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Figure 1. Storm reports from the storm prediction center (SPC) website for the 24 hour period ending at 1200 UTC 19 July. Types are color coded as in the key in the lower left of the images.

15 July 2012 Severe Weather Event By

Richard H. Grumm and Elyse Colbert National Weather Service State College PA 16803

1. Overview

Severe weather affected the Mid-Atlantic area on 18 July 2012 (Fig. 1). This was the second significant severe weather event over the Mid-Atlantic in 3 days. A large ridge with above normal 500 hPa heights was over the western Atlantic (Fig. 2a) and a series of waves in the implied fast flow in the gradient in the fast flow north of the retrograding ridge over North America was slowly carving out a trough in the northeastern United States (Fig. 2b-f). Impulses in this fast flow were the likely triggers of the convection over Pennsylvania on 15 July 2012 and 18 July 2012. The net effect of the waves and convection was to return the 500 hPa heights to normal of the northeast and establish a strong ridge over the western plains (Fig. 2f).

The larger scale 850 hPa thermal pattern (Fig. 3) and precipitable water (PW) pattern (Fig. 4) indicated warm air moving over the ridge in North America and a surge of high into the central and eastern United States on the flanks of the western Atlantic ridge. The event of 15 July was associated with the plume of high PW (Fig. 4b). A stronger frontal system with cooler and drier air was associated with the event of 18 July 2012 (Fig. 4d-e & Fig.3d-e). Clearly, there was significantly cooler air at 850 hPa over southern Canada by 0000 UTC 19 July 2012 accompanied by drier air. The Mid-Atlantic heat episode of mid-July 2012, several cities in Pennsylvania and the Mid-Atlantic1 region reached 100F on 18 July, ended with a severe weather event.

The convection began around 1600 UTC and moved to the south and east. At 1800 UTC the 1.5 and 5km shear was relatively weak and the lifting condensation level height (LCL) was relatively high (Fig. 5). The conditions did not favor strong and persistent supercell storms and the shear and relatively high LCL heights implied a low risk of

1 The 850 hPa temperatures were 20-21C with 2 standard deviation above normal anomalies. 100F temperatures typically require 20C or greater at 850 hPa. This shows up better in 6-hourly data not shown here.

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tornadoes. The CAPE however was forecast to peak over 2400JKg-1 in the NAM and the GFS. The high CAPE implied with significant lift large updrafts could be realized. The -20C level was around 26kft suggesting that pulse storms with 60dBZ above 26kft could produce downbursts or large hail.

This paper will document the severe event of 18 July 2012. The focus is on the overall conditions, the mode of convection and radar evolution of key severe storms.

2. Data and Methods

All radar data used were obtained from the local real-time AWIPS archive and were displayed using AWIPS. Storm reports were obtained using the Storm Prediction Center database.

The large scale pattern was reconstructed using the NCEP GFS and NAM model 00-hour forecasts. Some convective parameters required using 3-hour forecasts to estimate the condition. All model data were displayed using GrADS (Doty and Kinter 1995).

3. Regional parameters The shear, LCL height and CAPE at 18/1800 UTC (Fig. 5) showed relatively weak shear

and high LCL heights. High instability was a critical issue and the potential for strong updrafts. The 850 hPa winds (Fig. 6) were relatively week and from the west during the time of the convection (1600-2200 UTC). The weak 850 to 10m shear (Fig. 5 & Fig. 7) was due in part to the weak winds at 850 hPa (Fig. 6).

The weak shear and somewhat unidirectional shear is evident in the NAM profiles near Harrisburg, PA at 1800 and 2100 UTC (Fig. 8). The 1800 UTC profiles shows deep instability and CAPE over 400 JKg-1, and weak shear. The sounding was very unstable. The 2100 UTC sounding still showed instability and some drying in the mid-levels. The winds near 500 hPa are out of sync with earlier and later times, perhaps implying some local convection in the model.

4. Radar

The initial storms of the event were pulse storms. Several warnings were issued for storms which produced 60dBZ cores from 26 to 32kft. However, in south-central Pennsylvania the storms began to produce strong rear-flank downdrafts on the southwest flanks of the storms. These storms persisted longer than the earlier pulse storms and produced both large hail and wind damage. The KCCX radar showing two such storms at 1808 UTC is shown in Figure 9. The hook and strong rear-flank downdrafts on the storms persisted for nearly an hour (Fig. 10). The warnings were based on the KLWX radar (Fig. 11) which had strong winds and strong rotation in the storm relative products (not shown).

