This page shows some of the distinct radar signatures associated with the severe weather event of 30 July 1999. This event was dominated by southeast moving supercell thunderstorms which produced large hail and damaging winds over east-central PA during the afternoon and late evening hours on Friday, 30 July 1999.

To see a quick summary of the severe weather please read the local storm report (LSR). This product has been edited to show radar signatures associated with several of the severe weather reports. A list of all the LSR’s issued during the event can be reached here too.

The supercell thunderstorms, which affected east central PA, moved out of New York State, entering PA in northeastern Tioga County. Successive cells moved across the same region several times. At least 3 distinct supercell thunderstorms moved over the Mansfield area of Tioga County. Reports of large hail were associated with at least 2 of these storms. These storms continued to split and redevelop as they moved southward across Williamsport, Selinsgrove, Harrisburg, and York. Hail was the main threat from these storms over the northern half of the region. Over Perry, Cumberland, Dauphin, and York counties, high winds were the main severe weather problem. The tornado reported in the LSR was never verified. Damage in the area where the tornado was reported appeared to be from straight-line winds.

Interesting aspects of the case included the unusually large hail, high VIL values (75 VIL), the appearance of hooks and appendages on virtually all the supercells, and the almost low-precipitation like appearance of some supercell storms which occurred on this date. One other curious aspect was that all the hooks and appendages were observed on the northwest flank of these storms, probably due to the weak northwest flow aloft and low-level southwesterly inflow into these storms.

Archive II data were used to examine these storms from our RIDDS tapes. Storm reports came from locally collected data.

The discussion focus is on reflectivity data with a few VIL products shown. However, several velocity images were made along with velocity cross sections. All the available radar images can be accessed here.

1. Tioga County family of supercells

Around 1700 UTC, thunderstorms over central NY began moving toward northern Pennsylvania. The first storm entered PA in northern Tioga County around 1740 UTC.

The base reflectivity image at this time showed a small supercell thunderstorm north of Wellsboro. This storm continued to track to the south and east passing between Wellsboro and Mansfield around 1820 UTC. It was about this time that Ping-Pong ball sized hail was reported in Mansfield. Note the second supercell thunderstorm north of this storm in southern NY State. This storm continued southward and by 1844 UTC was south of Mansfield with a second supercell, to the north, moving toward the Mansfield region.

By 1918 UTC, the second cell was over Mansfield and the leading storm was dissipating over southern Tioga County. This storm intensified by 1928 UTC as shown in the plan view and reflectivity cross section. This storm weakened as it moved south of Mansfield as shown in the 1942 UTC image. In this latter image, it is interesting to note the "family of supercells over southern NY and central PA and the notable outflow boundary from the cells over Lycoming County. The 1 inch hail report logged at 328 PM was likely associated with this storm as it moved over Mansfield.

At 2017 UTC a third supercell thunderstorm moved over the Mansfield area.

VIL values with these storms ranged from 49 at 1746 and 52 at 1752 UTC to 69 over Mansfield at 1918 UTC. It appeared that the two well-developed supercells produced severe weather.

2. Lycoming and mid-Susquehanna Valley supercells

The storms moving out of Tioga County seemed die as they moved into Lycoming County. However, a large cell began to rapidly develop over Lycoming County around 1918 UTC. By 1928 UTC, this storm was a bonafide mini-supercell bearing down on Garden View and Williamsport. This continued to intensify, by 1937 UTC it had a distinct appendage echo and a weak echo notch. A cross section taken through the storm revealed a strong, upright updraft in the storm. This storm continued to move south southeastward through the county and was just west of Williamsport by 1952 UTC and 1957 UTC.

By 2028 UTC this first storm had moved over Union County and a second large supercell storm had moved out of Tioga County over Lycoming County. The white line in the figure shows the orientation of the reflectivity cross section taken on the southern storms. The southern storm was just blowing up. Note the strong hook echo on the larger supercell in eastern Union County to the north of Milton. The VIL in this storm was in the low 60s between 2022 and 2028 UTC.

At 2028 UTC, yet another supercell can be seen well south of these storms over northern Dauphin County.

The 2038 UTC image showed the family of supercells extending from near Williamsport southward to the Millersburg area. The white line shows the area of the cross section taken through the northern Williamsport storm. Note that this storm and the storm near Milton have distinct hook echoes. The cross section through the Williamsport storm showed a strong, upright supercell thunderstorm. This storm had a 70 VIL value around 2040 UTC as it moved over southern Lycoming County. Numerous reports of large hail (1.75 inches in diameter) came from the Williamsport area at this time.

The 2058 UTC image showed yet a third strong supercell thunderstorm moving across Lycoming County with several other supercell thunderstorms to the south. Well-defined hooks or pendants can be seen on at least 5 cells in this image. The northern Lycoming storm moved over Jersey shore around 2130 UTC. By 2202 UTC, all the activity ceased in Lycoming County was focused over Union and Snyder Counties.

The image at 2232 UTC shows a pronounced hook echo over Snyder County. A cross section through this storm showed the strong updraft core. This storm continued to move southward over Perry and Cumberland Counties and other location of the Lower Susquehanna Valley.

3. Lower Susquehanna Valley including the York Storms

By 2251 UTC the supercell storms had moved over the densely populated Lower Susquehanna Valley region of the State. In the 2311 UTC image, all the towns and cites are removed to clearly show the storms. The 2316 UTC image shows a supercell thunderstorm over central York County, near the city of York and a second, smaller storm to the north over Perry County. Hail flares can be seen in this image on the southeast side of the supercells in York County

By 2221 UTC only the Perry and York City storm are present. Hail flares are still visible in the York storm. A cross section through the Perry County storm shows this was a very tall and vertical storm. The strong updraft contained an elevated area of 70dBZ echoes. This storm began to collapse as seen in the 2336 UTC image. However, it produced large hail and damaging winds over eastern Perry County. In this image, the York storm continued to move southeastward. This storm achieved a VIL of 75 over York as shown in the 2316 UTC VIL product. This storm continued to produce hail as it moved over northern Maryland.

CONCLUSIONS:

1. This was a big supercell day with many mini-supercells and several large supercell thunderstorms.
2. Over the northern 2/3rds of central PA, the main threat was from large hail
3. As the storms moved southward, the threat included damaging winds and hail
4. Virtually all the storms had hook or pendants echoes. These features were located on the NW flank of the cells.
5. These storms had strong updraft cores and were very upright.
6. As with other storms, the counties on the RFD side of the storms had an extremely high probability of experiencing severe weather.
7. This event spanned approximately 6 hours as the storms moved into northern PA around 1740 UTC and exited around 0000 UTC.