Western Pennsylvania Derecho and Snake Springs Tornado 11...

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Western Pennsylvania Derecho and Snake Springs Tornado 11 June 2014

By Richard H. Grumm

National Weather Service State College, PA 1. Overview

The combination moist air (Fig. 1), a strong low-level jet (Fig. 2), and instability (Fig. 3) combined to produce severe weather (Fig. 4) over western Pennsylvania during the late afternoon and early evening hours of 11 June 2014. The clustering of wind damage reports in southwestern Pennsylvania was associated with a mini-derecho which moved southwest to northeast across the region. The tornado and hail reports were associated with a supercell thunderstorm which developed well ahead of the mini-derecho.

The strong 850 hPa winds (Fig. 2) and the high CAPE late in the afternoon produced favorable shear and instability for the evolution of convective storms. The high PW air contributed to low LCL heights which, when below about 1000m often increase the probability of tornadoes when supercells develop. The high PW air also contributed to heavy rainfall (Fig. 5) and some locally heavy rainfall produced flooding during the event. The lone supercell in Bedford County produced heavy rainfall as it moved to the north-north east into Huntingdon County. The white arrow in Figure 5 parallels this feature. The larger area of 32 to 64 mm or rainfall over western Pennsylvania was enhanced by the fast moving derecho which moved out of West Virginia. The rainfall data implies two enhanced regions of rainfall with the westernmost area closely linked to the system which developed over West Virginia.

The supercell thunderstorm; which developed near the Maryland border near Centerville in southern Bedford County; was captured by at least 3 individuals. These videos showed funnels which at times intersected the ground. A storm survey near (Fig. 6) where the videos (Fig. 7) were taken found no visible damage. However the region had swaths of shredded leaves and reports of quarter to golf ball size hail. The mini-derecho downed numerous trees and left over 2000 customers without electricity.

2. Mini-derecho

Figure 8 shows the evolution of the GOES-IR and lightning data during the afternoon and evening hours of 11 June 2014. These data show that the initial convection and cold cloud tops developed over West Virginia and southeastern Ohio. Figure 9 shows the bow as it appeared over southwestern Pennsylvania and raced to the northeast. The windspeeds increased as the storm moved deeper into Pennsylvania, likely the result of sampling lower in the storms and the evolution of the stratiform rain to the rear of the convection. The stratiform rain area is often indicative the evolution of the strong rear-inflow jet.


The 0.5 degree (Fig. 9:lower 2 panels) velocity data with this storm showed winds over 50kts and at times over 60 kts. In Clearfield County, two enhanced areas of cyclonic rotation developed (Fig. 9 lower panel). These short-lived vortices (Fig. 10) can produce short-lived tornadoes in QLCSs. However, despite enhanced wind damage a survey of these features did not show tornadic damage.

3. Snake Spring Tornado

The Z and V at associated with the small supercell which produced the Snake Spring Tornado1 are shown at 2121 UTC, 2157, and 2237 UTC 11 June 2014 (Fig. 11). The storm moved into Bedford County, PA around 2121 UTC and moved to the north-northwest. The 2157 UTC image was taken at as it corresponded with the time of two videos of the funnel and rotating clouds. The velocity data was over 50kts on the inbound side of the storm at this time. By 2237 the storm was close to Everett, just south of the areas which did show signs of wind damage from the storm.

The Z,V, ZDR, and ZDP at 2246 show to storm over Everett. The base velocity detected outbounds and continued to show the strong inbounds on the southeastern edge of the storm. ZDR was relatively high in the front flank of the storm, likely the rain drops with another smaller area of higher ZDR on the southern flank of the storm. Between the main area of rain and trailing rainfall, ZDR was less than 1. The storm survey team received reports of quarter to golf ball size hail along the 26 mile track region surveyed. South of the damage swath (Fig. 6) by the winds associated with the strong inflow into the mesocyclone, there were regions of shredded leaves implying large hail at least as far south as Centerville. However, south of Everett the only discernable damage was a field where the grass appeared to have been flattened by wind.

4. Summary

The combination of strong southerly flow in the lower 0 to 3km of the atmosphere and instability, convection developed in West Virginia and Ohio. A derecho developed with this feature racing across southwestern and west-central Pennsylvania producing wind damage. Reports indicated at least 2000 customers lost power due to downed wires as the system swept across the region.

Ahead of the more pronounced derecho, a supercell developed in western Maryland and move into Bedford County and eventually dissipating over southern Huntington County, PA. This storm produced large hail, in the 1 (US quarter) to 1.5 inches (Golf ball) and for a 12 mile swath it produced sporadic wind damage, mainly as inflow into the broader and weaker mesocyclone.

Both systems produced heavy rainfall and flooding. As indicated in Figure 5, the bow echo and supercell contributed significantly to the overall rainfall totals.

1 Named after the creek valley it affected.


Figure 1. RAP-13km precipitable water (mm) and precipitable water anomalies in 2-hour increments from a) 1800 UTC 11 June through f) 0400 UTC 12 June 2014. Return to text.


Figure 2. As in Figure 1 except for 850 hPa winds (kts) and v-wind anomalies. Return to text.


Figure 3. As in Figure 1 except for lowest 1800 hPa most unstable convective available potential energy. Return to text.


Figure 4. Storm reports from the Storm Prediction Center showing severe weather colorized by type as indicated in the color key in the bottom left of the image. Return to text.


Figure 5. Stage-IV estimated rainfall for the period from 1800 UTC 11 June through 0600 UTC 12 June 2014. Values in MM, The white arrow shows the path of the supercell and its enhancement to the rainfall over Bedford and southern Huntingdon Counties. Return to text.


Figure 6. Map of the track of the funnel cloud and Snake Spring Tornado. All visible wind damage with the tornado was surveyed near and north of route 30. The region to the south, where multiple videos indicated funnels and possibly a tornado on the ground had no discovered wind damage. Hail swaths were visible in regions where the hail shredded leaves. Return to text.


Figure 7. Screen shot of video clip of mesocyclone near Centerville, PA. Several audio portions implied debris in the circulations. Return to text.


Figure 8. GOES IR showing the cold cloud tops in West Virginian and lightning and the northward track of the system into western Pennsylvania. The lower right panel shows the two areas of enhanced cold cloud tops which relate back to the two areas of heavy rainfall in Figure 4. The time stamps were not saved in these images. Return to text.


Figure 9. KCCX 0.5 degree Z and V showing the bow echo evolution in Pennsylvania at 2213 and 2326 UTC 11 June and lower panels show 0018 UTC 12 June 2014. Return to text.


Figure 10. KCCX 0.5 degree velocity at 0022 and 0027 UTC showing the enhanced circulations along the bow. Return to text.


Figure 11. KCCX 0.5 degree Z and V from top to bottom at 2121 UTC, 2157, and 2237 UTC. Return to text.


Figure 12. As in Figure 10 except for Z, V, ZDR, and KDP at 2246 UTC. Return to text.


Photos : From Watters Works (home site was affected by the storm).

http://wattersworks.smugmug.com/Other/SSV-Tornado-June-2014/

Western Pennsylvania Derecho and Snake Springs Tornado 11...

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