Eastern United States Big Chill of February 2016 by Richard H. Grumm National Weather Service State College, PA 16803

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1 1. Introduction Eastern United States Big Chill of February 2016 by Richard H. Grumm National Weather Service State College, PA and Anne Balogh A deep 500 hpa trough (Fig. 1) accompanying arctic air brought two surges of cold weather to the eastern United States 10 to 15 February The first surge of cold air brought cold weather from the Ohio Valley into Florida with several record low minimum temperature records set February and a larger number of record minimum-maximum temperatures set on February (Table 1). The second surge of cold air was focused over the northeastern United States with the cold peaking on 14 February when 118 record lows were set or tied. This airmass set new record lows in New York City (-1F) and Boston (-9F) for the date with both cities going below zero Fahrenheit for the first time in years. New York City had not fallen below zero since 19 January 1994 and Boston recorded its coldest low 15 January Table 1 lists the number of low temperatures set or tied during the first 15 days of February On 14 February 2016 Watertown, New York fell to -37F besting the old record of -30F making New York City seem relatively balmy in comparison. The short-lived extreme cold snap was due to the deep 500 hpa trough (Fig. 1) and deep arctic air as indicated in the 850 hpa temperature field (Fig. 2). The -36C contour at 850 hpa was visible at 1200 UTC 13 February (Fig. 2d) as the air mass was moving across the eastern Great Lakes and into the northeastern United States. It is interesting to note that during this period of intense cold and the deep trough in the East, the larger ridge in the West (Fig. 1) was producing extreme warmth over the western United States. This paper will examine the meteorological setting for arctic outbreak and cold snap of February The focus is on the pattern, anomalies in the pattern, and the extent of the cold air as characterized by the size and area of the -25 and -30C contours at 850 hpa during the arctic intrusions. Forecasts from the NCEP GEFS suggest that the cold outbreak and quick return of warmer air was relatively well forecast. The return flow cyclone is not discussed though this system produced extreme temperature swings sub-zero temperatures on the 14 th were replaced with +50F readings on the 16 th with cyclone. 2. Methods and data The climate forecast system re-analysis data was used to reconstruct the patterns and the standardized anomalies associated with the storm. A focus was on several key fields often used to anticipate the intensity of a storm. The NCEP GEFS was used to show the forecasts of the arctic outbreak.

2 NCDC was used to compute the number of record lows and record minimum maximum temperatures used in Tables 1 & 2. The raw temperature data were recovered from National Weather Service Public information statements (NWSPNS) and NCDC. Though all data in the Tables were from NWSPNS. 3. Results a. The pattern The 500 hpa pattern (Fig. 1) showed the deep 500 hpa trough which was over the eastern United States on 10 February The height anomalies in the trough were on the order of -2 to -3σ. In the West and over the western Atlantic there were large ridges with positive height anomalies. The positive height anomalies were on the order of +-2 to -3σ above normal in the Western United States and Canada. A second wave in the trough was clearly visible in central Canada at 0000 UTC 12 February; it was this feature that brought the second more significant surge of cold air. The initial cold surge brought the coldest air into the Ohio Valley and into the southeastern United States. The second and colder surge, moved into the northeastern United States. The core of this cold air moved over New York and New England (Fig. 2). The core of the cold air, viewed in 6-hour increments over the eastern United States (Fig. 3) had temperatures in the -30 to 34C range as it moved into northern New York and New England between 1800 UTC 13 February and 0000 UTC 14 February 2016 (Fig. 3b-c). This blob of extreme cold air was over southern New England by 0600 UTC 14 February (Fig. 3d) and then moved over the adjacent western Atlantic (Figs. 3e-f). The -24C contour affected a larger area which included Pennsylvania, New Jersey, and Delaware. The NCDC data indicated that record lows and record low-maximum temperatures were set primarily over the northeastern United States. The records were set in close proximity to the 850 hpa cold blob. The surface pressure pattern (Fig. 4) in the CFSR indicated there was a surface wave along the arctic front and a large anticyclone behind this frontal boundary. The surface pressure anomalies in the anticyclone were 1 to 2σ above normal. The center of the surface anticyclone was already along the coast by 0000 UTC 15 February (Fig. 4f). The 850 hpa temperatures return to near normal by 1200 UTC 15 February 2016 (Fig. 2f) showing how short-lived the episode of extreme cold was. b. GEFS forecasts The NCEP GEFS 850 hpa temperatures forecasts from 6 GEFS cycles initialized at 0000 UTC from 08 to 13 February 2016 are shown in Figures 5-7. The forecasts valid at 0000 UTC 14 February (Fig. 5) show the cold pool over the northeast several later forecasts show a closed - 32C pocket and a broader area of -20C air. Beneath the -28 to -32C contours the standardized anomalies were -3 to 4σ below normal.

