Upper Midwest heavy rain and floods of 23-24 September 2010 By

Richard H. Grumm National Weather Service Office

State College, PA 16803

1. INTRODUCTION Heavy rains impacted the upper Midwest on 23-24 September 2010 (Fig. 1). The heaviest rain fell between 1800 UTC 22 September through 1800 UTC 23 September 2010. The initial heavy rainfall was near the border if Iowa and Minnesota and then moved northward. The heavy rainfall was associated with convection. There were numerous reports of severe weather over Nebraska, Iowa, and southern Minnesota during the onset of the event. Most of the severe convection was on 22 September (Fig. 2). It will be shown that the pattern for this event was generally well predicted and thus the models and ensemble forecast systems (EFS) showed a high probability of heavy rain. The details remained elusive in this case. In this, the NCEP models and EFS clearly showed a bias placing the heavier rainfall north and west of the region where it was observed. This general northwest bias is a common error in many heavy rainfall events. This paper will provide an overview of the conditions associated with this event. A basic forecast methodology is presented showing how standardized anomalies and probabilities can be used to aid in improving situational awareness. The details may be elusive but we have considerable skill in relative to the patterns and probabilities. 2. METHODS

The pattern was reconstructed used the NCEP GFS and NAM. All data were plotted in GrADS (Doty and Kinter 1995). The higher resolution NCEP NAM is used to show the conditions during the event. The anomalies were computed from the NCEP/NCAR re-analysis data (Kalnay et al 1996) as describe by Hart and Grumm 2001 and Grumm and Hart 2001. Unless otherwise stated, the base data was the GFS or NAM and the means and standard deviations were computed by comparing the NAM to the NCEP/NCAR 30-year climatological values. For brevity times are referred to in the format of 23/1800 for 23 September 2010 at 1800 UTC.

3. METHODS i. Rainfall patterns and analysis The overall rainfall for the event was shown in Figure 1. The heaviest rainfall occurred in southern Minnesota and extended northeastward into central Wisconsin. Locally 160 to over 192 mm of rain were sensed by the Stage-IV data. As shown in Figure 3, the heaviest rain fell after 22/1800 UTC. The heaviest rain in southern Minnesota fell between 22/1800 and 23/0600 UTC as indicated by the heavier amounts for two periods ending at 23/0000 and 23/0600 UTC in Figures 3a & 3b respectively. Though not shown, a band of heavy rain impacted central Minnesota and northern Wisconsin between 23/1800

and 24/0000 UTC. This contributed to heavy rainfall upstream of the regions which had already experienced heavy rainfall. The area of 64 mm of rain in Figure 1 and the extensive area of the 32 mm contour depicts this latter region of heavy rainfall.

ii. Large scale pattern The large scale pattern, depicted by the 500 hPa heights and anomalies is shown in Figure 4. A large subtropical ridge was present over the eastern United States. A closed 5940 m contour was presented between 24/0000 and 24/1200 UTC along the southern eastern coast. Clearly, the flow about the subtropical ridge played a role in this event. In addition to the strong subtropical ridge, a 500 hPa shortwave over the Pacific Northwest had moved over the impacted region (Figs. 4a-i). The 250 hPa winds (Fig. 5) and wind anomalies better defined the upper-level flow over the ridge and the enhanced jet-streak moving into the western portions of the ridge. The appearance of a coupled jet circulation was north of the region of heavy rainfall. iii. Regional l pattern The larger scale pattern played a significant role in setting up the conditions favoring heavy rainfall. The flow about the subtropical ridge (Fig. 4) brought a surge or atmospheric river (AR :) of high precipitable water into the region (Fig. 5). GFS PW anomalies were 3 to 5SDs above normal in the affected region. The PW pattern clearly showed an east-west boundary north of the ridge (Figs. 5a-f) which the upper-level short-wave (Fig. 4) and more north-south oriented frontal boundary then “rolled up”. The raw PW values were 45 to 55 mm over northern Iowa and Minnesota from 23/0000

through 23/1800 UTC (Fig. 6). This high PW air slide over Wisconsin and Michigan thereafter as the entire system became more progressive.

The 850 hPa low-level jet (LLJ) evolutions are shown in Figure 7. These data show the easterly flow in the jet entrance region over southern Canada and a strong southerly jet into the east-west boundary in Figure 6. The surge of high PW was associated with this strong LLJ. Total wind anomalies near 6SD were present at 23/1800 through 24/1200 UTC (Figs. 7f-i). Earlier in the event LLJ wind anomalies were on the order of 3 to 4SDs above normal. The surface fields showed the evolution of a cyclone with the upper-level short-wave (Fig. 8). Pressure anomalies were on the order of -1 to -2SD below normal with the surface cyclone. To the south and east the strong subtropical ridge, with positive pressure anomalies was evident.

iv. Forecasts

GEFS and SREF QPFs from 9 runs are shown in Figures 9 & 10 respectively. Comparing these data to Figure 1 it is clear that both EFSs produced the heavy rainfall generally north and west of the verifying location. On the positive side, the general pattern of the rainfall and the timing was relatively good. The total QPF in the GEFS (Fig. 9) runs was generally around 64 mm in the axis of the heavy rainfall. Slightly lower amounts were indicated in early runs verse latter runs. This is likely the result of uncertainty. Furthermore, these data show the ensemble mean QPF and maximum output from a single member is not shown. The forecast initialized at 22/0000 UTC attempted to bring the heavy rainfall further south and closer to

where it was observed relative to earlier forecasts. The SREF (Fig. 10), despite higher spatial resolution, showed a similar mean QPF. However, the SREF had lower overall QPF totals than the GEFS. Similar to the GEFS, the SREF had a distinct north and west displacement bias. This northward displacement can be explained in part due to the surge of high PW in the SREF (Fig. 11) and the GEFS (not shown) too far north of the observed position. The SREF PW and PW anomalies show a well predicted pattern with PW values and anomalies close to those observed. However, these forecasts pushed the warm moist air too far north relative to observations. The 850 hPa winds (Fig. 12) too were well predicted but they too were showing a surge of strong winds slightly farther north than observed.

