois University, Davis Hall 118, DeKalb, the winter of 1978/79, more than 90% of the Great IL 60115. Lakes were ice covered by mid-February (Great Lakes E-mail: [email protected] Environmental Research Laboratory ‘‘Historical Ice

DOI: 10.1175/JAMC-D-15-0084.1

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Cover’’; http://www.glerl.noaa.gov/data/ice/), which in- persistence of cold weather, the frequency and amount creased a typical 3-day commercial shipment through the of snow, and the amount and persistence of snow on the Great Lakes to 9 days (Guarino 2014). By mid-February ground. To evaluate the severity of the 2013/14 winter, of 2014, approximately 5.5% of all U.S. flights had been the accumulated winter season severity index (AWSSI; canceled as a result of conditions of extreme cold or Boustead et al. 2015) was used. The AWSSI uses com- snowfall, which was the highest rate in 25 years (Quirk monly available daily climatic data—maximum and 2014). By March, the city of Chicago had spent $2.8 million minimum temperature, snowfall, and snow depth. It on filling approximately 240 000 potholes. The continu- does not account for wind or freezing rain. The advan- ous subfreezing air temperatures experienced in the 2013/ tages of such an index are that it allows for the creation 14 winter caused soil temperatures to drop below freezing of a historical database for any location with daily to depths of 1.2–1.5 m in the Midwest (data obtained from temperature, snow, and snow-depth data and allows http://www.ncdc.noaa.gov/data-access/land-based-station- for comparisons of season-to-season severity at one lo- data/land-based-datasets/cooperative-observer-network- cation in the context of the climatological mean of that coop). These conditions caused water in underground location or between locations. This study utilized pipes and water mains to freeze, leaving entire sub- available AWSSI calculations during the 1950–2014 divisions without water for periods from days to weeks period for 12 midwestern locations to assess the severity (see, e.g., La Salle, Illinois: Collins and Abbey 2014; Stella of the 2013/14 winter. 2014). Communities such as Moline, Illinois, and Des The AWSSI was used to assign a score to each daily Moines, Iowa, experienced their all-time highest number maximum temperature, minimum temperature, snow- of broken water mains because of the severe cold. fall, and snow depth (Table 1). The scores were summed The goal of this research was to describe and explain throughout the winter, which resulted in a score that the 2013/14 midwestern winter. The two primary ob- integrated the temperature and snow character of the jectives were to document the climatological extremes, winter as well as its length (Boustead et al. 2015). At any putting them into a historical perspective, and to identify location, the winter season began when the first of any and discuss the meteorological features and tele- one of the following instances occurred: first measurable connections that were associated with those climato- snowfall ($0.254 cm), maximum temperature at or be- logical conditions. low 08C, or 1 December (i.e., latest start to season). The winter season ended at the last occurrence of any of 2. Approach the following: last measurable snowfall ($0.254 cm), last day with 2.54 cm of snow on the ground, last day a. Climatological assessment with a maximum temperature of 08C or lower, or on the Surface climatological data for the period from 1950 date 28 or 29 February (i.e., earliest conclusion to sea- through 2014 were obtained from the Midwestern Re- son). At a specific location, the final winter score was gional Climate Center’s Application Tools Environ- then compared with other winter scores and ranked. ment, known as ‘‘cli-MATE’’ (http://mrcc.isws.illinois. b. Meteorological conditions examined edu/CLIMATE/), to characterize the 2013/14 winter and place it in historical perspective. Daily and monthly This study examined a number of atmospheric vari- temperature, precipitation, snowfall, and snow-depth ables, sea surface temperature (SST), and teleconnections information was obtained from National Weather (TC) in an effort to explain the climate conditions asso- Service first-order and cooperative stations across the ciated with the severe 2013/14 midwestern winter. Surface Midwest for the period from December 2013 through weather features such as winter cyclone tracks were March 2014. Station and climate-division (CD) averages classified and counted using daily weather maps (obtained and anomalies were determined and mapped using at http://www.esrl.noaa.gov/psd/data/composites/day/). cli-MATE and the ‘‘Climate at a Glance’’ tools from Monthly upper-atmospheric height anomalies for the National Centers for Environmental Information 500-hPa pressure fields were examined across the (http://www.ncdc.noaa.gov/cag/). Northern Hemisphere. Determining the appropriate climate conditions that Northern Hemispheric modes were related to the create a severe winter was challenging. A winter at a North American atmospheric pattern that continuously location may have much-below-normal temperatures affected the Midwest. Atmospheric pattern descriptions but below-normal snowfall. Is this situation more severe were defined in terms of TCs and custom bounded than a winter with temperatures that are slightly below geographical boxes of specific weather variables that normal and snowfall that is much above normal? were calculated with NCEP–NCAR reanalysis data The severity of a winter is related to the intensity and (Kalnay et al. 1996). Modern TC indices (Wallace and

