National Weather Service-Pennsylvania State University Weather Events

Transcription

1 National Weather Service-Pennsylvania State University Weather Events March Madness: The 2 March 2012 super outbreak-draft by Richard H. Grumm National Weather Service State College PA and Kyle Imhoff The Pennsylvania State University Abstract: A vigorous short-wave swept across the Mid-Mississippi and Ohio Valleys on 2 March 2012 resulting in one of the largest early spring severe weather outbreaks since This system produced over 900 reports of severe weather, including an estimated 144 tornadoes. This was the second deadly severe weather and tornado event to affect the central United States in less than three days. The strong trough brought a brief surge of warm moist air into the Mid-Mississippi and Ohio Valleys. The warm air produced unusually high CAPE along with strong low-level winds. Above the warm moist air was relatively dry air which led to an elevated mixed layer over the region. Many of the regional soundings and model forecast soundings showed the classic Type-I or load-gun soundings often associated with many high end severe weather and tornado events. This paper will document the deadly tornado and severe weather event of 2 March The focus is on the pattern, the elevated mixed layer, and the value of standardized anomalies to aid in identifying extreme high impact events.

2 1. INTRODUCTION A vigorous short-wave (Fig. 1) swept across the Mid-Mississippi and Ohio Valleys on 2 March 2012 resulting in one of the largest early spring severe weather outbreaks since 1950 (Table 1). The low-level flow ahead of the vigorous 500 hpa wave produced strong low-level flow which pulled a plume of moisture into the eastern Mid-Mississippi (MMV) and Ohio Valleys ( Fig. 2). The precipitable water (PW) values surged to 25 to 30 mm over the Ohio Valley between 1200 UTC 2 March and 0000 UTC 3 March PW anomalies were on the order of 2 to 3σ above normal at the height of the event. The deep low-level moisture and southwesterly flow aloft produced an elevated mixed layer (EML: Carlson et al. 1983) which likely contributed to the high number of severe reports and tornadoes over the MMV and Ohio Valley (Fig. 3). With over 900 reports of severe weather, this event was clearly a significant severe weather event and will surpass 14 March 2006 (Table 1) as the largest single severe day in March. The concept of significant severe weather is often described as the occurrence of hail greater than 5.1cm, convective wind gusts over 33ms-1 and/or tornadoes (Banacos and Ekster 2010; Doswell et al. 2005). The event of 2 March 2012 had large hail, damaging winds and tornadoes in the EF0 to EF4 range. In addition to meeting this more general requirement, there were hundreds of reports over the MMV and Ohio Valley which met this requirement. Significant March severe weather outbreaks typically have 200 or more severe reports with only 7 events having over 300 reports. The event of March 2006 had 410 and 518 severe reports respectively, (Table 1) making it the event of record for March. A method to better identify significant or High Impact weather events (HIWE) was demonstrated by Grumm and Hart (2001). This study showed the value of using re-analysis climate data (R-Climate) to characterize an event. 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 in reference to analysis and forecasts which use re-analysis climate data to diagnose or forecast the departures from normal. This term is used here to aid in discriminating between re-analysis climate, based on re-analysis data and internal model climate data. 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 record setting severe weather and tornado outbreak of 2 March The focus is on the R- Climate based standardized anomalies as a tool to both analyze and predict this and similar extreme convective events and the conditions over the MMV and Ohio Valley that produced this

