Severe Weather Event of

09 July 1999

By

Richard H. Grumm

And

Mike Dangelo

National Weather Service State College PA 16803

Introduction:

The severe weather event of 09 July was marked by 3 distinct phases. Initially, a lone supercell thunderstorm moved across north central Pennsylvania during the later afternoon hours. This storm produced large hail and downed a few trees in its path. Later, a line of thunderstorms moved across the northern half of Pennsylvania. Initially, this line was composed of several rotating supercells, but as their circulation’s merge, a distinct line-echo wave pattern (LEWP) developed. Several mini-bow echoes developed in this LEWP.

This page shows some of the distinct radar signatures and significant storms associated with this event. To see a summary of the severe weather please read the local storm report (LSR). A graphical summary of the type and location of the damage is provided in the graphic in our main webpage overview document. The letter W denotes wind damage and T possible Tornadoes. The time, local time, to the nearest minute is provided on the map.

There are a few points that should be noted about this event. First, the NWS in State College lost communications with the WSR-88D for about 70 minutes during the first phase of the event. Archive III and IV data are not available. We may have archive II data for the missing 70 minutes. At the current time, our RIDDS data is missing the same 70-minute window. The radar and the radar communications functioned admirably during the second and third phase of this event.

Phase I:

The Elk-Cameron-Clinton-Centre-Union County supercell

Note: the orientation of this storm over Elk County provided excellent images of the structure of a supercell thunderstorm. Composite reflectivity and storm relative velocity data were used when possible.

This storm formed to the west of the State College Forecast Office’s County warning area over Venango and Clarion Counties around 1900 UTC. A composite reflectivity image of the developing supercell at 1939 UTC shows the developed supercell entering western Elk County, at this time, it already had a well developed and deep updraft core. At 1944 UTC the storm was in Elk County, the white line shows the slice used for the reflectivity and velocity cross sections.

 

This storm continued to strengthen as it moved across Elk County. By 1949 UTC, the storm had 70 dBZ cores within it. These cores extend well up into the storm, supported by the strong updraft (due to the storms orientation, the outbound velocity data depicted the updraft quite nicely). The storm continued to strengthen as it approached Ridgeway around 1954 UTC suspending up to 70 dBZ cores to around 30 kft and showing a strong updraft. It is also interesting to note the development of the descending rear flank downdraft in this storm, shown in the inbound velocity data. Hail with diameters the size of quarters was reported south of Ridgeway shortly after the time of this image. Nickel diameter size hail was reported in Clearfield County a short time later.

By 2034 the storm was over Cameron County, still showing an intense vertical structure with 70 dBZ cores suspended to around 21 kft in the storm. Velocity data was not as useful due to the angle of the storm to the radials.

The storm continued to move to the southeast, but due to communication problems with the radar, no data were available again until 2153 UTC. AT this time, the storm was over Union County. The remnants of the storm could still be seen over southern Northumberland County at 2232 UTC. Like many supercell thunderstorms, this storm persisted for over 3 hours.

 

Phase II:

Supercells

This phase was marked by a persistent supercell thunderstorm over Warren County, called the Tidioute storm. The reflectivity data at 2327 UTC (730 PM). The data show a strong thunderstorm over southwestern Warren County, near Tidioute. The corresponding velocity and storm relative velocity (SRM) data area also shown. The radar detected mesocyclone can be seen just northeast of Titusville in the SRM image. The SRM showed a distinct rear flank downdraft in this storm. The base velocity data showed windspeeds up to 64 kts in this rear flank downdraft (RFD).

A velocity cross section showed a deep core of strong winds in the RFD. The SRM cross section clearly showed the mesocyclone and the depth of the mesocyclonic circulation. This SRM section was taken SW to NE across the mesocyclone. Unfortunately, this strong supercell downed trees leading to a fatality in Warren County.

The supercell phase of the event continued near the New York border for the next 2 hours. The archive II data showed two distinct supercells along the New York border, one of which affected extreme northern Tioga. However, in Pennsylvania, the character of the convection changed as thunderstorm circulation’s merged. A LEWP developed and moved across the northern 2/3’s of the Commonwealth.

The Tioga county supercell can clearly be seen at 0002 UTC along the NY border. Note that the "hook" is in northwest Tioga County. Also note the second supercell skirting along the NY/Potter County border at this time. A reflectivity cross section revealed how strong and deep this storm was. The echo top was over 45 kft and the 50-dBZ core was over 30 kft in the storm.

The SRM showed the mesocyclone and the WSR-88D produced a tornadic vortex with this storm. The rotation and strong winds were apparent in the velocity data too.

This storm retained its structure as it zipped across northern Tioga County as seen in the 0007 UTC, 0020 UTC, and 0025 UTC reflectivity images. Note the hook echo just inside Tioga County and the trailing supercell to the west. A reflectivity cross section at 0025 shows how this storm remained large and strong. A time series of cross sections did reveal that this storm pulsed several times during its life.

PHASE III:

LEWP Phase

The LEWP phase began in Warren County and southward. A southwest to northeast oriented line of storms developed. Initially, the cells were connected in a nearly continuous belt of 45-50 dBZ’s. Later, large gap areas formed between individual cores. During this later phase, the line produced little or no damage.

A distinct bow echo formed just west of Clearfield County around 0056 UTC (Just before 9 PM). The bow quickly moved across southern Clearfield County and by 0126 UTC was entering Centre County. The distinct bow moved across State College around 0150 UTC. By 0200 UTC the bow had moved past State College toward Mifflin County. A reflectivity cross section revealed a distinct tilt to the storm. A corresponding velocity cross section revealed a strong rear inflow jet into the bow. The SRM revealed several distinct mesoscale circulations’ along the distinctive bow echo.

For all images click here.