by
Richard H. Grumm
National Weather
Service State College
A cold front approached western Pennsylvania during the early afternoon hours of 02 June 2000. Ahead of this cold front, a line of thunderstorms developed. The first cells developed along the lake breeze front off from Lake Erie. Two of these storms became supercells as they moved across Warren County. The lead supercell persisted for over 2 hours ending in Potter County. To the west of these storms, a line echo wave pattern developed over Ohio and western Pennsylvania. This line moved eastward across Pennsylvania during the afternoon and evening. Along the northern edge of this line, the lake breeze convergence zone enhanced storms along the northern extent of the line. This produced a long-lived bow-echo, which moved across the northern tier of Pennsylvania. From forecast point of view, the models showed the flow of warm moist air into Pennsylvania ahead of the frontal zone. Moisture variables at 850 and 700 hpa were forecast to be 1-2 standard deviations above the seasonal normals. Unseasonably dry air was behind the front. The forecast aspects of this case may be worthy of an independent study. Most of the severe weather was associated with the supercell storm, distinct bows along the line, and the outflow that moved off the Allegheny Plateau. In Clearfield and Centre Counties, the outflow outran the thunderstorms. High winds associated with the outflow created power outages due to falling trees. In most cases, weak F0 damage was observed due to falling or topping of not so healthy trees. However, a few spots appeared to suffer real widespread damage of the strong F0 and F1 type. Areas of Warren County, near Youngsville, and Lycoming County, near Hughesville, appeared to suffer widespread and significant damage. The Warren County report indicated over 1000 down trees and should be investigated for possible tornadic damage with the supercell. A downburst or gustnado from a bow echo are the likely causes of damage in Lycoming County. The remainder of this report will show radar data related to different times or geographic locations associated with the event. All radar data shown is from our local RIDDS tape and is the full resolution archive II data.
The first cells of the day formed in New York to the east of Lake Erie. The initial convection developed as the Lake Erie breeze converged with the warm moist air over northwest PA and southwestern New York. The first cells appeared in Cattaraugus County NY around 1400L along the Lake Breeze front. The cold lake air hit the warm southwesterly flow, igniting the first convection of the day.
By 1500 UTC, more cells formed on the south of this in Erie County PA . These storms moved rapidly eastward reaching NW Warren County around 1530 UTC. A cell along the southern flank of the line began to develop an appendage in the base reflectivity data and took on supercell characteristics in the velocity data around 1600 UTC. A strong inbound area was visible in the velocity image at this time .
The WSR-88D storm-tracking algorithm began tracking this as shown in the base reflectivity product . A second storm formed to the west of this storm and it too began tracking across Warren County . Both storms appeared stronger viewed from the KBUF radar. Operationally, the severe thunderstorm warning and following tornado warning were issued based on the base velocity and SRM data from KBUF. A mesocyclone was detected around1618 UTC in the velocity data in the lead supercell thunderstorm. 1628 UTC detected a TVS. The storm developed deep mesocyclone in the SRM .
A line of shower and thunderstorms began to develop to the west of the two supercells in Warren County around 1645 UTC. By
1700 UTC this line and the two supercells can clearly be seen in the base reflectivity image. A reflectivity cross-section taken through the supercell at this time showed the storm was pulsing over McKean County.
The line of showers and thunderstorms to the west enhanced along its northern edge as it moved eastward. This probably was the result of the Lake Breeze converging with the more general line. Cells along this line entered Warren County around 1727 UTC The leading supercell kept moving to the east-southeast. Although not shown, the storms along the line had relatively low VILS in the 20-30s.
A wave like pattern, similar to those seen in prior events (29 May 1998) began to develop along the line in western PA. The wave showed up in the reflectivity and in the velocity data by 1747 UTC. Note the strong inbounds along the line and the presence of a Rear Inflow Jet (RIJ) in Crawford County. A trailing area of
stratiform rain was visible in the reflectivity cross section . Note that the first, long-lived, supercell was now chugging along into Potter County. This wave kept moving eastward as seen in the 1831 UTC reflectivity and reflectivity cross-section. By this time, there was a forward tilt to the storm. This was seen in the May 1998 and July 1999 events. The base velocity data showed strong winds along the line up to 64 kts. The SRM showed strong convergence along the line.