3 event. 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 Prediction s (NCEP) Global Forecast System (GFS) was 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. Soundings were obtained from the University of Wyoming website and model soundings were taken form GFS and NAM analysis and forecasts using the GrADS Skew-T program. The Skew-T program was modified to read NAM and GFS GRIB files to produce skew-ts. The severe weather reports were obtained from the real-time Storm Prediction Center (SPC). Comparative data from were also 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 02/1200 UTC and 03/0000 UTC which would be 1200 UTC 2 March 2012 and 0000 UTC 3 March 2012 respectively. Fully qualified dates are limited to comparative data from times outside of January The Storm system and impacts i. The large and regional scale pattern The 500 hpa pattern (Fig. 1) showed the short-wave which moved over from the Pacific northwest to the MMV between 01/0000 and 03/0000 UTC. The strong southwesterly flow ahead of this system likely aided in pulling dry air off the higher terrain of the southwestern United States and Mexico into the MMV and Ohio Valley. As the 500 hpa wave entered the MMV the height anomalies were on the order of -2 to -3s below normal (Figs. 1e-f). The low-level response to the approaching short-wave, including the surge of high PW air (Fig. 2), the strong southwesterly flow (Fig. 4) and the strong 850 hpa moisture flux (MFLUX:Fig. 5 ). The 850 hpa wind anomalies were on the order of 2 to 3σ above normal over the region at the height of the event (Fig. 4c-e), PW anomalies were in the 2 to 3σ range, and the 850 hpa MFLUX was on the order of +4 to 5σ above normal in the MMV and Ohio Valley. Soundings and model soundings showed an EML over the MMV and Ohio Valley. The 02/1800 UTC NAM soundings, for a point near West Liberty, Kentucky, valid at 1800 and 2100 UTC 2 March 2012 (Fig. 6) show the moist boundary layer with a mixed layer above this layer. Both soundings show deep southwesterly flow and a curved hodograph. The NAM CAPE was forecast to 939 and 2316 JKg -1. These CAPE values are impressive for mid-april let alone 2 March. Similar to the NAM, the NCEP GFS with 27km resolution also showed the moist boundary layer and the EML above it (Fig. 7). The closest proximity sounding was Nashville (KBNA) at 03/0000 UTC (Fig. 8) which showed the EML somewhat displaced aloft. Initially, no backward trajectory analysis was

4 conducted to track the EML back to the elevated terrain of the southwestern United States and Mexico. However, an examination of soundings upstream showed the EML move over stations to the south and west. The 02/1200 UTC sounding at Little Rock, AR (KLZK) shows the EML over the shallow moist layer. Once it was clear there was an EML to the west, the ARL trajectory site was used to compute a backward trajectory from the GDAS using Louisville as the end point. These data revealed that the air near 3000 m (700 hpa) came from the elevated plateau region of northern Mexico (Fig. 9) and the air near 850 hpa originated over the Gulf of Mexico. This event was a textbook example of the air streams associated with severe convection. ii. Forecasts The larger scale pattern, to include the deep trough and strong southerly winds and surge of high PW air were relatively well forecast. This could be inferred from the short-range forecast soundings from the NAM and GFS (Figs. 7&8). For brevity some key SREF forecasts and 4km NAM forecasts are provided. SREF forecast focus on stability and shear. The 01/0300 UTC SREF (Fig. 10) shows the 850 hpa wind and v-wind wind anomalies over the Ohio Valley at 02/2100 UTC, the PW and PW anomalies, CAPE and CAPE anomalies, and the surface pressure with surface pressure anomalies. These data show that at 02/2100 UTC the SREF predicted at the 70% level or higher forecast which departed from R-Climate by 1.5s or more for the more normally distributed fields such as wind, PW, and mean sea-level pressure. This forecast implied a strong signal for a pattern and parameter associated with severe weather. The less Gaussian CAPE fields showed CAPE of JKg-1 over the region which represented a 6s event in many locations. Early March is a time of extremely low CAPE and these CAPE values were significantly above normal for the time of year. A more probabilistic display from the 01/0300 UTC SREF (Fig. 11) showed the midlevel lapse rates, CAPE exceeding 800 and 1200JKg-1 and shear. The shear values exceeded the contour normally used as the shear was extremely high and was likely to exceed 1.2x10^2s-1. These forecasts (Figs. 10&11), 36 hours prior to the bulk of the severe weather showed nearly ideal conditions for severe weather to include strong winds, strong shear, and instability. The extremely strong shear has been shown by Grunwald and Brooks (2011) to be a good discriminator between high winds and tornadic events. The SREF initialized at 02/0300, 24 hours later showing conditions at 02/1800 and 02/2100 UTC (Figs. 12 & 13) showed a similar pattern as the earlier SREF forecasts with some subtle differences. The CAPE values were slightly higher and the probability of exceeding both 800 and 1200JKg-1 was higher in these shorter range forecasts. Clearly, this is the impact of lower spread in shorter-range forecasts. Two NAM 4km forecasts are shown in Figure 14. The forecasts initialized at 02/0600 UTC valid at 03/0200 show the implied multi-cellular line of storms extending from southwestern Pennsylvania southward to northern Alabama. The red line shows the path of the implied relatively large storm which originated in southern Illinois around 2000 UTC 2 March 2012 in the 4km NAM. The forecasts initialized at 02/1200 UTC show a similar line of storms but the locations of stronger elements are distinctly different. A cell that developed at 02/2000 UTC in southern Illinois in the 02/1200 UTC cycle (Fig. 15) is the element in southeastern Ohio at 03/0200 UTC in the lower panel in Figure 14. The details in these and other forecast cycles (not shown) all showed a good concept of a convective line but how the line evolved was quite differently. iii. Observations The SPC severe reports were shown in Figure 3 and the data from showing significant March severe weather events was shown in Table 1. These data imply, not accounting for social media report inflation, that this was the largest March severe weather event and the most prolific March tornado event since A rigorous analysis of EF2 and greater tornadoes

