Early January 2007 Warm Episode and Record Warmth of 6 January By Richard H. Grumm, Ron Holmes, And Michael Connelly National Weather Service Office State College PA 16803 1. INTRODUCTION The winter of 2006-2007 got off to an unseasonably warm start over most of the eastern United States. As illustrated by the temperature trace at Harrisburg, PA (Fig. 1) daily highs were often above normal and experienced periods where the high temperature was 1 to 2 standard deviations above normal and 2 periods where the high temperatures approached 3 standard deviations above normal. The daily low temperatures rarely fell to normal and there were several periods where the overnight lows were close to the normal daytime high. This pattern can be found at cooperative observing stations along the East Coast and in other parts of the United States, east of the Rocky Mountains. The warm weather was not unique to eastern North America as the warm to near record warmth, including a lack of snow was a observed over most of Europe. The warmth in the Mid-Atlantic region peaked on 6 January 2007 when many sites in the region tied record high temperatures for the date and several locations tied or exceeded long standing all-time monthly high temperature records (Table 1). Kennedy International airport, NY (KJFK) and Portland, Maine (KPWM) set new monthly record highs. Daily high temperature records were set at most climatic sites in the region. An examination of the data in Table 1 suggests that there have been several warm episodes in January including the 1998 and 1950 event which accounted for a number of the records. Both of these dates are included in the Global re-analysis data, providing the opportunity to examine the large-scale conditions associated with these events and contrast them to the event of 6 January 2007. Several of the monthly high temperature records from 1950 were set on the 4 th and 26 th of the month. The conditions associated with this event are explored in the context of potential culprits. The most obvious would be global warming and ENSO. However, the impact of a persistent positive phase to the NAO is also examined. Interestingly, few studies have documented extended warmth over the eastern United States with the warm phase of ENSO. Several positive ENSO or El Nino years, such as 1998 have been warm in the eastern United States. However, the teleconnection maps imply that the warmth is typically farther west over the plains of North America. Global warming is a
Previous Daily Record Year of Previous Record City/Location New Record January Maximum Date of January Maximum Allentown,PA 70 60 1998 Atlantic City,NJ 60 60 1998 Georgetown,DE 66 62 1972 Philadelphia,PA 73 71 1950 Reading,PA 70 63 1950 Trenton,NJ 72 68 1950 Wilmington,DE 71 70 1950 Central Park,NY 72 63 1950 72 January 26, 1950 LaGuardia,NY 72 59 1998 72 January 26, 1950 Kennedy Airport,NY 71 57 1998 69 January 26, 1950 Islip,NY 65 55 1998 Bridgeport,CT 64 53 1949 Newark,NJ 72 62 1950 Boston,MA 69 62 1913 72 January 26, 1950 Bradley/Hartford,CT 72 58 1929 70 January 14, 1932 Providence,RI 64 61 1913 Worchester,MA 66 57 1929 67 January 26, 1950 Albany,NY 71 60 1929 60 January 13, 1932 Binghamton,NY 60 56 1998 Scranton,PA 64 54 1998 Syracuse,NY 62 59 1998 Harrisburg,PA 67 66 1890 Willimsport,PA 64 59 1946 Washington,DC 73 72 1950 Baltimore,MD 72 69 1984 Sterling,VA 71 66 1998 Burlington,VT 62 56 1890 Montpelier,VT 65 46 1949 Portland,ME 67 53 1949 64 January 26, 1950 Grey,ME 67 53 1949 64 January 26, 1950 Table 1. Locations of record reports in the northeastern United States for 6 January 2007. The high for the date and previous record high and year provided. Where applicable the monthly record and previous dates are shown. For most 1950 monthly records many sites showed the 4 th and 26 th as the highest known maximum. Only the latter date is shown here. difficult culprit to pin down. Studies and reports on the effects on high latitudes suggest there are longer periods of open water and thinner ice in the arctic. But the direct feedback and implications on eastern United States winters are not well documented. The ultimate cause aside, the winter of 2006-2007 has been very warm over most of eastern North America. This paper will document the warm episode of 6 January 2007 and the overall warmth experienced during the first week of January 2007. The event will be compared to previous events using the NCEP re-analysis data. The goal is to better understand the conditions which favor warm episodes, which are more commonly referred to as heat waves when they are observed in the warm season where they are accompanied by high heat index values. 2. METHODS
