Pulse Storms of mid-June 2005 By

Greg DeVoir And

Richard Grumm National Weather Service Office

State College, Pennsylvania

1. INTRODUCTION An extremely warm and humid air mass moved over the Mid-West and northeastern United States during the week of 6 June 2005. This air mass persisted for nearly 7 days. The precipitable water values hovered around 40 mm most of the time, which was in the 1.5 to 2 standard deviation above normal range. This warm moist air mass, north of the subtropical ridge over the southeastern United States and western Atlantic, produced ideal conditions for pulse type thunderstorms. Thunderstorms developed over central Pennsylvania on the 9th, 10th, 11th, and 13th of June. The main threat from most of these slow moving storms was locally heavy rain. However, several storms developed to be very tall and pushed 50-60dBZ cores over 20KFT. In some instances, such as the storm on 13 June 2005, the storm had 60dBZ cores near 28KFT, above the -20C layer. Several storms that had 60 dBZ cores at or above the -20C level produced downbursts. The freezing level was very high and no hail was reported from these storms. This note will review the conditions associated with several of the pulse storms which produced damaging winds during the period of 9-13 June 2005.

2. METHODS

All imagery shown was derived from the 7-day archive on AWIPS. The focus is on model output soundings and radar data.

3. RESULTS

i. 9 June 2005 Convection began earlier in the afternoon with minor flooding due to slow moving thunderstorms in a weakly sheared environment (Figure 1). With weak shear and light winds, the initial storms developed along the terrain, and then on the resulting outflow boundaries (OB). The OB became numerous during the mid and late afternoon. Visible GOES satellite showed the outflow boundaries and resulting cell interactions. The first severe thunderstorms were observed over York County when storm produced 65dBz to around 15KFT near Dover with echo tops around 40KFT (Figure 2). As this cell collapsed, it produced several reports of wind damage in and north of Dover. This storm was one of a cluster of storms over York County between 1900 and 2100 UTC and the cluster is clearly visible at 2006 UTC (Figure 3.). The downburst produced an observed wind of 53 mph at Enola. The potential for more 50 mph wind gusts existed for Dauphin,

Figure 1 LAPS sounding over southeastern Pennsylvania valid at 2000 UTC 9 June 2005. White line shows the 0C and -20C contours. Return to text.

Cumberland, Perry and Lebanon Counties as the storm and an attendant cluster of storms exited York County. Additional reports of downed trees from the initial storm were received from Dauphin and Cumberland Counties. These storms prompted the issuance of severe thunderstorm warnings in Perry and Cumberland Counties. High winds were observed in both counties but no severe weather has been attributed to these storms.

The Lemon (1980) criteria of 50 dBZ near 27KT was not met by most of the storms on 9 June. The criteria is flexible and in some areas can be as high as 30KFT. More recent work has focused on the height of the 50dBZ cores and the -20C level. However, the -20C level near 26KFT and 0C level near 14.5KFT (Figure 1) likely contributed to the potential of strong wet microbursts with 60-65 dBZ near 15KFT. On 13 June, a storm with 65dBZ cores near 19KFT and

Figure 2 Cross section of the storm in York County at 2011 UTC 9 June 2005. Return to text.

55 Figure 2dBZ cores near 24KFT produced no severe weather in Centre County. One difference was the higher -20C level on 13 June 2005, closer to 28KFT.

55 Figure 2dBZ cores near 24KFT produced no severe weather in Centre County. One difference was the higher -20C level on 13 June 2005, closer to 28KFT. The storms on the 13th mainly produced heavy rainfall. Many reports and radar estimates of 2 inches of rainfall were received. With PWAT values near 40-50 mm, (1.8inches in Figure 1.

The storms on the 13 mainly produced heavy rainfall. Many reports and radar estimates of 2 inches of rainfall were received. With PWAT values near 40-50 mm, (1.8inches in

th

Figure 1.) thunderstorms produced 50mmhr-1 rainfall rates. Only minor flooding problems were reported in association

with these slow moving storms of 9 June 2005.

ii. 13 June 2005

Two warnings were issued on this day. The warning for Centre County did not verify, while the warning for western Cumberland County did verify with extensive wind damage near Shippensburg. The sounding over the affected region is shown in Figure 4. These data show a slightly higher -20C level than observed on 9 June, with the -

Figure 3 Storm cluster over York County at 2006 UTC 9 June 2005. Return to text.

