2 July 2013 Flash Flood Event

Transcription

1 2 July 2013 Flash Flood Event By Richard H. Grumm and Charles Ross National Weather Service State College, PA 1. Overview A retrograding 500 hpa cyclone and anticyclone (Fig. 1) set up deep southerly flow over the eastern United States from 1 to 5 July The deep easterly flow brought a plume of deep moisture (Fig. 2) with 2-3 σ above normal precipitable water (PW) values into the eastern United States. The result was a series of severe weather events and locally heavy rainfall events over the eastern United States. This paper will focus on the heavy rain and flash flood event over New England on 2-3 July The impressive features at 500 hpa at this time included the close 5940 m high over the western United States, the deep 500 hpa low over the southern United States, and the 5940 m anticyclone over the western Atlantic. Each of these feature contributed to significant weather to include the western United States heat wave and fires; the cool period over the southern United States from North Carolina to Oklahoma, and the warm and wet period over the eastern United States. The pattern, with the deep southerly flow between the anomalous trough to the west and the anomalous anticyclone to the east is a well-known and documented heavy rainfall event pattern (Junker et al. 2008; Bodner et al 2011). The key to the pattern is the amplified flow and the anomalies aid in identifying systems which have higher potential to produce higher end events (Junker et al 2009). This paper will document the heavy rainfall event of 2-3 July The focus is on the pattern and key features which produced the favorable synoptic scale environment. This case is one of many which shows the power of standardized anomalies and subtropical anticyclones in producing high impact weather. 2. Data and Methods The large scale pattern was reconstructed using the 00-hour forecast of the NCEP Global Forecast System as first guess at the verifying pattern. The standardized anomalies were computed in Hart and Grumm (2001). All data were displayed using GrADS (Doty and Kinter 1995). Rainfall was obtained from the gridded State-IV data and compared to local observations and the NMQ Q2 data. The NMQ data were used to examine hourly evolutions of rainfall and radar echoes. It should be noted that neither gridded dataset adequately captured the locally heavy rainfall which in some locations exceed 6 inches in 3-6 hours.

2 Storm reports were obtained from the Storm Prediction Center. Processed SPC data was used to overlay on the anomalies and the base SPC images were used. 3. Pattern over the region The larger scale pattern indicated that the plume of deep moisture (Fig. 2) and above normal PW air moved up the western flanks of a strong subtropical ridge over the western Atlantic, west of the deep 500 hpa trough over the central United States (Fig. 1). This pattern, with the enhanced flow between anomalous troughs and ridges has been shown to be a key heavy rainfall pattern (Junker et al 2008; Junker et al. 2009; Bodner et al 2011). The 6-hour PW evolution during the primary period of heavy rainfall (Fig. 3) showed a plume of +2 to +3σ PW anomalies along the East Coast. In New England, the PW anomalies remained over +3 σ above normal for a prolonged period of time. The 13km PW data from the RAP analysis showed more detailed structures with enhanced values of PW and PW anomalies in the Hudson-Chaplain Valleys and even the Connecticut River valley (Fig. 4). These data shows some promised for high resolution models to capture locally high moisture fields. The 2-day rainfall showing areas of 12.5mm or more QPF (Fig. 5) shows areas of heavy rainfall over Virginia, western Pennsylvania, New York, and New England. Areas of rainfall over 100 mm were observed during this period in western New Hampshire near the Connecticut River valley. The 24-hour rainfall maximum zoomed in over New England (Fig. 6) showed the two regions of heavy rainfall over New Hampshire. The purple dot is near Westmoreland where several bridges and roads were severely damaged by flash flooding 1. The southern area shows mm or rainfall while local reports indicated around 6 inches of rainfall, most of which fell between 00 and 0600 UTC 3 July The locally heavy rainfall in southwest New Hampshire, based on NMQ Q2 and radar, began around 03/0000 UTC with the heaviest rain falling from 03/0200 and 03/0300 UTC (Fig 7). Most of the rain heavier rainfall north of the band in southwestern NH fell from 02/ /0600 UTC when 1-3 inches of rain was estimated followed by additional rainfall from 02/1800 through 03/0600 UTC. The towns of Westmoreland and Walpole had significant flooding (Channel 9 Manchester) Overall the data in Figures 5 & 6 show that mesoscale areas of heavy rainfall were observed in the synoptic scale plume of deep moisture. 4. Summary A plume of deep moisture between a strong subtropical anticyclone to the east and a deep trough produced locally heavy rains in the eastern United States from Virginia to Maine. The heavy 1 News reports and personal observations. While vacationing nearby author photographed washed out areas of River Road and route 63; and Route 12 in Westmoreland. 2 Personal observation from Spofford 3-4 miles where 1.5 inches was observed about 3-4 miles south of flash flooding region and based on NMQ Q2 and radar data.

