Mid-Atlantic Severe Weather Event of 23 June 2015

Transcription

1 Mid-Atlantic Severe Weather Event of 23 June 2015 By Richard H. Grumm National Weather Service State College, PA 1. Overview A widespread severe weather event occurred in the eastern United States on 23 June 2015 (Fig. 1). This event was the second phase of a two-day event which included severe weather from Minnesota to western New York on 22 June 1. The severe weather occurred in a pattern with fast northwesterly flow (Fig. 2) in the tight gradient over a building subtropical ridge over the southern United States (Fig. 2c-f). A short-wave moving over the ridge was clearly evident from 1200 UTC 22 through 1200 UTC 24 June A plume of deep moisture if PW anomalies (Fig. 3) of over 2 to 3σ above normal were present ahead of the short-wave and on the northwest side of the building 500 hpa ridge. The Climate Forecast System-Reanalysis (CFSR) data showed a region of high CAPE in the plume of high PW which moved over the Mid-Atlantic Region between 1200 UTC 23 to just prior to 0000 UTC 24 June 2015 (Fig. 4). In this area of high CAPE in the CFSR there was strong westerly flow at 850 hpa. The westerly winds at 850 hpa were has high as 4 σ above normal (Fig. 5c) and were generally 2 to 3σ above normal as the wave moved through the Mid- Atlantic region. There was strong generally linear shear and high CAPE. Regional the NCEP 3km HRRR analysis (Fig. 6) at 1500 UTC 23 June 2015 showed strong 850 hpa shear (Fig. 6a) over northern Pennsylvania with high CAPE to the south (Fig. 6d). The high CAPE and shear regions shifted eastward by 1800 UTC (Fig. 7). The CAPE and shear did not line up well. The shear was focused on the relatively cool side of the frontal boundary implied in the PW field (Fig. 3) and the 850 hpa temperature field (not shown). In addition to the multi-day severe weather event produced (Fig. 1) as the short-wave moved over the ridge (Fig. 2) the convection produced areas of 25 to 75 mm of rain. The stage-iv data showed a strip of heavy rain in northern Illinois; one from southern Michigan which extended into eastern Ohio. Farther north there was a strip of heavy rain across Lake Ontario into northern New York and New England. It will be shown that the broken stripe of rain from western Pennsylvania into New Jersey and southern New England was associated with persistent convective cluster which produced a linear stripe of wind damage (Fig.1) from central Pennsylvania into southern New England. 1 Technically ending at 1200 UTC 23 June.

2 2. Long Lived cold pool storm and outflow Figure 9 shows the cluster of storms which tracked from eastern Ohio, across Pennsylvania and ended in eastern New England. The broken line of wind reports in Figure 2 lined up well with this system. The strong larger scale QLCS which developed in southeastern Pennsylvania can be seen as it cross the New Jersey coast and was entering the Washington, DC metropolitan area. This section will show some of the details of the radar as the system crossed central Pennsylvania. It was one of 3 systems which cross the region (LOOP). The first went across the northern tier of Pennsylvania and southern New York early in the morning and dissipated. A second followed the path the third one did but it dissipated over central Pennsylvania. The severe reports (Fig. 2) in Clearfield and Centre counties are mix of damage from those 2 events. The third mini-derecho of the day is shown using the KCCX radar with 0.5 base reflectivity (Z) and velocity (Z). Overall, base velocity is extremely useful for linear convective systems. Figures show the evolution of the convection. The key to the severe weather was the strong lowlevel velocities with the cold pool near and to the rear of the convective towers shown in the Z data. The winds at times were between 50 and 75kts. The 1531 UTC image (Fig. 10) the cluster of storms and the strong inbounds was over Clearfield County. Winds were 50 to 70 kts in the V as the system moved through Philipsburg. By 1613 UTC (Fig. 11) the system was blowing through State College. Numerous reports of trees and wires downed by the storm were reported from Bellefonte southward into State College. The one report that showed up at 1618 UTC (not shown) was for county wide damage. By 1623 UTC (Fig. 12) the system was bearing down on Millheim and Rebersburg. The latter town does not show up in the image but is just north of Millheim on the other side of along ridge line. Wind damage was reported and there were reports of a tornado. The tornado report was likely due to concentrated areas of embedded stronger downbursts which uprooted trees and did minor damage to some structures. The area from Rebersburg and Aaronsburg was surveyed to assess the damage swaths. By 1639 UTC the mini-derecho had pushed east of Millheim. Strong outbound V were still present with wind over 60 kts. The system did sporadic wind damage along its path. The last image from KCCX is from 1821 UTC showing the 3 cores and outbounds in northeastern Pennsylvania where it produced additional wind damage. Beyond this point it was tracked using the MRMS sites composite reflectivity used to make the 20 minute loops and the starting and end points in Figure 9. The larger wind and hail producing system which produced a more significant swath of wind and some hail reports (Fig. 2) in southeastern Pennsylvania, New Jersey, and Maryland developed in southeastern Pennsylvania. KCCX radar images of the system at 2029 and 2130 UTC are shown