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TABLE 1. AWSSI point thresholds for temperature and snow.

Snow depth Points Max temperature [8C(8F)] Min temperature [8C(8F)] Snowfall [cm (in.)] [cm (in.)] 1 From 23.9 to 0.0 (25–32) From 23.9 to 0.0 (25–32) 0.25–2.4 (0.1–0.9) 2.5 (1) 2 From 26.7 to 24.0 (20–24) From 26.7 to 24.0 (20–24) 2.5–5.0 (1.0–1.9) 5.1 (2) 3 From 29.5 to 26.8 (15–19) From 29.5 to 26.8 (15–19) 5.1–7.5 (2.0–2.9) 7.6 (3) 4 From 212.2 to 29.6 (10–14) From 212.2 to 29.6 (10–14) 7.6–10.1 (3.0–3.9) 10.2 (4) 5 From 215.0 to 212.1 (5–9) From 215.0 to 212.1 (5–9) — 15.2 (6) 6 From 217.8 to 215.1 (0–4) From 217.8 to 215.1 (0–4) 10.2–12.6 (4.0–4.9) 22.9 (9) 7 From 220.5 to 217.9 (from 25to21) From 220.5 to 217.9 (from 25to21) 12.7–15.1 (5.0–5.9) 30.5 (12) 8 From 223.3 to 220.6 (from 210 to 26) From 223.3 to 220.6 (from 210 to 26) — 38.1 (15) 9From226.1 to 223.4 (from 215 to 211) From 226.1 to 223.4 (from 215 to 211) 15.2–17.7 (6.0–6.9) 45.7 (18) 10 From 228.9 to 226.2 (from 220 to 216) From 228.9 to 226.2 (from 220 to 216) 17.8–20.2 (7.0–7.9) 60.1 (24) 11 — From 231.6 to 229.0 (from 225 to 221) — — 12 — — 20.3–22.8 (8.0–8.9) — 13 — — 22.9–25.3 (9.0–9.9) — 14 — — 25.4–30.4 (10.0–11.9) — 15 ,228.9 (,220) From 237.2 to 231.7 (from 235 to 226) — — 18 — — 30.5–38.0 (12.0–14.9) 91.4 (36) 20 — ,237.2 (,235) — — 22 — — 38.1–45.6 (15.0–17.9) — 26 — — 45.7–60.0 (18.0–23.9) — 36 — — 60.1–76.1 (24.0–29.9) — 45 — — $76.2 ($30.0) —

