The Devastating Mid-Mississippi Valley Floods of 1-2 May 2010 By Richard H. Grumm National Weather Service

The Devastating Mid-Mississippi Valley Floods of 1-2 May 2010 By Richard H. Grumm National Weather Service 1. INTRODUCTION A devastating heavy rainfall event affected the Mid-Mississippi Valley (MMV) from 1-3 May 2010. The heavy rain and associated severe thunderstorms killed 29 people in the impacted region. As shown in Figure 1, the heaviest rains extended from northwestern Mississippi across western Tennessee and into southwestern Kentucky. Western Tennessee had the heaviest rainfall with over 200 mm inches in the gridded datasets and reports of over 400 mm based on spotter reports. The City of Nashville was devastated by floods which closed the historic district and impacted famous sites such as the Grand Old Opry House. Nashville set a single day record on 2 May 2ith 7.25 inches exceeding the record 6.60 inches set on 13 September 1979. This followed the now third wettest day on record, 1 May 2010 when 6.32 inches of rain was observed. The 2-day rains made May 2010 the wettest May on record for the city. The 2-days of heavy rainfall set many new record rainfall records across portions of western Tennessee and Kentucky. The largest report to date was 17.73 inches at Camden, Tennessee. Bowling Green, Kentucky, a location with over 140 years of records, set a 2-day record with 9.67 inches of rainfall. In addition to the heavy rainfall, the event contained severe weather. The combination of heavy rain and severe weather caused massive power outages over the region. As shown in Figure 2, the severe weather peaked on 1 May over the heavy rainfall region with tornadoes and high winds being the primary severe weather observed. There were 146, 81, and 50 severe reports on 30 April, 1 May and 2 May respectively. Figure 2 only shows the severe weather on 1-2 May 2010. There were 3 tornado related fatalities on 1 May including one in Pocahontas, Tennessee. Bodner et al. (2011:Figure 4) showed that heavy rain events in the central United States during the historic floods of 1993 and 2008 occurred with an anomalously deep trough over the Rocky Mountains and a strong ridge over the eastern United States. This produces strong southerly flow allowing Gulf moisture to surge into the region. Not surprisingly, they found 850 hpa v-wind anomalies and precipitable water anomalies in excess of +2 standard deviations above normal with nearly all of this significant events (Bodner et. al 2011:Tables 2 &3). The event of 1-2 May 2010 had persistent and anomalous moisture and accompanying anomalous southerly flow, key ingredients for a significant heavy rainfall event (Doswell et. al 1996) and for historic events (Bodner et. al 2011). An important conclusion reached by Bodner et. al (2011) was that most of the events were associated with slow moving quasi-stationary fronts and along which weak surface waves developed. Not surprisingly, most of the historic events of 1993 and 2008 contained mesoscale convective system (MCS:Maddox 1979) associated with them. Carbone et al. 2002;Fritsch et al. 1986; Ashley et al. 2003; Junker et al. 1999 and Schumacher and Johnson 2006) all demonstrated the significant role MCS s play the total warm season precipitation over the Midwest and their impact on flash flooding over the region. From a prediction perspective, it will be shown that the NCEP models and ensemble prediction systems correctly predicted the pattern and significant anomalies associated with heavy rainfall. The value of ensembles and anomalies of key ingredients associated with heavy rainfall were demonstrated by Junker et al (2009). Other studies have shown the value of anomalies in identifying and predicting heavy rainfall events (Hart and Grumm 2001; Grumm and Hart 2001; Graham and Grumm 2010; Stuart and Grumm 2006;Junker et al 2008). This case is a classic