The SRM ,
SRM cross section and base velocity showed min bow echoes along the line and strong front to rear
flow, indicative of a stable RIJ behind the line. This probably indicated the line would be long-lived and that straight-line
wind damage was a strong possibility. Note the weak echo notch in the mini-bow
echo along the Elk-McKean County border in the
reflectivity data. By 1941 UTC, the
northern wave along the line had developed a strong stratiform rain area over
the RIJ. New spearhead like echoes were
visible over Pittsburgh’s County warning area as shown in the 1941 UTC reflectivity image. This
stratiform rain area and the weaker echo returns behind the line appear to be a
significant indicator of a stable long-lived convective system.
The northern Portion of the line would move eastward maintaining about 2-3 mini-bow echoes. The center part would follow a similar pattern as would the
southern part. By this time, supercell activity had ended.
Northern History images:
Base reflectivity at 1950 UTC
Base reflectivity at 2015 UTC –nice bow over NW Lycoming County, stratiform rain echoes over Tioga.
Base reflectivity at 2041 UTC – bow echo over Williamsport, note rear inflow notch (RIN) with others to the north and to the south over Clinton County
Base reflectivity at 2106 UTC
Base reflectivity at 2130 UTC
As the line moved eastward, it took on a more multi-cellular appearance.
These storm refer to the storms along the line that moved through the center of the State College County Warning area.
The storms moved rapidly across Indiana and Jefferson County and then into Clearfield County. By 1950 UTC the center of the line could be seen across Clinton, Elk, and Clearfield Counties. More important at this time was the velocity data , which showed the outflow boundary out-running the actual convection in Clearfield County. The velocity cross-section showed the RIJ associated with this system.
The outflow boundary was better observed in the SRM and reached the RDA around 2021 UTC . The base reflectivity data showed the strong convection well west over Clearfield County, but a find line running close to the position of the gust front was visible in these data.
By 2141 UTC the line had moved over State College as seen in the velocity , srm and reflectivity
Note the main convection was still about 20 miles to the west. The fine line in the reflectivity data is clearly seen moving southwestward out of State College, not to be confused with the more SW-NE oriented ridgelines just to the south. A velocity cross-section through the line taken from Lewistown to Clearfield, showed the strong RIJ associated with the convection.
As the gust front moved eastward, new convection developed. A bow echo developed over Mifflin County and moved rapidly east-southeastward reaching northern Dauphin County by 2210 UTC. A series of images is shown below:
The State College radar went down for 30 minutes, but the storms crossed Lebanon and Lancaster Counties around 2300 UTC.
2325 UTC storm exited. No storms crossed York County due to the 280-290 winds at midlevels.
The term southern refers to the southern tier counties and the storms along the line that moved through this area.
Cambria
and Somerset
Well-defined echoes formed to the west and moved over Cambria and Somerset Counties. The KCCX radar depicted a TVS over Westmoreland County around
2120 UTC (VolScan200). This storm would move across Somerset County between 2145 and 2220 UTC. It developed a distinct bow shape by 2301 UTC (note this was during a 25 minute radar coverage gap).
Southeast:
The southeastern storm was the last clearly identifiable mini-bow echo of the event. This bow was really an extension of the Westmoreland and Somerset
County storm. It can be seen moving out of Fulton and into Franklin County at 2306 UTC. The storm entered Adams County around 2335 UTC. This storm retained a nice bow signature in the velocity and SRM imagery. The SRM showed convergence along the leading edge of the bow.
The storm moved through the Gettysburg area round 2345 UTC. The storm entered York County around 0006 UTC just west of Hanover. The velocity data showed the strong winds and a more distinct bow pattern then did the reflectivity data The velocity data showed the strong winds and more distinct bow pattern then did the reflectivity data. The bow can still be seen over southern York County at 0016 UTC.
This storm exited southwestern York County southeast of Hanover, around 0021 UTC.
Conclusions:
Two supercell storms formed early in the event. These storms formed ahead of the line echo wave pattern.
Once the LEWP dominated, most of the severe weather was associated with distinct bow echoes along the line. Most storms tracked from about 280 degrees. In eastern Clearfield and Centre Counties, the outflow and its associated winds arrived about 30 minutes before the convection and heavy rains.
The lake breeze was associated with the initial supercells development and may have contributed to the persistent bow echo along the northern edge of the LEWP in northern Pennsylvania.
This system is a close analog to the 29 May 1998, 9 July 1999, and 16 August 1999 events.