3 By 0600 UTC most of the forecasts (Fig. 6) showed the cold pocket, slightly less intense over the same general region and farther east. By 1800 UTC (Fig. 7) the cold air had moved offshore and at 850 hpa there was a rapid rise in the 850 hpa temperatures. These forecasts implied an intense surge of deep cold air of limited duration. Though not shown these forecasts indicated a return of near normal 850 hpa temperatures to all of the northeast by 1800 UTC 15 February The 500 hpa forecasts (Fig. 8) showed the deep trough associated with the forecast short-lived arctic blast. Several GEFS forecasts had a closed 5040m contour in the mean over northeastern North America and New England at 0000 UTC 14 February c. NAEFS The NAEFS is a combination of the CMC and NCEP ensemble forecasts systems. Similar to the GEFS the NAEFS forecast the deep trough and the arctic outbreak. The 500 hpa forecasts from 6 forecast cycles (Fig. 9) show the pattern over the United States. Forecasts from 04 February were not as robust with the deep trough (Fig. 9a) but slowly converged toward deeper trough by forecasts issued at 0000 UTC 8 February implying at least 6 days advance forecasting of a the arctic surge into the northeastern United States. The corresponding 850 hpa temperatures also showed the surge of cold air into the northeastern United States (Figs. 10 & 11). The forecasts showed less skill 8-10 days in advance (Figs. 10a-b,11a-b) and they showed similar skill in suggesting the deep cold would be short-lived. 4. Conclusions A surge of arctic air with a strong and deep 500 hpa trough brought a short-lived shot of extremely cold air to the eastern United States on February The pocket of coldest air had 850 hpa temperatures values between -30 and 32C. In recent years, there has been a dearth of large pockets of sub -30C air at 850 hpa. The crème de la crème of the cold air at 850 hpa used to denote arctic is in North America has been fleeting in recent years. This contraction of the cold air has been document by Martin (2014). This short-lived but intense arctic air mass focused over New York and New England. This led to temperatures below 0F at New York City, Providence, and Boston. New York had not seen a sub-zero reading in decades. Temperatures were well below zero over much of the eastern United States from northern Pennsylvania to Maine with readings in the -20 to -30F range over northern New York and New England. The NCEP GEFS and the joint NCEP-CMC NAEFS should considerable skill in predicting this intense arctic outbreak to include the short duration of the event. The larger scale features to include the large ridge to the west were relatively well forecast.

4 The cold pool over the Northern hemisphere was examined using Python to find the areas -20 and -30C. Total area covered by the -20C contour peaked over North America at 0000 UTC 14 February with a deep cold pocket over eastern North America and Greenland. The total area was about 6.46% of the area of the Northern Hemisphere. The area covered by the -30C air was smaller, about 0.74 percent of the entire Northern Hemisphere. The two notable cold pockets are over Siberia and eastern North America with a third smaller area over northern Greenland. The area of Siberia persisted for several days while the pool over North America quickly dissipated. 5. Acknowledgement PSU for data access and processing 6. References Martin, J.E 2015: Contraction of the Northern Hemisphere, Lower-Tropospheric, Wintertime Cold Pool over the Past 66 Years. Journal of Climate,28,

5 7. Date Potential Tied Broken Total Region/Regions 1-Feb West 2-Feb West 3-Feb West 4-Feb West 5-Feb West 6-Feb SWEST 7-Feb SWEST 8-Feb SWEST/FL 9-Feb HI 10-Feb FL/AL 11-Feb East/FL 12-Feb East 13-Feb East 14-Feb East/NE 15-Feb East Table 1. List of dates and number of record minimum temperatures set or tied on the date. The general region where most of the records were set is noted along with the number of possible record which could have been set or tied. Data from NCDC records site. Return to text. Date Potential Tied Broken Total Region/Regions 10-Feb Appalachians OH Valley 11-Feb Appalachians OH Valley 12-Feb SE/Nengland 13-Feb EAST 14-Feb MIDWEST 15-Feb EAST 16-Feb NC Table 2. As in Table 1 except for record low-maximum temperature records set or tied. Return to text.