Gutzler 1981; Mo and Livezey 1986; Barnston and and averaged spatially into a monthly and seasonal Livezey 1987; Thompson and Wallace 2000), such as the time series for evaluation purposes. The North Pacific tropical Northern Hemisphere pattern (TNH) and the SST pool (SST1) dataset was defined by the November North Atlantic oscillation (NAO), were defined by mean SST area within the bounds of 358–508Nand the Climate Prediction Center (CPC) and used to ana- 1808–1308W. The DJFM North Pacific SST pool (SST2) lyze their influence on North American winters. These was defined by the same procedure as was used for the TC datasets (i.e., NAO and TNH) are numerical time SST1 dataset but the seasonal mean of DJFM was ac- series that were standardized for the most recent cli- cumulated. The next two datasets involved DJFM matological 30-yr base period. The polarity and magni- mean 500-hPa height values across different areas. The tude of TCs are indicative of the type of atmospheric North Pacific heights (NPHS) dataset was bounded by blocking or weather pattern that a certain region is likely 558–658Nand1608–1258W, and the Hudson Bay heights to incur. Loading patterns (500 hPa) associated with (HBAY) dataset was bounded by 558–658Nand958– these specific modes are shown in Fig. 1. 758W. NCEP–NCAR reanalysis information was ac- The eastern Pacific oscillation (EPO) index was de- quired to calculate the EPO and the four bounded indices veloped to determine the mean monthly and December– (i.e., HBAY, NPHS, SST1, and SST2) for the period from March (DJFM) seasonal time series using two different January 1950 to March 2014 (Kalnay et al. 1996). These specific coordinate boxes (208–358N, 1608–1258Wminus were used to describe how the 2013/14 winter evolved. 558–658N, 1608–1258W) of the area-weighted mean For the period of 1950–2014, the magnitudes of TC in- 500-hPa geopotential height fields. The EPO index dices were compared with severe winters as defined by was created using data from the NCEP–NCAR re- the AWSSI scores for the Midwest. analysis (Kalnay et al. 1996), was calculated on the basis of an EPO formula from the NOAA/Earth System 3. 2013/14 climatological conditions Research Laboratory (http://www.esrl.noaa.gov/psd/ a. Climate summary data/timeseries/daily/EPO/), and was standardized for climatological studies (30-yr mean). The ‘‘east Pacific– Temperature rankings were determined using CD North Pacific’’ TC, derived from the CPC, was not used data for the period from December 2013 to March 2014 because the time series does not include December for the contiguous 48 United States (Fig. 3). The extent values. of the cold during the winter of 2013/14 was sizable, Four indices were calculated using area-weighted with much of the central and eastern United States ex- weather variables bounded by a coordinate box (Fig. 2) periencing below-normal and/or record-below-normal

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FIG. 1. Loading patterns (1 mode) for winter NAO, PNA, and TNH teleconnections as defined by the CPC and Climate Diagnostics Center. (The EPO loading-pattern correlation graphic was provided by the ESRL Physical Sciences Division; http://www.esrl.noaa.gov/psd/.) temperatures. The core area of the ‘‘much below nor- winters in the late 1970s were the defining winter experi- mal’’ (i.e., CDs experiencing one of their 10 coldest ence in the past 40 years. The winter of 2013/14 was similar winters) and record-cold region was over the central in respect to temperature. The DJFM midwestern tem- United States from the Midwest down to the lower perature for 1976/77 was 24.98C (23.28F), for 1977/78 it Mississippi River valley. At the state level for the DJFM was 26.68C (20.18F), and for 1978/79 it was 26.38 (20.78F), period, both Michigan and Wisconsin experienced their as compared with 2013/14 when it was 26.38 (20.78F). third coldest period ever, Illinois and Indiana had their fourth coldest, Missouri had its fifth coldest, Iowa and Minnesota had their sixth coldest, and Ohio experienced its ninth coldest period ever. A closer examination of CD temperature anomalies for the Midwest (Fig. 4) shows the magnitude of the winter temperature departures. Areas in eastern Min- nesota, much of Wisconsin, eastern Iowa, northern Illi- nois, and northwestern Indiana were 3.98–4.58C below normal for the DJFM period. The rest of the Midwest was 1.68–3.98C below normal for the period. Seven CDs in the Midwest experienced their coldest DJFM dating back to 1895.

The people of the Midwest had not experienced FIG. 2. Locations of area-weighted mean weather variables that such a cold winter in over a generation (Fig. 4). The three compose the SST1/SST2, NPHS, and HBAY datasets.

