Tornado and Derecho Threat from Midwest to Appalachians

A near-classic early-summer sequence of potentially tornadic storms followed by destructive straight-line winds is in the cards for the Midwest on Wednesday afternoon and evening, with the wind-packing storms possibly approaching the mid-Atlantic states on Thursday morning. In its Day 1 outlook updated at 12:30 PM EDT Wednesday, NOAA’s Storm Prediction Center is calling for a moderate risk of severe weather (the second highest of SPC’s five risk categories) from northern Illinois into western Ohio. Lesser risk categories extend to the Washington, D.C. area. The event will unfold along a warm front extending east-southeast from a surface low in Iowa through the risk corridor. A very humid, unstable air mass lodged against the front will provide plenty of fuel for severe storms, and a strong jet stream oriented parallel to the front will keep the storms moving at a rapid pace as they eventually congeal into a large mesoscale convective system (MCS).

While there is a real risk of strong tornadoes on Wednesday, especially in and near northern Illinois, an even larger area will be vulnerable to potentially damaging winds produced by the expected MCS. The high-resolution HRRR model suggests a large MCS will be consolidating and pushing rapidly southeast across Ohio around midnight Wednesday night (see Figure 2 below). This MCS may produce a derecho, a large area of thunderstorm-generated high winds that propagates rapidly over distances of 250 miles or more. Longer-range models bring the MCS into West Virginia and Pennsylvania overnight, with the potential for storms moving into or regenerating across the Washington, D.C./Delmarva area on Thursday morning. Depending on how the first batch of storms evolves, another round of severe storms could pop along the front from Kentucky to the mid-Atlantic on Thursday (NOAA/SPC has a slight risk for this area in its Day 2 outlook).


Figure 1. WU depiction of NOAA/SPC severe weather risk areas as of mid-morning Wednesday, June 22, 2016, valid through 8:00 AM EDT Thursday, June 23.


Figure 2. In its run from 14Z (10:00 AM EDT) Wednesday, June 22, 2016, the high-resolution HRRR model depicts a large thunderstorm complex moving rapidly across Ohio at 05Z (1:00 AM EST) Thursday, June 23. Such model predictions are not intended to be exact depictions of where thunderstorms will actually be located. Image credit: www.tropicaltidbits.com

A whopper of a wind-producer
The term “derecho” gained wide notice after an especially powerful one moved from northern Illinois to the mid-Atlantic coast in a matter of hours on June 29-30, 2012 (see Figure 3). That derecho, one of the strongest documented in North America, caused almost $3 billion in damage and took 28 lives. Millions were affected by power outages that lasted for days in some areas, and countless trees were uprooted. At first glance, it might seem that models are painting a scenario for Wednesday night bearing some resemblance to the 2012 pattern. However, that event was fed by low-level heat and moisture unprecedented for late June; only a few hours before the high winds struck, Washington, D.C., had set a monthly record high of 104°F. Surface temperatures along the front won’t be as extreme in this case, which suggests less chance of a similarly potent event. The timing of the upper-level impulse that will propel Wednesday night’s storms is also less favorable for bringing the MCS across the Appalachians before the typical early-morning reduction in atmospheric instability. As shown in Figure 5 below, derechos are much more common from Illinois to Ohio than further east.


Figure 3. Composite radar image showing the progress of the intense derecho that swept from the Chicago area to Washington, D.C. on the night of June 29, 2012. Covering about 600 miles in just 10 hours, the derecho produced hundreds of severe wind gusts, with peak winds of 80 to 100 mph. Image credit: Greg Carbin/NWS Storm Prediction Center.


Figure 4. A thunderstorm complex associated with an incipient derecho moves into LaPorte, Indiana, on the afternoon on June 29, 2012. Image credit: Kevin Gould/NOAA, courtesy NASA Earth Observatory

Tonight’s derecho threat is right down the climatological alley
In a weather.com article published on Wednesday morning, Jon Erdman shows us just how well today’s event lines up with climatology. Erdman spotlights a new study by Corey Guastini and Lance Bosart (University at Albany, State University of New York), published in the April 2016 issue of Monthly Weather Review, that examines 256 U.S. warm-season derechos between 1996 and 2013. Figure 5 below shows the preferred corridor, extending from northern Illinois to Ohio, with much less frequent activity further east. “Northeastern Illinois is ground zero for warm-season progressive derechos," Bosart said.


Figure 5. The number of derechos observed within 10,000-square-kilometer boxes (about 60 by 60 miles) during the May-August interval from 1996 to 2013. Image credit: Corey T. Guastini and Lance F. Bosart, Analysis of a Progressive Derecho Climatology and Associated Formation Environments. Monthly Weather Review, April 2016, (c) American Meteorological Society.

What exactly is a derecho?
In Spanish, derecho has several meanings, including “straight.” Gustavo Hinrichs adopted the term in 1883 to describe a type of thunderstorm-related wind he dubbed “the straight blow of the prairies." [Derecho contrasts with the Spanish tornar, "to turn," as in a tornado--but we don't know if Hinrichs intended this contrast.] Hinrichs published at least one scientific article on derechoes, but the phenomenon did not get widespread research attention until a century later. Eminent severe storms forecaster Bob Johns played a key role in reviving the concept in a landmark 1987 paper with William Hirt. It was only after reviewing a number of “northwest flow events” that Johns and Hirt discovered and adopted the earlier term. In a 2007 essay, Johns describes how the derecho concept originated and evolved. The current definition of “derecho” in the AMS Glossary is “a widespread convectively induced straight-line windstorm. Specifically, the term is defined as any family of downburst clusters produced by an extratropical mesoscale convective system.” In order to place an event as a derecho in the NWS Storm Data archive, wind damage and/or wind gusts of at least 50 knots (57 mph) must extend along a band at least 250 miles long, with at least three reports of gusts reaching 65 knots (74 mph) separated by at least 40 miles.

In the June 2016 issue of the Bulletin of the American Meteorological Society, SPC’s Stephen Corfidi and colleagues propose narrowing the definition to focus on progressive-type derechos, the ones that carve out a single, well-defined swath of high wind, as opposed to other thunderstorm-driven high wind events that tend to be scattered over larger areas. The latter could be called “squall-line windstorms,” they suggest. “Used in this way, “squall line” would realize a renaissance of sorts; the term also would come to be associated with a more distinct meteorological phenomenon than in years past.”

We’ll be following the severe weather this afternoon and evening in a WU liveblog.

Bob Henson