End of heat-event severe event of 7 July 2012

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1 End of heat-event severe event of 7 July 2012 Richard H. Grumm And Elyse Colbert National Weather Service Office State College, PA Overview A widespread severe weather event affected Pennsylvania and New Jersey during the afternoon and evening hours of 7 July 2012 (Fig. 1). A bow-echo developed in northern Pennsylvania and raced to the southeast, this feature accounted for a significant percentage of the severe reports in Pennsylvania and New Jersey. The path length of this bow-echo was estimated to be about 293 miles, with wind damage and wind gust over 58kts along its path meeting the definition of a derecho. The system also produced some reports of large hail. A short-wave in the fast flow on the poleward size of the broad ridge (Fig. 2) over the central United States provided the lift and tapped the instability producing the event. The ridge was retrograding and a trough was developing over the eastern United States as indicated by the change in 500 hpa heights from 1200 UTC 8 July relative to 1200 UTC 6 July. The eastern United States heat wave of June-July 2012 was coming to an end as the heat and ridge moved westward. This was the second derecho to affect Pennsylvania during the heat event of June-July The derecho of 29 June 2012 brought severe weather to the region during the early morning hours of 29 June The high impact derecho that brought severe weather from Indiana to Washington, DC was of minimal impact to Pennsylvania. The fast flow in the northern periphery about ridges is an excellent location to for ridge-roller MCS development (Galarneau et al 2006; Galarneau 2008). With deep instability and strong shear, some of the MCS s produce derechoes and the summer of 2012 was a summer marked by several derecho producing MCS s in the eastern United States. This paper will examine the pattern associated with the derecho producing MCS of 7 July The focus will be on the larger scale pattern and standardized anomalies to describe the pattern. Radar data is used to show the key features which produced reports of severe weather. Like many bow echoes, the low-level velocity data is often useful in monitoring and making warning decisions on the advancing bow. 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.

2 The pattern was determined using the NCEP GFS 00-hour forecasts as the first guess as to the pattern. These data were then compared to the 30-year re-analysis fields to compute the standardized anomalies of the fields shown when standardized anomalies were used. 3. Regional Pattern A strong 250 hpa jet (Fig. 3) was present over the large ridge (Fig. 2). The ridge peaked at over 3s above normal from 06/0000 through 07/0000 UTC. At lower levels, a strong 700 hpa jet was present (Fig. 4). This feature and the implied strong shear moved out of Ontario (Fig. 4a) and across New York State and northern Pennsylvania between 07/0600 and 08/0000 UTC. A weaker wind maximum was present at 850 hpa but was not shown as the 700 hpa feature was significantly stronger. In addition to the strong 700 hpa jet, a surge of high precipitable water (PW) air came over the ridge, crossing Pennsylvania and New York on 7 July (Fig. 5) the PW peaked over the region around 1800 UTC on 7 July. The high PW air combined with the strong winds produced strong moisture flux at both 700 hpa (Fig. 6) and 850 hpa (Fig. 7). The strong moisture flux implied relatively strong forcing for lifting the warm unstable air. The winds also implied strong shear. The shear in the 10m to 850 hpa (Fig. 8a) and 10m to 500 hpa layer (Fig. 8b) depicted strong shear in a deep layer. The LCL heights were low and the CAPE was high over Pennsylvania. Though not shown, the high CAPE moved across southern areas of Pennsylvania from 07/1800 through 08/0000 UTC. 4. Satellite An MCS crossed Lake Ontario in the early morning hours and moved across New York. The incipient system was over Ontario at 07/1140 UTC (Fig. 9a) which raced over Lake Ontario and was over western New York by 07/1345 UTC (Fig. 9b). An area of convection to the west of this feature developed an incipient area of cold cloud tops as seen in the 07/1710 UTC GOES IR image (Fig. 9c). The older MCS is still visible to the east. Two cold cloud top centers were visible around 07/1825 (Fig. 9d) as the system began to rapidly intensity and move to the southeast. The developing MCS had an expansive cold cloud tap area by 07/1925 UTC and was quite expansive by 07/1955 UTC (Fig. 10a). Another area of convection began to develop over Ohio around 07/1800 UTC based on cold cloud tops on satellite (not shown). This more northsouth area of convection was associated with another organized area of convection moving to the east and was over western Pennsylvania by 07/1955 UTC (Fig. 10a). The northern developing bow echo has a massive and expansive cold cloud area. The remnants of the earlier MCS were moving over southern New England and Long Island 1. 1 Personal observation on eastern Long Island the clouds and very light rain limited the high temperatures for the day. Temperatures fell the mid-90s in Greenport NY to about 83F as the clouds moved over the region around 4 PM local time close to the time of the 1955 image.

