Severe Weather with a strong cold front: 2-3 April 2006 By Richard H. Grumm National Weather Service Office State College, PA 16803 1. INTRODUCTION A strong cold front brought severe weather to much of the United States east of the eastern plains, Mississippi and the Ohio Valley s on 2 April 2006 (Figure 1). There were 871 severe weather reports and 85 tornadoes. Over 29 people lost their lives in this deadly early-spring tornadic episode. The event of 3 April was not as impressive in the eastern United States, with severe 205 reports and no tornadoes. The differences between the two day were considerable. Warmer air with higher moisture and convective available potential energy (CAPE) dominated in the central United States on the second. A strong low-level jet was also present. On the 3 rd, the CAPE was lower. The stability likely played a significant role in limiting convection in the Mid-Atlantic region and the northeastern United States. Over Pennsylvania, despite above normal CAPE, on the order of 300 to 600 JKG -1 little severe weather developed. This case will provide an overview of the event. The focus will be on the convection and general lack of severe weather in central Pennsylvania. 2. METHODS Short-term ensemble (SREF) data were retrieved from NCEP in near-real time. Some images were reproduced after the event. Ensemble data is shown in a variety of products including probabilities, Figure 1 Storm Prediction Center reports of severe weather for 2 and 3 April 2006. Types of severe events are as shown in the key on each image. ensemble means, and the spread. In several images, the ensemble mean is compared to the 30-year climatology. These data are displayed in standard deviations form normal. Storm reports were retrieved from the Storm Prediction Center (SPC) website. Images are courtesy of the SPC.
Radar and satellite data were retrieved from the real-time AWIPS data feed. The images were made after the event using the 7-day archive playback feature. 3. RESULTS a.) Mid western United States SREF forecast initialized at 02/0900 forecast high CAPE in the region affected by the severe weather on 2 April. A large region of 1200 JKg -1 and a smaller region of 2400 JKg -1 of CAPE was forecast by the SREF. These values were well above the Climatological norms as little CAPE is routinely present on April second over the eastern United States north of the Gulf Coast. Thus, the departures from normal are meaningless. However the large values of CAPE, on the order of 1200 JKg -1 and greater would support large updrafts any time of year. The salient point was that large CAPE was both forecast and present over the middle of the United States on 2 April 2006. The 02/0900 UTC SREF CAPE forecasts are valid at 02/1800 UTC are shown in Figure 2. The high CAPE was well aligned with the area of above normal moisture (Figure 3). Precipitable water (PWAT) anomalies and high CAPE in the warm sector of cool and spring season storms nearly always are in good agreement. In addition to the high values of CAPE, the SREF forecast strong winds with an above normal southwesterly jet in the region impacted by the severe weather (Figure 4). Figure 2. SREF forecasts of CAPE (JKg -1 ) initialized at 0900 UTC 2 April 2006 valid at 1800 UTC 2 April 2006. Upper panel shows the spread and each members 600, 1200, and 2400 JKg -1 contours. The ensemble mean is thick black. Lower panel shows the ensemble mean position of the 600, 1200, 1800, and 2400 JKg -1 contours. Shading shows departure from normal in standard deviations. This translated into relatively high helicity as shown in Figure 5. The extremely high values of helicity, in the 400-500s-2 range were likely associated with the frontal system associated with the surface system (Figure 6). This severe weather event, like many previous events was associated with an anomalous surface cyclone passing to the north and west of the affected region. b.) Mid-Atlantic region
A comparison of the 2 days in Figure 1 reveal that the severe weather event of 3 April was considerably smaller than the event of 2 April. In Pennsylvania a problem with this event, despite strong winds and considerable shear, was the low values of CAPE. In Pennsylvania, the highest CAPE was confined to western areas where SREF forecasts showed CAPE on the order of 700 JKg - 1 this was close to the ensemble mean. The forecast for Pittsburgh is shown in Figure 7. Farther east, CAPE was forecast to be around 100 JKg -1 around Harrisburg (not shown). The peak intrusion of CAPE was at 03/1800 UTC (Figure 8) where a small area of 2400 JKg -1 was forecast in North Carolina. The area covered by the 2400 JKg -1 contour and high probability of 1200 JKg -1 was considerably smaller the day before. The are of high CAPE did outline the general area of severe weather quite well. Strong winds and considerable shear were present over the region. The 850 hpa jet was stronger north of the region affected by the severe weather (Figure 9). The strongest helicity was close to the front and jet, generally north of the region of maximum instability (not shown). Interestingly, the PWAT and PWAT anomaly lined up well with both the area of severe weather and higher CAPE (Figure 10). c.) Pennsylvania Despite the low CAPE, the CAPE over the region was above normal. Along with the strong shear, thunderstorms did develop. Few reached severe limits and only one County in central Pennsylvania had severe Figure 3 As in Figure 2 except SREF precipitable water (mm) and precipitable water anomalies. weather. Many locations reported subsevere size hail. The composite reflectivity at 03/1900 UTC showed line of strong echoes over west-central Pennsylvania. One are of 65-70 dbz was present in southwestern Elk County (white arrow). A mini-bow echo was apparent at this time moving through Dubois. The observation still showed southeast winds, but they clearly would have shifted. Temperatures were mainly in the 50s with dew points around 50 in central Pennsylvania. Warm advection was still visible to the east and northeastern Pennsylvania still had dew points in the 40s.
