Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: 24hourprof , 12:27 PM GMT on October 27, 2013
I couldn't let October pass without writing a blog about the EF-4 tornado (You-Tube video) that struck the town of Wayne in Wayne County, Nebraska, on the 4th (the tornado's track). I had to wait to write about this event because I couldn't access Level-III radar data from NCDC due to the government shutdown. Needless to say, it's been a while since the tornado outbreak occurred, so I promise to refresh your memory about what happened. Of course, there's a method to my madness, and I have a specific point to make about the EF-4 tornado. The media characterized the EF-4 twister as "rare," but I take issue with this characterization (surprise, surprise), and I'll explain my reasoning as this blog unfolds.
The six-hour surface forecast issued by the Weather Prediction Center at 1322Z on October 4, 2013. The forecast was valid at 18Z (1 P.M. CDT) on October 4, 2013. Larger image.
First, a review of what happened. The six-hour surface forecast (above; larger image) from the Weather Prediction Center issued early on the 4th (valid at 18Z, which is 1 P.M. CDT) showed a fairly expansive warm sector located over the Middle West (the warm sector lay ahead of a Pacific cold front and south of a stationary front). The Storm Prediction Center's 12Z Convective Outlook (below), issued at 06Z early on the 4th, indicated a moderate risk of severe weather over the Upper Middle West contained within a larger region with a slight risk.
The Day 1 Convective Outlook issued by the Storm Prediction Center at 06Z on October 4, 2013. The outlook was valid from 12Z (7 A.M. CDT) on October 4 to 12Z on October 5, 2013. Courtesy of the Storm Prediction Center.
Unless you've read SPC's specific criteria for these levels of risk, you might fall into the trap of interpreting them from a purely probabilistic perspective. Believe me when I tell you that this overly simplistic interpretation dramatically misses SPC's intent to convey the threat of severe weather to the general public. So please take a moment to digest SPC's description of the levels of risk for severe thunderstorms.
The 21Z Rapid Refresh model analysis of mixed-layer CAPE (red contours) on October 4, 2013. The units of MLCAPE are Joules per kilogram. The blue shaded regions indicate areas with convection inhibition (expressed in Joules per kilogram). The EF-4 tornado struck Wayne, NE, shortly after 22Z (5 P.M. CDT). Courtesy of the Storm Prediction Center.
Okay, let's look at the potential outbreak of tornadic thunderstorms from a forecaster's point of view. The surface-based CAPE and vertical wind shear between ten meters and an altitude of six kilometers over the region east of the Pacific cold front in Nebraska and south of the stationary front in northern Iowa were sufficiently high to warrant the moderate risk of tornadoes (read more about CAPE). To see what I mean, check out the 21Z (4 P.M. CDT) Rapid Refresh analyses of Mixed-Layer CAPE (above) and the surface-6 km wind shear (below). For reference, here's the 21Z surface analysis from WPC. Keep in mind that the EF-4 tornado struck Wayne, NE, shortly after 22Z (5 P.M. CDT)...check out the 2215Z mosaic of composite reflectivity.
The 21Z Rapid Refresh model analysis of bulk vertical wind shear between ten meters and six kilometers. Wind barbs indicate direction and magnitude (in knots) of the bulk wind shear, while blue contours are isotachs labeled in knots. The EF-4 tornado struck Wayne, NE, shortly after 22Z (5 P.M. CDT). Courtesy of the Storm Prediction Center.
For the record, Mixed-Layer CAPE (MLCAPE) allows for mixing, unlike the less realistic surface-based CAPE (SBCAPE). I'll write a future blog with more details about MLCAPE...for now, just keep in mind that relatively high values of MLCAPE (1000 to 2000 Joules per kilogram in northeastern Nebraska) indicated the potential for strong updrafts in thunderstorms. And the magnitude of the vertical wind shear (in the neighborhood of 50 knots or greater) favored discrete (or semi-discrete) thunderstorms to be supercellular.
The reports of severe weather on October 4, 2013. Courtesy of the Storm Prediction Center. Read more.
Yet the area affected by tornadic supercells was dramatically smaller than forecasters might have anticipated (see the storm reports for October 4 above). One of the reasons, I think, for the suppression of strong storms during the afternoon and evening of October 4 was a mesoscale convective system that developed over the region during the nighttime hours. I annotated the MCS on the 0730Z (2 A.M. CDT) mosaic of base reflectivity, and inserted (below) a loop of radar images from 00Z to 10Z on October 4. This MCS likely stabilized the affected area, especially across east-central Nebraska, southeast Nebraska, and southwest Iowa.
A loop of radar mosaics of base reflectivity from 00Z to 10Z on October 4 (7 P.M. CDT on October 3 to 5 A.M. CDT on October 4). This long-lived MCS helped to stabilize the lower troposphere over much of Nebraska and Iowa. Courtesy of NCAR and NEXLAB-College of DuPage.
