National Weather Service-Pennsylvania State University Weather Events

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1 National Weather Service-Pennsylvania State University Weather Events Heavy rainfall of 23 November 2011 by Richard H. Grumm National Weather Service State College PA Abstract:. A fast moving mid-tropospheric trough brought a surge of moisture into the Mid- Atlantic region. The above normal moisture and strong low-level flow produced heavy rainfall in eastern Pennsylvania into southern New England. A broad area of in excess of 32 mm of rainfall covered most of Pennsylvania and New York. Portions of eastern Pennsylvania saw over 48 mm of rainfall as did an area of eastern New York and adjacent western Vermont. Locally heavier rainfall amounts, in excess of 64 mm were observed in portions of eastern Pennsylvania. The rainfall resulted in some minor flooding in portions of eastern Pennsylvania. The rainfall was generally well predicted.

2 1. INTRODUCTION A modest 500 hpa trough moved through the eastern United States on November 2011 producing a period of heavy rainfall in the Mid-Atlantic region. Rainfall totals of 32 to 64 mm were common from southeastern Pennsylvania northward into New York and eastern Pennsylvania (Fig. 1). The rain cause 16 river points in the Mid- Figure 1.StageIV rainfall data showing total accumulated rainfall (mm) for the period of 0000 UTC 22 to 24 November Atlantic region to reach flood stage. One point, Shirleysburg on Aughwick Creek, reached moderate flood stage and another point, Frederick, Maryland on the Monocacy river reached major flooding, all other points reached minor flood stage. This was not a high impact weather event. This was the first flood event of November 2011 in the region, denoting a relatively quiescent period and was the seventeenth flood event for the year in This paper will document the pattern and the rainfall associated with the minor flood event of 23 November Methods and Data 1 Based on MARFC flood data provided by the MARFC.

3 The NCEP GFS is used to re-produce the conditions associated with the storm to include the large scale pattern. Though not used herein, the JRA-25 data (Onogi 2007) were used for comparison purposes. 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 forecast systems. The NCEP short range ensemble forecast system (SREF) are used herein to show the probability of heavy rainfall during this event. GFS and NAM data were also examined to see how the event was forecast from a short-range perspective. The Stage-IV rainfall data was downloaded from NCEP to produce images of the rainfall and to conduct some verification of the rainfall forecasts. 3. The Storm system and impacts i. The large scale pattern The large scale pattern over eastern North America and the adjacent western Atlantic showed somewhat zonal flow over the western Atlantic at 22/1200UTC (Fig. 2a). A modest 500 hpa trough moved into the eastern United States from November and was associated with the rainfall (Fig. 2a-f). This relatively fast moving short-wave brought a surge of high precipitable water (PW) air (Fig. 3) into the region which quickly exited the region by 23/1800 UTC (Fig. 3f). The heaviest rainfall was observed with the influx of 30 to 35mm PW values and the +2 to +3σ PW anomalies. A strong 250 hpa wind maximum was present in the ridge of the western Atlantic (Fig. 4) and indicated a potential jet entrance region along the East Coast. The overall conditions supported a surge of high PW air into this jet entrance region indicating a pattern conducive for heavy rainfall. ii. Regional pattern and key anomalies A modest surface cyclone and retreating anticyclone were associated with the rainfall event of November 2011 (Fig. 5). The surface cyclone tracked across the Midwest and Pennsylvania and then up the East Coast. The regional view of the PW field suggested a quasi-east-west frontal boundary over the region (Fig. 6). Though not shown, strong easterly winds were present on the cold side of the boundary over Michigan and Ontario at the height of the event. The 850 hpa winds indicated strong southsouthwesterly flow over Pennsylvania and southern New York between 23/000 and 23/0600 UTC (Fig. 7c-d). The period of strongest low-level 850 hpa winds coincided with strong moisture flux at 850 hpa (Fig. 8). The period of strong wind southerly winds and the surge of high PW air between 23/0000 and 23/0600 UTC coincided with the period of strongest moisture flux (Fig. 8c-d). The +5 to +6 moisture flux peak in southeastern Pennsylvania was in close proximity to the region of heaviest rainfall in southeastern Pennsylvania (Fig. 1).

