The Severe Weather Event of 7 August 2013 By Richard H. Grumm and Bruce Budd National Weather Service State College, PA 1. INTRODUCTION and Overview A fast moving short-wave (Fig. 1) with -1σ 500 hpa height anomalies and a modest 250 hpa jet (Fig. 2) produced a severe weather event over eastern Ohio and western Pennsylvania. It will be shown that a bow echo developed in northwestern Pennsylvania and moved southeastward across the State and produced the majority of the severe reports. To the southwest, supercells produced a mix of hail and wind reports and one confirmed EF0 tornado. The one short-lived EF0 tornado was observed in Quemahoning, Jenner and Lincoln Townships in Somerset County, Pennsylvania, south of the bow echo track. The bow echo, which produced many of the wind reports is one of many such systems be observed over recent years which at times dominate severe events (Grumm and Colbert 2012). These distinct systems and their ability to persist, Figure 3. Storm reports for the 24 hour period ending at 1200 UTC 8 August 2013. Data from the Storm Prediction Center and severe weather is color coded by type as in the key on the lower left of the image. producing significant number of severe reports and semi-linear features in SPC storm reports (Grumm and Colbert 2013) has been observed repetitively in central Pennsylvania. There is some potential here in understanding how these systems develop and if they can persist to improve warnings related to these systems. The short-wave and strong 250 hpa jet (Fig. 2) cause high precipitable water air to surge into eastern Ohio and western Pennsylvania (Fig. 4) with modest 850 hpa winds (Fig. 5). The lowlevel moisture and the cold air in the approaching trough (Fig. 1) produced a surge of high convective available potential energy (CAPE: Fig. 6) over portions of eastern Ohio and western Pennsylvania. Though not shown, there was some modest cold air damming over eastern Pennsylvania and thus relative stable air to the east. Over most of Pennsylvania the low-level air was relatively dry which may have contributed to the production of a cold pool and subsequent bow echo evolution.
The infrared satellite imagery shows the cold clouds and lightning with the bowing system over western Pennsylvania at 2002 UTC (Fig. 7). Note the lack of cold clouds and relatively warm cloud tops in central and eastern Pennsylvania associated with the low-clouds caught in the inversion (not shown) associated with the cold air damming. The bow echo did not penetrate deeply into the cold air and the satellite imagery suggests few traces of it after 2325 UTC as the cold cloud shield expanded to the southwest with the stronger convection in that region, close to the location of the lone tornado. KCCX radar shows the early stages of the bow echo over western Pennsylvania (Fig. 8) with the enhanced reflectivity and high winds over northwestern Pennsylvania. There were few returns in the cool stable air over central and eastern Pennsylvania. From a velocity perspective, KCCX had a great viewing angle of this storm as the motion was nearly down-radial. Peak winds were over 50kts in the 0.5 degree velocity at 2018 UTC. The cold tops on satellite implied that the convection had already produced a substantial anvil cloud (Fig. 8). Though not shown, the CC data implied hail and rain in the storm. The winds peaked over 60kts at 2050 UTC as the bow moved toward Clearfield County (Fig. 8 lower). The ZH and V both looked impressive on radar as the storm entered Clearfield County shortly before 2104 UTC (Fig. 9). The convection continued eastward but the winds weakened as the convection began to ingest the warm stable air to the south and east (Fig. 9-lower). The southern flank of the bow was able to begin to ingest warm air to the south and west (Fig. 10) and the southern flank of the bow began to intensify around 2343 UTC, the system persisted (not shown) nearly 2 more hours and accounted for a significant percentage of the severe weather reports from western Pennsylvania into southeastern Pennsylvania (Fig. 3). A around 2200 UTC a supercell thunderstorm developed in the warm humid air over western Pennsylvania. The storm had strong outbound winds (Fig. 11) in the hook region of the storm as viewed on KPBZ radar. This storm moved into Somerset County around 2254 UTC (not shown) and tracked across the County over the next 30 minutes. This storm was the storm which likely produced the tornado near Raphton, Pennsylvania at about the time of the 2303 UTC image (Fig. 11-lower). Another supercell developed north of this storm (Fig. 12) and as viewed on KCCX the storm had strong returns at lower levels (Fig. 12-upper left) and a 3-body scattering signal in the CC data (Fig. 12-lower right)1. The deep blues on the southwest edge of the storm showed the signal for a 3-body scattering and implied some larger hail. This signal persisted for about 9 volume scans indicating a very strong updraft core. The signal peaked between 2109 (not shown) and 2118 UTC (Fig. 13). The hail signal in the storm was visible through about 2224 UTC. This storm too had rotation in both the storm relative velocity (SRM: not shown) and base velocity data. No SRM data was shown here. Not surprisingly the rotating storms in southwestern Pennsylvania produced reports of large hail, wind damage, and one confirmed EF0 tornado. 2. Summary The combination of cold air damming to the east and an approaching short-wave to the west (Fig. 1 & 2) produced a severe weather event in western Pennsylvania. The surge of high PW air and the arrival of high CAPE with the moisture served as fuel for the convection. The strong jet 1 All images use here were 4 panels most images had the dual-pol data removed to match the text.