Figure 13 is a better display concept for the SREF data showing 3 forecasts of the probability of 4 inches (100 mm) or more QPF. The ensemble mean masks these higher end data and uncertain issues or differences between ensemble members limit the impact of these higher amounts on the mean. But clearly, the SREF had several members 30 to 60% of its member with 4 inches (100mm) of QPF. Though not shown, the GEFS had a similar number of members predict 3 inches (75 mm) of QPF.

4. CONCLUSIONS

A surge of high PW air with 3 to 5SD PW anomalies ahead of an approaching front brought heavy rainfall to northern Iowa, Minnesota, and Wisconsin on 23-24 September 2010. A widespread area received over 60 mm of rain (2.4 inches) with a

significant area of receiving over 96 mm (4 inches). Rainfall amounts in excess of 200 mm (8 inches) were observed in several locations. The heavy rainfall produced flooding. The larger scale pattern was one that is often associated with heavy rainfall and flooding. The pattern which produced the event was relatively well predicted by the NCEP GEFS with a lead-time on the order of days. The shorter range SREF also produced reasonable forecasts indicting the potential for heavy rainfall, both in the overall pattern and in the probabilities. However, both EFSs produced both the pattern and probabilities too far north of the location of observed heavy rainfall. This event clearly had a strong v-wind component. The combination of strong southerly winds and an accompanying surge of high PW were signals to the potential for an extreme rainfall event. This generalized pattern was shown in the GFS 00-hour forecasts and was present in both SREF and GEFS forecasts, though the placement of key features was not as indicated in the analyses. The GEFS and SREF QPF forecasts showed a typical bias with axis and probability of heavy rainfall being displaced north and west of the area which received heavy rainfall. The SREF PW and 850 hPa wind fields implied that the pattern, though well predicted had subtle errors which affected the placement of the QPF in the EFS. The pattern and probabilities appeared favorable for heavy rainfall. However the observed rainfall fell south and east of the areas predicted by the guidance. Clearly, there are too distinct forecast issues at hand, first is that the pattern and the probabilities indicated the potential for heavy rainfall and second is where. Clearly, with large anomalies and a high probability outcome, the forecaster is

forced to leverage known bias and focus on a relatively large threat area.

5. Acknowledgements

6. References

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Hydrology, American Meteorological Society, San Diego, CA, 9-13 January 2005, Paper 1.2.

Figure 1. Stage-IV precipitation data showing total accumulated rainfall from 1200 UTC 22-25 September 2010. Values in mm. Return to text.

Figure 2. Storm Prediction Center Storm reports for the period ending at 1200 UTC on 23 September 2010. Return to text. There were no reports in this region on 23 September. Return to text.

Figure 3. Stage-IV rainfall data (mm) for the 6-hour periods ending at a) 0000 UTC b) 0600 UTC, c) 1200, and d) 1800 UTC 23 September 2010. Return to text.

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Figure 4. GFS 00-hour forecasts of 500 hPa heights (m) and 500 hPa height anomalies (standardized anomalies) valid from a) 1200 UTC 22 September 2010 through 1200 UTC 24 September 2010. Return to text.

Figure 5. As in Figure 4 except for GFS 250 hPa winds and 250 hPa wind anomalies. Return to text.

Figure 6. As in Figure 4 except for GFS precipitable water (mm) and precipitable water anomalies. Return to text.

Figure 7. As in Figure 5 except for 850 hPa winds and 850 hPa wind anomalies. Return to text.

Figure 8. As in Figure 5 except of GFS mean sea level pressure (hPa) and pressure anomalies. Return to text.

Figure 9. NCEP GEFS ensemble mean QPF (mm) from forecasts initialized at a) 1200 UTC 19 September, b) 0000 UTC 20 September, c) 1200 UTC 20 September, d) 1800 UTC 20 September, e) 0000 UTC 21 September, f) 0600 UTC 21 September, g) 1200 UTC 19 September, h) 1800 UTC 21 and i) 0000 UTC 22 September 2010. Values of QPF in powers of 2 mm. Return to text.

Figure 10.. NCEP SREF ensemble mean QPF (mm) from forecasts initialized at a) 0300 UTC 21 September, b) 0900 UTC 21 September, c) 1500 UTC 21 September, d) 2100 UTC 21 September, e) 0300 UTC 22 September, f) 0900 UTC 22 September, g) 1500 UTC 22 September, h) 2100 UTC 22 and i) 0300 UTC 22 September 2010. Values of QPF in powers of 2 mm. Return to text.

Figure 11. As in Figure 10 except for SREF precipitable water (mm) and precipitable water anomalies valid at 1200 UTC 23 September 2010. Return to text.

Figure 12. As in Figure 11 except for 850 hPa winds (kts) and wind anomalies. Return to text.

Figure 13. NCEP SREF forecasts of the probability of 4 inches or more QPF in 48 hours ending 1200 UTC 24 September 2010. Upper panels show the probability of 4 inches or more QPF in 24 hours and the lower panels show the mean QPF and each member’s 4 inch contour. Return to text.

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