5 might show otherwise but no such tests were conducted herein. The storm reports appear to show a streaky nature of the severe weather implying persistent or continuously evolving cells dominated the severe reports, to include the hail streak from south-central Illinois into western Ohio, and to the south, the high volume of strong winds and tornadoes which developed over southern Illinois and tracked across southern Indiana and Ohio. Similar streaks can be implied farther south. This streaky nature to the severe weather showed up in the total estimated precipitation field (Fig. 16) and the 6-hourly precipitation field (Fig. 17). Clearly, the hail producing echoes were well aligned with the 12.5mm and greater rainfall from central Illinois into northern Ohio and the wind and tornado producing streak in Figure 3 from southern Illinois into southern Ohio was well aligned with the 12.5mm contour just north of the Ohio River (Fig. 16). Similar streaks in Figure 15 line up well with the severe reports and the track of echoes in the 4km NAM (Figs. 14 & 14). The 6-hourly precipitation (Fig. 17) implies that the convection initially organized in Missouri, there were severe reports, mainly hail, in Missouri prior to 02/1200 UTC (not shown). The convection and the rainfall showed more organization by 02/1800 UTC and 03/0000 UTC. Similar to the 24 hour precipitation data, these data too imply the streaky nature of the rainfall implying dominant cells or storms persisting as they tracked to the east. The NCEP 4km NAM was quite capable of replicating the general concept of streaky rainfall and implying dominant storms tracking eastward over the MMV and Ohio Valley on 2-3 March 2012 (Fig.18). 4. Conclusions A vigorous short-wave (Fig. 1) swept across the Mid-Mississippi and Ohio Valleys on 2 March 2012 resulting in one of the largest early spring severe weather outbreaks since 1950 (Table 1). The low-level flow ahead of the vigorous 500 hpa wave produced strong low-level flow which pulled a plume of moisture into the eastern Mid-Mississippi (MMV) and Ohio Valleys ( Fig. 2). The precipitable water (PW) values surged to 25 to 30 mm over the Ohio Valley between 1200 UTC 2 March and 0000 UTC 3 March PW anomalies were on the order of 2 to 3σ above normal at the height of the event. The deep low-level moisture and southwesterly flow aloft produced an elevated mixed layer (EML: Carlson et al. 1983) which likely contributed to the high number of severe reports and tornadoes over the MMV and Ohio Valley (Fig. 3). With over 800 reports of severe weather, this event was clearly a significant severe weather event and will surpass 14 March 2006 (Table 1) as the largest single severe day in March. The concept of significant severe weather is often described as the occurrence of hail greater than 5.1cm, convective wind gusts over 33ms-1 and/or tornadoes (Banacos and Ekster 2010; Doswell et al. 2005). The event of 2 March 2012 had large hail, damaging winds and tornadoes in the EF0 to EF4 range. In addition to meeting this more general requirement, there were hundreds of reports over the MMV and Ohio Valley which met this requirement. Significant March severe weather outbreaks typically have 200 or more severe reports with only 7 events having over 300 reports. The event of March 2006 had 410 and 518 severe reports respectively, (Table 1) making it the event of record for March. The GFS 00-hour forecasts used to diagnose this event suggest that many key parameters often associated with severe weather were present. This includes abnormally strong low-level (Fig. 4) flow which produced strong shear and curved hodographs (Fig. 7&8). R-Climate based Standardized anomalies are useful in identifying the pattern and whether key features associated with severe weather are exceeding climatologically known values. The NCEP SREF stability (Fig ) and shear forecasts showed a pattern often associated with severe weather. Additionally, the SREF showed a high probability of high CAPE exceeding 800 and 1200JKg-1 with strong shear. Based on the work of Brooks et al. (2003) and Grunwald and Brooks (2011) the strong shear with respectable CAPE is a good indicator of

