Severe Weather Event of 13 July 2014

Transcription

1 Severe Weather Event of 13 July 2014 By Richard H. Grumm and Elyse M. Colbert National Weather Service State College, PA 1. Overview Severe weather affected the eastern United States (Fig. 1) from northwestern Connecticut southwestwards into Ohio and Kentucky. Over Pennsylvania, there were two distinct semi-linear damage paths. One was over western Pennsylvania and one was over central Pennsylvania. It will be shown that each of these observable patterns was associated with persistent convective features containing bow echoes. The severe weather was observed (Fig. 2) on the northern flanks of an eroding 500 hpa ridge and an unseasonably strong 500 hpa trough which moved out of central Canada and over the Great Lakes. The 500 hpa height anomalies were -2 to -3σ below normal with this trough. This trough would usher in some of the coldest mid-summer air experienced in several years into the eastern United States. Though not relevant to this paper, the cold air would set several record low-high temperature and record low temperatures for July in many locations. An examination of the storm reports and radar over Pennsylvania and New York (Fig. 3) showed several distinct quasi-linear bands of severe weather. The three most linear damage patterns were located near Pittsburgh, over the Maryland-West Virginia panhandles, and across the southern tier of New York. Though not as clear, a distinct bow echo produced most of the damage in central Pennsylvania and another mini bow-like feature produced the linear damage swath north of the Maryland border. These semi-linear damage patterns and radar echoes associated with them have been documented by Grumm and Budd (2013) and Grumm and Colbert (2013 and 2012). The following sections examine the set-up for the convection and two of the features which produced a significant percentage of the severe weather in Pennsylvania. The features in New York and Maryland are referenced but not shown here. 2. The pattern The large scale pattern at 500 hpa (Fig. 2) showed the deep trough approaching the region. The strong gradient between the trough and ridge to the south produced a strong 250 hpa jet (Fig. 4). The strong jet pulled in a surge of deep moisture and a plume of high precipitable water (PW: Fig. 5) air into the region ahead of the cold front. This deep plume of moisture was associated with a strong 850 hpa jet with +2 to +3σ v-wind anomalies (Fig. 6) peaking in the afternoon and evening hours over Pennsylvania. The CAPE in this region of high PW and strong 850 hpa

2 winds peaked at 600 to 1800 JKg-1 around 1800 UTC over Ohio and western Pennsylvania and in the 1200 to 1800 JKg-1 range over southern Pennsylvania at 0000 UTC 14 July (Fig. 7). The environment contained sufficient CAPE and shear to support some storm organization. The environment was not favorable for tornadoes. 3. Western Pennsylvania and Pittsburgh Area Storm Figure 8 shows the 0.5 degree reflectivity (Z) and base velocity (V) at 20:24 and 20:35 UTC as well as the severe weather reports collected over this region on 13 July The Z data does not show how the cold pool with this storm became organized, and the storm and potential for wind damage is easier to follow using the features in the V data. Storm relative winds were not as revealing and are not shown. The base winds showed the mini-bow echo or spearhead echo feature which had strong outbound (red) winds. The KPBZ viewing angle was not down-radial so the winds were likely much stronger than recorded values of 35 to 45kts. At 2045 UTC (Fig. 9) the Z data looked unorganized, though the strong cold pool and associated outflow continued to race to the east and produced wind damage along the path. This feature never looked well-organized in the Z data, but the low-level V data showed an organized system that produce sporadic damage into Cambria County (not shown). The velocity data is often one of the better tools to find distinct bowing features and storms with strong cold pools. In this case, this spearhead echo clearly dominated the severe weather reports south of Pittsburgh and Greensburg on 13 July Central Pennsylvania Bow Echo The central Pennsylvania bow echo also accounted for a significant portion of the damage in central Pennsylvania (Fig. 10). However, the northern cold pool and enhanced winds initially propagated slightly to the north and east as the system moved eastward, and several shorter-lived cold pools with enhanced winds developed to the south. This damage path was not as linear as those observed in western Pennsylvania and in southern New York. The upper panel of Figure 10 shows the initial strong winds with the developing cold pool. Dissimilar to the western Pennsylvania storm, the Z data showed a better bowing structure and hints of a rear-inflow notch (RIN). Additionally, the system relative motion was nearly downradial. The lower panel (2226 UTC) shows the bow as it moved east of the radar and the strong winds were outbounds. By 2247 UTC (Fig 11 upper) the Z data showed a quasi-linear system with enhanced outbounds near the wind damage report northeast of the radar. The two wind reports to the south had not yet

3 occurred. The southern edge of the bow echo became enhanced and produced wind damage around 2318 UTC as the enhanced feature moved over that region (Fig. 11 lower panel). The last damage report in the State College warning area for this system (Fig. 12) occurred around 2355 UTC as the bow echo, with some enhanced outbound winds, past of the region near the lone severe weather report. An examination of composite reflectivity data only implied that this bow moved into eastern Pennsylvania and may have caused the wind damage in eastern Pennsylvania. 5. Summary A strong mid-summer 500 hpa shortwave moving over a ridge brought a surge of moisture, instability and shear into the Mid-Atlantic region resulting in a semi-linear severe weather event (Fig. 1) from southwestern New England into the Ohio Valley. Radar returns implied this event was dominated by Quasi-Linear Convective Systems (QLCS). Several distinctly linear, generally west to east damage swaths were observed during the event. The concentrated damage swath in southwestern Pennsylvania was associated with a strong cluster of storms which developed a mini-bow or spearhead echo. This persistent feature and its associated damage appeared linear in nature on the local and SPC plots of damage for 13 July The QLCS system over central Pennsylvania looked linear in both the Z and V data. However, the areas of damage were better defined in the V data than the Z data. The northern cold pool and winds produced most of the damage and may have produced damage along a 100 mile path. The damage areas were more sporadic, perhaps a population density issue, and did not appear as linear as the damage in southwestern Pennsylvania. Additionally, farther south along the larger QLCS line of storms, a second short-lived enhanced bow produced 2 reports of wind damage well to the south. The damage in New York State was not closely examined. However composite radar loops implied a single persistent supercell likely produced most of this damage (not shown). The concentrated area of damage in the panhandle of West Virginia and Maryland was also associated with a bowing segment (Fig. 13) with enhanced winds along the bow echo. As this feature moved eastward, it could be tracked on KLWX (Fig. 14 & 15) and matched to the wind damage. Similar to the KCCX and KPBZ data, the velocity data was more revealing than the reflectivity data. Lessons learned from this and similar events include knowing the environments in which cold pools can and will develop. Many studies show that Downdraft Convective Available Potential Energy (DCAPE Emanuel 1994) can serve as a proxy for the development of cold pools. If a storm or cluster of storms; the southwestern Pennsylvania event had a good cluster of storms; the