Figure 1 Plot of high and low temperatures at Harrisburg, PA from 10 December 2006 through 9 January 2007 The thick red (blue) lines shows the mean high (low) while orange (blues) and red (purples) dashed lines show the value of 1 standard deviation from normal. Only 1 standard deviation below (above) normal is display for high (low) temperatures. Record high temperature data were extracted from National Weather Service (NWS) record event reports for the climate sites (Table 1). The search was limited to sites in the Mid-Atlantic region spanning from Washington DC to Portland, ME. Other sites to the south and west also set records on the 5-6 th but an exhaustive search for all sites was not conducted. Plots from decoded METAR observations are used to show the high temperatures observed on during the January warm episode. NWS Cooperative observations sites (COOP) were used to show the 24-hour high and low temperatures at select points, such as Harrisburg, PA (Fig. 1). COOP data is available in plan view and time sections for over 100 stations in and around Pennsylvania. Model and re-analysis data were used to plot the conditions and comparative conditions associated with previous January warm episodes. 3. RESULTS i. previous events The record event data (Table 1) provided the dates of many previous records including several record high temperatures observed along the East Coast on 6 January 1998 and 1950. These reports also showed that several sites all-time January monthly records were set on the 4 th and/or the 26 th of January 1950.
Figure 2 Global re-analysis data valid at 1800 UTC 4 January 1950 showing a) mean sea-level pressure (hpa) and pressure anomalies, b) precipitable water (mm) and anomalies, c) 925 hpa temperatures (K ) and anomalies, and d) 500 hpa heights (m) and anomalies. Anomalies are from the 21-day centered mean and standard deviations from 1970-2000. Figures 2 & 3 show the conditions, as observed in the GR data, associated with the 4 and 26 January 1950 warm episodes. Typical of many cold season record high temperature events, the highest temperatures were observed with a surge of warm air ahead of a surface trough or cold front. The 26 January 1950 (Fig. 3) event showed to anticyclones with pressure anomalies 1 to 2 SDs above normal, a surge of high precipitable water (PW) ahead of the front, and 925 hpa temperature anomalies on the order of +2 SDs above normal in the northeastern United States. The 500 hpa heights were also above normal over the affected region. Similar features were observed in the 4 January 1956 event. Though not shown, both events also showed 2 to 3 SD above normal temperatures at 850 hpa. Figures 4 & 5 show the conditions associated with many of the 6 January daily high temperature records set in 1950 and 1998. Both events showed above normal PW anomalies and 500 hpa heights. Strong thermal anomalies were present at 925 hpa with a broad area of +2 SD above normal values over much of the northeastern United States. Critical features with the record events appeared to be a combination of unseasonably high PW, 500 hpa heights, and low-level temperatures. ii. Teleconnections Table 2 lists the monthly values of the NAO for the winters and the ENSO signal. A positive NAO dominated the winters of 1950 and 1998 and the NAO
Figure 3 As in Figure 2 except valid at 1800 UTC 26 January 1950. was strongly positive in early January 2007. These 3 winters appear to suggest a casual association with warmth and a positive NAO. The ENSO signal is not as clear. The winter of 1950 was a La Nina episode with below normal SST values while the 1998 El Nino was a significant warm episode and the current 2007 event is a weak to moderate warm episode. iii. 6 January 2007 The warm episode of 6 January 2007 was relatively well forecast by the NCEP models and ensemble prediction systems. Most forecasts showed above normal PW and low-level temperatures days in advance, suggesting that the 6 th would likely be the warmest day in the eastern United States. Figures 6 & 7 show the GEFS and NAM forecasts