6 tornadoes. The CAPE provides the updraft potential and the potential for damaging winds, but the research implies that the extremely high shear values correlated well with tornadoes and stronger tornadoes. Thus, the SREF provided excellent guidance suggesting that the larger scale pattern supported severe weather and a high probability of tornadoes and potentially strong tornadoes. The 4km NAM forecasts suggested the development of a strong line of convection would develop around 02/2000 UTC (Fig. 15) and sweep eastward. A comparison of the 03/0600 and 03/1200 UTC forecasts show despite these useful forecasts, there were significant difference in the forecasts. The earlier initialized 02/0600 forecasts showed a stronger storm over southern Illinois (Fig. 14) and tracked it into Pennsylvania (Fig. 15). The 02/1200 UTC cycle showed a smaller enhanced echo region and a slower tracking cluster. As seductive as these synthetic radar images appear, for specific forecasts they offer relatively random approximations. They should be used to infer the timing and the character of the convection but will likely only by chance have a potential storm or line of storms at the right location at the correct time. Clearly, storm scale ensembles are required to account for the significantly different outcomes produced by storm cycle models and a more probabilistic tool is required to make better use of these convective scale forecasts. The 4km NAM QPFs (Fig. 18) relative to observations (Fig. 16&17) imply that high resolution models in strongly forced situations can simulate what will evolve in the real atmosphere to include the streaky nature of the QPF. The only problem is the details as to how much QPF and where the QPF will fall varies markedly. 5. Acknowledgements Thanks to the Storm Prediction Center for real-time data access and images and for the 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. The University of Wyoming provided access to sounding and the Air Resource Laboratory provided access to HYSPLIT for trajectories. 6. References Banacos, Peter C., Michael L. Ekster, 2010: The Association of the Elevated Mixed Layer with Significant Severe Weather Events in the Northeastern United States. Wea. Forecasting, 25, LINK Brooks, H.E., Lee, J.W., Craven, J.P., The spatial distribution of severe thunderstorm and tornado environments from global reanalysis data. Atmos. Res , Carlson, T. N., S. G. Benjamin, G. S. Forbes, and Y. F. Li, 1983: Elevated mixed layers in the regional severe storm environment: Conceptual model and case studies. Mon. Wea. Rev., 111, Doswell, C. An III, H. E. Brooks, and M. P. Kay, 2005: Climatological estimates of daily local nontornadic severe thunderstorm probability for the United States. Wea. Forecasting, 20, 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, Ferro, C., 2007: A probability model to verify deterministic forecasts of extreme events. Wea. Forecasting, 22: Grumm, R.H 2011: The Central European and Russian Heat Event of July-August 2010.BAMS, 92,

7 Grumm, R.H. and R. Hart. 2001: Standardized Anomalies Applied to Significant Cold Season Weather Events: Preliminary Findings. Wea. And Fore. 16, 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 Hart, R. E., and R. H. Grumm, 2001: Using normalized climatological anomalies to rank synoptic scale events objectively. Mon. Wea. Rev., 129, Lalaurette, F., 2003: Early detection of abnormal weather conditions using a probabilistic extreme forecast index. Quart. J.Roy. Meteor. Soc., 129, Legg, T.P. and K.R. Mylne 2004: Early Warnings of severe weather from ensemble forecast information. Wea. Forecasting, 19, National Weather Service: 2009: Service Assessment: Super Tuesday Tornado Outbreak of 5-6 February, 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, United Press International 2012: US Tornado death toll reaches 37. UPI 3 March Verbout, Stephanie M., Harold E. Brooks, Lance M. Leslie, David M. Schultz, 2006: Evolution of the U.S. Tornado Database: Wea. Forecasting, 21, doi: /WAF910.1

8 Figure 1. NCEP GFS 00-hour forecasts of 500 hpa heights (m) and height anomalies in 12 hour increments from a) 0000 UTC 01 March 2012 through f) 1200 UTC 03 March Heights every 60m and anomalies in standard deviations based on the color bar. The green dot is near West Liberty, KY. Return to text.

