Mid-Atlantic Derecho event of 29 June 2012

Mid-Atlantic Derecho event of 29 June 2012 Richard H. Grumm Charles H. Ross And Elyse Colbert National Weather Service Office State College, PA 16803 1. Overview An area of convection developed over eastern Michigan around 0015 UTC 29 June 2012, raced across Lake Erie, entered western Pennsylvania around 0415 UTC, reaching central Pennsylvania at 0615 and then crossed the State (Fig. 1) before racing out and over the western Atlantic. The Mesoscale convective system (MCS) created a swath of wind damage with some embedded hail reports from northwestern Pennsylvania southeastward and into Maryland, Delaware, and New Jersey (Fig. 2). This MCS met the definition of a derecho in that it was a widespread convective windstorm with a concentrated area with winds over 50kt lasting spanning at least 240 miles (400km). This derecho was a classic ring-of-fire event occurring in the ideal spot on the northern fringes of a massive mid-tropospheric anticyclone (Fig. 3). Several other fast moving MCS s classifying as derechoes or ridge-rollers have been identified (Galarneau and Bosart 2006; Galarneau et al. 2008). In this case, the MCS developed on the edge of the +1σ height anomalies in southern Michigan and fairly parallel to the 5880 m contour. The northwest flow side of a ridge is an ideal location of MCS evolution. As seen in Figure 3, the system lowered the heights slightly over the Mid-Atlantic region. The large ridge and massive area impacted by extreme heat in the Mid-Mississippi Valley was likely the more new worthy event of 28-29 June 2012. The regional view of the precipitable water (PW) fields showed the surge of high PW air over the Great Lakes on 28 June (Fig. 4). As the PW surge moved to the southeast it peaked near +3s above normal (Fig. 4d) at 0000 UTC 29 June as the surge of warm moist air raced to the southeast. There were hints in several model and ensemble runs of potential convective development over the region. Forecasts for overnight convection were generally indicating a 60% chance of thunder and the possible development of an MCS. The higher resolution 4km models were not particular clear on this evolution and its exact details. This case study will document the Mid-Atlantic derecho event of 29 June 2012. The radar data showed a textbook bow echo with impressive winds behind the line with 50-70kts of winds in the lower elevation slices. This was one of the stronger MCS observed on KCCX and moved through extremely fast. Based on the wind data in the radar and the damage swath, this event is classified here as a derecho. 2. Data and Methods

Satellite and radar data were obtained using AWIPS. All data shown were produced from AWIPS. Model data and standardized anomalies were produced using GrADS (Doty and Kinter 1995). Storm reports were taken locally and the Storm Prediction Center plots are displayed herein. The definition of a derecho was met here but this term and MCS are used interchangeably. 3. Regional Pattern Figure 3 suggested the Mid-Atlantic region was in the ring-of-fire in the northern periphery of a strong ridge. A surge of deep moisture (Fig. 4) moved over this ridge. The surge of high PW air was associated with a strong low-level jet (LLJ) at 850 hpa (Fig. 5). The wind anomalies peaked at 2-3σ above normal at the height of the event over south-central Pennsylvania and Virginia (Fig. 5e). The MCS developed on the northern edge of this feature (Fig. 1). The combination of deep moisture and strong winds produced high values of 850 hpa moisture flux and large moisture flux anomalies (Fig. 6). Typically, high moisture flux anomalies are good indicators of heavy rainfall, though in this case and other convective cases, it serves as a good proxy for forcing for strong convection. The lack of heavy rainfall was likely related to the speed at which the system moved. The strong moisture flux and winds were associated with a surge of high CAPE air. CAPE values typically peak at 1800-2100 UTC in the Mid-Atlantic. However the models forecast (not shown) and analyzed (Fig. 7) CAPE to exceed 3600JKg-1 from 0600 to 1200 UTC 29 June 2012. Could this keep be realized, it favored large updrafts and the wind shear, over 20kts was in the favorable high end potential damage range. CAPE over 1200 JKg-1 and shear in excess of 20kts is a good indicator for severe convection in the Mid-Atlantic region. The more linear wind profiles supported a strong bow-echo rather than supercell evolutions. The 700 and 500 hpa winds were from the north-northwest at 15m-s (30kts) and 30ms-1 (60kts: Fig. 8). 4. Radar As impressive as the satellite data and wind data in the GFS 6-hour analysis looked, the radar signal with this event was rather incredible. The fast moving derecho generated a cold pool and a strong bow echo signature on radar. Down radial winds of 60 to 80kts moved from northwest to southeast across Pennsylvania. This was a classic case where velocity data was critical to maintain high situational awareness and to reflectivity data was of limited value at times. Unless stated otherwise all reflectivity and velocity data are from the 0.50 degree elevation cut. The KCCX radar showing reflectivity and velocity at 0601 UTC show the echoes over Clarion County with the strong signal in the velocity data. Winds in the velocity in the blue