valid at 1800 UTC 6 January 2007. The GEFS forecasts were initialized at 0000 UTC 2 January 2007 and show the anomalously warm 925 hpa temperatures over the region during the afternoon of 6 January. The NAM forecasts (Fig. 7) duplicate the images shown for the historic events. These 48 hour forecasts show that the NAM correctly forecast an anomalously warm situation and high PW values over the region. The pattern and anomalies fit the historic event types (Fig 2 for example) quite well. Figure 8 shows the 00-hour NAM forecast valid at 1800 UTC. These data show that the NAM initialized unseasonably warm conditions at both 925 and 850 hpa over the northeastern United States. Similar to the historic events, in addition to the large thermal
Figure 4 As in Figure 2 except valid at 1800 UTC 6 January 1950. Figure 5 As in Figure 2 except valid at 1800 UTC 24 January 1950. anomalies, high PW anomalies were also
present. The pressure fields showed the trough sweeping through the region. Areas to the west, such as State College in central Pennsylvania saw high temperatures peak in the late morning and early afternoon as the cooler air began moving into the region during the afternoon hours. The lower temperatures to the west can be seen in Figure 9. This plan view image of METAR site high temperatures; based on the highest hourly value; shows that the warmest temperatures were confined to the easternmost areas. The timing of the weak cold advection behind the trough may have limited high temperatures farther to the west. iv. Global warming According to the National Climatic Data Center (NCDC), 2006 was the warmest year in the lower 48 states. It was slightly warmer than the previously record warm year of 1998. Along the East Coast, many states had abnormally warm weather in December with five states experiencing record warmth. Other NCDC 1 statistics of note include the fact that 9 of the past 25 years have been the warmest years on record across the United States. The average temperature Nationwide is about 1F warmer than 100 years ago. Not surprisingly, the British Meteorological Office 2 (BMO) issued a forecast 1 NCDC website contains details at http://www.ncdc.noaa.gov/oa/climate/research/2 006/ann/ann06.html 2 See report at http://www.metoffice.gov.uk/corporate/pressoffi ce/2007/pr20070104.html suggesting that 2007 would be warmer than 2006. Short term data from NCDC clearly suggests that we are in a longer term warm period. The BMO report suggests this trend will continue providing a warmer year in 2007. Climatological studies beyond the scope of this report would be required to definitively show that this global warming trend shall continue and determine the root causes of the recent warmth. The Intergovernmental Panel on Climate Change (IPCC) report on climate change is due out later in 2007. The impact of Greenhouse gases is the leading suspect in the overall warming around the globe. Insufficient data is available to address this issue within the context this document. Figure 11 shows the snow cover over the northern hemisphere as of 9 January 2007. These data show a snow deficit over eastern North America and Europe. This reinforces the concept that conditions have been warm on both sides of the Atlantic. Though not shown, there has been a slight snow deficit over the hemisphere most of this winter. The snow deficit over eastern North America and Europe could signal the impact of global warming. The lack of ice north of Norway could be related to this. But the clearer signal may be the impact of the persistently positive NAO over the past 4-6 weeks. It would appear a strongly positive NAO favors warm conditions over both regions and may contribute to the snow deficits. 4. CONCLUSIONS A warm start to the winter of 2006-2007 was punctuated by a warm episode along
