26 July 2012 Derecho Events

Transcription

1 1. Overview 26 July 2012 Derecho Events By Richard H. Grumm And Charles H. Ross National Weather Service State College, PA On 26 July 2012 two lines of thunderstorms (Fig. 1) brought severe weather (Fig. 2) to the eastern United States from Connecticut southwestward through Kentucky on 26 July The data suggest that strong and damaging winds dominated the severe reports. Of the 473 severe reports there were 7 tornadoes3 of which were in Pennsylvania and 4 New York State respectively. Over Pennsylvania there were several long-lived bow echoes, several of these raced across northern Pennsylvania between 1800 and 2100 UTC 26 July The conditions over the Mid-Atlantic region (Fig. 3) were nearly ideal of multi-cellular storms and the potential for persistent storms. The 0-1km shear (Fig. 3a) was modest and mainly under 10ms-1 however the shear in the 0-5km layer (Fig. 3b) was well over 20ms-1 with relatively high lifting condensation (LCL) heights and every high convective available potential energy (CAPE). Persistent supercells require deep and strong shear. Based on the work of Craven and Brooks (2004); Brooks et al. (2003) and Grunwald and Brooks (2011) the potential for rotating storms was focused in northern Pennsylvania and New York though the low-level shear was relatively weak and the LCL heights were relatively high. These latter two conditions did not favor tornadoes which the Brooks studies have shown often require low LCL heights and strong shear in the lower 1km of the atmosphere. Local studies in Pennsylvania also suggest that most tornadoes occur with LCL heights below 1000m and low-level shear over 12ms-1 and stronger tornadoes requiring low-level shear in excess of 20ms-1. The storms produced widespread wind damage in Pennsylvania. The northern tier was particularly hard hit by strong winds over the entire region and several strong and destructive microbursts. Power outages affected wide swaths of McKean, Potter, and Tioga Counties. Trees were down nearly continuously from the intersection of route 219 in McKean County, south of Lewis Run along US 6 to Smethport. In Smethport, there were numerous trees down and trees on houses. Port Allegany was the only town along US 6 with power on 27 July. The sporadic tree damage continued from Smethport to Coudersport. The damage continued to Galeton, which had also suffered power outages 1. This paper will document the severe event of 26 July 2012 with an emphasis of the large scale conditions over Pennsylvania and adjacent States. The focus is in the pattern and key 1 Survey team drove this route and had difficulty getting fuel due to the lack of power and thus close business and fueling stations. The downed power lines limited cell phone connection to coordinate with the office and emergency responders. A gas station in Galeton had a generator and was used to refuel and return to the WFO.

2 feature associated with the severe weather. Radar data is used to show the character of the convection and the dominant convective modes. 2. Data and Methods All radar data used were obtained from the local AWIPS archives within the office. The large scale pattern was reconstructed using the 00-hour forecasts of the GFS and NAM. To fill in 3-hour periods the NAM 3-hour forecast data was used. Many of the fields are shown using the NCDC re-analysis climatology. All of these data were displayed using GrADS (Doty and Kinter 1995). 3. Pattern over the region The 500 hpa pattern over the United States between 25/000 and 27/1200 UTC (Fig. 4) showed a trough over western Canada which would move to the southeast over the period shown, deepening the trough in the Mid-West. Ahead of this trough there was weak ridge which would push some extremely warm air into the Mid-Atlantic region, ahead of the approaching short-wave. The ridge and a trough to the northeast produced a strong gradient at 500 hpa (Fig. 4c-e) over the Mid-Atlantic region and Northeast. Strong 850 hpa (Fig. 5) and 700 hpa (Fig. 6) wind maximum were shown to pass over Pennsylvania and New York associated with this region of implied confluence. The strongest 850 hpa winds (Fig. 5c) were forecast to move across the region around 26/1200 UTC. These enhanced winds may have been associated with a convective region the model predicted would move across the region around this time. The 700 hpa winds showed a similar pattern but had a second wind maximum mover across northern Pennsylvania at 26/1800 UTC and then showed strong winds over southwestern Pennsylvania and West Virginia at 27/0000 and 27/0600 UTC (Fig. 6e-f). The winds were shown to be 3 to 3s above normal during the time of the highest probability of convection. Ahead of the 500 hpa shortwave (Fig. 4) there was a surge of warm moist air. The 850 hpa temperatures were well above 2-3σ (Fig. 7) and there was a surge of PW values over 50 mm which was 2-3σ above normal (Fig. 8). The warm moist air surged into the region ahead of a cold front associated with the approaching short-wave (Fig. 4). The front and upper divergence likely provided the lift required to tap the strong instability. The instability at 26/1800 and 26/2100 UTC (Figs. 3 & 9) showed CAPE in the NAM above 1800 JKg-1 over most of Pennsylvania. The high CAPE was relatively well aligned with the surge of high PW air over the same region (Fig. 8). The shear and instability rapidly moved to the south and east (not shown) and the CAPE was below 600 JKg-1 over all except the extreme southeastern portion of Pennsylvania by 27/0000 UTC. Not surprisingly the convective activity diminished rapidly after 26/2300 UTC (next section).