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FIG. 3. The CD mean temperature rankings over the contiguous United States for December 2013–March 2014 relative to the historical record dating back to 1895. Record-coldest CDs are denoted by thick borders. (Obtained online at http://www.ncdc.noaa.gov/cag/ mapping.)

The other factor that made the 2013/14 winter stand runs of days with temperatures, snowfall, and snow out was the above-normal snowfall accumulation depth at different values; and the length of the season. throughout the Midwest. Snowfall for DJFM period The winter score for any selected winter could be com- (Fig. 5) was 100–150 cm (40–60 in.) above normal in pared with the median (i.e., 1950–2014) winter score northeastern Illinois, northern Indiana, northwestern for a location to determine the relative severity of a Ohio, and western Michigan. Large portions of the rest winter. Therefore, the higher the AWSSI score is, the of the Midwest were 25–100 cm (10–40 in.) above nor- colder and snowier are the winter. In a speciﬁc year, the mal for snowfall. This resulted in a wide band of 2–3 raw AWSSI scores for a number of stations can be used times the normal snowfall stretching from Missouri, to understand winter severity across the region. The through Illinois, Indiana, and Ohio as well as northern median AWSSI (1950–2014) score for Duluth, Minne- Kentucky and southern Michigan. sota, is 2000.0, whereas the median for Chicago, Illinois, is 543 (Table 2), indicating that the average winter is b. Historical climate perspective of the 2013/14 winter much colder, snowier, and likely longer in Duluth than using the AWSSI in Chicago. To determine the relative severity of the The coldest weather in the contiguous 48 United States, seasonal AWSSI, along with the median, the 25th and relative to normal, encompassed an area extending from 75th AWSSI percentiles were calculated for each loca- the northeastern three-quarters of Minnesota through tion (i.e., they can be used to compare AWSSI values Wisconsin, the Michigan Upper Peninsula, and northern between locations). Illinois (Fig. 3). Snowfall departures, in terms of percent The AWSSI 1950–2014 median, 25th-percentile of normal, were greatest in the southern half of the values, and 75th-percentile values and the 2013/14 Midwest, the Ohio valley, and far northern Minnesota AWSSI values were determined for 12 selected mid- (Fig. 5). western locations (Table 2). The rank of the AWSSI The AWSSI tracks daily climate variables throughout (Table 2) with respect to all winters for the period the winter season, in effect accounting for extremes; 1950–2014 indicated that 2013/14 was the most severe

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FIG. 4. The CD mean DJFM 2013/14 temperature anomalies (8C) from the 1981–2010 average covering the Midwest. Record-coldest CDs are denoted by yellow borders. winter for both Detroit (Michigan) and Duluth (tied vortex over Hudson Bay into southeastern Canada. with 1995/96 for Duluth). The AWSSI score for Detroit Hudson Bay experienced its second-lowest average was 1277, which is more than 500 above the 75th- winter 500-hPa-level geopotential height (5069 gpm) percentile AWSSI score, exceeding the score of 1046 set within the NCEP–NCAR reanalysis dataset (Kalnay in the winter of 1977/1978. For all 12 locations, the et al. 1996). Arctic air masses associated with this trough winter of 2013/14 was one of the 10 most severe since created the cold conditions experienced in the upper 1950, with eight cities ranked third most severe plains, Midwest, and many areas generally east of the or higher. Mississippi River (Fig. 3). The air masses were not modiﬁed much as they traversed from Siberia, the 4. Meteorological assessment of winter 2013/14 Arctic, and Canada because of widespread and persistent snow cover over much of Canada and the Midwest a. North American 2013/14 winter characteristics in 2013/14. The primary atmospheric feature attributed to the The frequent incursion of unusually cold air masses severe winter of 2013/14 involved a prolonged period into the Midwest was often associated with the occur- of atmospheric blocking that was associated with the rence of surface ‘‘Alberta clipper’’ systems. Clipper polar jet stream over Alaska and northwestern Canada. events occurred more frequently over the Midwest As a result, a persistent trough developed downstream than did any other storm track (Table 3), which was over central North America (Fig. 6). This trough be- notably different from what typically occurred during came an anchor for the 500-hPa-level low pressure the severe winters of the late 1970s (Changnon et al.

