System aims to predict risky ‘superfog' conditions
Published: Sunday, July 29, 2012 at 6:06 p.m.
Last Modified: Sunday, July 29, 2012 at 6:06 p.m.
One meteorologist dubbed it superfog. Others, whiteout.
It's a phenomenon that happens when smoke from a smoldering fire combines with fog or other moisture in the air and makes a dense mixture that cuts visibility to a few feet. It typically occurs in the hours before dawn. It can have disastrous results for nearby roads.
Superfog seems unpredictable, covering roads minutes after they were completely clear. But U.S. Forest Service meteorologists developed a system. They gathered information from Florida highway crashes where smoke and fog were present to develop an index predicting those conditions. One meteorologist is now working on a more precise way to determine when superfog is likely to happen.
"From the motorist's perspective, an encounter with superfog is equivalent to a bridge collapse," said Gary Achtemeier, the forest service research meteorologist who came up with the superfog name and who's developing a screening model for it.
"If you're driving up the road and all of a sudden there's no bridge there, that's the time you have to get stopped. It's the same way with superfog," he added.
Florida Highway Patrol supervisors are supposed to be trained in the low visibility occurrence risk index, or LVORI, which was created to predict the probability of reduced visibility due to fog or smoke.
FHP Lt. John Gourley, who reopened Interstate 75 before a series of crashes happened on Jan. 29 amid smoke and fog, later told investigators he was unfamiliar with LVORI. Records show that he received the training in March 2008.
U.S. Forest Service meteorologist Lee Lavdas developed LVORI using highway patrol records from the late '70s to early '80s. He used 3,000 accident reports that mentioned the presence of fog, smoke or both. Rather than widespread fog that slowly blankets an area, he focused on the localized fog that suddenly obscures visibility. He based LVORI on relative humidity and the atmospheric dispersion index, which measures the atmosphere's ability to ventilate smoke.
Land managers now use the 10-point LVORI scale to determine whether controlled burns risk conditions that could shroud roadways. The higher the number, the greater the risk.
"If I burn tomorrow, am I going to smoke up a road tomorrow night? That is the main question in the land manager's mind," Achtemeier said. "LVORI was intended to answer that question."
A National Weather Service advisory sent at 3:19 p.m. on the day before the I-75 crashes predicted a maximum LVORI that night of 6 for Alachua, Baker, Bradford and Union counties. FHP guidelines say a 7 or higher poses the biggest risk.
The weather service, however, was unaware of the Paynes Prairie wildfire. The NWS might have issued an advisory for an area closer to Paynes Prairie had it been told, said Steve Letro, meteorologist in charge of the weather service's Jacksonville area.
FHP's policy for shift commanders includes a checklist for smoke and fog incidents that requires a spot forecast from the weather service to be obtained, which also didn't happen.
LVORI has its flaws. Letro said it provides an average over an area and is used for planning rather than on-the-spot decisions. It tends to overpredict fog during the summer and is more accurate during colder weather, Achtemeier said.
Since helping Lavdas on publishing a paper on the index, Achtemeier has further studied the conditions leading to dense fog and smoke.
While a prevailing theory was that particulates in the smoke cause the lack of visibility, he found that the water in smoke from a smoldering fire can also be an issue. Such warm, moist smoke can combine with the humid, cool air of the early morning hours to create superfog.
There doesn't even have to be fog present for it to happen, Achtemeier said.
"Essentially, the conditions for superfog are best met in those few hours before sunrise — which, of course, augments the disaster," he said.
The kind of conditions in the early morning I-75 crashes also happened in the deadly predawn crashes on I-4 in Polk County four years earlier, said James Brenner, fire management administrator for the Florida Forest Service. Rather than superfog, he prefers to simply refer to the phenomenon as a whiteout. "It's a complete lack of visibility … You just simply cannot see," he said.
Achtemeier developed a superfog screening model comprising 16 different measures of temperature and moisture from smoke. He's working on an index that he retroactively applied to the night of the I-75 crashes.
He found 75 percent of the measures predicted an hour's worth of superfog around the time the highway was reopened. It rose to 100 percent shortly after the crashes started.
Indexes alone won't prevent crashes, Achtemeier conceded. Observers on the ground must determine whether there's a fire, its proximity to roads, the topography of the area and whether wind is pushing smoke in that direction.
For drivers, he said, conditions on the ground change so quickly that there's little chance of defense. Motorists who avoided crashes when superfog clouded Interstate 4 in 2008 had to drive off the highway and up the embankment as far from the road as possible.
"That's for survival. This is how serious superfog is," he said.
Contact staff reporter Nathan Crabbe at 338-3176 or firstname.lastname@example.org.
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