Figure 1 Total observed liquid precipitation (mm) from 1200 UTC 01 May through 1200 UTC 02 May 2010. From the unified precipitation data set. case on the value of anomalies in identifying a potentially significant heavy rainfall event. The NCEP short range ensemble forecast system (SREF) is used to show the probability of heavy rainfall associated with this event. Though the 32km SREF would, in fact did, under predict the local maximum rainfall, it did well in predicting the pattern, distribution and probability of heavy rainfall. As expected it missed the exact areas impacted and the amounts observed. This paper will document the historic and devastating rainfall and flooding over the MMV on 1-2 May 2010. The focus is on the pattern and anomalies associated with this meteorologically and climatologically significant event. Some forecasts from the NCEP models and ensemble forecast systems (EFS) are presented to show the value of ensembles in the forecast process. 2. METHODS AND DATA The 500 hpa heights, 850 hpa temperatures and winds, other standard level fields were derived from the NCEP GFS, NAM, GEFS, and the NCEP/NCAR (Kalnay et al. 1996) reanalysis data. The means and standard deviations used to

compute the standardized anomalies were from the NCEP/NCAR data as described by Hart and Grumm (2001). Anomalies were displayed in standard deviations from normal, as standardized anomalies. All data were displayed using GrADS (Doty and Kinter 1995). The standardized anomalies computed as: SD = (F M)/σ ( ) Where F is the value from the reanalysis data at each grid point, M is the mean for the specified date and time at each grid point and σ is the value of 1 standard deviation at each grid point. Model and ensemble data shown here were primarily limited to the GFS and GEFS. The 1.25x1.25 degree JMA data may be used when it becomes available. The NAM and SREF data were also available for use in this study. Displays will focus on the observed pattern and some forecast issues associated with the pattern. For brevity, times will be displayed in day and hour format such a 02/0000 UTC signifies 2 May 2010 at 0000 UTC. Figure 2. Storm reports from the Storm Prediction Center (SPC) showing event types by color for all reports on 1 May and 2 May 2010. Courtesy of the Storm Prediction Center. i. Synoptic scale pattern 3. RESULTS

Figure 3. NAM 00-hour forecasts of 500 hpa heights (m) and 500 hpa height anomalies (standard deviations) from NAM forecast initialized at a) 0600 UTC 1 May, b) 1200 UTC 01 May, c) 1800 UTC 1 May, d) 0000 UTC 2 May, e) 0600 UTC 2 May and f) 1200 UTC 2 May 2010. Figure 3 shows the large scale 500 hpa pattern over the region for the period of 01/0600 UTC through 02/1200 UTC 2 May 2010 from the NCEP NAM. The key feature is the deep and anomalous 500 hpa trough over the western United States. In addition to the deep trough a strong subtropical ridge was present to the east. The negative height anomalies were generally around -4SDs below normal in the trough with a minimum reached at 02/0000 UTC at -5SDs below normal. Positive height anomalies were over 2SDs above normal in the subtropical ridge. Strong southwesterly flow was present over the central United States. The 250 hpa winds (Fig. 4) showed the strong upper-level jet with wind anomalies in excess of 3SDs above normal, in the gradient between the strong ridge and deep trough (Fig. 3). The jet was initially quite diffluent (Figs. 4a-c) over the MMV but increased over time to a strong core of high winds over the region. Then an exit like feature began to move over the region around and after (not shown) 02/1200 UTC (Fig. 4f). ii. Regional pattern and anomalies The PW and PW anomalies over the southern United States are shown in Figure 5. These data show the surge of high PW air, an Atmospheric River (AR: Neiman et al. 2002 & 2008)

