Heavy Rainfall and Flooding of 23 July 2009 By Richard H. Grumm And Ron Holmes National Weather Service Office State College, PA 16803
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1 Heavy Rainfall and Flooding of 23 July 2009 By Richard H. Grumm And Ron Holmes National Weather Service Office State College, PA INTRODUCTION Heavy rains fall over Pennsylvania and eastern New England on the 23 rd of July 2009 (Fig.1) ahead of an upper level trough. The heavy rainfall over Rhode Island and Connecticut draw ones attention in Figure 1. Over Pennsylvania, the rain fell mainly in the late afternoon and evening hours from slow moving thunderstorms. The heaviest rainfall was observed in Cumberland and York Counties where reports of 5.7 inches were observed at Bloserville and Carlisle Springs and around 5.8 inches near Siddonburg. Heavy rains and flooding from 3 to 4 inches of rainfall was observed near and just west of State College, PA. The official COOP sites showed 4.10 inches in Cumberland County. The COOP data is heavily used in the UPD data shown in Figure 1. The locally heavy rainfall produced considerable urban and street flooding. Figure 1. The UPD rainfall data for the periods of 12Z 23 July 12Z 24 July 2009 (mm) and the mean 500 hpa heights (m) for the same period.
2 Figure 2. Stage-IV rainfall (mm) for the period of 1800 UTC 23 July through 1200 UTC 24 July The 6- hourly Stage-IV data were used to produce this image. This led to a sink hole on a road in State College. The heavy rainfall in the Cumberland County area caused flooding along streams and Creeks. Yellow Breeches creek near Camp Hill rose to around 10.50n at 0900 UTC on 24 July which is 3.5 feet above flood stage. The rapid rises and quick flooding were related to the rapid rainfall rates. This document will summarize the meteorological conditions associated with the heavy rainfall and flooding of 23 July The focus is on the pattern and causes of the event. Japanese re-analysis data (JMA) which is on a 1.25x1.25 degree grid. Anomalies were produced using the NCEP 21-day centered mean values. For example, the JMA heights were subtracted from the GR mean values for the date and time and then divide by 1 standard deviation for that date and time. This provided standardized anomalies which are shown in the following section. The mean pattern was obtained by summing all the times, in 6-hour increments from the data and dividing by the number of time periods employed. 2. METHODS and DATA Atmospheric data used here, to produce the patterns, were obtained from the The unified precipitation data (UPD) was used to produce the daily accumulated data shown in Figure 1.
3 Figure 3. As in Figure 2 except Stage-IV data for the 6 hour periods ending at a) 1800 UTC 23 July, b) 0000 UTC, c) 0600 UTC and d) 1200 UTC 24 July Values in mm with 25mm approximately 1.0 inches. The 6-hourly stage IV data is used to show the timing of the rainfall. 100 mm bull s eye in south-central Pennsylvania. 3. RESULTS i. rainfall pattern The Stage-IV high resolution precipitation data focused over the Mid- Atlantic region is shown in Figure 2. These data were summed over the 3 6- hourly periods when the rainfall was observed. The axis of heavy rainfall shows a general east to east path of training cells. Several 50 to 75 mm areas of rainfall are visible in these data which were not as well defined in the coarser UPD data. There is also a 75 to The 6-hourly data are shown in Figure 3. These data show very light rainfall from 1200 to 1800 UTC on the 23 rd. The heavy rainfall was evident from 1800 to 0000 UTC on the 24 th (Fig. 3b) with some residual rainfall though 0600 UTC on the 24 th. Note in New England that the heavy rainfall moved in later. These New England data were on the limbs of our selected projection, but over 50 mm of rain was indicated over eastern Long Island by 24/0600 UTC. ii. large scale pattern
4 presence of an east-west boundary. The PW field implied a more north-south frontal zone in Pennsylvania and a more east-west zone in New England. PW values were high, near 40 mm in New England implying above normal moisture. Figure 4. As in Figure 1 except JMA 850 hpa isotherms (C ) and the UPD rainfall. Figure 6 shows the NAM 00-hour forecast at 1800 UTC on 23 July These data reveal a strong low-level easterly jet along the coast and over the western Atlantic with a mesoscale circulation off the Delaware coast (Figs 6a-c). There was also a surge of high PW air with the low-level jet with PW values near 40 mm in eastern Pennsylvania which were 1 to 2.5 SDs above normal (Fig. 6d). The larger pattern showed high PW air to the north and east too. Figure 5. As in Figure 1 except JMA precipitable water (mm) and the UPD rainfall. The 500 hpa pattern and precipitation was shown in Figure 1. These data showed an upper-level trough moving over the region. At lower-levels there was an east-west baroclinic and moisture zone. Figures 4 & 5 show the 850 hpa isotherm and precipitable water patterns respectively. These data imply the Though not shown, the 500 hpa trough was -1 to -2SDs below normal and centered over Indiana at 23/1800 UTC and there was atypically strong jet streak over the region. These features moved eastward and are shown at 24/0000 UTC in Figure 7. The strong jet and deep 500 hpa trough are clearly abnormally strong features for late July. At lower levels at 24/0000 UTC (Fig.8), the strong low-level jet had moved over Long Island and into eastern
