By:
Bob Henson
, 4:35PM,GMT on March 23,2015
Last fall the SPC risk categories underwent their first substantial revision in 33 years. The main change was to add two categories on either side of the “slight risk” designation.
Marginal: a stray severe storm is possible, but probabilities are too low to merit a slight risk. (This replaces the label “SEE TEXT”, which had been used to steer readers to the discussion accompanying each outlook for more details.)
Enhanced: activity may be more widespread than implied by a slight risk, but a moderate risk is not justified.
Each of the five categories used by SPC relates to a certain level of probability of various types of severe weather. These probabilities are issued by SPC forecasters in Norman, Oklahoma, based on a wide range of guidance from national- and storm-scale computer models and observations. Although the five risk categories aren’t used beyond Day 3, SPC does provide probabilities and discussions out to days 4 through 8, providing an important heads-up on threatening situations that show up consistently in long-range guidance. For example, the risk of a multi-day severe weather episode across the south central states was highlighted five days ahead of the devastating Super Outbreak that peaked on April 27, 2011.
Figure 1. Top: the Day 1 convective outlook issued by NOAA’s Storm Prediction Center at 1630 GMT on April 27, 2011. Bottom: how this outlook would have appeared in the newly expanded five-tier structure, with the categories “marginal” (MRGL) and “enhanced” (ENH) now included. Image credit: NOAA/SPC.
The pros and cons of risk terminology
For those with enough technical savvy, probabilities are very helpful, but other users need or prefer a more concise message. This is where the worded categories come in. It’s widely acknowledged that the terminology isn’t perfect--to an untrained ear, “slight” could sound dismissive, and “moderate” may not carry the gravity it deserves--but SPC opted to refine a familiar, widely trusted system rather than uproot it. As they explain: “The categorical words Slight, Moderate and High have been used by SPC for nearly 35 years and are generally understood by the weather risk communication community. Making measured changes, we believe, is more effective than a wholesale change.”
A separate concern with the risk categories is that the extent of severe storms doesn’t necessarily correspond with how strong they are. Sometimes you have enough widespread wind or hail to merit a moderate risk, but there are few tornadic thunderstorms and little major damage. Conversely, an isolated supercell might produce a destructive tornado even if it’s located in a slight-risk area. SPC attempts to capture these distinctions on Day 1 by breaking the probabilities for each threat into two categories: general severe weather and “significant” severe weather (see Figure 2 caption). The probabilities in each tier then determine which of the five risk categories is in effect, using the somewhat complicated table shown in Figure 2.
Figure 2. The risk categories (colored boxes) that correspond to various percentage probabilities of severe weather (left axis of figure) in the SPC Day 1 convective outlooks. Slightly different tables are used for Day 2 and Day 3 outlooks. When more than one category is valid in a given area, the highest category is used (e.g., an area with 45% probabilities of all three types of severe weather would be designated as high risk, but if the 45% probability only existed for hail and wind, the risk designation would be moderate). A severe thunderstorm produces hail at least 1” in diameter, wind gusts of at least 58 mph, and/or a tornado. “Significant” severe events include 2” diameter hail, wind gusts of at least 75 mph, and EF2 tornadoes. Image credit: NOAA/SPC.
How are they used?
“The convective outlooks are very important products to emergency managers,” says Kenneth Galluppi, associate director of the Advanced Technology Innovation Center at Arizona State University. Galluppi and colleagues have interviewed many hundreds of emergency managers over the years to analyze how they receive and communicate information. Weather is just one of the many threats an EM has to deal with, Galluppi notes, so there’s major pressure to look for clear break points that distinguish more-serious from less-serious weather days. His work suggests that that many EMs wait until a moderate risk is in effect. This is especially true in storm-prone parts of the country, where slight-risk days are commonplace in the spring and summer. The new “enhanced” category provides one way to alert EMs and others that a situation falling short of the moderate designation still deserves attention. However, it may take time for EMs and other users to adapt to the new system. In a few informal tests carried out at conferences, Galluppi found that EMs gravitated to the moderate-risk zones even when the new “enhanced” area was present. “In terms of triggering action, ‘moderate’ seems to be the flag,” says Galluppi.