Figure 4. As in Figure 3 except for NAM 00-hour forecasts of 250 hpa wind and 250 hpa wind anomalies. extending from the Gulf of Mexico into the MMV. At 01/1200 UTC the PW anomalies in western Tennessee peaked at hover 5SD above normal, a rarely observed event. The high PW anomalies were associated with a strong southerly jet (Fig. 6) which surged into the MMV with 40KTS and greater winds and 2 to 3 SD v-wind anomalies at times. The v-winds peaked near 4SDs above normal on the over the Gulf States (Fig. 6f). The NAM surface pressure pattern showed low pressure over the MMV with 1 to 3 SD MSLP anomalies. Similar to Bodner et al. (2011), the pressure field suggested weak surface iii. disturbance moved along the frontal zone on 1 and 2 May 2010. One wave, over Arkansas as 02/0000 UTC was associated with -3SD MSLP anomalies. This wave was over southeastern Missouri by 02/0600 UTC (Fig. 7e). Forecasts-GEFS The GEFS members all predicted the large scale pattern quite accurately and thus the GEFS was able to predict the heavy rainfall over the MMV. Due to timing and resolution issues, the GEFS was only capable of predicting a heavy rainfall event but lacked the ability to correctly predict exactly where and when the heavy rainfall would occur.

Figure 5. As in Figure 3 except for NAM 00-hour forecasts of precipitable water (mm) and precipitable water anomalies. For brevity, 9 GEFS mean QPFs are shown in Figure 8, all verifying at 03/1200 UTC. The GEFS ensemble mean QPFs show that the GEFS correctly predicted a heavy rainfall event over the MMV and western Ohio Valley region. Forecasts from 28 April 2010 clearly predicted the heaviest rainfall too far north and west of the verifying region (Figs. 8a-c) and grossly under predicted the total amount as only 64 mm of QPF were shown in Figure 8a-b. There was a slow and steady progression of the axis of heavy rainfall to the south and east and an increase in the total QPF (Fig. 8c-f) and the introduction of the 96 mm contour near the impacted region. The 29/1800 UTC GEFS ensemble mean QPF showed a closed 128 mm contour just north and west of Nashville (Fig. 8f). Interestingly, most of these GEFS forecasts had 96 mm of total QPF for the period and consistently predicted a southwest to northeast axis of the heavy rainfall area. For 75km resolution members these forecasts were quite useful and clearly outlined the generalized threat area quite well. The forecasts initialized at 30/1200 UTC showed a decrease in the total rainfall. This cycle was curiously dry relative to the subsequent GEFS forecasts initialized at 30/1800 UTC and 01/0000 UTC (not shown). iv. Forecasts-SREF The SREF QPFs are shown in Figure 9. With 32 km resolution, the SREF forecasts showed more details and sharper edges than the GEFS. At times,