3 rainfall areas were mesoscale in nature implying the lack of strong synoptic scale forcing and reflecting the convective nature of the event. The resulting convection produced areas were 1-6 inches of rain fell in 1-3 hours, resulting in flash flooding. The flooding in southwestern New Hampshire on the evening hours of 2 July, which spanned 03/0000 UTC through 03/0400 UTC, produced locally heavy rainfall in Westmoreland and Walpole, New Hampshire. The resulting rainfall resulted in flash flooding which washed out bridges, the shoulders of roads, and culverts designed to handle lower volumes of water than produced by the intense down pours. Farther north, heavy rainfall affected the Alstead area, most of the heavy rain in that region was observed overnight on 1-2 July followed by additional rainfall during the evening hours of 2 July. This rainfall too resulted in flash flooding and considerable damage due to flooding in and around Alstead. This case shows the power of surge of high PW air between large anticyclones and deep troughs. Despite the ideal pattern and moisture plume, the rainfall pattern was mesoscale in nature, extremely localized, and the periods of intense rainfall followed a distinct diurnal pattern with most of the rainfall occurring after 0000 UTC. 5. Acknowledgements 6. References Bodner, M. J., N. W. Junker, R. H. Grumm, and R. S. Schumacher, 2011: Comparison of atmospheric circulation patterns during the 2008 and 1993 historic Midwest floods. Natl. Wea. Dig., 35, [Abstract] Doty, B.E. and J.L. Kinter III, 1995: Geophysical Data Analysis and Visualization using GrADS. Visualization Techniques in Space and Atmospheric Sciences, eds. E.P. Szuszczewicz and J.H. Bredekamp, NASA, Washington, D.C., Junker, N.W, M.J.Brennan, F. Pereira,M.J.Bodner,and R.H. Grumm, 2009:Assessing the Potential for Rare Precipitation Events with Standardized Anomalies and Ensemble Guidance at the Hydrometeorological Prediction Center. Bulletin of the American Meteorological Society,4 Article: pp Junker, N. W., R. H. Grumm, R. Hart, L. F. Bosart, K. M. Bell, and F. J. Pereira, 2008: Use of standardized anomaly fields to anticipate extreme rainfall in the mountains of northern California. Wea. Forecasting,23,

4 Maddox, R.A., 1980: Mesoscale convective complexes. BAMS, 61,

5 Figure 1. The GFS 00-hour forecasts of 500 hpa heights and height anomalies in 24 hour increments from a) 0000 UTC 01 July 2013 through f) 0000 UTC 6 July 2013Return to text.

6 Figure 2. As in Figure 1 except for precipitable water and precipitable water anomalies. Black dot is near Westmoreland, NH. Return to text.

7 Figure 3. As in Figure 2 except for 6-hourly data from a) 0000 UTC 2 July through f UTC 3 July Return to text.

8 Figure 4. As in Figure 3 except for the 13km RAP analysis in 2-hour increments from a) 1800 UTC 2 July through f) 0400 UTC 3 July Return to text.

9 Figure 5. Stage-VI QPE (mm) for the 24 hour period ending at 0000 UTC 4 July Values lower than 12.5 mm (0.50 inches) have been filtered out. Return to text.

10 Figure 6. As in Figure 5 except for 6-hourly accumulations for the period ending at a) 1200 UTC 2 July, b) 1800 UTC 2 July, c) 0000 UTC 3 July, and d) 0600 UTC 3 July Return to text.

11 Figure 7. NMQ Q2 data showing 1-hour rainfall ending at 2300 UTC 22 July near the onset time, 0300 UTC and the 3-hour totals from 0100 through 0400 UTC when most of the rainfall accumulated.

12 Photos Route 63. Photo WMUR website.

13 Regional map of affected area. NWS State College Case Examples