Correction (July 7, 2012: 7:36 p.m.): The name of the agency that issued the warning before Hurricane Katrina hit has been corrected. The original version of this story was incorrect on this point | Our corrections policy

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See a timeline of June 29 weather events

Eighteen minutes.

That's the least amount of time the National Weather Service wants to transpire between when a severe thunderstorm or tornado warning is issued and when the storm affects a warned location.

A severe thunderstorm warning was issued for Roanoke at 8:26 p.m. on June 29. At 9:08 p.m., an 81 mph wind gust rocked the Roanoke Regional Airport. Even allowing for a few earlier gusts topping the severe threshold of 58 mph, that's at least half an hour lead time.

But did everybody get the warning who should have, in a manner they could understand? That's the much larger question the weather service is grappling with, a question that Dave Wert, meteorologist in chief at the National Weather Service in Blacksburg, says extends beyond isobars and Doppler radar to sociology and psychology.

Despite the Blacksburg office exceeding all of the "internal performance metrics" for warnings during the damaging derecho, Wert takes seriously any comments floating around the blogosphere that the storms "hit without warning."

"It's one thing for the National Weather Service to say we got the warning out 49 minutes in advance, we did our job," Wert said. "But obviously some people didn't get it, or they wouldn't make that comment."

"It goes to the societal response to examining how people prepare for severe weather, how they receive and integrate information, and ultimately how they act or don't act on it — enhancing our technologies, methodology, approach and method of service to best address our nation's needs," Wert said.

The weather service continues to expand the means by which it gets information out, moving beyond just television and commercial radio as it did in years not so long past. The Internet, social media and smartphones have created new ways to transmit and receive information.

National Oceanic and Atmospheric Administration (NOAA) weather radio, with settings that can automatically turn on the radio when a warning is issued for a given locality, is still the method most pushed by the weather service.

Nationwide, there have been experiments with how to craft warnings that make people really take notice.

Forecasters in Missouri and Kansas tried out a new three-level tornado warning system this past spring, with enhanced wording about potential destruction and death. Research revealed that many people in Joplin, Mo., knew about the tornado warnings but didn't immediately act on them when 162 people were killed there amid the atomic bomb-like devastation of the May 22, 2011, tornado.

In 2005, the National Weather Service used apocalyptically worded warning text to get the attention of New Orleans residents before Hurricane Katrina.

The warnings for the derecho didn't break any new ground nationally, but were unusual for Southwest Virginia.

Typically, in Southwest Virginia, severe thunderstorm warnings are small polygon-shaped areas covering a part of a large county or maybe a city or two. They usually carry boilerplate wording about quarter-sized hail and 60 mph wind gusts being possible, near the lower limits of what is considered "severe" for a thunderstorm.

The warnings issued on June 29 were issued in large blocks covering multiple counties, and contained wording that warned of "destructive winds in excess of 80 mph."

"When you see a huge list of counties, it almost inevitably means it's a monster," Wert said.

"We had an unusually high indicator of wind in the statements, in excess of 80 mph. We don't normally do that. ... Never in my 10 years here am I aware of another time when we categorically blanketed a warning with 80 mph winds."

Forecasts earlier that day did not raise high probabilities of storms, let alone the historic wind blast that wrecked the region's power grid. But as the derecho took shape across Indiana and Ohio, precipitation probabilities were increased and the wording in forecasts was gradually intensified as it became more obvious the storms would reach Southwest Virginia.

The development of the derecho was not a total shock for forecasters. For days, the Blacksburg office's area forecast discussions had mentioned the potential for storm clusters to develop in the Upper Midwest or Ohio Valley and slide southeastward, around the big dome of hot air to the south, toward Virginia. But forecast models were imprecise in picking out the timing and location of such storm clusters.

Wert described the difference between the agency's "probabilistic" and "deterministic" forecasts. The area forecast discussions, read avidly online by weather enthusiasts and those with technical weather background, touch on all the possibilities of what can happen. They are considered "probabilistic" in weighing different plausible scenarios against one another.

It's an approach that I employ on the Weather Journal blog — which did mention the word "derecho" as a possibility as early as 9:50a.m. on June 29.

But the public forecasts — the ones most people click online or hear on NOAA weather radio — are deterministic, a best guess of what actually will happen, not a discussion of all that possibly can happen.

In time, public forecasts may become more probabilistic than deterministic, Wert said.

Derechos hit Southwest Virginia about once every four years on average, Wert said, but one the intensity of June 29 is more of a once-in-20-year event.

Twenty years ago, the warnings might not have been as timely.

"This was a highly unusual event that's not easily diagnosed because it's so extreme and unusual," Wert said. "Can we do better in the future? Yes, absolutely. In the past, we wouldn't have caught it till it was too late.