the SREF did remarkably well predicting the area where the heaviest rains would fall. However, the largest contour produced by the SREF was about 64 mm, only a few runs showed some areas of 96 mm of rainfall. The lack of a 96 mm contour reflects the details and flaw of averages with regards to displaying QPF data. Probabilistic displays are required to truly capture the range of potential maximum rainfall. Figure 10 shows the SREF mean PW and PW anomaly field from the members were used to show the rainfall in Figure 8. These data show that the SREF correctly predicted the surge of high PW air and 2 to 3 SD PW anomalies into the region. The orientation of the implied frontal zone and the axis of high PW air aligned reasonably well with the southwest to northeast axis of heavy rainfall in Figure 8. Clearly, the SREF was able to capture the larger scale pattern quite well. Though not shown, the SREF did remarkably well with the 850 hpa winds and v-wind anomalies too. Figure 11 shows the 30/0900 and 30/2100 UTC SREF 4 inch or greater QPFs. The upper panels show the probability of 4 inches or more in the 48 hour period ending at 03/1200 UTC and the lower panel s shows the ensemble mean QPF and each member s 4 inch contour. This and other forecasts revealed that the location of heavy rainfall in the SREF varied markedly from member-to-member and thus limited the ability of the mean field to depicted 96 mm or more QPF. 4. CONCLUSIONS A devastating rainfall event impacted the MMV on 2-3 May 2010. The heavy rains set many daily, 2- day and monthly rainfall records in portions of western Kentucky and Tennessee. The heavy rains produced flooding and some of the worst flooding in recent memory in portions of Kentucky, Tennessee, and Mississippi. In addition to the heavy rainfall, the event produced widespread severe weather. Most of the severe weather was observed on 30 April and 1 May 2010. This event shared many of the characteristic found in previous studies of heavy rainfall events. Using standardized anomalies (Grumm and Hart 2001) the data shown here illustrated the patterns of some of the key fields associated with heavy. As shown in Figure, a deep 500 hpa trough was present in close proximity to that described by Bodner et al. (2011) during the historic flooding events of 1993 and 2008. Additionally, the PW anomalies over 4SDs above normal (Figure 5) were observed in the impacted region during this event. Bodner et al. (2011) showed that during the historic rain events of 1993 and 2008, PW anomalies with all of these events were on the order of 2 to 4SDs above normal. In addition to the high PW anomalies, this event had large v-wind anomalies, and a series of surface waves moved along the frontal zone. Thus, this multi-day event shared many of the key characteristics identified by Bodner et al. (2011). This event shared some of the key paramaters associated with previously documented heavy rainfall events (Table 1) including: A plume of high PW air into the affected region with over +2.5SD PW anomalies. A deep trough with -1 to -2SD height anomalies to the west of the affected area A strong low-level jet at 850 and 925 hpa with v-wind anomalies over 2 SDs above normal A strong upper-level (250 hpa jet near the affected region A strong frontal zone to focus convection, in this case a strong southwest to northeast oriented frontal zone as defined by the PW field. Figure 12 shows the SREF flood threat potential based on anomalies from the SREF initialized at 30/0900 UTC. These data show the high probability of a +2SD or greater PW anomaly over the region (Fig. 12c) where the heavy rain fell. These data also show the probability of the 925 hpa winds exceeding 2.5SDs above normal (Fig. 12b). These two panels clearly defined a threat of a Maddox Synoptic event type with a strong southerly jet and a surge of high PW air into the region. Though not shown, the GEFS predicted a similar pattern. Products such as these may help define the patterns and probabilities of significant

rainfall events. One should not that Figure 12a, with u-wind anomalies were implemented to aid in determining the potential for a frontal type rain event. In addition to recognizing the pattern it is important that our models and EFS can predict these patterns. The data presented here suggested that the NAM (Fig. 13), GEFS, and SREF each was able to predicted the pattern and orientation of the axis of heavy rainfall. Figure 8 demonstrated how the GEFS did relatively well in predicting a wide area of 96mm or greater QPF over the MMV. It would be useful to know the internal climatology of 96 mm rainfall events over 24, 48, and 72 hour periods in the GEFS in order to gauge how climatologically significant this event was within the model and thus apply some value to it in the terms of potential impact. Figures 9 and 11 suggest that using a mean field (Fig. 9) may obfuscate the true magnitude and potential of the heavy rainfall potential. The probabilistic approach (Fig. 11), which to show all 9 runs in Figure 9 would require many additional 2-panels Figures. But the probability and spaghetti plots showed that SREF was predicting over 4 inches (100mm) of QPF but that the location varied considerably limited the appearance of the 96 mm contour when displaying the mean alone. It is important to know the range to include the maximum rainfall potential. Similar to the GEFS, it would be useful to know the return period within the SREF system of 64 and 96 mm of QPF. Knowing when the EFS and when a model is predicting a record event is important forecast and decision support information. Clearly, it would be of considerable value to know the climatology of heavy rainfall in the SREF and within the GEFS and thus know when the respective EFS is forecasting a record event. This is an important piece of forecast information that is missing in the forecast process. The NAM total QPF from the 30/1200 UTC cycle is shown in Figure 13. These data show a large close 128 mm contour in the general region where the heavy rainfall was observed. The NAM, run at 12km resolution, should be able to produce higher QPF amounts than the 32km SREF and the 75 km GEFS. Forecasters should be aware that finer resolution models typically produce higher values of QPF relative to their coarser resolution cousins. One problem with finer resolution models is that they convey little information about uncertainty unless their output is temporally and spatially adjusted to produce uncertainty information. 5. Acknowledgements 6. REFERENCES Bodner, M.J., N.W Junker, R.H. Grumm and R.S Schumacher 2011: Comparison of Atmospheric Circulation Patterns durng the 2008 and 1993 Historic Midwest floods. Submitted to WAF April 2010.

