Mid-Atlantic Severe Event of 1 June 2012

Mid-Atlantic Severe Event of 1 June 2012 1. Introduction An unseasonably deep midtropospheric ridge (Fig. 1) brought a strong cold front into the Mid-Atlantic region on 1 June 2012. A surge of warm moist air ahead of this system produced severe weather from South Carolina to Pennsylvania (Fig. 2). The strong low-level shear and relatively low lifting condensation levels (LCL) likely contributed to the 17 reported tornadoes in Pennsylvania 1 (3) and Maryland (14) with an additional 8 tornadoes in Virginia. The association of high shear and low LCL heights is a potential indicator of increased tornado potential (Brooks et al 2003; Grunwald and Brooks 2011; Craven and Brooks 2004). Supercell tornadoes typically form in the rear-flank downdraft or hook-echo portion of supercells (Markowski 2002) and the downdraft air is the likely source of the low-level vorticity (Markowski and Richardson 2009). Thus, the low LCL heights are theorized to aid in producing relatively warm rear-flank downdrafts which are the likely source of vorticity for the tornadoes produced by supercell thunderstorms. It should be noted that all of the tornadoes on 1 June were weak tornadoes and most occurred in the deep warm air well ahead of the frontal system. Figure 2.. Storm Prediction Center storm reports showing severe weather by type for the periods ending 1200 UTC 02 June 2012. Return to radar. This study examines the event of 1 June 2012 and compares some of the key synoptic features and parameters to literature related to severe storms and severe storm modes. In eastern Pennsylvania and Maryland, early in the event the conditions favored rotating storms and most of the tornadoes were associated with discrete cells. However, there were 3 non-supercell tornadoes in western Pennsylvania which occurred in a strongly sheared environment with convective available potential energy below about 800JKg -1. 1 Storm Reports showed 2 entries for the Ligonier tornado in Westmoreland County and the Indiana County tornado though on the page was not verified as of 14 June 2012.

To the west, a quasi-linear convective system (QLCS: Atkins et al. 2005; Atkins et al. 2009) evolved producing at least 3 distinct bowing segments along the longer line of convection. One small cell developed ahead of this line producing the first 2 tornado reports in western Pennsylvania. The distinct bow echoes appeared to account for all of the wind and the lone tornado report in west-central Pennsylvania. Archived radar data was used to classify the storm types and the resulting severe weather. The 3 primary storm types were those defined by Trapp et al. (2005) and Grams et al. (2012). In this event most of the storms in the east which produced tornadoes were discrete cells the western storms, closer to the front, were more linear such that the Quasi-Linear Convective Storm (QLCS) type dominated in that region. This paper will document the severe event of 1 June 2012 with a focus on the patterns and features aiding in discriminating the severe weather threat. Data from 1 June 2012 is shown in reference to the large scale pattern, but the focus is on the severe weather of 1 June 2012. 2. Data and Methods All radar data used were obtained from the National Climatic Data Center (NCDC) and displayed using the Gibson-Ridge software package (GR2-Analysist). Storm reports from the Storm Prediction Center (SPC) and the NCDC storm databases were used here. The NCDC storm data were put into a Placefile which were then used as overlays in GR2Analyst to match storm types to tornado reports. The NCDC storm report latitude and longitude had to be converted to degrees from degree and minute data and local times were converted to UTC. The large scale pattern was reconstructed using the NCEP NAM data. These data were displayed using GrADS (Doty and Kinter 1995). The focus was on the pattern to diagnose fields which departed significantly from climatology and on fields shown to aid in discriminating storm type. 3. Pattern over the region The 500 hpa pattern (Fig. 1) showed a deep trough moving across the Mid-West at 01/0600 UTC (Fig. 1a) which entered Pennsylvania by 02/0000 UTC. The height anomalies in the trough were -1 to -3σ below normal, implying a strong trough for early June. This trough brought a surge of warm air and deep moisture into the region. The surge of high precipitable water (PW) into the Mid-Atlantic region was fast and short-lived (Figs. 3a-f). The PW values peaked near 40mm and in the 1 to 3σ above normal range near the peak time of the convection (Fig. 3d). The deep moisture surge was accompanied by a strong southerly low-level jet as shown in the NAM 00-hour 850 hpa wind field (Fig. 4). The 850 hpa winds peaked at over 50 kts around 02/0000 UTC with 3 to 4σ above normal wind anomalies.