"We're doing better, and will do better in the future."

June 29 weather timeline

(Official National Weather Service text in all caps.)

>> 4:29 a.m.
The first hazardous weather outlooks issued by the National Weather Service in Blacksburg for June 29 carry no mention of severe storm potential for the day or night.The outlook including Roanoke focuses on heat.TEMPERATURES AROUND 100 DEGREES AND MODERATE HUMIDITY LEVELS WILL CREATE HEAT INDEX VALUES FROM 100 TO 105 THIS AFTERNOON.

>> 6 a.m.
Roanoke’s morning low temperature never gets below 84 degrees. If it stands through midnight, it would be Roanoke’s highest low temperature ever recorded, smashing the previous record of 80 set four times in the 1930s and ’40s.

>> 11:25 a.m.
A report of estimated 60 mph winds just southeast of Rochelle, Ill., west of Chicago is the first severe storm report of what would become a massive derecho through the Ohio Valley, central Appalachians and mid-Atlantic regions. Over the next half-hour, numerous trees and power lines are reported downed in nearby Kane County, Ill.

>> 11:50 a.m.
The first severe thunderstorm watch is issued by the Storm Prediction Center for the Chicago area and northern Indiana.

>> 2:54 p.m.
A 91 mph gust is clocked at Fort Wayne (Ind.) International Airport. This would be the highest officially recorded wind gust recorded during the derecho.

>> 3 p.m.
Roanoke reaches 100 degrees under clear skies.

>> 3:05 p.m.
A severe thunderstorm watch is issued for extreme southeastern Indiana and central and southern Ohio. The Storm Prediction Center’s watch discussion text mentions the potential for the storms, forming a “bow echo” on radar, to continue into the evening.
A DEVELOPING MCS/BOW ECHO ACROSS INDIANA IS EXPECTED TO MOVE ESEWD THROUGH THE AFTERNOON/EVENING AT ROUGHLY 50 KT. THE STORM ENVIRONMENT DOWNSTREAM FEATURES STRONG INSTABILITY AND SUFFICIENT DEEP-LAYER FLOW/SHEAR TO MAINTAIN AN ORGANIZED MCS…AND THERE WILL BE THE POTENTIAL FOR WIDESPREAD DAMAGING WINDS /SOME SIGNIFICANT/ WITH THIS BOWING SYSTEM THROUGH THIS EVENING.
(MCS stands for mesoscale convective system, which is a large cluster of thunderstorm cells operating as a single unit.)

>> 4:12 p.m.
A high temperature of 104 degrees is recorded at Roanoke Regional Airport — a record high temperature for the date, tied for the record high temperature for the month of June, and Roanoke’s highest temperature since 1983.

>> 4:25 p.m.
The hazardous weather outlook from the weather service office in Blacksburg is updated to include a mention of possible severe weather for the Roanoke and New River valleys.
ISOLATED TO SCATTERED STRONG TO SEVERE THUNDERSTORMS MAY BE POSSIBLE THIS EVENING INTO TONIGHT.

>> 5:03 p.m.
The Storm Prediction Center issues a “mesoscale discussion” that includes much of central and western Virginia, including the Roanoke and New River Valleys. The storm is described as a “derecho” for the first time.
SUMMARY…WIDESPREAD/LOCALLY SIGNIFICANT WIND DAMAGE WILL CONTINUE WITH THE ONGOING DERECHO.
REPORTS OF WIND DAMAGE AS WELL AS OBSERVED GUSTS IN EXCESS OF 80 MPH HAVE BEEN RECEIVED. WITH AN EXTREMELY UNSTABLE DOWNSTREAM AIRMASS WHERE SURFACE TEMPERATURES HAVE HEATED INTO THE LOW 100S IN MANY LOCATIONS…EXPECT THE DERECHO TO CONTINUE -- CROSSING THE OH VALLEY AND MOVING INTO THE APPALACHIANS OVER THE NEXT 1-2 HOURS. EXPECT THAT WIND DAMAGE POTENTIAL WILL CONTINUE INTO VA/MD THIS EVENING…GIVEN THE ORGANIZATION OF THE SYSTEM AND THE FAVORABLE THERMODYNAMIC ENVIRONMENT. THUS...NEW WW DOWNSTREAM OF WATCH 436 WILL BE ISSUED WHICH WILL EXTEND EWD TO THE CHESAPEAKE BAY.

>> 5:55 p.m.
Hazardous weather outlooks for the Roanoke and New River valleys are updated to include the following wording:
A FAST MOVING LINE OF STRONG TO SEVERE THUNDERSTORMS CONTAINING STRONG OR DAMAGING WINDS MAY IMPACT THE AREA BETWEEN SUNSET AND MIDNIGHT.

