Sunday, May 21, 2017

The History of 'High Risk' Severe Weather Outlooks

Jon Erdman
Published: May 18,2017


It's one of the most urgent warnings NOAA's Storm Prediction Center can give hours before a severe weather outbreak: the "high risk" severe weather outlook.
Only issued by the SPC when there's high confidence in a volatile setup of severe weather for any given day somewhere in the country, high risks catch the attention of meteorologists every time.
An example of a high-risk severe thunderstorm outlook (area shaded in pink) issued by NOAA's Storm Prediction Center for May 18, 2017.
(MORE: Tornado Central)
When the nation's best severe weather forecasters issue such a high risk, they're concerned about one of the following in the area, according to the SPC website:
  • A tornado outbreak featuring "numerous intense and long-tracked tornadoes."
  • "A long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage."
(MORE: What the SPC's Risk Categories Really Mean)
Documentation on past high-risk outlooks is most thorough for those issued this century, but there were documented cases in the 1980s and 1990s.

Only a Few Each Year

From 2000 through May 2017, an average of three to four days are covered by an SPC high risk each year.
Both 2010 and 2003 had the most in any year – six – while a few recent years didn't have a single high risk. From June 2014 until late January 2017, no high risks were issued, the longest stretch this century.
(INTERACTIVE: Experience the Formation of a Tornado)

Two Dangerous Months

April and May combined to account for nearly two-thirds of all high risk days since 2000, with 21 and 18, respectively. These months frequently have the right volatile mix of enough warm, humid air overlaid by a still-active spring jet stream.
There have been high risk outlooks issued in the fall and winter months, including a first-ever high risk in late January 2017.
Only August and September have not seen a single high risk outlook, usually thanks to a more inactive jet stream required to prime the atmosphere for large-scale tornado outbreaks or derechoes – large-scale convective wind-damage events.
That's not to say you can't get a rash of tornadoes with, say, a landfalling hurricane in those months, as we saw with Hurricane Ivan in 2004.
(MORE: The Future of Tornado Warnings)

Either in Plains, Midwest or South

This century, all high risks have covered either parts of the Midwest (Mississippi Valley, Ohio Valley, Great Lakes), Plains (Dakotas to Texas) or the South (Arkansas and Louisiana to Virginia and the Carolinas).
(MORE: Different Types of Tornadoes)
SPC High Risks By Region
2000 - May 2017

(Note: Some high risks covered multiple regions)
South/Gulf Coast27
However, one high risk was issued for parts of the Northeast in association with one of the most prolific derechoes of recent times – the May 30-31, 1998 Great Lakes and Northeast derecho.

A Solid Track Record

We examined each of the 59 high risk outlooks from 2000 through April 2017 to determine how many of those forecasts rang true to the dire nature of the outlooks.
There are various shades of gray in assessing the accuracy of these forecasts, somewhat analogous to grading the drafts of professional sports teams years after the fact.
Nearly three-quarters (44) of the high-risk forecasts were solid.
President George W. Bush aboard Marine One surveys tornado damage in Lafayette, Tennessee, on Feb. 8, 2008, following the Super Tuesday outbreak.
(Jim Watson/AFP/Getty Images)

These not only included obvious cases, like the late-April 2011 Superoutbreak, but also some tougher, more out-of-season forecasts, such as the November 2013 Midwest outbreak and the February 2008 Super Tuesday outbreak.
Only seven of these high-risk forecasts could be clearly categorized as busts, primarily from a lack of tornadoes in the high-risk zone.
As SPC forecasters explained in a discussion regarding the May 18, 2017 high risk, a number of factors, such as too many competing severe thunderstorms interfering with each other, can lead to a busted high risk.
The most recent example of this was on April 27, 2014, when a small part of Arkansas was highlighted in the afternoon, but relatively few tornadoes occurred in that zone.
(MORE: Why Rating Tornadoes in Rural Areas is Challenging)
Another eight cases weren't clearly either a hit or bust.
One example was April 5, 2017 in the Southeast. The high risk issued around midday certainly seemed to capture a long-track supercell spawning tornadoes in the southern half of Georgia. However, one could argue the density of reports of severe weather was higher in areas surrounding the high risk.
That case points out that while a high-risk area is typically denoted as "particularly dangerous" in the lexicon of meteorologists, you should still take a moderate, enhanced or even a slight risk of severe thunderstorms seriously.
You can check the Storm Prediction Center's outlooks here.

Not Panic, But Preparation

If your area is in a high risk, the government's most experienced severe weather forecasters are confident of either a tornado outbreak or widespread damaging thunderstorm winds, and you should pay particularly close attention to the weather situation that day or night.
Have a dependable method of getting severe weather watches and warnings, whether through local media, NOAA weather radio, social media or a smartphone or tablet app. If using an app, make sure your phone and app settings are such that you can be awakened from a sound sleep.
Know where to take shelter if a warning is issued. If you live in a mobile home, find out if your mobile home park has a safe shelter underground, or find somewhere else where you can take shelter ahead of time.
(MORE: Your Odds of Being Hit by a Tornado)
It is more likely in areas covered by a high risk that any tornado warnings will be for confirmed, large and destructive tornadoes, rather than false-alarms based on radar-indicated rotation alone. Planning ahead for these dangerous severe weather days can save your life.
Jonathan Erdman is a senior meteorologist at and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.
MORE: Tornadoes in History

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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