National Weather Service-Pennsylvania State University Weather Events Eastern United States Winter Storm and Severe Event of 28-29 February 2012 by Richard H. Grumm National Weather Service State College PA 16803 and Kyle Imhoff The Pennsylvania State University Abstract: A mid-tropospheric trough moved into a strong ridge over the Gulf States. Strong southerly flow between the systems produced a surge of above normal precipitable water into the eastern plains and the Mid-Mississippi Valley resulting in a two-day severe weather event. North of the surface cyclone associated with the wave heavy snow was reported from the upper-midwest into New England. The warm air and strong winds produced 36 tornadoes over 2 days and in excess of 400 reports of severe weather. Initial data from the Storm Prediction Center suggests 13 people were killed in the tornado outbreak with 6 deaths in Harrisburg, Illinois. This was one of the largest severe weather and tornado outbreaks in the Midwest since 1950. The last major February event was the Super Tuesday event of 6 February 2008 which had over 500 severe reports and 93 tornadoes. This paper will document the event of 28-29 February 2012. The focus is on the pattern and the value of standardized anomalies to aid in identifying extreme high impact events.
1. INTRODUCTION A vigorous 500 hpa trough moved across the Rocky Mountains and into the Plains on 28 February 2012 (Fig. 1). The low-level flow between the 500 hpa wave and the 500 hpa ridge to the east produced strong low-level flow which pulled a plume of moisture into the eastern Plains and Mid-Mississippi Valley (MMV: Fig. 2). Precipitable water anomalies of 2 to 3σ above normal were present in this moisture plume along with a strong 850 hpa jet. The combination of warm moist air and strong winds produced a late-winter severe weather event. Similar to many winter severe events, north of the accompanying surface cyclone, heavy snow was observed from the upper-midwest into New England. This event produced the first documented February tornadoes in Nebraska (Fig. 3) and over 440 severe reports over the two day period of 28-29 February. There were 48 tornadoes and at least 13 reported fatalities related to the severe weather (AP 2012; St. Louis Post Dispatch 2012). Grumm and Hart (2001) showed the value of using re-analysis climate data (R-Climate) to characterize an event. The term R-Climate is used in reference to analysis and forecasts which use re-analysis climate data to diagnose or forecast the departures from normal They used standardized anomalies to show how the pattern with significant weather events may aid in discriminating ordinary from extraordinary events. The general method to compute and use R- Climate was described in Hart and Grumm (2001). Studies using R-Climate data (Graham and Grumm 2001; Stuart and Grumm 2006; Hart and Grumm 2001;Junker et. al 2008) have shown the value in using these data to anticipate many significant weather events over the past 60 years. Several of these previous studies have demonstrated that these data can be employed with model and ensemble forecast system data to better anticipate high event events. Junker et al. (2008) specifically showed how significant large R-climate anomalies in model and ensemble forecasts of patterns associated with heavy rainfall, when combined with model quantitative precipitation forecasts can aid in predicting heavy rainfall. The term R-Climate is used here to aid in discriminating between re-analysis climate, based on re-analysis data and internal model climate data. Specifically, the European Center for Medium Range forecasting (ECMWF) uses a 15-year internal ensemble forecast system climatology to aid in predicting high impact weather events (Lalaurate 2002, Legg and Mylne 2004,Ferro 2007). The ECMWF produces an Extreme Forecast Index (EFI) based on this internal model climatology. It should be noted in the United States severe weather is typically associated with convective based weather. The term is used more generally in Europe and other countries to include many high impact weather and it is not a term focused on convectively driven severe weather. This paper will document the pattern and the R-climate anomalies associated with the lower severe weather and tornado event of 28-29 February 2012. The focus is on the R-Climate based standardized anomalies as a tool to both analyze and predict this and similar extreme convective events. The forecast section of this paper will show the value of using climate data and internal ensemble prediction system climate data to better anticipate and predict extreme weather events. 2. Methods and Data The National Centers for Environmental Predictions (NCEP) Global Forecast System (GFS) were used to re-produce the conditions associated with the event including the large scale pattern. The standardized anomalies are displayed in standard deviations from normal as in Hart and Grumm (2001) and are computed using the climatology from the NCEP/NCAR global