>> 6:35 p.m.
Severe thunderstorm watch No. 438 is issued for all of Virginia except the extreme southeast and southwest corners.
FAST MOVING AND DAMAGING DERECHO OVER OH HAS RESULTED IN WIDESPREAD DAMAGING WINDS. THIS SYSTEM WILL CONTINUE TO TRACK EAST-SOUTHEASTWARD ACROSS THE WATCH AREA THIS EVENING. STORMS MAY LOSE SOME INTENSITY AS THEY MOVE ACROSS THE APPALACHIANS…BUT MOIST AND VERY UNSTABLE AIR MASS OVER VA SUGGESTS THAT THREAT OF AT LEAST SCATTERED SEVERE WIND GUSTS WILL CONTINUE.

>> 7:49 p.m.
The National Weather Service in Blacksburg issues its first severe thunderstorm warning for Greenbrier, Mercer, Monroe and Summers counties in West Virginia.AT 746 PM EDT…DOPPLER RADAR INDICATED A LINE OF SEVERE THUNDERSTORMS CAPABLE OF PRODUCING DESTRUCTIVE WINDS IN EXCESS OF 80 MPH. … THIS STORM HAS A HISTORY OF PRODUCING WIDESPREAD WIND DAMAGE ACROSS CENTRAL WEST VIRGINIA.

>> 8:26 p.m.
A severe thunderstorm warning is issued until 9:30 p.m. for Alleghany, Bath, Botetourt, Craig, Giles, Montgomery, Pulaski and Roanoke counties, plus the cities of Roanoke, Salem, Covington and Radford.The warning notes the storms are CAPABLE OF PRODUCING DESTRUCTIVE WINDS IN EXCESS OF 80 MPH.

>> 8:47 p.m.
A severe thunderstorm warning is issued until 9:45 p.m. for Bland, Giles, Pulaski, Smyth, Tazewell and Wythe counties.
THIS STORM HAS THE POTENTIAL TO CAUSE SERIOUS INJURY AND SIGNIFICANT DAMAGE TO PROPERTY. MOVE TO A BASEMENT OR TO AN INTERIOR ROOM ON THE LOWEST FLOOR OF A STURDY SHELTER NOW. EVACUATE MOBILE HOMES AND MOVE INSIDE A STURDIER STRUCTURE. IF NONE IS AVAILABLE SEEK SHELTER IN A DITCH OR OTHER LOW SPOT AND COVER YOUR HEAD.

>> 8:51 p.m.
An update for the 8:26 p.m. warning that includes most of the Roanoke and New River valleys includes the following wording.
AT 847 PM EDT…NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A LINE OF SEVERE THUNDERSTORMS CAPABLE OF PRODUCING DESTRUCTIVE WINDS IN EXCESS OF 80 MPH. THE LINE WAS MOVING SOUTHEAST AT 55 MPH. THE MAIN WINDS ARE OCCURRING WELL AHEAD OF THE RAINFALL.

>> 8:57 p.m.
A severe thunderstorm warning is issued until 10 p.m. for Amherst, eastern Botetourt, northwestern Campbell, Bedford and Rockbridge counties, plus the cities of Buena Vista, Lexington and Lynchburg.
The warning notes the storms are CAPABLE OF PRODUCING DESTRUCTIVE WINDS IN EXCESS OF 70 MPH, and also states the line is moving southeast at 65 mph.
THIS STORM HAS A HISTORY OF PRODUCING WIDESPREAD WIND DAMAGE ACROSS WEST VIRGINIA AND WESTERN VIRGINIA. THIS IS AN EXTREMELY DANGEROUS SITUATION. SEEK SHELTER NOW INSIDE A STURDY STRUCTURE AND STAY AWAY FROM WINDOWS.

>> 9:02 p.m.
A severe thunderstorm warning is issued until 10 p.m. for southeastern city of Roanoke plus Carroll, Floyd, Franklin, southern Montgomery, southeastern Roanoke County and southeastern Pulaski County. The warning notes storms CAPABLE OF PRODUCING DAMAGING WINDS IN EXCESS OF 60 MPH. Storms are reported to be moving at 90 mph, a speed almost unheard of for storm motion in Southwest Virginia.

>> 9:08 p.m.
An 81 mph gust is recorded at Roanoke Regional Airport. Power flashes are visible as the winds blow trees into power lines and blow transformers. Numerous reports of wind damage have already been received from localities west of Roanoke.

>> 9:58 p.m.
The update for the warning issued southeast of Roanoke notes: THE LINE OF STORMS WHICH PROMPTED THE WARNING HAVE MOVED OUT OF THE AREA.

>> 11:15 p.m.
Roanoke’s temperature drops to 73 degrees in the storm-cooled air, supplanting the morning’s potential record-shattering 84 as the low for the date.

Weather Journal

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