9 Figure 2. As in Figure 1 except for precipitable water and precipitable water anomalies focused over the Ohio Valley. Precipitable water is every 5mm. Return to text.

10 Figure 3. Storm reports for the Storm prediction center (SPC) for 2 and 3 March Data are color coded by event type. Images courtesy of the Storm Prediction Center. Return to text.

11 Figure 4. As in Figure 1 except for 850 hpa winds and 850 hpa wind anomalies. Winds are in knots. Return to text.

12 Figure 5. As in Figure 2 except for 850 hpa moisture flux and moisture flux anomalies. Return to text.

13 Total Tornado Hail Wind Date /13/ /15/ /12/ /9/ /1/ /9/ /13/ /30/ /26/ /5/ /31/ /31/ /4/ /29/ /27/ /7/ /29/ /6/ /30/ /8/ /27/ /22/ /31/ /3/ /10/ /30/ /26/ /22/ /30/ /31/1993 Table 1. The dates of significant severe weather events by date sorted by the total number of severe events. Data include the total number of reports of severe weather, tornado reports, hail reports, and wind reports by date. Data span from the Storm Prediction Center data base. Return to text.

14 Figure 6. NCEP 12km NAM soundings at a point near West Liberty, KY from the 1800 UTC 2 March NAM. Dry bulb is red and wet bulb curve is in green. Winds color coded by speed. Hodograph is in upper right with key convective parameters in the table to the right of the Skew-T. Return to text.

15 Figure 7. As in Figure 6 except for 27.5km NCEP GFS soundings. Return to text.

16 Figure 08. Rawinsonde Skew-T for Nashville, TN (BNA) valid at 0000 UTC 3 March 2012 and Little Rock, AR (LZK) at 1200 UTC 2 March Sounding courtesy of the University of Wyoming. Return to text.

17 Figure 9. Backward trajectory from a point near Louisville, KY ending at 1800 UTC 2 March Trajectories for 5760 m (red), 3000 m (blue) and 1500 m (green) were selected from the GDAS courtesy of the Air Resource Laboratory website. Return to text.

18 Figure 10. NCEP SREF forecasts initialized 0300 UTC 1 March 2012 showing forecasts valid at 2100 UTC 02 March 2012 showing a) 850 hpa winds and the probability of 850 v-wind anomalies greater than 1.5s above normal, the ensemble mean winds are in ms-1, b) ensemble mean precipitable water (mm) and the probability of precipitable water 1.5s above normal, c)ensemble mean CAPE and the probability of CAPE exceeding 6s above normal, and d) ensemble mean mean-sea level pressure (hpa) and the probability of pressure -1.5s below normal. Return to text.

19 Figure 11. As in Figure 10 except for a) hpa lapse rates greater than 6.5Ckm-1, b), CAPE greater than 800 JKg-1, c) CAPE greater than 1200 JKg-1, and d) shear greater than 1.2x10^s-1. Values of exceedance are in percent. Return to text.

20 Figure 12. As in Figure 11 except for SREF initialized at 0300 UTC 2 March 2012 valid at 1800 UTC 2 March Return to text.

21 Figure 13. As in Figure 12 except forecasts valid at 2100 UTC 2 March Return to text.

22 Figure 14. 4km NAM forecasts of synthetic radar valid at 0200 UTD 3 March The red line in the upper image shows the track of a simulated large reflectivity area that developed over Illinois around 2000 UTC 2 March Return to text.

23 Figure 15. As in Figure 14 except forecasts valid at 2000 UTC 2 March Arrows show the initial strong echo in Illinois which tracked west in both forecast cycles. Return to text.

24 Figure 16. Stage-IV precipitation data (mm) showing the total precipitation for the period of 1200 UTC 2 March through 1200 UTC 3 March Shading as in key to the right and contours are 12.5, 25, 50 and 75mm. Return to text.

25 Figure 17. As in Figure 16 except for precipitation in 6-hour increments ending at a) 1200 UTC 2 March, b) 1800 UTC 2 March, c) 0000 UTC 3 March, and d) 0600 UTC 3 March Values shaded by color bar to right of each image. Return to text.

26 . Figure 18. As in Figure 15 except for 4km NAM accumulated quantitative precipitation forecasts (mm) from the 0600 and 1200 UTC 2 March 4km NAM. Accumulation periods end at 1200 UTC 3 March Return to text.

27