Wednesday, July 31, 2013

Historical Case Study of July 1994 Limerick Tornado

I intended to write about July 27, 1994 - the day that 25 tornadoes touched down in Virginia, Maryland, Delaware, and Pennsylvania, including the F3 in Limerick, Pa. - and use images to show how it was a classic tornado day.

Unfortunately, there is no such thing as a classic tornado day in the Northeast.

However, I still have plenty of images to share.

(A quick note about data: The only files I could access to make these maps were from 12, 18, and 21 UTC on the 27th. The first tornado of the day - an F0 northwest of Richmond, Va. - touched down at 1725 UTC (1:25 pm EDT), so I thought that 18 and 21 UTC maps would work well enough to depict the atmosphere that produced these tornadoes.

Also, radar data is mostly missing. KDIX (Mount Holly) does not have any data from before 1995, and KLWX (Sterling, VA) is missing data for the 27th both for Level II and Level III. However, if anyone has radar data that I didn't find, I would gladly add it and cite them.)

Ok...on to the maps!

Surface maps:

There was no obvious surface Low in the immediate area and surface winds remained light throughout the day.


However they did change direction during the morning.

The caption is supposed to say TEMP (F) here as well.

Wind:

These images show the wind at three different levels of the atmosphere - at the surface (in black), 850 mb (in blue), and at 500 mb (in red). Areas where the wind veers with height, from southeasterly at the surface to southwesterly at 500 mb would be favored for tornadoes.


While there are many areas that exhibit classic veering direction, 850 mb winds are uncharacteristically slow. Most significant tornado days will have wind of at least 25-30 knots at 850 mb.



CAPE:

Although the temperature gradient didn't appear very tight, the tight CAPE gradient shows that there is either a warm front or cold front running roughly along I-95. Given that there are lower surface pressures to the southwest over Virginia, and that the surface wind has a distinct easterly component later in the day, the gradient is most likely a warm front.



Sure enough, the area of instability has expanded northward throughout the day. It's a warm front.

However, notice that even by 21 UTC, the area around Limerick - where one of the two F3 tornadoes struck - is still only in an area of 250-500 J/Kg of CAPE.

Helicity:


Helicity increases significantly through the morning and into the afternoon. The axis of highest values also expands northeastward through the afternoon.


Still, for 0-3 km Helicity, these values are a bit lower than one would expect. It is possible that they increased by 8 p.m. and 11 p.m., when the strongest tornadoes struck. 

However, we can see that the area where the axis of CAPE overlaps the axis of Helicity is where all of the tornadoes were:



Many diligent youtubers have uploaded segments of the news from the day after, like this one of WCAU (back when it was still a CBS affiliate.)

Before I finish, I have to say thank you to Kevin Goebbert for assisting me with these maps.

Either Monday or Wednesday of next week I'll be back with a look at another Philadelphia-area tornado day: May 31, 1998. And that discussion will include radar.

Editor's Note: One of the many videos that recapped the damage from Limerick on YouTube is available below. This features footage from WPVI, WCAU, and The Weather Channel. This does include some radar, not GR2 or GR3, but it's radar nonetheless.