3 in Figures 15 & 16 respectively. This multi-cellular line with strong winds and merging cold pools produced wind damage in the Harrisburg region as it moved to the south and east. The strong convergence line (Fig. 16) and elongated multi-cellular line of storms showed that this system produced extensive wind damage from near Reading southward into Maryland. This large system moved to the southeast, had deeper cores in the cells and produced far more wind damage then the mini-derecho which had cross the region moving from west to east. 3. Summary A series of organized convective systems moved across Pennsylvania on 23 June The strong shear and high CAPE did not quite align well but the two ingredients worked well to produce a significant Mid-Atlantic severe weather event. A majority of the wind damage reports were associated with 3 distinct mini-derechoes which crossed northern, central, and central and eastern Pennsylvania respectively on 23 June. The fourth system was a significant larger QLCS system which produced a significant number of both wind and some hail reports from southeastern Pennsylvania into New Jersey, Maryland, and Delaware. All of these convective system required the production of cold pools to be long lived. Though not shown, downdraft CAPE was relatively high on this day, in the 800 to 1200JKg-1 range at times in central Pennsylvania. This may have been a clue that the storms could produce cold pools. Additionally, the NCEP 3km HRRR produced a series of convective lines and tracked mesoscale cold pools with these systems. Examples from the 1400 UTC 23 June 2015 HRRR are shown in Figure 17. The HRRR did not explicitly forecast each convective event but it showed the potential for cold pools and for long-lived cold pools which were forecast at time to reach the coastal plain. These cold pools were associated with areas of implied convection in the model simulated radar. The red arrows in Figure 17 highlight such a feature. This case implies there is some incredible forecast potential in the HRRR as it evolves and evolves into and ensemble forecast system. 4. Acknowledgements NSSL MRMS page and NSSL on demand page 5. References

4 Figure 1. Storm reports by type from the Storm Prediction center for the 24 hour periods ending at 1200 UTC 23 and 24 June Return to text.

5 Figure 2. CFSR 500 hpa heights and height anomalies for the period of a) 0000 UTC 22 June through f) 1200 UTC 24 June 2015 in 12 hour increments. Return to text.

6 Figure 3. As in Figure 2 except for precipitable water and precipitable water anomalies. Return to text.

7 Figure 4. As in Figure 2 except for CAPE from the CFSR in 6-hour increments. Return to text.

8 Figure 5 As in Figure 4 except for 850 hpa winds and u-wind anomalies. Return to text.

9 Figure 6. NCEP HRRR analysis of 850 hpa shear, 850 hpa lapse rates, LCL height and CAPE valid at 1500 UTC 23 June Return to text.

10 Figure 7. As in Figure 6 except for valid 1800 UTC 23 June. Return to text.

11 Figure 8. State-IV rainfall data for the period of 1800 UTC 22 June to 0600 UTC 24 June showing the total rainfall and the impact of the convection described in the text. Return to text.

12 Figure 9. Composite radar showing the cluster of storms near their origin at 1320 UTC and the remenants fo the trackable echoes at 2200 UTC. The white line shows the approximate track. Return to text. Loop:

13 Figure 10. KCCC 0.5 degree reflectivity (Z) and velocity (V) showing the mini-derecho at 1531 UTC. Towns are referred to in text. Storm reports are plotted based on the time window of the report and radar and may not include reports were numerous damage reports occurred over a region. Return to text.

14 Figure 11. As in Figure 10 except valid at 1613 UTC. Return to text. NWS State College Case Examples

15 Figure 12.. As in Figure 10 except valid at 1623 UTC. Return to text. NWS State College Case Examples

16 Figure 13.. As in Figure 10 except valid at 1639 UTC. Return to text. NWS State College Case Examples

17 Figure 14.. As in Figure 10 except valid at 1824 UTC. Return to text. NWS State College Case Examples

18 Figure 15. As in Figure 14 except valid at 2029 UTC focused over southeastern Pennsylvania. Return to text.

19 Figure 16. As in Figure 15 except valid at 2130 UTC. Return to text. NWS State College Case Examples

20 Figure UTC 3km HRRR showing 2m temperatures (F) in 1 hour increments. The red arrow highlights the moving cold pool associated with convection produced by the HRRR. Return to text.

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