3 By 2040 UTC (Fig. 10c) There were clearly two convective areas moving over Pennsylvania. The northernmost one was moving to the southeast and the western one was moving slightly to the south of east. The northern MCS with associated with main derecho (next section) was over New Jersey at 07/2310 (Fig. 10c). By 07/2345 the larger MCS was off the Jersey coast and would continue to race out over the Atlantic for several more hours, the northern edge would bring some rain and thunderstorms to Long Island. The MCS which developed from the convection over Ohio was over southeastern Pennsylvania by 07/2345. This system would produce most of the severe weather from Pittsburgh to Harrisburg as shown in Figure 1 and it would move over Delaware and southern New Jersey as it trekked to the coast. 5. Radar The IR imagery (Fig. 11) showed the initial convection with the bow echo over New York State. This feature entered Pennsylvania around 07/1842 UTC (Fig. 11) based on the 0.50 degree reflectivity and velocity data. This feature clearly showed a bowing segment in the inbounds at 1908 UTC. The maximum inbound winds were in the 50 to 74kt range between 1842 and 2000 UTC. As the system moved south and east at closer ranges an outflow boundary became evident (Fig. 12b). Due to viewing angles, the KCCX radar did not see the mid-level circulation on the eastern edge of the line as well as KBGM (Fig. 13) and KDIX (Fig. 14). The strong circulation was associated with the near linear damage swath (Fig. 1) in the storm report data. The more focused strong winds were with this feature. Though not shown, a weaker broken line of storms moved west to east across southcentral Pennsylvania and producing a swath of wind and hail reports. It also interacted with the outflow from the leading bow echo. 6. Summary A strong short wave came over the ridge producing several MCS s over the eastern United States on 7 July One MCS developed over western New York and raced across north-central Pennsylvania and southeastward into New Jersey. This MCS qualified as a derecho as it produced a swath of wind gusts over 58kts and down trees over a 290 mile long path. Most of the damage was close to a strong circulation on the eastern edge of the derecho producing MCS. A second area of convection developed over Ohio and moved eastward across southcentral Pennsylvania producing both wind and hail reports in its path across the State. Both convective areas produced expansive cold cloud tops identifying each MCS. The strong ridge of June-July 2012 created favorable conditions for derecho producing MCS s. The derecho of 29 June was a remarkable event. It would be interesting to determine if the strong ridge which produced the record warm July of 1936 produced some similarly impressive derechoes. Storm reports and radar data were not available during this period.

4 However, newspaper accounts might shed light on the production of derechoes and MCS during that historic event. This would be a worthy research endeavor in the event that predictions of 1 in 20 year heat wave become 1 in 2 year events. If this were to prove true there could be a 10 fold increase in the number of MCS and derecho producing derechoes in the eastern United States as the climate warms. The shear in the 10m to 850 hpa (Fig. 8a) and 10m to 500 hpa layer depicted strong shear in a deep layer. The weaker low-level shear implied that conditions did not favor supercell storms and tornadic storms.. The LCL heights were low in northern Pennsylvania which is a good indicator for tornadoes when accompanied by strong low-level shear and a height potential for supercells. Conditions did not favor supercells though the deep ~6km shear implied storms could persist and organize. The strong shear likely contributed to the hail reports with this event. 7. References 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 Galarneau, T. J., Jr., L. F. Bosart, and A. R. Aiyyer, 2008: Closed anticyclones of the subtropics and middle latitudes: A 54-yr climatology ( ) and three case studies. Synoptic-Dynamic Meteorology and Weather Analysis and Forecasting: A Tribute to Fred Sanders, Meteor. Monogr., No. 55, Amer. Meteor. Soc., [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 Wea Forecasting, 21, 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,

5 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: Figure 1. Storm reports from the Storm Prediction Center. Return to text.

6 Figure 1. GFS 00-hour forecasts showing 500 hpa heights (m) and height anomalies (standard deviations from normal) in 12 hour increments from a) 0000 UTC 6 July 2012 through f) 1200 UTC 8 July Return to text.

7 Figure 3. As in Figure 2 except for 250 hpa winds (ms-1) and 250 hpa wind anomalies. Return to text.

8 Figure 4. As in Figure 2 except for 700 hpa winds and wind anomalies. Return to text.

9 Figure 5. As in Figure 3 except for precipitable water (mm) and precipitable water anomalies. Return to text.

10 Figure 6. As in Figure 3 except for 700 hpa moisture flux and moisture flux anomalies. Return to text.

11 Figure 7, As in Figure 6 except for 850 hpa moisture flux and moisture flux anomalies. Return to text.

12 Figure 8. NAM 00-hour forecast valid at 1800 UTC 7 July of a) 10m to 850 hpa shear, b) 10m to 500 hpa shear, c) LCL heights in meters, and d) surface based convective available potential energy. Return to text.

13 Figure 9. GOES IR images all valid on 7 July clockwise from top a) 1145 UTC, b) 1345 UTC, c) 1710 UTC, and 1825 UTC. Data courtesy of Lindsey of CIRA, Fort Collins, Colorado. Return to text.

14 Figure 10. As in Figure 9 except for valid at a) 1955, b) 2040, c) 2310 and d) 2345 UTC. Return to text.

15 Figure 11. KCCX radar showing 050 degree reflectivity and velocity at 1842 and 1980 UTC. Return to text.

16 Figure 12. As in 11 except 1934 and 2133 UTC. Return to text. NWS State College Case Examples

17 Figure 13. As in Figure 12 except for KBGM radar at 2100 and 2121 UTC. Return to text.

18 Figure 14. As in Figure 12 except for KDIX radar at 2130 and 2221 UTC. Return to text.

19 Old Figure 9. GOES IR images valid at a) 1725 UTC and b) 1925 UTC. Return to text.

20 Odl Figure 10. As in Figure 9 except for 2140 UTC and 2332 UTC. Return to text.

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