reanalysis data (Kalnay et al. 1996). The focus is on the pattern and anomalies associated with the storm. The value of EFS and anomalies with EFS data are presented. Ensemble data shown here are from the NCEP Global Ensemble forecast system which is run at 75km in horizontal resolution. The emphasis here is on products which may aid in predicting high wind events. This includes the probability of strong winds at various levels to include 10m, 850 hpa, 700 hpa and 500 hpa. These data were also used to examine the pattern using the 27.5km NCEP GFS 00-hour forecasts. The pattern and standardized anomalies followed the methods outlined in Hart and Grumm (2001) and the GFS 00-hour forecasts were used to establish the pattern and standardized anomalies. The severe weather reports were obtained from the real-time Storm Prediction Center (SPC). Comparative data from 1950-2011 were also retrieved from the SPC website and were put into MySQL for obtaining statistics and comparative data. For brevity, times are presented as day and hour in the format 28/1200 UTC and 29/0000 UTC which would be 1200 UTC 28 February 2012 and 0000 UTC 29 February 2012 respectively. Fully qualified dates are limited to comparative data from times outside of January 2012. 3. The Storm system and impacts i. The large scale pattern The 500 hpa pattern (Fig. 1) showed the short-wave which moved over the eastern Plains on 28 February 2012. The short-wave was associated with a strong 250 hpa jet with winds in excess of 100kts in the jet core (Fig. 4). The 250 hpa winds and anomaly pattern at 29/1200 UTC showed a near text book divergent pattern over the southern plains with a +2 to +3σ jet core in that region (Fig. 4d). This divergent pattern persisted and was still evident at 01/0000 UTC (Fig. 4e). The wind anomalies in the jet and in the jet entrance over the Great Lakes (Fig. 4b) weakened over time. Between the approaching short-wave and ridge to the east, low-level southerly flow developed (Fig. 5) in the plains and moved eastward. This strong low-level jet (LLJ) first appeared over western Kansas and Nebraska at 27/1800 UTC (not shown) then slowed and strengthened as it moved eastward. The data in Figure 5 show these winds in close proximity of the time bracketing the severe weather and tornadoes. The precipitable water (PW) field shows the surge of high PW into the region with the strong LLJ. At 28/1800 UTC PW anomalies peaked at over 3s above normal in Kansas and Nebraska (Fig. 6c). The combination of the strong LLJ and the above normal moisture (MFLUX) resulted in high values of 850 hpa moisture flux (Fig. 7). MFLUX values peaked over 6s above normal in Kansas and Nebraska at 28/1800 UTC and this region of unusually high MFLUX and MFLUX anomalies moved eastward as the system progressed to the east. ii. Forecasts For brevity 2 SREF runs are shown to illustrate the strong shear and instability which was forecast over the Plains and MMV on 28 and 29 February 2012 (Figs. 8-11). The focus here is on displays of strong low-level winds, shear, and instability. The CAPE images show the probabilities of 400 and 800JKg-1. Brooks and Craven (2003) and Grunwald and Brooks (2011) showed the importance of CAPE and shear and the critical need for shear to produce stronger tornadoes. Thus, SREF images focus on shear and instability. The severe weather on 28 February (Fig. 3) developed in Kansas and Nebraska and moved eastward. The 27/2100 UTC SREF forecasts valid at 29/0000 UTC (Fig. 8) the 850 hpa LLJ was over 25 ms-1 in this region and the ensemble mean 400JKg-1 CAPE axis reached into
southern Nebraska. The shear was predicted to be over 1.2x10^2 s-1 in this region. The initial severe weather developed in this modest CAPE, though sufficiently strong CAPE for later February. The convection then moved eastward. Though not shown, the SREF also developed a north-south QPF axis which moved eastward implying precipitation. The strong winds and shear persisted overnight but the CAPE slowly diminished with only small area left by 29/0900 UTC (Fig. 9). Despite this the convection kept progressing eastward. The CAPE continued to fall as shown in the forecasts initialized at 28/0900 valid at 29/0900 UTC (Fig. 10) and 29/1200 UTC (Fig. 11). These forecasts imply the convection was forecast to develop in a modestly unstable environment with very strong southerly winds and strong shear. This strong forcing created a signal in the NCEP NAM. The 4km NAM initialized at 28/1800 UTC (Fig. 12) showed the initial enhanced simulated echoes from South Dakota to Kansas. At 29/0300 UTC the convection held together through 29/0600 UTC and still had a strong cell implied along the line in Illinois by 29/1100 UTC before weakening at 29/1200 UTC. Other NAM cycles showed a line of storms but the intensity and locations varied markedly. The 28/1800 UTC cycle showed the strongest signal after 0600 UTC and were shown in Figure 13. But the 28/1200 cycle had a weaker line in Illinois by 29/1100 UTC. The forecasts initialized at 29/0000 and 29/0600 UTC were also relatively weaker with the Illinois line. There may be spin-up issues (Fig. 13) with these high resolution runs and some type of storm scale ensemble would likely improve these types of forecasts. These data can directly be compared to the composite reflectivity data (Figure 14) which shows that despite the 4km NAM forecasts, the line remained rather strong as it crossed southern Illinois during the morning hours on 29 February 2012. iii. Observations The severe