Figure 4 As in Figure 2 except 850 hpa winds with upper panel showing U-wind anomalies and lower panel showing V-wind anomalies. The line grew more impressive as shown in the 1955 UTC (Figure 12) image. Despite the intense echoes, the storms were not very deep. Cross sections revealed that most storms had 50-55 dbz cores to about 12-15KFT. Only a few storms in western Pennsylvania had 50 dbz cores to around 20KFT. This was confined to regions of higher CAPE. Despite the general lack of severe weather. The strong shear did allow for storm organization. Bow echoes were observed. Figure 13 shows the 0.5 degree reflectivity and storm relative velocity data. The Figure 6. SREF forecasts of mean-sea level pressure valid at 1800 UTC 2 April 2006 showing a) ensemble mean and spread and b) ensemble mean and departure from normal. distinct bow echo at 1905 UTC was well defined in the SRM data. It produced some damage to the west but only 35 KTS of wind at the airport in Dubois, Pennsylvania.. 4. CONCLUSIONS A strong cold front produced severe weather over a large portion of the eastern United States on 2 and 3 April 2006. The high CAPE and shear on the 2 nd produced over 800 severe weather reports and many tornadoes to include several killer
tornadoes. Lower CAPE on the 3 rd appeared to limited the area and number of severe weather reports. The high CAPE and helicity were present with the large and damaging severe weather event of 2 April 2006. The SREF forecasts showed CAPE values above and 2400 JKg -1 in the region affected by the tornadoes and severe weather. Strong winds, as depicted by the low-level jet were also associated with this event. Similar to many Mid-Atlantic region events, a strong surface cyclone passed northwest of the region affected by the severe weather. The lower CAPE and the smaller area affected by the high CAPE appeared to limit the area affected by severe weather on the 3 rd. Most of the severe weather was confined to regions were the CAPE was forecast reach around 1200 JKg -1 and greater. An examination of NAM initializations suggest this area was well forecast by the SREF. The concentration of severe weather in North Carolina and Virginia was in close proximity of the highest CAPE forecast on the third. Figure 5. SREF helicity (s-2) valid at 1800 UTC 2 April 2006. Contours show ensemble mean helicity and shading shows the spread. severe potential of this event in Pennsylvania. 5. REFERENCES In Pennsylvania, the CAPE was forecast be mainly less than 1000 JKg -1 and in central and eastern sections not much above 100 JKg -1. These values are much above the seasonal normals, thus suggesting the potential for thunderstorms. The strong shear, and helicity forecast to over 200s -2 suggested some organization to the convection. A limiting factor was the instability which likely limited the updrafts thus limiting the storms development. The strong shear did organize the convection and several bow echoes did develop. However, the lack of sufficient instability likely limited the
Figure 7 SREF CAPE forecasts at a point near Pittsburgh from forecasts initialized at 0900 UTC 3 April 2006. Thin lines show each ensemble member. Thick line is the ensemble mean. Figure 8. SREF forecasts of CAPE (JKg -1 ) initialized at 0900 UTC 3 April 2006 valid at 1800 UTC e April 2006. Upper panel shows the spread and each members 600, 1200, and 2400 JKg -1 contours. The ensemble mean is thick black. Lower panel the same contours and the probability of exceeding 1200, JKg -1.
Figure 9. As in Figure 4 except SREF forecasts initialized at 0900 UTC 3 April 2006 valid at 1800 UTC 3 April 2006. Figure 10. As in Figure 3 except SREF PWAT forecasts initialized at 0900 UTC valid at 1800 UTC 3 April 2006.
Figure 11 KCCX composite reflectivity at 1905 UTC and surface observations around 1900 UTC 3 April 2006. Figure 12. As in Figure 11 except valid at 1955 UTC.
a.) 1905 b.) 1905 c.) 1925 d.) 1925 Figure 13. Base reflectivity and storm relative velocity data from the KCCX the panesl show a) base reflectivity (Z) and b) SRM valid at 1905 UTC and c) base reflectivity and d) SRM at 1925 UTC.