Why the storms in the southeast began to rotate is unclear. The westernmost storm, in Franklin County (Fig.9-11) showed accelerating winds as the storm came off the higher terrain

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into the valley. There may have been a terrain effect that changed the character of these storms. Most of the rotating storms produced large hail.

One pulse storm developed east-northeast of the radar producing a down draft and a microburst which had a divergent pattern in the wind field (Fig. 12). The storm also put 60dBZ cores over 32kft. The downbursts produced 3 reports of downed trees. The storm produced a long-lived outflow boundary which triggered some weaker new cells to develop. The outflow was clearly visible in the reflectivity and velocity data through 2041 UTC (Fig. 13) and later.

5. Summary

A severe weather event developed in the Mid-Atlantic region and brought damaging winds and large hail (Fig. 14) to central Pennsylvania. The event occurred as a short-wave and accompanying frontal boundary moved through a strong ridge over the eastern United States. The instability ahead of this system produced strong updraft cores and a mix of pulse and weakly rotating storms developed. Several pulse storms produced microbursts. Closer to the Maryland border several storms took on weak supercell characteristics. These storms produced both wind damage and large hail.

This case shows the value in using the -20C level in conjunction with 60dBZ cores to warn or identify significant pulse storms. In total 6 pulse-like storms were warned on. Three clearly produced some wind damage. The last pulse storm of the day (Fig. 13) produced 3 known reports of wind damage.

Despite the relatively weak shear, several storms in southern Pennsylvania began to rotate. At least 3 of these storms attained a strong rear flank downdraft as the storms moved from the higher terrain into the valleys. This implies terrain may have played a role in the weak rotation. Several of the rotating storms, such as those in Figures 9-11 produced large hail and damaging winds. Though not shown, another weakly rotating storm produced the large hail in Dauphin County (Fig. 14); a spotter photographed the hail in the town of Dauphin in Dauphin County and posted it on the office Facebook page. Figure 14 was obtained from the office Facebook page.

6. References

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

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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.

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

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Figure 2. GFS 00-hour forecasts of 500 hPa heights and 500 hPa height anomalies in 24-hour increments from a) 0000 UTC 15 July through f) 0000 UTC 20 July 2012. Return to text.

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Figure 3. As in Figure 2 except for 850 hPa temperatures and temperature anomalies. Return to text.

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

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Figure 5. NAM 00-hour forecasts valid at 1800 UTC 18 July 2012 showing a) 850 hPa to 10m shear (kts), b) 500 hPa to 10m shear (kts), c) the height of the lifting condensation level (m), and d) convective available potential energy. Return to text.

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Figure 6. As in Figure 2 except for GFS 00-hour forecasts of 850 hPa winds (kts) and wind anomalies in 6-hour increments from a) 0000 UTC 18 July 2012 through f) 0600 UTC 19 July 2012. Return to text.

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Figure 7. As in Figure 6 except NAM 3-hour forecasts valid at 2100 UTC 18 July 2012. Return to text.

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Figure 8. 1800 UTC NAM profiles for a point near Harrisburg, PA showing the profiles and severe weath4er parameters at (top) 1800 UTC and (bottom) 2100 UTC. Return to text.

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Figure 9. KCCX radar showing 0.5 degree reflectivity and base velocity at (top) 1800 UTC and (bottom) 1818 UTC. Return to text.

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Figure 10. As in Figure 9 except valid at 1832 and 1855 UTC. Return to text.

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Figure 11. As in Figure 9 except for KLWX valid at 1820 and 1828 UTC. Return to text.

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Figure 12. As in Figure 9 except valid at 1950 and 1959 UTC. Red curve shows approximate outflow boundary. Return to text.

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Figure 13. As in Figure 12 except valid at 2023 and 2041 UTC. Return to text.

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Figure 14. Hail stones from the town of Dauphin in Dauphin County Pennsylvania 18 July 2012. Image was posted to the office Facebook page.. Return to text.