20C level near 27.3KFT. The freezing was also higher, near 15.7KFT. Compared to Figure 1, the PWAT was a bit higher, nearly 2 inches and there was pronounced westerly winds. This storm did show weak rotation at times and as shown in Figure 5, it moved westward across Franklin County before producing damage in western Cumberland county. Figure 6 shows a reflectivity cross section of the storm near Shippensburg. The 50 dBZ cores are near 29KFT, higher than both the -20C level (Figure 4) and the established Lemon (1980) criteria. Though not shown, at times this storm showed weak cyclonic rotation on both KCCX and KLWX radars. Additionally, a 60dBZ core was

observed on KLWX radar near 28KFT, which prompted the issuance of the warning for Cumberland County. This storm produced widespread reports of strong and damaging winds in and around the Shippensburg region. One report, received about 15 hours after the event read (minor edits) as : We obviously had a very severe, localized, event here yesterday ~1600 (2000 UTC). Hoping someone reported that. Just on my drive thru town to my home I saw one power pole snapped (no tree limb involved), a soccer goal, with netting flipped over and carried on top of another structure some 20 yards away, and the machine barn of a farm pretty much destroyed. All the damage was

Figure 4. As in Figure 1 except sounding for 2100 UTC 13 June 2005. Return to text.

within radius 1/4 mile. These two damage patterns seem to be headed in DIFFERENT DIRECTIONS . I'll look again soon. Apparently there was much damage on campus, though I have not been there yet. Phones still out there. I'll report back with more detail if you like.

4. CONCLUSIONS During a 5 day period of time, in a weakly sheared environment, several pulse like

thunderstorms produced strong and damaging winds. As shown in the cross sections, the storms were relatively high topped storms for Pennsylvania. The -20C level was unseasonably high for mid-June. In the two cases presented the storms had a deep area of >50dBZ echoes between the 0 and -20C contour. These storms had many characteristics typically associated with storms which produce wet microbursts. In both cases, the precipitiable water values were high, around 2 inches. Though not

shown, these values are around +2 standard deviations above normal. Both cases were associated with relatively high CAPE, with values in excess of 1500 and 2400 JKG-1 respectively on the 9th and 13th respectively (Figures 1 & 4). The large CAPE is required to produce the storm and significant precipitation loading. High PWAT values likely made these storms efficient precipitation producers as well as contributing to the overall CAPE. High CAPE and PWAT values are parameters which can be forecast 12-60 hours in advance. In addition to CAPE and PWAT, several storms on these days quickly produced highly reflective cores. These cores covered a vertical wide range between the 0 and -20C levels. The Shippensburg storm also showed some signs of mid-level convergence several volume scans prior to severe weather. The convergence, weak

rotation, and 60 dBZ cores over 25KFT in this storm made it a high probability storm to produce severe weather. With 50dBZ cores well over the standard Lemon criteria (Lemon 1980), this was a high probability storm to produce severe weather. Unfortunately, the time scales required to find the suspended cores, the descending cores, and the convergence in the storm is on the order of minutes. This does not produce long lead-times in storms of this nature.

5. REFERENCES Lemon, L. R., 1980: Severe thunderstorm radar identification techniques and warning Criteria. NOAA Technical Memorandum NWS NSSFC-3. NOAA National Severe Storms Forecast Center, Kansas City, MO.

Southern Region Technical Attachment SR-185 1995: Techniques for issuing severe thunderstorm warnings.

a) 1920 UTC

b) 1946

Figure 5. Composite reflectivity of the evolving storm over Franklin County at a) 1920 and b) 1946 UTC. This storm would later produce strong winds at Shippensburg. Return to text.

Figure 6 Reflectivity cross section of a storm in Cumberland County at 1950 UTC 13 June 2005 taken near the time of severe weather in Shippensburg, Pennsylvania. Return to text.