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Figure 7. As in Figure 3 except for NAM 00-hour forecasts of mean sea level pressure (hpa) and mean sea level pressure anomalies.

Figure 8. NCEP GEFS ensemble mean QPF (mm) showing accumulated QPF valid at 1200 UTC 3 May 2010 from forecasts initialized at a) 0000 UTC 28 April 2010, b) 1200 UTC 28 April, c) 0000 UTC 29 April, d) 0600 UTC 29 April, e) 1200 UTC 29 April, f) 1800 UTC 29 April, g) 0000 UTC 30 April, h) 0600 UTC 30 April and i) 1200 UTC 30 April 2010.

Figure 9. As in Figure 8 except for SREF mean QPFs from SREF members initialized at a) 2100 UTC 29 April 2010, b) 0300 UTC 30 April, c) 0900 UTC 30 April, d) 1500 UTC 30 April, e) 2100 UTC 30 April, f) 0300 UTC 30 April, g) 0900 UTC 30 April, h) 1500 UTC 30 April and i) 2100 UTC 30 April 2010.

Figure 10. As in Figure 9 except showing SREF precipitable water and precipitable water anomalies from the ensemble mean valid at 1200 UTC 2 May 2010.

Figure 11 SREF forecast of 4 inches or more QPF in the 48 hour period ending at 1200 UTC 3 May 2010 from forecasts initialized at 0900 and 2100 UTC 30 April 2010. Upper panels show the probability (shaded) of 4-inches or more QPF and the mean 4 inch contour. Lower panels shows the ensemble mean QPF for the period and each member s 4 inch contour.

Figure 13. NAM forecasts initialized 1200 UTC 30 April 2010 showing a) mean sea level pressure (hpa) and pressure anomalies and b) total accumulated precipitation (mm) for the period ending at 1200 UTC 3 May 2010.

. Figure 12. SREF threats of heavy rainfall patterns from the NCEP SREF initialized at 0900 UTC 30 April 2010. Data shown include the probability of a) 925 hpa u-winds less than -2.5SDs below normal, b) 925 hpa v-winds greater than 2.5SDs above normal, c) precipitable water values greater than 2SDs above normal and the 25 and 50 mm contour from the ensemble mean, and d) the probability of the 925 hpa winds being - 1.5SDs below normal.

Date Region 24 hour Max (mm) PW anomlies 850 v- wind 500 hpa low to west 2-Apr-79 TN 96 2 1 Y 4-Apr-79 AL 96 2 1 Y 12-Apr-79 MS 96 2 2 Y 13-Apr-79 AL 96 2 2 Y 16-Nov-87 AR 128 2 3 Y 25-Dec-87 TN 128 3 2 Y Comments SW-NE PW axis Table 1. List of events by date and the location of the heavy rainfall based on the UPD dataset. Data include the State where the heaviest rainfall was observed, the value of the PW anomaly and the value of the v-wind anomaly and if a 500 hpa trough was to the west. Comments provided as needed.

Figure 13. JRA data showing the conditions associated with the heavy rains and flooding of April 1979. Data include a) 500 hpa heights (m) and height anomalies, b) 850 hpa winds and v-wind anomalies, c) precipitable water (mm) and precipitable water anomalies, and d) mean sea-level pressure (hpa) and pressure anomalies.

Figure 14. As in Figure 13 except for 1200 UTC 16 November 1987. Case taken from Corfidi, Junker, and Glass 1990 Study of the Louisianna/Mississippi Floods of 15-16 November 1987 study.