6 Station Name County State Min T Station Name County State Min T MOUNT WASHINGTON Coos NH E BETHLEHEM Grafton NH -25 Watertown Airport Jefferson NY -37 FIRST CONNECTI Coos NH -25 BIG MOOSE Herkimer NY NNE CLARKSVI Coos NH -25 BIG BLACK RIVER Aroostook ME -32 LAKE FRANCIS - NH Coos ESE FINE St. Lawrence NY WSW PITTSBUR Coos NH -25 WANAKENA St. Lawrence NY -31 NASHVILLE 1 E Chittenden VT S COPENHAGEN Lewis NY E EAST CRAFTS Orleans VT -24 ADIRONDACK AIRPORT Franklin NY E ORANGE Orange VT -24 MOUNT VAN HOEVENBERG Essex NY E EAST CRAFTS Orleans VT -24 UNDERHILL STATE PARK Chittenden VT NW HANKSVIL Addison VT NNW CANTON St. Lawrence NY SE WATERTOW Jefferson NY W STRATTON Bennington VT -28 HOULTON INTER Aroostook ME -24 GRIFFISS AIRFIELD AP Oneida NY -28 DICKEY Aroostook ME W OTISCO Onondaga NY -28 PRESQUE ISLE AI Aroostook ME NNE COPENHAGEN Lewis NY NNW WILMING Windham VT ESE EAGLE BAY Hamilton NY -27 ESTCOURT STATI Aroostook ME -23 GALLUP MILLS NEPP Essex VT NNW GALWAY Saratoga NY -22 WALDEN Caledonia VT WNW LITTLE FA Herkimer NY -22 Fort Drum Airfield Jefferson NY SE INDIAN LAK Hamilton NY -22 NINE-MILE BRIDGE Aroostook ME ESE CHARLESTO Montgomery NY -22 POTSDAM AIRPORT St. Lawrence NY WNW READSBO Bennington VT -22 OGDENSBURG AIRPORT St. Lawrence NY NNE STRATTON Bennington VT -22 DELTA LAKE Oneida NY NE FLEISCHMA Greene NY N SHERBURNE Chenango NY SE GRAFTON Rensselaer NY ESE CICERO Onondaga NY SE ADAMS Berkshire MA -21 HAMILTON Madison NY -26 DOLGEVILLE Herkimer NY -21 DELTA DAM (THIS IS SITE LOC Oneida NY WSW EAST WIN Berkshire MA NE LOWVILLE Lewis NY ESE JOHNSTOW Fulton NY -20 TUPPER LAKE Franklin NY W KNOX Albany NY ESE TUPPER LAKE Franklin NY NW DUANESBU Schenectady NY -20 FINE St. Lawrence NY WNW ARLINGT Bennington VT -20 SUTTON Caledonia VT -25 YORK-THOMASV York PA -10 MASSENA AIRPORT St. Lawrence NY -25 SOUTH BOSTON Suffolk MA -9 1 ESE CLAY Onondaga NY -25 NEW YORK New York NY -1 1 WSW CICERO Onondaga NY -25 Table 3 Listing of select low temperatures on 14 February 2016.

7 Figure 1. CFRSR 500 hpa heights and anomalies in 24 hour increments from a) 0000 UTC 10 February 2015 through f) 0000 UTC 15 February Contours every 60m and anomalies in standard deviations as in the color bar. Return to text.

8 Figure 2. As in Figure 1 except for 850 hpa temperatures and temperatures anomalies in 24 hour increments from a) 1200 UTC 10 February through f) 1200 UTC 15 February Isotherms every 2C. Return to text.

9 Figure 3. As in Figure 2 except for the 850 hpa temperatures and anomalies in 6-hour increments from a) 1200 UTC 13 to f) 1800 UTC 14 February Return to text.

10 Figure 4. As in Figure 5 except for mean sea level pressure (hpa) and pressure anomalies in 12 hour increments from a) 1200 UTC 12 February through f) 0000 UTC 15 February Isobars every 4 hpa. Return to text.

11 Figure 5. NCEP GEFS forecasts of 850 hpa temperatures valid at 0000 UTC 14 February 2016 from 6 GEFS fore cycles initialized at 0000 UTC every 24 hours from a) 8 February 2016 through f) 13 February Return to text.

12 Figure 6. As in Figure 5 except for valid at 0600 UTC 14 February Return to text.

13 Figure 7. As in Figure 5 except valid at 1800 UTC 14 February Return to text.

14 Figure 8. As in Figure 5 except for 500 hpa heights and anomalies valid at 0000 UTC 14 February Return to text.

15 Figure 9. As in Figure 5 except for the NAEFS forecasts of 500 hpa heights and anomalies valid at 0000 UTC 14 February Forecasts were initialized at 0000 UTC a) 4 February, b) 6 February, c) 8 February, d) 10 February, e) 11 February and f) 12 February. Return to text.

16 Figure 10. As in Figure 9 except for the NAEFS-bias corrected data showing 850 hpa temperatures valid at 0000 UTC 14 February Return to text.

17 Figure 11. As in Figure 10 except valid at 1200 UTC 14 February Return to text.

18 Figure 12. Plot of CFSR 850 hpa temperatures (K) over the Northern Hemisphere at 0000 UTC 14 February White areas are where the temperatures were -20C (253K) or lower and the area encompassed by the -20C air is plotted at the top of the image as a percent of the area of the Northern Hemisphere. Return to text.

19 Figure 13. As in Figure 12 except for the -30C air at 850 hpa. Return to text.