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b. The PNA, North Pacific anomaly, and other teleconnections A positive temperature anomaly developed over the NPHS SST pool during November of 2013 with an SST1 index value of 12.7 standard deviations s. A warm SST pool over the North Pacific is a common occurrence when the Pacific decadal oscillation is in a long-term negative phase (Mantua et al. 1997) but is far from common when compared with the other episodes of extremely positive SST anomalies. The SST2 index measured at 13.7s above the climatological mean for DJFM 2013/14 (Fig. 7). The SST1 domain experienced near-record warmth in November of 2013, and SST2 experienced record warmth in DJFM 2013/14. How did the warm North Pacific SST anomaly affect the 2013/14 winter over North America? Oceanic SSTs covering the prescribed North Pacific area can force circulation patterns that lead to changes within the height-field distribution pattern over the Northern Hemisphere through bottom-to-top (ocean to atmosphere) processes (Namias 1969, 1970; Kushnir and Lau FIG. 5. Midwest DJFM 2013/14 snowfall (cm) departure from the 1992; Hartmann 2015). North Pacific SSTs are able to climatological mean. modulate the Pacific–North American (PNA) TC pattern and PNA-like pattern variants (Frankignoul and 1980) and what occurs in an average winter (Changnon Sennéchael 2007). Surface heat flux was the main driver 1969). Weak clipper systems rotated cyclonically around between SSTs and geopotential height fields, although the semipermanent Hudson Bay low pressure feature, in some instances atmosphere–ocean feedback (cloud frequently providing light-to-moderate midwestern cover, evaporation, radiation, etc.) can occur to balance snowfall amounts of generally less than 15.2 cm (6 in.). the heat flux budget (Frankignoul et al. 2004). The Farther south over the mid-Mississippi River basin, west- SST2 anomaly was likely the primary reason for the to-east-moving extratropical cyclones also brought snow persistent blocking ridge over the Bering Sea, Alaska, events across eastern portions of the Midwest. Although and northwestern Canada (NPHS anomaly) during the the Midwest experienced fewer heavy-snow-producing 2013/14 winter. The SST2 anomaly acted as a trigger to cyclone tracks (i.e., Colorado lows and Gulf lows), many displace the polar vortex over central and southern parts areas still received above-average snowfall (Fig. 5)be- of Canada (Hartmann 2015). The mean NPHS (Fig. 2) cause of the increased frequency of Alberta clippers. geopotential height for DJFM 2013/14 was observed at

TABLE 2. The median AWSSI (1950–2014), 25th- and 75th-percentile values (1950–2014), AWSSI score for 2013/14, and rank of the 2013/14 AWSSI by location. The parenthesized T denotes a tie in rank.

Median AWSSI 25th/75th-percentile AWSSI Winter AWSSI Rank of AWSSI Location for 1950–2014 for 1950–2014 2013/14 2013/14 Chicago, IL 543.0 465.8/749.0 1106 3 Columbus, OH 393.0 321.5/514.3 638 6 Des Moines, IA 693.0 567.5/810.0 931 10 Detroit, MI 566.5 455.5/703.5 1277 1 Duluth, MN 2000.0 1666.5/2314.0 2822 1 (T) Indianapolis, IN 403.5 328.0/501.8 821 2 Louisville, KY 220.0 152.5/297.0 375 3 Madison, WI 880.5 718.8/1156.0 1393 6 Milwaukee, WI 748.5 617.8/921.0 1280 3 Minneapolis, MN 1223.5 900.5/1545.5 1820 3 Sault Ste. Marie, MI 1946.0 1595.0/2122.0 2570 2 St. Louis, MO 298.0 240.5/371.0 463 7

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FIG. 6. The 500-hPa geopotential height anomaly (gpm) map for DJFM 2013/14. The red ‘‘L’’ deﬁnes the area of the Hudson Bay low, and the blue ‘‘H’’ centered around the Bering Sea/ Alaska/Gulf of Alaska represents the North Paciﬁc high pressure zone. (The image was provided by the ESRL Physical Sciences Division; http://www.esrl.noaa.gov/psd/.)