The evolution of the 10m to 850 hpa (~1km bulk shear) and 10m to 500 hpa (~6km bulk) bulk shear; the LCL height and surface based CAPE at 1200 UTC 1 June through 0000 UTC 2 June 2012 is shown in Figures 5-7. These data show strong shear over the region in the approximate 1km layer with over 15ms-1 of shear over parts of the region for the 12 hour period with over 15ms-1 by 01/1800 (Fig. 6a) and some areas of over 20ms-1 by 02/0000 UTC (Fig. 7a). The 6- hourly CAPE data in the NAM analysis showed the higher CAPE over Maryland and Virginia, peaking in the 1200-1800JKg-1 range around 01/1800 UTC before moving to the east. Though not shown, analyzed CAPE and RAP analysis showed a surge of 1200JKg-1 into southeastern Pennsylvania around 01/2100 UTC. The 01/1800 UTC NAM 3-hour forecast predicted about 800JKg-1 in this region at 2100 UTC (Fig. 8). In western Pennsylvania, the CAPE was rather anemic and did not support large updrafts. However the strong winds favored the potential for strong winds and bowing segments in any convection which did develop. The western area was more indicative of a cool season event with modest CAPE and strong winds, a known QLCS like pattern supporting bow echoes and the potential for non-supercell tornadoes. 4. Radar evolution and severe weather The large scale radar perspective at 2100 UTC 1 June (Fig. 9) shows the organizing QLCS over western Pennsylvania and the discrete cells over southeastern Pennsylvania, Maryland, and Virginia. These data line up well with the majority of tornadoes (Fig. 2) occurring with the discrete cells. The discrete cells ahead of the frontal system and developing QLCS over Maryland and southern Pennsylvania are shown in Figure. 10. The data show the radar reflectivity and the severe weather observed during the event. The 5 western tornadoes (red symbols) ending with a tornado over York County, Pennsylvania were associated with a series of supercells and all but one report likely came from the lead storm of the two which followed a similar track. The hook with the northern cell is evident at 1916 UTC with the tornado report at the northernmost location in Maryland. The storm over Maryland entered Pennsylvania producing wind damage and a brief EF0 tornado around 2025 UTC (Fig. 11). The radar indicated a hook or appendage in the reflectivity and a mesocyclone. The damage reports appear to be displaced well northwest relative to the expected location based on the radar. The QLCS entering the State College warning area at 2113 UTC is shown in Figure 12. A bowecho like structure was present over southwestern Pennsylvania on KCCX radar, the KPBZ radar showed a stronger signal (not shown) with winds near 55kts. At 2113 UTC a mesocyclone was present near the observed tornado in Westmoreland County.

There were at least 3 distinct bow-echo like structures along the line. The image at 2223 UTC (Fig. 13) shows the 0.5 degree winds as the bow was moving across Cambria County though only 1 report of wind damage is shown 2. This bow moved to the northeast and was associated with 4 reports of wind damage (Fig. 14 red line). There was a short-lived tornado near the weak cyclonic bookend vortex (Fig. 13 red arrow). This southern bow produced the wind report in extreme western Somerset County. The central bow echo swept through the RDA and is shown in the outbound winds at 2329 UTC (Fig. 14). This image shows two distinct bow echoes, the northern one also produced several reports of wind damage. 5. Summary A strong upper-level short-wave and accompanying frontal system helped trigger a severe weather event in the Mid-Atlantic region on Friday 1 June 2012. Well ahead of the front in the more region of strong low-level wind shear with CAPE in the 800-1800JKg -1 range discrete storms developed. Several of these storms were relatively long lived and at least one discrete storm produced 3 tornadoes. Farther west, with lower CAPE but strong shear, a QLCS like system developed. Distinct bowing segments along this larger line of convection produced most of the severe weather. The supercells in the warm air occurred with strong low-level shear, modest CAPE, and relatively low LCL values. This likely favored the evolution of discrete cells with large updrafts and the potential for rotation. The relatively low LCL heights may have increased the probability of these rotating storms to produce tornadoes. A recent study of tornadoes in Pennsylvania examining low-level shear and LCL heights suggests that many tornadoes are observed with relatively weak 0-1km shear, generally less than 8ms -1 (15kts). The 10-15ms -1 shear in the 10m to 850 hpa layer was close to the range of most tornado events in Pennsylvania. It should be noted that for F2-F4 tornadoes the shear is often over 20ms -1. The LCL heights with most tornadoes are under 1000m. The QLCS system which developed and moved across western and central Pennsylvania produced at least 3 identified bowing segments. All of the reports of wind damage were associated with these bowing segments. The lowest elevation slice behind these bows was in the 40 to 50kt range with a few down-radial values approaching 60kts. One tornado was associated a cyclonic bookend vortex (Fig. 13). Just ahead of the QLCS a small rotating storm likely produced the 1 verified tornado report in Westmoreland County. Despite relatively low CAPE, under 600JKg -1 an updraft developed and the strong shear produced a short-lived min-supercell thunderstorm (Fig. 12). 2 The Cambria 911 center had 12 reports of downed trees and wires only one was recorded in storm reports.

