Flooding and Severe weather of 27 June 2013

Transcription

1 Flooding and Severe weather of 27 June 2013 By Richard H. Grumm National Weather Service State College, PA 1. Overview Flooding and severe weather affected Pennsylvania on 27 June 2013 (Fig 1). The severe weather included at least 2 confirmed tornadoes one of which occurred in close proximity of the National Weather Service Office in State College. The event occurred on the northeastern edge of a strong subtropical ridge (Fig. 2) which was centered over the southwestern United States. The 500 hpa heights implied a strong mid- and upper-tropospheric jet stream moving over the ridge which produced strong low-level southerly flow (Fig. 3) allowing a surge of deep moisture (Fig. 4) into the Mid-Atlantic region. The 850 hpa winds (Fig. 3) indicate that the 850 hpa cyclone centers moved across Pennsylvania between 1200 UTC 27 June through about 0600 UTC 28 June It will be shown that the severe weather and tornadoes were observed in close proximity to the deep lower tropospheric cyclone. The heavy rainfall caused flash flooding in several locations in central Pennsylvania which corresponded well with the regions of heavy rainfall (Fig. 1). The Stage-IV data underestimated the point value maximum which shows mm (2.5 to about 4 inches) near the green dot by Dubois. Heavy rain and flooding was observed near Beech Creek and State College. The estimates showed 48 to 64 mm or rainfall in those areas where point maximums and radar estimates were around 6 inches. Most of the rainfall fell within a 3 hour period which was a significant contributor to the flash flooding. This paper will examine the severe weather and flash flood event of 27 June Focus is on the pattern and key areas of 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). Rainfall data were obtained from the NCEP stage-iv data archives and compared to NMQ-Q2 and local WSR-88D precipitation estimates. The stage-iv data grossly underestimated the rainfall in areas of significant flooding. 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.

2 All gridded data were displayed using GrADS (Doty and Kinter 1995). 3. Pattern over the region The large scale pattern (Figs. 2, 3, 4) showed the strong flow over the tropical ridge (Fig. 2) which allowed for a surge of high precipitable water (PW) into the eastern United States to include Pennsylvania. The 500 hpa pattern implied a relatively strong 500 hpa trough (Fig. 2c-f) developed along the East Coast as the subtropical ridge amplified in the western United States. The strong height gradient between these two features produced a strong northerly 250 hpa jet (Fig. 5). Despite the strong dynamics, the instability was relatively low with meager values of CAPE over Pennsylvania during the event (Fig. 6). The steady low-level southeasterly flow (Fig. 7) and high PW air (Fig. 8) which slowly crossed Pennsylvania produced the multi-hour long periods of heavy rainfall and isolated severe weather. The 850 hpa wind field shows the progression of the low-level cyclone over the region (Fig. 7) and the moisture flux field (Fig. 9) showed the relatively high moisture flux (MFLUX) on the east side of the cyclone. The stronger MFLUX and higher MFLUX anomalies were displaced well to the south and east. From a strongly forced perspective, the RAP data implied strong MFLUX over the Delmarva Peninsula. The 6-hour rainfall estimates (Fig 10) show good agreement with the forcing and surge of high PW air ahead of the 850 hpa cyclone and the regions of locally heavier rainfall. There was a relative dearth of rainfall in the region of extremely high MFLUX over the Delmarva. 4. Rainfall The total estimated rainfall for the high impact portion of the event (Fig. 1) and the 6-hour rainfall amounts from the Stage-IV data (Fig. 10) showed the general areas affected by heavy rainfall. These data tended to under estimate the maximum rainfall as estimated by the radar and by point gages. Point values reports during the event indicated areas of over 150 mm (6 inches) of total rainfall, most of which fell in 1-3 hours which produced localized flash flooding in several locations. The NMQ Q2 data gage corrected radar (Fig. 11) showed the regions of heavier rainfall. A point in area 1 showed 6.59 inches of rainfall. The 6-hour window of heavy rainfall was from 1600 through 2200 UTC (Fig. 12 upper). The 3-hour maximum rainfall in western Clearfield County (area A) was from UTC 27 June (not shown) where a broad region had in excess of 3 inches of rainfall and point values reach 5.97 inches 1. Hourly rainfall rates in western Clearfield County peaked near 3 inches an hour for the period ending at 1800 and 1900 UTC when point maxima were 2.77 and 3.14 inches (not shown). 1 Based on 1 and 3-hour NMQ Q2 rainfall estimates.