4 collective outflow can produce a meso-high. The resulting feature is typically best viewed using velocity data. The bow echoes in central Pennsylvania and West Virginia also showed a strong signal in the reflectivity data. Once the feature is present on radar it can produce a series of severe weather reports, thus creating the quasi-linear damage swaths (Fig. 1). In strongly forced environments the these long swathed liner mainly straight line wind damage reports are termed derechoes (Johns and Hirt 1987). These stronger systems often have coherent signals in both the reflectivity and velocity data. In weaker environment s, the systems tend to be smaller, have shorter damage swaths, and often do not appear as organized in reflectivity as they do on velocity data. These long-lived systems likely occur in regions with strong forcing. Model studies (Rotunno et al 1988; Weisman 1992, and Weisman 1993) show that sustained bow echoes with strong cold pools require about 20ms-1 of vertical wind shear within 2.5km of ground level and CAPE values in excess of 2200JKg-1. These conditions and parameters were idealized for model simulations. Johns (1993) suggested that the real-world conditions favoring development of bow echoes and derechoes is more complex. Model proximity sounding studies suggest that higher DCAPE is required to sustain bow echoes relative to strongly forced events. Coniglio et al (2007) showed the key parameters associated with sustained QLCSs. Wind shear is critical in developing and maintaining the cold pool. They found deep vertical wind shear to be critical factor. As shown in this case study these nearly ideal conditions can and clearly do exist in very regionalized if not localized environments as at least 3 mini-bow echoes produced locally linear swaths of damager across western and central Pennsylvania and West Virginia and Maryland. 6. Acknowledgements PSU for data access. 7. References Coniglio, Michael C., Harold E. Brooks, Steven J. Weiss, Stephen F. Corfidi, 2007: Forecasting the Maintenance of Quasi-Linear Mesoscale Convective Systems. Wea. Forecasting, 22, doi: Emanuel, K. A., 1994: Atmospheric convection. Oxford University Press, 883 pp. Evans, J. S., and C. A. Doswell, 2001: Examination of derecho environments using proximity soundings. Wea. Forecasting, 16, Johns, R. H., 1993: Meteorological conditions associated with bow echo development in convective storms. Wea. Forecasting, 8,

5 , and W. D. Hirt, 1987: Derechos: widespread convectively induced windstorms. Wea. Forecasting, 2, Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory for strong, long-lived squall lines. J. Atmos.Sci., 45, Weisman, M. L.,1992: The role of convectively generated rear-inflow jets in the evolution of long-lived mesoscale convective systems. J. Atmos.Sci., 49, , 1993: The genesis of severe, long-lived bow echoes. J. Atmos.Sci., 50,

6 Figure 1. Storm reports for 13 July from the Storm Prediction Center. Return to text.

7 Figure 2. GFS 00-hour forecast of 500 hpa heights (m) and 500 hpa height anomalies (sigma) in 6-hour increments from a) 0000 UTC 13 July 2014 through f) 0600 UTC 14 July Return to text.

8 NWS State College Case Examples Figure 3. KCCX radar and storm reports radar is valid at 1852 and 2226 UTC. Squares are wind damage and green triangles are hail reports. Times were picked for the start and mid-point of the event. Return to text.

9 Figure 4. As in Figure 2 except for 250 hpa winds and wind anomalies. Return to text.

10 Figure 5.. As in Figure 2 except for precipitable water and precipitable water anomalies. Return to text.

11 Figure 6. As in Figure 2 except for 850 hpa winds and wind anomalies. Return to text.

12 Figure 7. As in Figure 2 except for GFS analyzed mixed layer CAPE (JKg-1). Return to text.

13 Figure 8. KPBZ radar showing 0.5 degree reflectivity and velocity along with SPC storm reports for 13 July Upper images were valid at 2024 and lower images at 2035 UTC. Images were part of a loop of this feature. Return to text.

14 Figure 9. As in Figure 8 except for 2045 UTC. The yellow arrow shows the general path of the spearhead echo. Return to text.

15 NWS State College Case Examples Figure 10. As in Figure 8 except for KCCX radar at 2200 and 2226 UTC. Return to text.

16 NWS State College Case Examples Figure 11. As in Figure 10 except at 2247 and 2318 UTC. Return to text.

17 Figure 12. As in Figure 11 except 2355 UTC. Return to text. NWS State College Case Examples

18 Figure 13. As in Figure 11 except for KLWX radar at 2301 UTC showing the bow echo over West Virginia. Return to text.

19 Figure 14. As in Figure 13 except valid at 2339 UTC. Return to text. NWS State College Case Examples

20 Figure 15. As in Figure 13 except valid at 0004 UTC 14 July Return to text.