6. References Atkins, N.T., C.S. Bouchard, R.W. Przybylinski, R.J. Trapp, and G. Schmocker, 2005: Damaging Surface Wind Mechanisms within the 10 June 2003 Saint Louis Bow Echo during BAMEX. Mon. Wea. Rev., 133, 2275 2296. Atkins, N.T., and M. St. Laurent, 2009a: Bow Echo Mesovortices. Part I: Processes That Influence Their Damaging Potential. Mon. Wea. Rev., 137, 1497 1513. Atkins, N.T., and M. St. Laurent, 2009b: Bow Echo Mesovortices. Part II: Their Genesis. Mon. Wea. Rev., 137, 1514 1532. Brown, R. A., D. W. Burgess, and K. C. Crawford, 1973: Twin tornado cyclones within a severe thunderstorm: Single Doppler radar observations. Weatherwise, 26, 63-71. Craven, J. P., and H. E. Brooks, 2004: Baseline climatology of sounding-derived parameters associated with deep moist convection. Natl. Wea. Dig., 28, 13 24. 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., 209-219. Davies, J. M., 2006a: Tornadoes in Environments with Small Helicity and/or High LCL Heights. Wea. Forecasting, 21, 579 594. doi: http://dx.doi.org/10.1175/waf928.1 Davies, J.M.. (2006b) Tornadoes with Cold Core 500-mb Lows. Weather and Forecasting 21:6, 1051-1062 Online publication date: 1-Dec-2006. 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, 106-123 Online publication date: 1-Feb-2012. http://journals.ametsoc.org/doi/pdf/10.1175/waf-d-11-00008.1 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 2002. Wea Forecasting, 21, 288 306. 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, 1692 1721. Markwoski, P. M. 2002: Hook echoes and rear-flank downdrafts: A Review. MWR,130,852-876. Markowski,P.M and Y.P. Richardson 2009: Tornadogenesis: Our current understanding. Science Digest,93 3-10. Link 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, 327-341. 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, 3513 3535. doi: http://dx.doi.org/10.1175/2008mwr2315.1 Schoen, J.M W. S. Ashley. 2011: A Climatology of Fatal Convective Wind Events by Storm Type. Weather and Forecasting 26:1, 109-121. Online publication date: 1-Feb-2011. 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, 23 34. doi: http://dx.doi.org/10.1175/waf-835.1

Weaver, S. C., and S. Nigam, 2008: Variability of the Great Plains low level jet: Large scale circulation context and hydroclimate impacts. J. Climate, 21, 1532 1551.

Figure 1. GFS 00-hour forecasts of 500 hpa heights and height anomalies from a) 0600 UTC 01 June through f) 1200 UTC 02 June 2012. Contours every 60 m anomalies as in color bar in standard deviations form normal. Return to overview.

Figure 3. As in Figure 1 except for precipitable water (mm) and precipitable water anomalies form a) 0600 UTC 1 June to f) 1200 UTC 2 June 2012. Contours every 5 mm. Return to pattern.

Figure 4. As in Figure 3 except for 850 hpa wind and wind anomalies. Return to pattern.

Figure 5. NAM 00-hour forecasts valid at 1200 UTC 1 June 2012 of a) 10m to 850 hpa wind shear (ms-1), b) lapse rates 850 to 500 hpa (C/km) d) LCL heights (m), and d) surface based CAPE (JKg-1). Return to text.

Figure 6. As in Figure 5 except valid at 1800 UTC 1 June 2012. Return to text.

Figure 7. As in Figure 5 except valid at 0000 UTC 2 June 2012. Return to text.

Figure 8. As in Figure 5 except NAM initialized at 1800 UTC showing 3-hour forecast valid at 2100 UTC 1 June 2012. Return to text.

Figure 9. NMQ site composite reflectivity over the Mid-Atlantic region at 2100 UTC 01 June 2012. Data as per the scale on the bottom of the image. Return to text.

Figure 10. KLWX 0.5 degree reflectivity at 1854 and 1916 UTC 1 June 2012. Red tornado symbols show observed tornadoes and light blue boxes show wind damage reports. Arrow shows tornado icon and hook as in text. Red line parallels the tornado paths. Return to text.

Figure 11. As in Figure 10 except KLWX valid at 2025 UTC showing 0.5 degree storm relative motion and 0.5 degree reflectivity. This is the same storm the black arrow points to in Figure 10 just later in time. The blue square and red tornado show the location of reported severe winds and a tornado. Return to text.

Figure 12. As in Figure 11 except for KCCX radar at 2113 UTC. Return to text. NWS State College Case Examples

Figure 13. As in Figure 12 except for 2223 UTC Arrow shows cyclonic bookend vortex tornado location icon is better seen in the velocity image.the red line shows the wind damage point and the tornado produced by the southern bow echo. The northernmost tornado symbol was based on Storm Reports and was never verified. Return to text.

Figure 14.. As in Figure 12 except for 2339 UTC. Black line connects wind reports with central bow echo. Return to text.