5355 gpm, the third highest in the NCEP–NCAR re- on midwestern temperature and precipitation patterns analysis dataset (Kalnay et al. 1996), exceeding all other (Renken et al. 2014). DJFM seasons within the dataset record from 1948 to The TNH teleconnection was 11.1s from December March of 2014 except for two seasons. through February of 2013/14 (CPC does not calculate Three other teleconnections were examined for their the March TNH). Symptomatic of past severe winters relationship to the winter of 2013/14. The TNH pattern that experienced a warm North Pacific state, the positive affects central U.S. winter temperature and storm track TNH contributed to a strong Hudson Bay 500-hPa because its primary 500-hPa anomaly is located slightly low pressure system. The winter of 2013/14 was no dif- south of Hudson Bay, as seen in Fig. 1 (Rodionov and ferent, recording the second-strongest winter HBAY Assel 2000). The physical importance of the TNH on anomaly in the NCEP–NCAR reanalysis record midwestern winter climate conditions is related to its (Kalnay et al. 1996). A past study has also shown that spatial locality, impact on regional weather patterns, positive TNH values correlate positively with Great and relationship to winter Great Lakes ice cover (Assel Lakes ice cover (Assel and Rodionov 1998) and re- and Rodionov 1998; Rodionov and Assel 2000). The gional snow cover over the central United States. This NAO and EPO can aid in diagnosing the relationship feature was also related to the strong midlevel sub- between Atlantic and Pacific blocking and their influence sidence from California to the Gulf of Alaska. The

TABLE 3. Illinois storm track–type frequencies during the winter of 2013/14. Boldface font indicates dominant type for each month and the season.

Period Clippers Colorado/Oklahoma ‘‘hook low’’ Gulf of Mexico/eastern Texas low Upper midwestern low Dec 2013 7 410 Jan 2014 9 400 Feb 2014 5 401 Mar 2014 4 6 11 DJFM 25 18 2 2

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FIG. 7. Sea surface temperature anomaly (8C) for DJFM 2013/14. (The image was provided by the ESRL Physical Sciences Division; http://www.esrl.noaa.gov/psd/.) positive TNH affected the interior by supporting cold- c. Winter 2013/14 comparisons with past severe air outbreaks and securing the persistent western U.S. winters high pressure feature that helped to establish extreme drought over California (Wang et al. 2014). The severe midwestern winters in the late 1970s were The EPO can gauge North Pacific or Alaskan block- associated with Greenland-based blocking. The nega- ing regimes but can also be an indicator relating to the tive NAO during the winters from 1976 to 1979 domi- persistence and strength of the Hudson Bay low. Walsh nated the planetary circulation; this circulation pattern et al. (2001) described the origins of cold air masses is associated with midlatitude troughing over the that moved into the Midwest and other regions across eastern contiguous United States (Table 4). During the United States and Europe. The origins of these some winters (e.g., 1983/84 and 2013/14), the primary cold air masses were delineated very well by the type blocking mechanism was Pacific based and not Atlantic of blocking pattern imposed upon Alaska and north- based. The 2013/14 winter was dominated by Pacific- western Canada. Although Walsh and his colleagues did basedblockingassociatedwithanegativeEPO,as not discuss the EPO, it is evident by the sea level pres- opposed to the late 1970s when high-latitude blocking sure anomaly in Fig. 7 of Walsh et al. (2001) that the was closely affiliated with Atlantic-based circulation EPO spatial component exists. The winter of 2013/14 patterns (Shabbar et al. 2001). featured a November EPO at 21.4s and a December– A classification system for blocking that is denoted March mean EPO value measured at 21.4s (the sixth by the negative EPO, negative NAO, or a combination lowest since 1950). of both was created to categorize the Midwest’s most

TABLE 4. Most robust winters with regional AWSSI counts of two or more (i.e., number of stations that experienced one of its top five most severe AWSSI winters) with associated EPO and NAO values. Blocking classification for each winter is denoted on the right. Boldface font corresponds to the blocking classification.