3 4. Radar The first line of the event developed over southern Ontario and swept across lake Erie producing a line of storms in eastern Ohio and western Pennsylvania. The strong winds behind the convection in the 0.50 degree velocity were over 50kts and at times near 80kts. This prompted warnings in western Pennsylvania well ahead of the approaching in of storms (Fig. 10). KCLE down-radial and thus the data was well suited to evaluate the winds, despite the elevation over northwestern Pennsylvania. The radar captured the line and strong out bounds winds (Fig. 11) as far west as Cameron County (Fig. 11 bottom). As the line moved eastward KBUF and KBGM had better sampling of the storms. 2 Initially, the KBUF radar was nearly orthogonal to the storm motion and the reflectivity was used to see the leading edge of strong convection. The sampling improved as the storms moved into McKean County where there was extensive damage. The radar implied a mesocyclone near Smethport (Fig. 12) in the middle of McKean County at 1843 UTC. The KBUF and KBGM radars showed some rotation at and above 15kft over McKean and Potter Counties in the base velocity and stronger rotation in the storm relative data which prompted several tornado warnings. The damage in the northern regions across McKean County based on a survey team is shown in Figure 13. Tree damage falling from around 240 degrees was common. There were two concentrated areas of damage one in Lewis Run and the other in Smethport as indicated by the ovals in Figure 13. There were actually two distinct damage areas in Lewis run, one along and west of 219 where winds accelerated down a hill side snapping trees and damaging out building. Several homes sustained minor damage primarily from trees. The pine trees were most affected many were uprooted or snapped. In the village there was another region of enhanced damage mainly pine trees and several homes were damaged by falling trees. At least 5 homes in Lewis run had sustained visible tree damage. Along the survey route heading east in the trail of the high winds indicated in Figures there was damage all the way to Smethport. Smethport was another area of focused damage with many clusters of trees down, mainly parallel to the road. Several homes (estimates of 17) were damaged by falling trees and county wide the number was around 34 homes. The progression of the system on KBUF is shown in Figure 14. As the system moved into McKean County the system became outbound dominated and showed signs of rotation. The KBGM radar at this time also showed signs of rotation (not shown). This contributed to the damage swath across northern Pennsylvania and northward into New York and produced several tornado warnings for these northern storms along the line (Fig. 15). The KBGM radar showed the storm as it moved across Tioga County which too was hit by extensive tree damage and power outages. 2 KCCX radar was down from approximately 1800 through 2000 UTC.

4 The survey of the storms from Smethport to Coudersport and Genesee is shown in Figure 17. Concentrated damage was found northeast of Coudersport and in Genesee. Sporadic tree damage was seen all along both tracks to these locations. The extent of the damage east of Coudersport became more sporadic and less distinct. The damage swaths line up well with the key features on the KBGM and KBUF radars. In all 6 regions of intense damage were surveyed or examined along this track for possible tornado damage. Only the one location northeast of Coudersport showed clear and distinct rotation in the damage swaths. Clearly, intense downdrafts were produced by the system which produced extensive damage in the higher end EF0 and lower EF1 range. The focus of the activity moved south after 2000 UTC. These storms too produced damaging winds and bowing segments. The hardest hit region of in the lower Susquehanna Valley, though the damage was not as extensive as in the northern tier. 5. Satellite imagery The GOES IR image at 1732 UTC (Fig. 18) shows the initial storms after moving on shore across westernmost Pennsylvania and Ohio. These storms raced westward and the expansive cold cloud tops visible over northern Pennsylvania and New York at 1855 UTC. Coldest elements lined up well with the strong storms in northern Pennsylvania. The enhanced cold clouds over southwest Pennsylvania was associated with another west-to-east moving bow like feature which would bring wind damage into western Pennsylvania. 6. Summary A line of thunderstorms raced across northern Pennsylvania and southern New York during the afternoon hours of Thursday 26 June The northern line contained bowing segments with over 70kts of low-level velocity in the radar data. Several storms along this line developed mid-level mesocyclones which likely contributed to the enhanced damage, prompted tornado warnings producing 7 known tornadoes in New York and Pennsylvania. In addition to the tornadoes, there were several intense microbursts and a more general broad swath of sporadic wind damage across the region. The extensive wind damage downed trees and power lines leaving most of the northern tier of Pennsylvania with extensive power outages. The larger scale pattern was a pattern conducive to the production of severe weather to include strong winds (Fig. 6). A surge of deep warm moist air (Fig. 7 & 8) move into the region ahead of an advancing mid-tropospheric short-wave (Fig. 4) and attendant cold front (Fig. 8). This triggered the convection tapping the instability and shear (Fig. 3). The strong generally west to east 700 hpa winds has accompanied many bow echo and derecho events during the summer of There was a strong 700 hpa jet associated with the events of 29 June and 7 July 2012 (Grumm 2012).

