Introduction:

This cases started as a cluster of thunderstorms moved across western Pennsylvania
during the afternoon hours of Wednesday, 21 Jun 2000.  A distinct bow echo
developed over Elk County and moved eastward.  Several other small bow echoes developed
over Clearfield and Centre Counties and moved eastward. These storms were rather weak
in Centre and Clearfield Counties, taking on better characteristics to the east.

On bow echo moved across Perry and Cumberland Counties.  The the lee of Blue Mountain, this
bow spun-up a short-lived tornado. This proves once again that not all tornadoes come
from well defined supercell thunderstorms.  An article in  Weather and Forecasting
in 1999 discussed the limits of forecasting tornadoes that spin-up.  This study showed
that nearly 50% of all supercell thunderstorms with tornadoes have these spin-up tornadoes.
Virtually all bow echo tornadoes fall into this latter category. The implication in that
study was that the WSR-88D was well suited to forecast the less frequent, spin-up tornado.
The Cumberland County tornado appears to be the result of a bow echoes rear inflow jet
accelerating down a terrain features, and probably could be classifed as a spin-up
tornado.


Method:
The hard work and ability to anticipate this event led to a good tape RIDDS tape archive
and an archive IV CDROM.  All data shown here are from the raw archive II data displayed
using WATADS.

Results

Elk-Lycoming Bow echo

The images show the storm over Clinton County around  2044 UTC.
At this time, the storm had lost its distinct bow appearance. A cross section revealed
a strong foreward tilt and an area of trailing stratiform rain.  The latter often accompanies
well developed rear inflow jets (RFJ). The storm relative  image  at this time showed a weak circulation near the bookend vortex over southwestern Tioga 
County.  The winds were at a bad angle to the radar at this time, forcing the forecasters
to rely more on reflectivity signatures.  The base velocity did show a strong outbound maximum
in Tioga County but missed the strength of the winds moving into the convergence line.

As the storm moved farther eastward, the  velocity data  
and  SRM data  begin to reveal the strenght of the bow. Note the 60 KT outbound velocity near the cyclonic bookend vortex.  The reflectivity data
showed a developing weak echo channel in the  bow.
This feature continued to develop and can be seen just south of storm 75  at 2145 UTC.  The outbounds over central Lycoming were underestimated by the radar
and the storm speed was relatively fast, this probably lead to the weak outbounds in the
SRM making the RIJ hard to clear discern in the imagery. 

To show this, an  SRM  and Velocity image are shown. Note the stronger outbounds in the SRM in the bow over Union County
where the winds are blowing down a radial.

This southern mini-bow echo took on a well defined shape as it moved toward Selinsgrove
as seen in the  velocity  and   reflectivity images  at 2234 UTC.  This bow echo then moved across Northumberland, southernMontour and Columbia Counties.

At this time, farther south, a strong well defined bow echo was exiting Perry County
and moving into northwestern Dauphin County.  This feature can be seen in the 2319 UTC
 velocity ,  srm , ands  reflectivity data . 

If this were not enough, another bow was trucking across Franklin County as shown in the
as seen in the  reflectivity  and  SRM   data. 

Storm surveys and follow-up calls suggest these distinct bows all produced damaging winds.

Cumberland Bow and spin-up tornado

Current estimates suggest this storm struck north of Carlisle between
719 and 729 PM local time Wednesday evening.  A storm survey revealed a 
very short-lived F0 tornado formed from this storm.


base reflectivity data
 2314  |
 2319  |
 2324  |
 2329  |
 2334  |

Storm relative Velocity data

Velocity data
 2314  |
 2319  |
 2324  |
 2329  |
 2334  |



Conclusion: 
The majority of the strong F0 damage was done by distinct bow echoe.  As the storms moved
into the more unstable air the east, the took on better structure and more bow echoes
developed.

The relation of the winds to the RDA made using velocity and velocity based products difficult
in central Pennsylvania, especially just northwest to northeast of the radar from Clinton
into Lycoming Counties.

Finally, a spin-up tornado appeared to form as one of the bows crossed Blue Mountain, once
again showing the significance of terrain interacting with convective storms.