weather over the eastern Plains and MMV on 28-29 February (Fig.3) showed that the initial severe weather developed over Kansas and Nebraska and rapidly moved eastward. The severe reports and radar suggest that the convection began to develop after 28/2200 UTC. The first well organized cells appeared by 28/2200 UTC and rapidly organized as they moved eastward (Fig. 15). A broken line was evident by 29/0200 UTC which kept intact and was over southern Illinois by 29/1100 UTC (Fig. 14). This line was responsible for the severe weather shown in Figure 3. Based on radar (Fig. 13 & 14) and severe report times (not shown) the event developed late on 28 February and lasted less than 24 hours, but due to severe weather reporting times, the event was recorded on 28 and 29 February. With over 440 severe reports on 28-29 February 2012, the event was comparable to some of the larger severe events since 1950 (Table. 1). This event, which technically spans two days, is behind the event of 11 February 2009 and 6 February 2008 when 534 and 475 severe reports were recorded. The 28 February numbers put this event as the 4 th largest single day February event. 4. Conclusions A vigorous 500 hpa trough moved across the Rocky Mountains and into the Plains on 28 February 2012 (Fig. 1). The low-level flow between the 500 hpa wave and the 500 hpa ridge to the east produced strong low-level flow which pulled a plume of moisture into the eastern Plains and Mid-Mississippi Valley (MMV: Fig. 2). Precipitable water anomalies of 2 to 3σ above normal were present in this moisture plume along with a strong 850 hpa jet. The combination of warm moist air and strong winds produced a late-winter severe weather event. Similar to many winter severe events, north of the accompanying surface cyclone, heavy snow was observed from the upper-midwest into New England. The GFS 0-hour forecasts suggest that this event occurred with abnormally strong lowlevel flow and with a surge of above normal moisture. Model forecasts (Figs. 8-11) and
observations (not shown) suggests that the CAPE with this event was relatively low, about 400-800 JKG-1. However, cold season severe events often do not require large values of CAPE and 400-600JKG-1 of CAPE in a strongly sheared environment is often enough to produce upright convection. This may relate back to Brooks and Craven (2003) and Grunwald and Brooks(2011) which show that strong shear (Fig. 5&6: Grunwald and Brooks 2011) is often a better predictor of tornadoes, and of stronger tornadoes more specifically, than CAPE or LCL height. Given sufficientcape, low-level shear is extremely important in sustaining strong convection and producing tornadoes. Based on radar (Fig. 13 & 14) and severe report times (not shown) the event developed late on 28 February and lasted less than 24 hours, but due to severe weather reporting times, the event was recorded on 28 and 29 February. This is a worst case scenario and often occurs with strongly forced events as the life threatening weather occurs when people are the least connected and are likely to be asleep. This can increase the danger and the loss of life during an event. The data shown here implies that the 32km NCEP SREF did well in predicting the pattern favorable for convection and potentially severe convection. The SREF predicted a strong LLJ and strong shear comparable to the values shown by Grunwald and Brooks (2011). In addition to the SREF forecasts of a favorable large scale environment, the NCEP 4km NAM (Fig. 12 & 13) attempted to produce a line of showers with enhancements along the line that at times were similar to the echo patterns which appeared on radar (Fig. 14&15). The 4km NAM forecasts showed great variability and tended to under predict the strength of the convection by the morning hours (29/1100 UTC). There was considerable run-to-run variability in the 4km NAM forecasts and some suggestion of a spin-up issue with more recent forecasts. These forecasts imply the need for a good Storm Scale Ensemble to deal with severe convection. 5. Acknowledgements Thanks to the Storm Prediction Center for real-time data access and images and for the 1950-2011 data for climatological referencing. Thanks to the Pennsylvania State University and the National Weather Service in State College for support of student volunteers to conduct research and case studies. 6. References Brooks, H.E., Lee, J.W., Craven, J.P., 2003. The spatial distribution of severe thunderstorm and tornado environments from global reanalysis data. Atmos. Res. 67 68, 73 94. Graham, Randall A., and Richard H. Grumm, 2010: Utilizing Normalized Anomalies to Assess Synoptic-Scale Weather Events in the Western United States. Wea. Forecasting, 25, 428-445. Ferro, C., 2007: A probability model to verify deterministic forecasts of extreme events. Wea. Forecasting,22: 1089-1100. Grumm, R.H 2011: The Central European and Russian Heat Event of July-August 2010.BAMS, 92, 1285-1296. Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. and Fore., 16,736 754. Grunwald,, S. and H.E. Brooks, 2011: Relationship between sounding derived parameters and the strength of tornadoes in Europe and the USA from reanalysis data. Atmos. Res. 100. 497-488. Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, 2426 2442.