5 This event had relatively high LCL heights in the areas of convection making tornadoes and strong tornadoes a low probability outcome. Additionally, the 0-1.5km shear as estimated from 10m to 850 hpa winds was modest and thus did not favor strong mesocyclones and warm downdrafts near the surface. The shear was stronger to the north where several tornadoes did in fact develop and produce damage. The mid-level shear was particularly strong and may have contributed significantly to the rotating storms in northern Pennsylvania and southern New York State. However, the low LCL heights likely limited tornado activity. Several of the evolving supercells appeared to develop out of initial bow echo elements. The strongest supercell developed north of the Pennsylvania and New York border and produced a tornado in Elmira, New York (WFO-BGM PNS and SPC storm reports). The event of 26 June was a significant severe weather event from southern New York into the Ohio Valley (Fig. 2). The strong surge of high PW air ahead of the front provided the fuel and instability for the storms and the front provided the lift. Due to the strong and deep midlevel shear and the lack of strong low-level shear the majority of storm reports were of wind damage. 7. Acknowledgements Elyse Colbert assisted with the storm surveys and the survey maps in the Appendix of this document. The EMA s in McKean and Potter Counties facilitated the surveys and focused areas to examine. Additionally they provided summarized data on injuries, deaths, and the number of damaged homes. They also provided information on the towns that actually had power, few. 8. References Brooks, H.E., Lee, J.W., Craven, J.P., The spatial distribution of severe thunderstorm and tornado environments from global reanalysis data. Atmos. Res , Craven, J. P., and H. E. Brooks, 2004: Baseline climatology of sounding-derived parameters associated with deep moist convection. Natl. Wea. Dig., 28, 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., Davies, J. M., 2006a: Tornadoes in Environments with Small Helicity and/or High LCL Heights. Wea. Forecasting, 21, doi: Davies, J.M.. (2006b) Tornadoes with Cold Core 500-mb Lows. Weather and Forecasting 21:6, Online publication date: 1-Dec Abstract. Full Text. PDF (1512 KB) Grams,J.S, R. L. Thompson, D. V. Snively, J. A. Prentice, G. M. Hodges, L. J. Reames. (2012) A Climatology and Comparison of Parameters for Significant Tornado Events in the United States. Weather and Forecasting 27:1, Online publication date: 1-Feb Grams, J. S.,W.A.Gallus Jr., S. E.Koch, L. S.Wharton,A. Loughe, and E. E. Ebert, 2006: The use of a modified Ebert McBride technique to evaluate mesoscale model QPF as a function of convective system morphology during IHOP Wea Forecasting, 21,

6 Grunwald,S. and H. E. Brooks: 2011: Relationship between sounding derived parameters and the strength of tornadoes in Europe and the USA from reanalysis data.atmos. Research,100, Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic observations within rear-flank downdrafts of nontornadic and tornadic supercells. Mon.Wea. Rev., 130, Rutledge, G.K., J. Alpert, and W. Ebuisaki, 2006: NOMADS: A Climate and Weather Model Archive at the National Oceanic and Atmospheric Administration. Bull. Amer. Meteor. Soc., 87, Markowski, P, Y. Richardson, E. Rasmussen, J. R. Davies-Jones, R. J. Trapp, 2008: Vortex Lines within Low-Level Mesocyclones Obtained from Pseudo-Dual-Doppler Radar Observations. Mon. Wea. Rev., 136, doi: Schoen, J.M W. S. Ashley. 2011: A Climatology of Fatal Convective Wind Events by Storm Type. Weather and Forecasting 26:1, Online publication date: 1-Feb Abstract. Full Text. PDF (1569 KB) Trapp, R. J., S. A. Tessendorf, E. S. Godfrey, H. E. Brooks, 2005: Tornadoes from Squall Lines and Bow Echoes. Part I: Climatological Distribution. Wea. Forecasting, 20, doi:

7 Figure 1. Composite reflectivity at 2055 UTC 26 July showing the key lines associated with the widespread severe weather of 26 July Data from the NMQ Q2 site. Return to text.