So, even though it initially appeared that there would be sufficient uplift to get air parcels to the level of free convection (low-level convergence along the front), the stabilization from the earlier MCS likely meant that the initiation and maintenance of severe thunderstorms would require additional lift, especially over east-central Nebraska, southeast Nebraska, and southwest Iowa. Why not over northeast Nebraska and northwest Iowa, you ask? Well, the MCS did indeed affect this region...revisit (above) the loop of radar mosaics of base reflectivity from 00Z to 10Z on October 4. But any stabilization in the lower troposphere was overcome by strong low-level convergence along the front (see this 21Z RR analysis of MSL isobars and 10-meter winds), coupled with differential positive vorticity advection (see this 21Z RR analysis of 500-mb heights and 500-mb absolute vorticity) and the resulting upward motion at 700-mb below (dashed contours in microbars per second).
The 21Z Rapid Refresh model analysis of vertical motion at 700 mb expressed in microbars per second. Dashed contours represent upward motion at 700 mb (roughly three kilometers). Courtesy of Penn State.
Throw in upward motion in the exit region of a 250-mb jet streak (check out this RR analysis of 250-mb isotachs, labeled and color-filled in knots). These 250-mb isotachs are superimposed on 250-mb vertical motion, with upward motion designated by dashed contours in microbars per second. Note the relatively strong uplift at 250 mb over the area where the tornado outbreak occurred (revisit storm reports).
The bottom line here is that there was stronger uplift ("deeper" forcing) over northeast Nebraska and northwest Iowa, which effectively overcame any lower-tropospheric stabilization from the earlier mesoscale convective system. In contrast, I believe that stabilization from the earlier MCS negatively impacted the initiation and maintenance of tornadic supercells over northern to central Iowa and northeast Nebraska, which were included in SPC's moderate risk of tornadic supercells.
The 2214Z image of base reflectivity from the radar at Omaha, Nebraska (KOAX in the lower-right corner of the image). Courtesy of NOAA.
The 2214Z image of base reflectivity (above) shows the discrete supercell that produced the EF-4 tornado in Wayne County, Nebraska. The corresponding 2214Z image of storm-relative velocities (below) displays the velocity couplet associated with the tornadic supercell.
The 2214Z image of storm-relative velocities from the radar at Omaha, Nebraska (KOAX in the lower-right corner of the image). Courtesy of NOAA.
In the final analysis, forecasters researching this outbreak of tornadic supercells might be tempted to give the event a cursory and perfunctory look. If so, they might say, "Yeah, that's an easy one," without noticing the relatively large area with large CAPE and strong wind shear. If forecasters give this outbreak a closer look, they're likely to see the high-end potential for tornadic supercells over east-central Nebraska, southeast Nebraska, and southwestern Iowa and wonder about this sizable, "false-alarm" area. At this point in their investigation, they might discover the earlier MCS and connect the dots between stabilization and the negative impact on storm initiation and maintenance.
They might also note the elevated mixed layer surging east with the Pacific cold front. To see what I mean, check out the special 18Z radiosonde soundings at Omaha, Nebraska (shown below). For the record, an elevated mixed layer (EML) is a warm, dry (low relative humidity) layer of air from the high western deserts that sometimes gets advected eastward, forming a capping inversion and suppressing thunderstorms, unless, of course, the cap is broken by strong uplift. Which brings me to the other point I made...low-level convergence and the associated uplift were not sufficient to get air parcels to the level of free convection and, thus, overcome any stabilization from the earlier MCS.
The 18Z radiosonde soundings of temperature (red) and dew points (green) on October 4, 2013. At the time, an elevated, well-mixed layer of air with fairly low relative humidity was surging eastward atop the Pacific cold front. Courtesy of the Storm Prediction Center.
What becomes clear to me is that predictive tools such as CAPE and vertical wind shear can get forecasters only so far. That's because the third ingredient for predicting severe weather is the strength of uplift, which is difficult to assess in real time or in short-range forecasting.
Lastly, I want to address the media's use of the word, "rare," to describe the EF-4 tornado that struck Wayne, NE. I admit that strong tornadoes are indeed "rare" over eastern Nebraska and western Iowa during early autumn. But I had the distinct impression that the media's reference to "rare" included all autumnal outbreaks of strong tornadoes. If I interpreted the media's reference correctly, then I'm here to tell you that outbreaks of strong tornadoes east of the Rockies during autumn are far from rare. Indeed, they happen from time to time wherever a storm environment similar to the one on October 4 develops east of the Rockies (the location varies from year to year, of course). Such storm environments don't materialize every year, but they occur often enough during fall to warrant the media not using "rare" to describe these autumnal tornado outbreaks.
Moreover, if an outbreak of tornadic supercells occurs during fall in storm environments similar to the one on October 4, then nobody should have been surprised by the EF-4 twister. Indeed, the vertical wind shear over Nebraska and Iowa on October 4 was sufficiently strong to support violent tornadoes (EF-4 and EF-5). So I believe that describing the EF-4 tornado that struck Wayne, Nebraska, on October 4 as "rare" was not a valid characterization. In other words, the occasionally strong tornado outbreaks that occur during autumn are closer to the norm than they are to "rare." And yes, a similarly strong tornado outbreak will probably occur again in Nebraska during the early-fall season.
Many thanks to Steve Corfidi of SPC for his input on this tornado outbreak.
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