the East Coast of the United States on 6 January 2007. The warm episode saw many daily record high temperatures for the date fall to new record highs. Additionally, several sites tied or set new record high temperatures for the month of January (Table 1). Several of these monthly record highs were observed in 1998 and 1950 allowing a comparison of the features associated with this event relative to previous record warm days in January. Based on comparisons from the GR data and model output several critical features appeared to be associated with January warm episodes. These features included a combination of unseasonably high PW, 500 hpa heights, and low-level temperatures. Most of the variables were on the order of +2 SDs or more above normal in the affected areas. Several of the events also occurred ahead of an approaching cold front, suggesting the surge of warm air was focused in the southerly flow ahead of the low-level surface trough (See Fig 2 for example). There are many potential causes of the warmth experienced in January 2007. The culprits range from global warming to the flow pattern or perhaps a combination of each. It is interesting that the warm January of 1950 occurred during the negative or cold phase of ENSO while the warmth of January 1998 and 2007 were associated with the warm phase of ENSO. Connecting the 3 record warm episodes to ENSO shows a signal but not a definitive signal. Table 3 shows the ten warmest January temperature records in Pennsylvania for select sites. Most of these sites set several record highs during January 1950. Other years of note appear to be 1998 and 1995. Many sites have many top 10 warmest day records encompassing the past 10-20 years. Figures 12 & 13 show the plots of daily temperatures for January 1950 at Harrisburg, Warren, Philadelphia and Allentown, Pennsylvania. At these sites, and numerous other sites, the 25 th, 26 th and 4 th of January were exceptional warm days and these dates appear again in Table 3 and in many of the record event reports produced on 6 January 2006. The patterns shown in Figures 2 & 3 show the conditions associated with these previous record breaking January days. All 3 years where record high s for the month of January were set in the northeastern Unite States were associated with the positive phase of the NAO. The positive NAO signal may be a common thread, one that teleconnects well with warm conditions in both eastern North America and western Europe. This warmth due to the NAO may explain the snow deficit (Fig. 11) in these two regions. It appears that a positive NAO may be a good indicator of the potential for unseasonably warm episodes in the eastern United States and Western Europe. It should be noted that November 1950 was an extremely active weather month in the eastern United States. It should also be noted that the significant ENSO year of 1998 and the attendant warm winter was associated with significant spring time severe weather in the Mid- Atlantic region. However, there is insufficient data to draw any realistic conclusions on these data to predict the upcoming year.
preliminary data suggest that the annual average temperature for 2006 was 55 degrees F, 2.2 degrees F (1.2 degrees C) above the 20th century mean and 0.07 degrees F (0.04 degrees C) warmer than 1998. Previous record warm years of 1998 and 1934 were likely eclipsed by the robust warmth of December 2006. 5. REFERENCES Ambaum, M.H.P, B.J. Hoskins, and D. B. Stephenson 2001: Arctic Oscillation or North Atlantic Oscillation? Jour. of Climate.14, 5762 5776. Christopher G. Fletcher,C.G. and M. A. Saunders 2006.Winter North Atlantic Oscillation Hindcast Skill: 1900 2001.19,5762-5776. Figure 6. GEFS forecasts of 925 hpa temperatures initialized 0000 UTC 02 January 2007 valid at 1800 UTC 6 January 2007 showing a) each members 16, 8, and 0C contour and the spread about the mean and b) the ensemble mean and the departure in standard deviations form normal. The impacts of global warming may be affecting the large scale circulation and may play a role in the tendency for warm weather. As noted by the Climate Prediction Center report: The 2006 average annual temperature for the contiguous U.S. was the warmest on record and nearly identical to the record set in 1998. NCDC showed that 7 months in 2006 were much warmer than average, including December, which ended as the fourth warmest December since records began in 1895. ENSO Year JAN FEB MAR DJF 1950 0.92 0.4-0.36-1.8 1998 0.39-0.11 0.87 2.4 2006 + NA NA + Table 2. NAO value by year and month for past winters and the overall 3-month (Dec- Jan-Feb) seasonal value of ENSO. Data from the Climate Prediction Center. For 2006 the general sign of the NAO and ENSO is positive.
6. Figure 7. As in Figure 2 except NAM forecast initialized at 1800 UTC 4 January 2007 valid at 1800 UTC 6 January 2007. Figure 8 As in Figure 7 except NAM 00-hour forecasts valid at 1800 UTC 6 January 2007.