How the SPC system began
The use of a multi-tiered labeling system for storm risk has deep roots. The first such system was in place not long after the 1952 establishment in Washington, D.C., of the Weather Bureau Severe Weather Unit, which eventually became the National Severe Storms Forecast Center in Kansas City, MO, and later moved to Norman as SPC. According to SPC forecaster Stephen Corfidi, the original outlooks included the following four categories:
Isolated thunderstorms (expected to cover less than 15% of the marked area)
Widely scattered thunderstorms (15 - 30%)
Scattered severe thunderstorms (31 – 45%)
Numerous severe thunderstorms (more than 45%)
By the mid-1970s, these outlooks were being featured on NBC’s “Today” show as part of NOAA’s SKYWARN program. This was one of the few ways in which the outlooks reached the general public in those pre-Internet, pre-Weather Channel days.
As far as Corfidi can tell, the “numerous” category was never used, but on the morning of the epic Super Outbreak of April 3, 1974, forecaster Roy Darrah placed a large section of the Ohio and Tennessee Valleys fell under a “scattered” risk. “ ‘Scattered’ was the default ‘high’ risk of that period,” Corfidi told me. “It was a pretty gutsy call. The forecast was very good, both spatially and temporally.” (See Figure 3). A total of 148 tornadoes occurred on April 3-4--the largest 24-hour total until the catastrophic Super Outbreak of April 2011--and more than 300 people were killed, a toll unmatched until the 2011 outbreak. Almost all of the U.S. tornadoes occurred within the “scattered” risk area. Bill Murray (Alabama Weather Blog) offers an evocative retrospective on that day’s outlooks.
Figure 3. Convective outlooks issued early on April 3, 1974, for the period from 1200 April 3 to 0000 GMT April 4 (left) and 0000 – 1200 GMT April 4 (right). The crosshatching on the left map shows watches already in effect. Image credit: Stephen Corfidi, NOAA/SPC.
By 1977, forecasters and researchers began to ponder whether the coverage-based labels such as “few” or “scattered” might leave users confused over whether the terms applied to severe storms or to thunderstorms in general. The result was a new system, adopted in 1980, with new labels and percentages that applied specifically to severe storms:
Low: 2-5% coverage of severe storms
Moderate: 6-10% coverage
High: greater than 10% coverage
(The category “low” was changed to “slight” in 1981.)
The first-ever “high risk” outlook, and the first tornado watch to bear the label “particularly dangerous situation,” were issued on April 2, 1982. Later that day, one of the nation’s worst tornado outbreaks on record produced 56 tornadoes from Texas to Illinois, killing 30 people and injuring more than 300. The toll could have been much higher were it not for the unusually strong early notice of the situation and the substantial lead time for warnings of some of the worst tornadoes. A Silver Medal from the U.S. Department of Commerce went to the seven meteorologists behind the successful forecast, including Bob Johns, whose longtime role at SPC is outlined in a fascinating open-access paper, “A Forecaster’s Story: Robert H. Johns,” available in the E-Journal of Severe Storms Meteorology.
Figure 4. The TOR:CON outlook issued on Feb. 8, 2015 (not today!). Image credit: The Weather Channel.
TOR:CON--another way to describe tornado risk
Regular viewers of The Weather Channel and users of weather.com will recognize the phrase TOR:CON (Tornado Condition Index). This frequently updated alert system was created by severe weather expert Dr. Greg Forbes and colleagues in 2009 to capture the risk of tornadoes on a simple 1-to-10 scale. A TOR:CON value of 3 corresponds to a 30% chance of a tornado within 50 miles of a given point, whereas the analogous SPC tornado probabilities apply within 25 miles.
“Since the area is four times larger for a 50-mile radius than for a 25-mile radius, the implied TOR:CON probabilities are generally higher than those of SPC,” says Forbes. He adds: “TOR:CON values reflect my own forecast and are not just mathematical translations of the SPC forecast.”
This week’s outlook
Severe storms with large hail and damaging winds are possible on Tuesday from eastern Oklahoma into Missouri, with the zone stretching on Wednesday from north Texas into southern Illinois. As of this morning (Monday), the odds of widespread severe weather or strong tornadoes appear low. SPC has slight-risk zones in effect for Tuesday and Wednesday. TOR:CON values reach a maximum of 3 on both days. The Weather Channel maintains a TOR:CON update page in addition to its severe weather tracking page.
Bob Henson
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