3 The Beech Creek area (black oval Fig. 11) 3-hour rainfall maximum was around 5.43 inches between 2100 UTC 27 June and 0000 UTC 28 June Most of this rain fell in a window after 2100 UTC as prior to 2100 UTC only 0.20 inches of rain had been observed over the region. The 3-hour maximum values where 5.11 and 5.43 inches ending at 2300 UTC and 0000 UTC respectively. The region received extremely heavy rainfall over a 2 to 3 hour period. Hourly data suggest 1 to 2.5 inches fell from UTC and some areas near Beech Creek received 2.5 to 3.0 inches of rainfall from UTC. The heavy rainfall near the arrow in Figure 11 (Pine Grove Mills) fell in a 2 hour period from 1900 to 2100 UTC (Fig 13). Local rainfall maximums in the NMQ Q2 data were 4.20 and 4.09 for the 3-hour period ending at 2100 UTC and 2200 UTC. The hourly data showed 2.5 to 3.0 inches over this region from UTC (Fig. 13). The rapid and intense rainfall lead to rapid flash flooding over the region of Pine Grove Mills, located at the base Pine Grove Mountain. 5. Severe Weather As shown in Figure 1 and based on the CAPE (Fig. 6) this was not a high impact severe weather event and the flooding was meteorologically more significant. However the strong shear produced a several wind reports and two confirmed tornadoes in central Pennsylvania. The easternmost Tornado was associated with a strong and relatively persistent mesocyclone. The weak rotating mesocyclone was present by 1806 UTC (not shown) and peaked around 1828 UTC (Fig. 14) and the system became diffuse and weak after 1900 UTC (not shown). This system produced a confirmed EF1 tornado in Toboyne Township around 1820 UTC 28 June which had a 50 yard wide damage swath about 2 miles in length. The second tornado was associated with the rain band which produced flooding in both Pine Grove Mills and Beech Creek. It was one of several mesocyclones that developed near the larger scale tropospheric cyclone. One of a family of mesocyclones was east of Whipple Dam State Park (Fig. 15) around 1941 UTC. A strong mesocyclone quickly developed farther east in the larger rain band (Fig. 16) south east of Boalsburg as the air descended down Pine Grove Mountain. In bound winds were on the order of 40 to 45kts with weak outbound in both the storm relative and base velocity data. The tornado was likely on the ground around 2004 UTC (Fig. 17) and inbound winds peaked near 50kts east of Boalsburg. There was an implied low CC area (Fig. 17 lower right) implying non-meteorological targets east of Boalsburg. Some interpretations of these signals suggest a modest debris ball. The system rapidly weakened as it moved eastward. A distinct 75 yard wide and 1 mile long damage path was observed east of Boalsburg, PA (Fig 18). The weakly rotating and visible mesocyclone on the ground was observed moving westward and winds of 25 to 37kts were locally measured at the National Weather Service Office in State College. Radar indicated a weak and broad mesocyclone in the 0.5 degree cut (not shown) as the mesocyclone reach the NWS office. A third mesocyclone

4 developed as this system moved into Beech Creek. This mesocyclone produced numerous reports of funnel clouds but no significant reports of damage. 6. Summary Strong mid-tropospheric cyclone associated with a strong upper tropospheric jet brought a surge of moisture and high shear into Pennsylvania on 27 June 2013 resulting in a complex event which produced several areas of flash flooding. In addition to the areas of heavy rainfall, lasting 1-4 hours in duration, the strong shear and Helicity produced some severe thunderstorms and two confirmed tornadoes. Flash flooding near Beech Creek resulted in 1 fatality. The larger scale pattern implied strong forcing though the implied stronger forcing was to the south and east. Despite the strong winds and high MFLUX along the Delmarva Peninsula (Figs. 7-9) the heavier rainfall (Fig. 1) amounts fell in close proximity to the advancing low-level cyclone. This region had high PW values (Fig. 8) and relatively high MFLUX though considerably lower than the MFLUX values observed to the southeast. This implies that lift, moisture, and instability played a significant role in the more mesoscale distribution of the precipitation. The locally heavy rainfall in Pennsylvania which caused flash flooding was extremely mesoscale in nature. The periods of heaviest rainfall were on the order of 2-3 hours in duration. The flash flooding locations shared several common characteristics including enhanced convection, locations with hourly rainfall rates of 2.5 to 3.25 inches and 3-hourly rates in excess of 4 inches. Interestingly, in all the 3 locations, heavy rainfall did not last much longer than 3 hours with most of the rainfall at each flash flood location peaking in a 1-2 hour window. In addition to the flash flooding the event produced several reports of downed trees and two tornadoes. The tornado in State College had a short path of intense damage, despite this, the weak and visible mesocyclone on the ground cross most of the valley producing little or no damage. As the visible mesocyclone on the ground reached the NWS office, surface winds, which turned over 180 degrees in 1-2 minutes never exceed 37kts. 7. Acknowledgements Spotters for video of the Nittany Farm Tornado which were posted to the NWS Facebook page and calls of flooding to the NWS office from several weather enthusiast in our forecast area. 8. References Atkins N. T., J. M. Arnott, R. W. Przybylinski, R. A. Wolf, and B. D. Ketcham, 2004: Vortex structure and evolution within bow echoes. Part I: Single-Doppler and damage analysis of the 29 June 1998 derecho. Mon. Wea. Rev., 132,