4 iii. observations The total rainfall was shown in Figure 1. The 6-hour rainfall totals (Fig. 9) show that the rain shield moved from west to east across the region. Light rain was observed for the 6-hour period ending at 22/1800 UTC in southern Pennsylvania with heavier rain over the Mid-West (Fig. 9a). Rainfall increased in intensity over all of Pennsylvania for the period 22/1800 to 23/0000 UTC (Fig. 9b) and implied second period of moderate rainfall from 23/0000 to 23/0600 UTC (Fig. 9c). The rain moved eastward between 23/0600 and 23/1200 UTC (Fig. 9d) and was mainly east of the region after 23/1200 UTC with only 2-8 mm observed in extreme eastern Pennsylvania after 23/1200 UTC and is thus not shown. iv. Forecasts This event was relatively well predicted and thus only short-term SREF QPF from 3 cycles on 22 November 2011 are shown. These data show that the SREF correctly predicted the general area of 25 to 50 mm of precipitation. The area of 50 mm of QPF or greater was too far north and east of the region where amounts in excess of 48 mm or more QPF were observed. There were some issues with predicting the higher end QPF amounts (Fig. 1). Similar location errors were found in the NCEP NAM and GFS and are not shown. The overall pattern and amounts were of value but the location of the heaviest rainfall was not well predicted by the guidance. 4. Conclusions A modest rainfall event brought some flooding to the Mid-Atlantic region on November Widespread rainfall amounts in excess of 32 mm covered a large portion of the Mid-Atlantic region and New York. The relatively broad region of eastern Pennsylvania and New Jersey received in excess of 48 mm of precipitation (Fig. 1). Most of the rain fell in a 12 hour period from about 22/1800 through 23/0600 UTC (Fig. 9). During this time the 850 hpa winds and moisture flux values peaked suggesting this was the period of strongest synoptic scale forcing. The area of heavy rainfall was relatively well predicted by the NCEP models and SREF. However, the rainfall amounts in excess of 50 mm were predicted too far north and east of the observed location (Figs. 1 &11). The models and ensembles appeared to have difficulty getting the location of the heaviest rainfall correct. 5. Acknowledgements Rainfall data from NCEP and the MARFC provided data on flooding and flooding statistics. A special thanks to Charles Chillag of the MARFC for tables and data summaries of the flooding. 6. References

5 Figure 2. GFS 00-hour forecasts of 500 hpa heights (m) and height anomalies (sigma) from a) 1200 UTC 22 November 2011 through f) 1800 UTC 23 November 2011 in 6-hour increments. Return to text.

6 Figure 3. As in Figure 2 except for precipitable water. Return to text.

7 Figure 4. As in Figure 2 except for 250 hpa winds and total wind anomalies. Return to text.

8 Figure 5. As in Figure 2 except for mean sea level pressure (hpa) and pressure anomalies. Return to text.

9 Figure 6. As in Figure 3 except for over the Mid-Atlantic region. Return to text.

10 Figure 7. As in Figure 2 except for 850 hpa winds and total wind anomalies. Return to text.

11 Figure 8. As in Figure 2 except for 850 hpa moisture flux and moisture flux anomalies. Return to text.

12 Figure 9. As in Figure 1 except for precipitation in 6-hour incements ending at a) 1800 UTC 22 November, b) 0000 UTC 23 November, c) 0600 UTC 23 November and d) 1200 UTC 23 November Return to text.

13 Figure 10. NCEP SREF forecasts of the probability of 25 mm of QPF in the 24 hour period ending at 2100 UTC 23 November Upper panels show the probability of exceeding 25 mm and lower panels show each forecast cycles prediction of the 25 mm contour, the mean 25 mm contour (thick black) and the ensemble mean QPF (shaded). Forecasts are initialized at a) 0300 UTC 22 November, b) 0900 UTC 22 November and c) 2100 UTC 22 November Return to text.

14 Figure 11. As in Figure 10 except for the probability of 50 mm or more QPF. Return to text.

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