Prelude: The Day that Started it All…
In the fall of 1999, I was a young, second-grade school kid with ambitions of being a ‘dinosaur man.’ But the course of events in the late afternoon hours of 13 October 1999 changed that completely. On that day, I saw my first tornado. For years after, I became distressed and petrified at the sound of tornado sirens. It wasn’t until I saw the analysis and coverage of Hurricane Katrina in 2005 that I became obsessed with weather, specifically tornadoes and lightning, severe thunderstorms, and hurricanes. Incidentally, I studied each of these in my undergraduate, master’s, and Ph.D., respectively.
In a retrospective, this short article will discuss the events of the October 1999 severe weather outbreak in the Ohio River Valley region that ‘kick-started’ my career.
As shown below, the Storm Prediction Center (SPC) storm reports for 13 October 1999 illustrate that this severe weather outbreak was heavily dominated by wind and hail reports. In fact, only one tornado report was included in the initial analysis. This tornado was not the tornado that I witnessed. The tornado in the SPC storm report occurred in western West Virginia. Towards the north, many hail reports covered the northeast corner of Ohio and western Pennsylvania.
This spread and orientation of storm reports is very common to this area. For the Ohio River Valley region, most severe weather tends to be associated with squall lines or embedded bow echoes along a strong, highly baroclinic zone with strong influence of frontal lifting.
However, I was still puzzled after looking at these reports. I specifically remembered a tornado in Fayette Co., Ohio on this day. To validate my memory, I went to the Tornado History Project webpage (which pulls data from the SPC archives) and, sure enough, I found my elusive tornado.
It was a weak and short-lived EF-0 tornado that touched down briefly in Bloomingburg, OH and in near-by Pancoastburg (I grew up in the small ~600 people village of Bloomingburg). I know, I know, this is NADER WEEK, why am I talking about a dinky EF-0 tornado when there are so many other, better tornadoes out in the Plains. I mean, you can only be a true severe weather guy/gal if you are obsessed with Plains tornadoes, right?!?!?! WRONG! Most people will remember the ’74 Xenia, Ohio tornado, but in general, strong tornadoes don’t happen in Ohio. However, a decent F3 tornado did occur further west near Circleville, OH .
II. Synoptic Conditions
The sounding for 12 UTC on the 13th of October showed minimal convective available potential energy (CAPE) prior to the severe weather. But, it is important to note that one doesn’t need high levels of CAPE in frontal lifting situations! The lifting condensation levels (LCLs) were pretty low at ~923 hPa (few 100 meters above the surface) and precipitable water values were appreciable.
It is also important to note how strong the winds were at around 850-800 hPa. Above 800 hPa, the winds weaken slightly, then increase in strength again aloft. While the speed shear was moderate, it was present near the surface and unidirectional. This low-level jet was crucial to organizing the severe weather. Further, tilting of horizontal vortex lines could have generated weak vertical vorticity, and aid in generating weak tornadoes or storm rotation.
While not as “easy” to obtain, the North American surface analysis in the Weather Prediction Center (WPC) archives. The 00 UTC analysis on 14 October 1999 shows that the severe weather across the state of Ohio was in fact driven by a dragging cold front from a surface low in Canada. Across the front was an approximately 5 – 10 degree gradient in temperature and a slightly higher gradient in dew point (Note: the colors on the map are topography). Once the front hit the western slopes of the Appalachians, most of the severe weather died out.
The severe weather formed in a band of convection along the front as seen in the radar reflectivity image below (image courtesy of the National Weather Service).
In the upper air analysis plots below (from the SPC), we see that there was an upper level trough with diffluence of the wind barbs to the east of the primary jet exit region (panel a). As the trough deepens, the jet picks up a more meridional orientation, placing Ohio in the left jet exit region. Incidentally, the northern tip of Ohio and Pennsylvania was also in the right jet entrance region (panel b). These areas are key areas for synoptic scale divergence (yellow contours) and subsequent vertical motion via quasi-geostrophic theory. At 925 hPa (panel c) on the 14th, there was significant warm air advection into areas of high dew point (surface analysis above, green contours in panel c). The 925 hPa analysis also shows the tilted trough beginning to close off and hit occlusion stage.
III. Concluding Remarks
While not as ‘attractive’ as the Great Plains supercells, severe thunderstorms and tornadoes do occur in the Ohio River Valley region. If you don’t think they are very impressive, just ask any of the locals of the monster ’75 Xenia tornado or the weak, but oh-so-rare Athens County, Ohio tornado in September 2010.
I remember watching the funnel cloud barely skirt the ground outside of my living room window as my father tore me away and whisked me down to the basement. The tornado did not touch down on my street, but did a few streets away. This single event is the reason I got involved in weather and continued from my undergraduate to my graduate career.
This specific severe thunderstorm and tornado was part of a frontally driven line of storms. The combination of strong low-level warm air advection, low LCL height, occluding surface cyclone, deepening upper level trough, upper-level divergence and diffluence, synoptic scale vertical motion, marginal CAPE, high dew point values, prior diurnal heating, and fairly unidirectional wind shear helped to drive strong surface wind gusts, large hail, and a rare late-season tornado to the Ohio River Valley region. The moral of the story is that you can’t ignore the presence of a good low-level jet in late-season severe weather outbreaks in the Ohio River Valley. While not prime chasing area, the combined synoptic and frontal characteristics are key to producing tornadoes in this region of the United States.