8 Figure 2. Storm Prediction center summary of severe weather for the 24 our period ending at 1200 UTC 27 July Data are color-coded as in the key in the lower left of the image. Data courtesy of the NWS Storm Prediction Center. Return to text.

9 Figure 3. NAM 00-hour forecasts valid at 1800 UTC 26 June 2012 showing a) 10m to 850 hpa shear, b) 10m to 500 hpa shear, c) LCL heights (m) and d) CAPE (JKg-1). Return to text.

10 Figure 4. GFS 00-hour forecasts of 500 hpa heights and 500 hpa height anomalies in 12 hour increments from a) 0000 UTC 25 July 2012 through f) 1200 UTC 27 July Heights every 60m. Return to text.

11 Figure 5. As in Figure 4 except for 850 hpa winds (kts) and wind anomalies in 6-hour increments from a) 0000 UTC 26 July 2012 through f) 0600 UTC 27 July Return to text.

12 Figure 6. As in Figure 5 except for 700 hpa winds and wind anomalies. Return to text.

13 Figure 7. As in Figure 5 except for precipitable water (mm) and precipitable water anomalies. Return to text.

14 Figure 8. As in Figure 5 except for 850 hpa temperatures (C ) and temperature anomalies Return to text.

15 Figure 9. As in Figure 3 except NAM 3-hour forecast valid at 2100 UTC 26 July Return to text.

16 Figure 10. KCLE radar showing 0.5 degree reflectivity and velocity at (upper) 1727 UTC and (lower) 1807 UTC. Return to text.

17 Figure 11. As in Figure 10 except for KCLE at 1822 and 1831 UTC. Return to text.

18 Figure 12. As in Figure 10 exept for KBUF at 1843 and 1901 UTC. Return to text.

19 Figure 13. Survey team region in McKean County. Red lines show the approximate route and there were sporadic trees down all along route 219 to Lewis Run. The oval in Lewis run denotes enhanced damage due to 2 downbursts in the town. Route 59 was used to connect to US 6 in Smethport. There were downed trees all along this route in a sporadic and chaotic nature nearly all trees fell or were snapped parallel to the road. Along 219 the trees fell across the road as the road is generally north-south. Guard rails were damaged due to the trees.. Detailed surveys are in the Appendix. Return to text.

20 Figure 14. As in Figure 12 except for KBUF radar at (top) 1822 (middle) 1847 and (bottom) 1843 UTC. Arrow shows enhanced velocity which created a mesocyclone in the storm relative velocity. Return to text.

21 Figure 15. As in Figure 14 except KBUF at 1857 and 1915 UTC. Tornado warnings in red. Return to text.

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23 Figure 16. KBGM SRM at 0.5 and 1.3 degrees storm motion 273 at 37kts valid at (top) 1951 UTC and (bottom) 2010 UTC. Return to text.

24 Figure 17. Map of estimated survey tracks from Smethport in east to Coudersport. Only Port Allegany had power. Orange track is main route. In Coudersport a diversion to Genesee was made. A 20 yd by 150 yd weak tornado swath was found to the lee of a hill near the red circle and extensive damage was found in Genesee focused in two areas separated by about 1.5 miles. Sporadic tree damage was visible along both tracks. Most trees fell from west to east. Detailed surveys are in the Appendix. Return to text.

25 Figure 18. GOES IR at 1732 and 1855 UTC. Return to text.

26 Appendix Survey Maps Firehouse Figure A1. Lewis Run microburst area. The yellow oval is region of tree damage. The two inner blue ovals are approximate regions of focused snapped trees and uprooted pine trees to the lee of the hill west of town and along main street south and west of the firehouse. There were trees down along the larger creekt mainly from the creek to the homes on Church Street. Blue lines show the basic orientation of the trees falling from about 240 degrees. Return to text.

27 Figure A2 Convergent damage swath on side of a hill northeast of Coudersport, PA south of Reese Hollow road. The most intense damage was on the side of the hill down to the dirt road. The damage was less distinct and intense east of the road and south of the bend in the road. Return to text.

28 Figure A3. Damage survey and estimates in Genesee, Pennsylvania. Outer red oval is region of tree enhanced snapped and uprooted trees, yellow oval is region of houses and trees along Grover Hollow road surveyed on foot. Yellow arrow is near where a tree fell on a camper killing one person and injuring another. Return to text.

29 Figure A4. Survey of Kinney road focused damage. Pine trees west of the house were uprooted and snapped. One tree hit the southwest corner of the house. Sporadic damage was all along Kinney road with this region of focused and intense damage from 220 to 240 degrees, all uprooted trees were from about 240 degrees. Arrow shows approximate location of downed utility poles were power company was working during the survey. Return text.