colors were 60 kts to as high as 73kts. The derecho did not look particularly impressive in the reflectivity data, however it was rather impressive in the velocity data (Fig. 9). The echoes along the derecho were at times appeared rather weak and disorganized. However the focused strong low-level jet clearly identified the feature through its life (Figs. 9-13). Once the cold pool and strong LLJ developed, the system was self-sustaining crossing all of Pennsylvania and exiting over the western Atlantic off Delare and New Jersey. The signal in the wind fields implied that at least from Clarion County into Maryland the system maintained 50 to at times 80kts of wind in the lowest elevation cut on the KCCX radar. This clearly meets the definition of a derecho. 5. Summary A rare and potent derecho or MCS affect the Mid-Atlantic region from around 0100 through 1000 UTC 29 June 2012. This was likely one of the fastest moving derechoes ever recorded on the KCCX radar and it had one of the more concentrated bands of high winds in excess of 50kts ever observed on KCCX radar. Derechoes of this intensity are quite rare in the Mid-Atlantic region. Though the derecho of 29-30 June 2012, which would affect Indiana through Virginia, would post an impressive example of how potent ridge-rollers can be likely the strongest event in the eastern United States since 15 July 1995. For the 29 June Pennsylvania case, the winds in the 0.50 degree slice often exceeded 70 and at time 80 kts on both sides of the State College County warning area. This derecho developed in the ring-of-fire of an intense ridge. Large ridges producing heat waves area areas where derechoes can and do develop. Some of the strong events of this nature have been term ridge roller (Galarneau and Bosart 2006). A famous ridge roller event occurred in the periphery of the ridge associate with the July 1995 heat wave, that event produced a derecho that came out of Canada devastating the forecasts of northern New York and into central New England. The event of 29 July shared many of the features often associated with derechoes and famous eastern United States derechoes. In this event, the PW, 850 hpa winds and moisture flux signal indicated good moisture and forcing for convection. Moisture flux is normally used for heavy rainfall but has shown good utility in predicting convection. Despite the high moisture flux, the lack of heavy rainfall was likely related to the speed at which the system moved. This also likely played a role in the diminished value of radar reflectivity data verse radar velocity data in diagnosing this event. The radar data shown here indicated strong winds with at time relatively weak to nondescript reflectivity data. The reflectivity data was at times of little value. As with most bowechoes and stronger derechoes, velocity data is often the most important warning tool. As systems move too fast the new elements come and go along the line too fast. Some steadfast guidance on warning on derechoes and bow echoes apply:

The structure and organization of derechoes and bow echoes typically are far better identifiable in winds data than reflectivity data. Down radial winds of 45 kts at lower elevation cuts are about a 50% probability warning criteria. Lower to the surface is better above 4kft the probability will fall. Down radial winds of 50kts are greater increase the probability of wind damage. Down radial winds over 60kts are rare and have a high probability of producing damage. Gust fronts at longer ranges are typically ahead of the signature on radar and can reduce lead-times Use the EAV tool and at longer ranges chose a radar that might be down radial or cutting the derecho at lower elevations Base warning decisions on V as it is more reliable when the system develops than the reflectivity data. Despite the strength of the winds this event did not produce broad swaths of damage as experienced in the 15 July 1995 event (Galarneau et al 2008) or anything similar to the event of 29-30 June 2012 (Fig. 14). Low-level stability based on the time of day may have been one factor limiting the damage produced by this event. Though it did produce a broad swath of damage. 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 Davies, J.M.. (2006b) Tornadoes with Cold Core 500-mb Lows. Weather and Forecasting 21:6, 1051-1062Online 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 Galarneau, T. J., Jr., L. F. Bosart, and A. R. Aiyyer, 2008: Closed anticyclones of the subtropics and middle latitudes: A 54-yr climatology (1950-2003) and three case studies. Synoptic-Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, Meteor. Monogr., No. 55, Amer. Meteor. Soc., 349-392. [Available at the AMS Online Store.] Galarneau, T. J., and L. F. Bosart, 2006: Ridge Rollers: Mesoscale Disturbances on the Periphery of Cutoff Anticyclones. Preprints, Severe Local Storms Special Symposium, Atlanta, GA, Amer. Meteor. Soc. 7pp. 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

Figure 1. Select GOES IR images at 0015 UTC, 0415 UTC, 0615 and 1045 UTC 29 June showing MCS evolutions. Insets show 0015 and 1045 UTC images. Return to text.

Figure 2. Storm reports from the Storm Prediction Center. Return to text.

Figure 3. GFS 00-hour forecasts of 500 hpa heights and height anomalies in 6-hour increments from a) 0600 UTC 28 June 2012 through f) 1200 UTC 29 June 32012. Heights every 60m and anomalies in standard deviations as in the color bar. Return to text.

Figure 4. As in Figure 3 except for regionalize view of precipitable water and precipitable water anomalies. Return to text.

Figure 5. As in Figure 4 except for 850 hpa winds (ms-1) and 850 hpa wind anomalies. Return to text.

Figure 6. As in Figure 5 except 850 hpa moisture flux and moisture flux anomalies. Return to text.

Figure 7.As in Figure 6 except for GFS 00-hour forecasts of CAPE contours every 1200 JKG-1 and shading as in the color bar. Return to text.

Figure 8. As in Figure 5 except for 500 hpa winds. Return to text. NWS State College Case Examples

Figure 9. KCCX radar at 0601 UTC (top) and 0606 UTC (bottom) left panels are reflectivity in dbz and right panels are velocity in kts as per color scales. Yellow boxes show warnings. Return to text.

Figure 10. As in Figure 9 except for 0620 and 0643 UTC. Darker blue velocity values are over 70kts. Return to text.

Figure 11. As in Figure 9 except valid at 0729 and 0739 UTC. Return to text.

Figure 12. As in figure 9 except valid 0748 and 0757 UTC. Return to text. NWS State College Case Examples

Figure 13. As in Figure 9 except for 0816 UTC. Return to text. NWS State College Case Examples

Figure 14. As in Figure 2 except valid for the 24 hours ending 1200 UTC 30 June 2012. Return to text.