Meteorological Regional AWSSI winter (DJFM) NAO (s) EPO (s) count Blocking classification 1977/78 20.8 21.4 9 Atlantic and Pacific blocking (2NAO/2EPO) 1978/79 21.1 21.2 9 Atlantic and Pacific blocking (2NAO/2EPO) 2013/14 0.5 21.4 8 Pacific only (2EPO dominant) 1976/77 21.4 20.1 7 Atlantic only (2NAO dominant) 1950/51 20.6 0.6 6 Atlantic only (2NAO dominant) 1981/82 0.0 0.2 6 No dominant blocking 1962/63 21.7 22.5 2 Atlantic and Pacific blocking (2NAO/2EPO) 1983/84 0.3 20.4 2 Pacific only (2EPO dominant) 1995/96 20.9 21.4 2 Atlantic and Pacific blocking (2NAO/2EPO)

Unauthenticated | Downloaded 09/26/21 02:23 PM UTC 2150 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 54 severe winters, which were determined by midwestern surface cyclone track was directly linked to the domi- AWSSI counts (Table 4). AWSSI counts were de- nant upper-level pattern, a large trough located from termined using the top five most severe winters for each Hudson Bay over the central and eastern United States, of the 12 selected midwestern cities (Table 2). The and produced light-to-moderate snowfalls and an ever- winters that experienced the greatest frequency (counts increasing snow depth into the middle of February of two or more cities) out of these 60 samples (i.e., top of 2014. five winters for the 12 selected cities) were chosen as The combination of severe cold and snowy conditions the most severe midwestern AWSSI winters (Table 4). led to a wide variety of impacts. The U.S. economy was Nine winters met the criteria for regional AWSSI status. negatively affected because of disruptions in production, The three winters in the late 1970s and 2013/14 ranked as construction, and shipments and reductions in home the four top severe Midwest winters. Table 4 provides and auto sales. Many workers lost income because of the regional AWSSI counts, rankings, and associated weather-related shutdowns, individuals who own and winters (defined by DJFM) with Atlantic, Pacific, or maintain homes and automobiles suffered losses, and both blocking types. The winter of 2013/14 was in- schoolchildren experienced multiple days off because teresting because of the degree of extreme cold and of the dangerously cold conditions. The transportation snow that occurred with only negative EPO being the sector (e.g., travel by ship, train, airplane, or automo- dominant blocking. Only two of these severe winters bile) was severely affected by the severe cold and fre- experienced a positive NAO: 1983/84 and 2013/14. quent snowfalls. An important consequence was that the winter provided another indicator that much of our municipal infrastructure (e.g., water supply systems) was 5. Conclusions rapidly aging and fragile. Although they are infrequent The winter of 2013/14 was the worst in the Midwest in occurrence, understanding the physical conditions since a series of severe winters in the late 1970s, and the and related impact information associated with severe resulting impacts affected nearly all weather-related winters is important as we continue to assess and man- sectors. The climatological and meteorological charac- age weather-related risks. teristics of the 2013/14 winter were evaluated, described, and put into historical context. Acknowledgments. The research group appreciates Average winter (December 2013–March 2014) tem- the support provided by the Midwestern Regional Cli- peratures ranked at or near the bottom (i.e., coldest) for mate Center/Illinois State Water Survey. We specifically many areas in the Midwest. The cold temperatures thank Zoe Zaloudek for creating maps related to the were persistent throughout the winter. This continuous research, Tyler Smith for compiling station AWSSI data, period of below-average temperatures experienced David Kristovich for providing comments, and Lisa across the center of the United States was related to two Sheppard for editing the manuscript. We appreciate the anomalous atmospheric features: 1) record warm North comments and suggestions from the anonymous re- Pacific SST forcing near record-high 500-hPa heights viewers. 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