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

6 Figure 1. Upper panels show the 24 hour rainfall focused over Pennsylvania with a green dot near Dubois where flash flooding and over 6 inches of rainfall was observed. The lower panel shows the Storm Prediction Center (SPC) sever reports color coded by type for the 24 hour period ending at 1200 UTC 28 June Return to text.

7 Figure 2. Return to text. NWS State College Case Examples

8 Figure 3. As in Figure 2 except for 850 hpa winds and wind anomalies in 6-hour increments from 0600 UTC 27 June through 1200 UTC 28 June Return to to text.

9 Figure 4. As in Figure 2 3except for precipitable water (mm) and precipitable water anomalies. Return to text.

10 Figure 5. Return to text. NWS State College Case Examples

11 Figure 6. As in Figure 4 except for CAPE (JKG-1). Contours every 600 JKg-1. Return to text.

12 Figure 7. As in Figure 4 except RAP 00-hour analysis of 850 hpa winds (ms-1) and total wind anomalies in 1 hour increments from a) 1700 UTC 27 June 2013 through f) 2200 UTC 27 June Return to text.

13 Figure 8. As in Figure 7 except for precipitable water and precipitable water anomalies. Return to text.

14 Figure 9. As in Figure 7 except for 850 hpa moisture flux (gkg-1 ms-1) scaled by 1000 and moisture flux anomalies. Return to text.

15 Figure 10. Stage-IV estimated rainfall (mm) in 6-hour increments for the 6 hour periods ending at a) 1200 UTC 27 June, b) 1800 UTC 27 June, c) 0000 UTC 28 June and d) 0600 UTC 28 June Return to text.

16 Figure 11. NMQ-Q2 radar and gauge estimated precipitation for the 24 hours ending at 0000 UTC 28 June The white oval area outlines Dubois and areas of Armstrong and Clearfield Counties where flash flooding was observed and referred to as area A in the test. The black oval area if the flood region near Beech Creek where. The arrow points to the local flooding southeast of State College where the town of Pine Grove Mills had flash flooding. Return to text.

17 Figure 12. As in Figure 11 except for 6-hour data showing the period of maximum accumulations near Area A in Figure 11. This 6-hour window was take 6 hours after the onset of rainfall. The lower panels shows the maximum period of rainfall near the black oval in Figure 11. This time window shows the maximum rainfall nearly all of which fell from 1600 to 1900 UTC. Timing was obtained from the NMQ Q2 site using 1, 3, and 6 hourly data. Return to text.

18 Figure 13. As in Figure 11 except for focused near the Arrow in Figure 11 showing the 3-hour and 1 hour rainfall maximum periods near Pine Grove Mills, PA. Return to text.

19 Figure 14. KCCX radar valid at 1833 UTC 27 June 2013 showing (clockwise form upper left) 0.5 degree cuts of Storm relative velocity, base velocity, horizontal reflectivity, and correlation coefficient. Return to text.

20 Figure 15. As in Figure 14 except valid at 1941 UTC centered near State College PA. Return to text.

21 Figure 16. As in Figure 15 except valid at 1955 UTC. Return to text. NWS State College Case Examples

22 Figure 17. As in Figure 15 except valid at 2004 UTC. Return to text. NWS State College Case Examples

23 Figure 18. NIttany Farms Tornado Path touchdown near pond just north of State Route 322 and the end point of continuous damage. Based on 28 June survey. Upper image is blow up area of the pond and farm. Most farm buildings and the house sustained some minor damage and a tree on the island in the pond was pulled from the ground. Return to text.

24 Appendix. Nittany Farms tornado damage. Small barn roof peeled back. Barn roof material wrapped about telephone pole and trees down near farm house and trees snapped across the corn fields behind the house, some of the corn too was flattened by the winds.