5 Figure 6. NAM 00-hour forecasts valid at 1800 UTC 23 July 2009 showing a) 850 hpa winds and v-wind anomalies, b) 850 hpa winds and u-wind anomalies, c) mean sea level pressure and pressure anomalies and d) precipitable water and anomalies. Pennsylvania. Over the Atlantic, a screaming southerly jet (Fig. 8a) was transporting a plume of high PW air into the region. A mesoscale cyclone was clearly present east of New Jersey. Drier air was moving over central Pennsylvania from the southwest (Fig. 8d). NAM forecasts valid for a short time period and ending at 24/1200 UTC. Clearly, the NAM had the large scale pattern and thus predicted heavy rainfall for New England (Fig. 10). However, the rainfall over Pennsylvania was grossly under predicted. Additionally, the details varied from cycle to cycle. By 24/0600 UTC the strong easterly winds, near -6SDs above normal had move into New England (Fig 9b). The deep cyclone was present east of Long Island New York. iii. Forecasts There are many forecasts aspects to this event to explore. The limit here is on The 23/0000 UTC (Fig 10 upper left) cycle showed the rainfall over New England east of the forecast location with about 12mm of rainfall over Pennsylvania. The 23/0600 UTC and 23/1200 UTC forecasts were more aggressive with the rainfall amounts and shifted the axis of heavy rainfall westward relative to the 23/0000 UTC forecasts. The 23/1200 UTC forecast had
6 Figure 7. As in Figure 6 except valid at 0000 UTC 24 July 2009 showing a) 250 hpa winds and v-wind anomalies, b) 250 hpa winds and u-wind anomalies, c) 500 hpa heights and anomalies d) 500 hpa temperatures and termperature anomalies. Return to text. generally less than 10 mm of rainfall over Pennsylvania. But the 72 mm over southern New England looked prescient. The 1800 UTC forecasts backed off the heavy rainfall in New England and lowered the expectations over Pennsylvania. A spin-up issue as the heaviest rains commenced at 23/1800 UTC. 4. CONCLUSIONS A heavy rainfall event impacted Pennsylvania and southern New England on the 23 rd of July The Heavy rainfall over New England was better forecast than the rainfall over Pennsylvania as presented in the NCEP NAM forecasts. The NAM grossly under forecast the rainfall over Pennsylvania. The key meteorological players in this event included a deep and unusually strong mid-tropospheric trough (Fig 1 & 7d), instability, above normal low-level moisture, and a trough moving across the region. Clearly, instability and upright convection played a role in the event over Pennsylvania. Though radar data was not presented, radar showed
7 low-core and slow moving thunderstorms over the region. In New England, the event was better predicted and had stronger forcing and patterns more typically associated with heavy rainfall. This included the strong low-level easterly jet, the strong surface cyclone, and the above normal PW surge into the region. Perhaps why we will need ever finer resolution ensemble. 5. References Mlodinow, Leonard, 2008: The Drunkards Walk: How Randomness Rules our lives. Pantheon Books, NY,NY. Despite the pattern being correct, the details in the heavy rainfall over New England were not well predicted. Figure 10 clearly showed significant variation in each successive 6-hour NAM forecast cycle. The shortest forecast, initialized at 23/1800 UTC may have been particular poor implying some model spin-up issues which caused an underestimation in the rainfall. Over Pennsylvania, the NAM was of no real value in getting a handle on the rainfall. Most forecasts showed 6-12 mm of rainfall, no where near the local 50 to 100 mm of observed rainfall. Perhaps high resolution models might help forecast there types of event, perhaps not. There was randomness to the convection and a randomness to where the convection trained to produce the locally heavy amounts and flooding. We can use patterns to help us forecast the weather. But patterns are often clearer in hindsight than in predictive mode (Mlodinow 2008). Some mesoscale events may not be predictable outside of using a radar to monitor the situation. One might find a good pattern in a single model run that may have predicted this event. But the run-to-run NAM inconsistencies imply we would only know the right pattern and the right run only after the event had occurred.
8 Figure 8. As in Figure 6 except valid at 0000 UTC 24 July 2009.
9 Figure 9. As in Figure 6 except valid at 0600 UTC 24 July 2009.
10 Figure 10. NAM precipitation forecasts (mm) for the period ending at 1200 UTC 24 July 2009 from forecasts initialized, clockwise from top at 0000 UTC, 0600 UTC, 1200 UTC and 1800 UTC on 23 July Return to text.
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