Figure 10 Observed hourly high temperatures on 6 January 2007. Temperatures in degrees F.
Figure 11 Observed snow cover (blue shaded) and the climatological extent of snow cover (green line) over the northern hemisphere. Courtesy of Dr Robert Hart of the Florida State University.
Station Day and Year Max Temp Station Day and Year Max Temp Station Day and Year Max Temp Station Day and Year Max Temp AOO 25-Jan-50 74 MDT 26-Jan-50 75 PHL 26-Jan- 50 74 ERI 25-Jan-50 72 AOO 4-Jan-97 68 MDT 4-Jan-00 68 PHL 29-Jan- 02 72 ERI 26-Jan-50 69 AOO 26-Jan-50 68 MDT 8-Jan-98 68 PHL 6-Jan-50 71 ERI 14-Jan-95 68 AOO 4-Jan-50 67 MDT 29-Jan-02 68 PHL 14-Jan- 95 70 ERI 5-Jan-97 66 AOO 20-Jan-51 67 MDT 4-Jan-50 67 PHL 15-Jan- 95 70 ERI 4-Jan-97 66 AOO 24-Jan-67 66 MDT 14-Jan-95 66 PHL 23-Jan- 99 69 ERI 22-Jan-33 66 AOO 25-Jan-67 66 MDT 4-Jan-98 66 PHL 24-Jan- 50 69 ERI 13-Jan-95 65 AOO 15-Jan-52 65 MDT 20-Jan-51 65 PHL 8-Jan-98 69 ERI 1-Jan-85 65 AOO 29-Jan-75 65 MDT 24-Jan-53 64 PHL 24-Jan- 67 69 ERI 18-Jan-29 65 AOO 8-Jan-98 65 MDT 9-Jan-98 64 PHL 4-Jan-50 68 ERI 14-Jan-32 65 STCP1 25-Jan-50 71 IPT 25-Jan-67 69 AVP 25-Jan- 67 67 PIT 31-Jan-02 72 STCP1 14-Jan-32 70 IPT 11-Jan-75 65 AVP 4-Jan-50 66 PIT 22-Jan-99 69 STCP1 26-Jan-67 69 IPT 14-Jan-95 64 AVP 11-Jan- 75 66 PIT 1-Jan-85 69 STCP1 25-Jan-67 68 IPT 4-Jan-50 64 AVP 8-Jan-98 66 PIT 13-Jan-95 68 STCP1 13-Jan-32 68 IPT 8-Jan-98 64 AVP 12-Jan- 75 66 PIT 13-Jan-72 68 STCP1 20-Jan-51 67 IPT 15-Jan-95 63 AVP 14-Jan- 95 66 PIT 21-Jan-59 67 STCP1 9-Jan-98 66 IPT 31-Jan-74 62 AVP 26-Jan- 50 66 PIT 14-Jan-95 67 STCP1 26-Jan-50 66 IPT 1-Jan-79 62 AVP 15-Jan- 95 65 PIT 4-Jan-93 67 STCP1 8-Jan-98 65 IPT 18-Jan-90 62 AVP 25-Jan- 50 64 PIT 2-Jan-00 66 STCP1 30-Jan-47 65 IPT 7-Jan-98 61 AVP 21-Jan- 59 64 PIT 5-Jan-97 66 Table 3. Top ten highest observed high temperatures for select sites across Pennsylvania. Station 3-letter IDs, the day and year along with the observed maximum temperature is provided for the 10 warmest days in the month of January.
a. b. Figure 52 As in Figure 1 except monthly high and low temperatures observed during January 1950 at a) Harrisburg and b) Warren Pennsylvania.
a. b. Figure 63 As in Figure 1 except monthly high and low temperatures observed during January 1950 at a) Philadelphia and b) Harrisburg, Pennsylvania.