Lalaurette, F., 2003: Early detection of abnormal weather conditions using a probabilistic extreme forecast index. Quart. J.Roy. Meteor. Soc., 129, 3037 3057. Legg, T.P. and K.R. Mylne 2004: Early Warnings of severe weather from ensemble forecast information. Wea. Forecasting, 19,891-906. National Weather Service: 2009: Service Assessment: Super Tuesday Tornado Outbreak of 5-6 February,2008. US Department of Commerce, Silver Spring, MD. 29pp. Trapp,R.J, S.A. Tassendorf, E.S. Godfrey, H. Brooks, 2005: Tornadoes from squall lines and bow echoes.part I: Climatological Distributions. WAF,23-34.
Figure 1. NCEP GFS 00-hour forecasts of 500 hpa heights (m) and height anomalies in 12 hour increments from a) 0000 UTC 28 February 2012 through f) 1200 UTC 01 March 2012. Heights every 60m and anomalies in standard deviations based on the color bar. Return to text.
Figure 2. As in Figure 1 except for precipitable water and precipitable water anomalies. Precipitable water is every 5mm. Return to text.
Figure 3. Storm reports for the Storm prediction center (SPC) for 28 and 29 February 2012. Data are color coded by event type. Images courtesy of the Storm Prediction Center. Return to text.
Figure 4. As in Figure 1 except for 250 hpa winds and 250 hpa wind anomalies. Winds are in knots. Return to text.
Figure 5. As in Figure 1 except for 850 hpa winds (kts) and total wind anomalies in 6-hour periods from a) 0600 UTC 28 February through f) 1200 UTC 29 February 2012. Return to text.
Figure 6. As in Figure 5 except for precipitable water (mm) and precipitable water anomalies. Return to text.
Figure 7. As in Figure 5 except for 850 hpa moisture flux and moisture flux anomalies. Return to text.
Figure 8. NCEP SREF initialized at 2100 UTC 27 February 2012 showing forecast valid at 0000 UTC 29 February 2012 including a) the mean 850 hpa wind and the probability of these winds exceeding 20ms-1, b) the mean CAPE and the probability of the CAPE exceeding 400 JKg-1, and c) the mean CAPE and the probability of the CAPE exceeding 400 JKG-1 and d) the 1000-850 hpa shear and the probability of the shear greater and then 0.012s-1. Winds are every 5ms-1, CAPE is contoured showing 400,800, 1200 and 2400 JKg-1, shear in powers of 2x10^2. Return to text.
Figure 9. As in Figure 8 except valid at 0900 UTC 29 February 2012. Return to text.
Figure 10. As in Figure 8 except for 0900 UTC 28 February 2012 SREF forecasts valid at 0900 UTC 29 February 2012. Return to text.
Figure 11. As in Figure 10 except valid at 1200 UTC 29 February 2012. Return to text.
Figure 12. Select 4km NAM synthetic radar images from the NAM initialized at 1800 UTC 28 February 2012 showing the forecasts valid at a) 0300 UTC, b) 0600 UTC, c) 1100 UTC and d) 1200 UTC 29 February 2012. Times match discussion in text. Return to text.
Figure 13. As in Figure 12 except valid at 1100 UTC 29 February from forecasts issued at 1200 UTC 28 February, 0000 UTC 29 February and 0600 UTC 29 February 2012. Return to text.
Figure 14. NMQ-Q2 site composite reflectivity (dbz) to match the times of the forecasts in Figure 13 and Figure 12. Return to text.
Figure 15. As in Figure 14 except for valid at 2200 UTC 28, 0000 UTC 29 and 0100 UTC 29 February 2012. Return to text.
Events Date 534 2/11/2009 475 2/6/2008 288 2/21/1997 275 2/10/1990 228 2/22/2003 224 2/26/1998 213 2/26/2008 197 2/21/2005 192 2/12/2008 175 2/10/1998 170 2/27/1999 164 2/24/2001 158 2/11/1999 156 2/9/2001 155 2/14/2000 149 2/18/2009 147 2/5/2008 146 2/17/1998 136 2/25/2001 124 2/16/2001 124 2/27/2009 122 2/13/2007 122 2/17/2008 120 2/25/2000 119 2/21/1993 119 2/16/1990 113 2/15/1987 109 2/21/1989 107 2/7/1999 106 2/18/2000 Table 1. List of number of Severe Storm reports and the date they were observed. Return to text.