aloft (Fig. 2) and the short-wave likely played a role in the favorable large scale environment and the lift. Many of the cells which developed in southwestern Pennsylvania quickly showed signs of rotation, like the result of the shear and the low-level frontal boundary associated with the cold air damming over central and eastern Pennsylvania. The northern edge of the severe weather, primarily wind damage, was associated with a bow echo. This system weakened as it ingested the cooler drier air to the east. As it moved farther south it was able to ingest some warm moist air and thus resurged. Farther south, the storms developed strong updrafts, developed rotation and showed signs (Fig. 12 & 13) that the updrafts produced large hail. The CC data clearly showed a supercell which had a three body scattering for a prolonged period spanning over 45 minutes. The two stronger supercells produced hail, wind, damage, and the southern supercell likely produced the EF0 tornado in Somerset County. It is interesting to note that the tornado occurred to the lee of the higher terrain to the west implying vortex stretching may have played a role in the tornadoes evolution. 3. Acknowledgements National Weather Service employees who worked the event, gathered and archived data. Spotters for reports and information on the storms.
Figure 2. 13km RAP 500 hpa heights (m) and height anomalies in 2 hour increments from a) 1400 UTC 8 June 2013 through f) 0000 UTC 8 June 2013. Return to text.
Figure 2. As in Figure 2 except for 250 hpa winds (kts) and wind anomalies. Return to text.
Figure 4. As in Figure 2 except for precipitable water (mm) and zoomed over Pennsylvania. Return to text.
Figure 5. As in Figure 4 except for 850 hpa winds and wind anomalies. Return to text.
Figure 6. As in Figure 5 except for surface based convective available potential energy. Return to text.
Figure 7. GOES ITR and 1-minute lightning data at 2002, 2155 and 2325 UTC. Return to text.
Figure 8. KCC radar showing 0.5 degree horizontal reflectivity and base velocity at (top) 2018 and (bottom) 2050 UTC 7 August 2013. Return to text.
Figure 9. As in Figure 8 except for 2104 UTC and 2238 UTC. Return to text.
Figure 10. As in Figure 9 except for 2343 7 August and 0006 UTC 8 August 2013. Return to text.
Figure 11. As in Figure 10 except KBPZ radar at 2214 and 2303 UTC. Return to text.
Figure 11. KCCX radar showing 0.5 degree slices at 2055 UTC, clockwise from top left horizontal reflectivity, velocity, correlation coefficient (CC), and specific differential phase (KDP). Return to text.
Figure 13. As in Figure 12 except valid at 2118 UTC. Return to text.
Appendices Tornado PNS and MAP of where tornado was observed NOUS41 KCTP 091849 PNSCTP PAZ033-092300- PUBLIC INFORMATION STATEMENT NATIONAL WEATHER SERVICE STATE COLLEGE PA 249 PM EDT FRI AUG 9 2013...TORNADO CONFIRMED 2 MILES EAST OF RALPHTON IN SOMERSET COUNTY PENNSYLVANIA... LOCATION...2 MILES EAST OF RALPHTON IN SOMERSET COUNTY PENNSYLVANIA DATE...AUGUST 7 2013 ESTIMATED TIME...APPROXIMATELY 720 PM EDT MAXIMUM EF-SCALE RATING...EF0 ESTIMATED MAXIMUM WIND SPEED...70-80 MPH MAXIMUM PATH WIDTH...75 YARDS PATH LENGTH...0.5 MILE FATALITIES...0 INJURIES...0 * THE INFORMATION IN THIS STATEMENT IS PRELIMINARY AND SUBJECT TO CHANGE PENDING FINAL REVIEW OF THE EVENT(S) AND PUBLICATION IN NWS STORM DATA....SUMMARY... THE NATIONAL WEATHER SERVICE IN STATE COLLEGE PA HAS CONFIRMED A TORNADO 2 MILES EAST OF RALPHTON IN SOMERSET COUNTY PENNSYLVANIA ON 08/07/13. A STORM SURVEY WAS CONDUCTED IN QUEMAHONING TOWNSHIP SOMERSET COUNTY. THE TORNADO TOUCHED DOWN 2 MILES EAST OF RALPHTON NEAR HORNER CHURCH ROAD. MINOR DAMAGE OCCURRED TO ONE HOME AND ONE BARN ROOF WAS PARTIALLY BLOWN OFF. 25-30 TREES WERE ALSO DOWNED OR DAMAGED. THANK YOU TO THE SOMERSET COUNTY EMERGENCY MANAGER AND THE STOYSTOWN FIRE DEPARTMENT FOR THEIR ASSISTANCE DURING THE SURVEY. THIS INFORMATION CAN ALSO BE FOUND ON OUR WEBSITE AT WEATHER.GOV/CTP. FOR REFERENCE...THE ENHANCED FUJITA SCALE CLASSIFIES TORNADOES INTO THE FOLLOWING CATEGORIES: EF0...WIND SPEEDS 65 TO 85 MPH. EF1...WIND SPEEDS 86 TO 110 MPH. EF2...WIND SPEEDS 111 TO 135 MPH. EF3...WIND SPEEDS 136 TO 165 MPH. EF4...WIND SPEEDS 166 TO 200 MPH. EF5...WIND SPEEDS GREATER THAN 200 MPH. $$ BUDD/COLBERT