Kevin Myatt's Weather Journal at roanoke.com

Storm chasers: Since May 14, Blacksburg's Dave Carroll trekked through tornado country. This week, he found exactly what he was looking for. Read his reports.

June 2, 2004

PROBABILITY OF UNDERSTANDING: 40 PERCENT?

We've heard it all our lives, but do we really stop to understand what it means? "There's a 40 percent chance of rain today." Does it mean:

A) That 40 percent of the area is going to get rain

B) That it will rain 40 percent of the time

C) There's a 40 percent chance it will be raining at any particular time during the day

D) None of the above

Probabability of precipitation, or "PoPs," as meteorologists sometimes call it in weatherspeak jargon, may be one of the most commonly misunderstood concepts in weather forecasting.

I found out recently while researching a reader's question about it that my own understanding of it was not accurate. I would have answered C. The correct answer is D. None of the three scenarios quite capture what it really means.

The probability of precipitation is the chance that it will rain at least .01 inch at a given location over the specified 12-hour period.

That 12-hour period roughly corresponds to between 4 a.m. and 4 p.m. as "today" and 4 p.m to 4 a.m as "tonight." Of course, the time period is shortened for "this afternoon" or some shorter update period.

So if there's a 40 percent chance of rain today in your forecast, the National Weather Service believes there is slightly less than even odds (50 percent) that you'll see .01 inch of rain or more on your doorstep. There's a slightly better chance it will not rain than it will rain.

Just a way of forecasters hedging their bets? Admittedly, yes. There's actually sort of a formula that the National Weather Service says forecasters use to figure out precipitation probability. It's this:

"Forecaster certainty that precipitation will form or move over the area multiplied by areal coverage of precipitation that is expected."

So, if there's an 80 percent certainty that rain will develop, but it's only expected to cover half the area, then 80 percent multiplied by that 50 percent gives you 40 percent.

The weather service places certain familiar words in forecasts to describe precipitation probabilities. If the probability is 20 percent or less, it's said there's a "slight chance." From 30 percent to 50 percent is just plain "chance." Raise the ante to 60 or 70 percent and you get the term "likely."

Beyond 80 percent, the weather service may not even insert the number into the forecast and you get something like: "Today: Rain." This is called "categorical PoPs." It is a declaration that it will rain or snow with virtual certainty. Sometimes, though, it doesn't.

Precipitation probabilities have to be taken with more and more grains of salt the farther out a forecast goes. A slight chance of snow 5 days out may be a winter storm warning or a sunny and dry forecast 1 day out. Declaring precipitation "likely" is rare beyond 48 hours because forecasting skill just doesn't allow that kind of certainty.

The time the precipitation lasts at a given location is not really a factor in the precipitation probability -- a 1-minute downpour of .01 and an all-day rain of more than an inch can both verify a 100 percent forecast. However, it's true that a 20 percent chance usually doesn't turn into a long-term rain event and a 90 percent chance isn't usually just showers.

The one question I still have: If there's a 40 percent chance of rain and it doesn't, does that mean the forecast is 60 percent wrong?

May 31, 2004

DON'T LOOK NOW, BUT IT'S HURRICANE SEASON

Atlantic hurricane season is under way.

It's never really clear cut when hurricane season begins, but atmospheric dynamics, sea temperatures and history have led the National Hurricane Center to designate June 1-Nov. 30 as such.

Sometimes, there's already been a named tropical storm before June 1. Sometimes, a tropical storm or hurricane will form in December. Other years, it seems that nothing gets going for months.

1992 was an example of such a slow-starting year. From June 1 to the last week of August, there was absolutely nothing of interest going on in the tropics. Then, finally, a hurricane formed. Perhaps you've heard of it: Andrew, the most destructive natural disaster in U.S. history in terms of monetary damage.

2003 was different entirely. It went into the record books as the first year on record when there were named tropical storms in the Atlantic both before and after the designated hurricane season.

While there were 16 named storms in 2003, there were only two landfalling U.S. hurricanes, neither of which would be considered a major hurricane. Isabel, however, was a large storm that affected a huge part of North Carolina and Virginia, becoming our state's most costly weather disaster on record. Most of the damage was in central and eastern Virginia, though.

Isabel briefly reached Category 5 status with winds topping 155 mph when it was still far out to sea, but weakened as she approached the North Carolina Outer Banks — thankfully. Even as a Category 2 with 100 mph winds, Isabel still literally ripped a hole in Hatteras Island.

Forecasts for this summer favor a busy tropical season. The National Hurricane Center is calling for 12-15 named storms in the Atlantic (including the Gulf of Mexico and Caribbean Sea) with 6-8 hurricanes and 2-4 major hurricanes (Category 3 or higher, or 111 mph winds or higher). Veteran hurricane forecast Dr. William Gray of Colorado State University, who practically invented the long-range tropical forecast and still scores amazing accuracy, is calling for 13 named storms, 7 hurricanes and 3 major hurricanes.

The major factor that could boost tropical activity in the Atlantic is a lack of an El Nino in the Pacific. When El Nino is firing, warm water tempeatures in the Pacific west of Mexico and Central America create excellent conditions for hurricane formation there, but seem to rob energy from the Atlantic. Atlantic water temperatures are running above average in many areas, another factor that seems to portend an active tropical season.

Here in western Virginia, hurricanes' biggest effects usually center on heavy rains that can cause flooding from runoff in our beautifully sculpted landscape. Perhaps counterintuitively, it would seem that hurricanes hitting the Gulf Coast often have a bigger impact on us than those hitting the Atlantic coast.

Hurricane Juan hit the Gulf Coast in 1985, then came inland and stalled along a front, causing Roanoke's massive flooding. As if Hurricane Camille's utter devastation of the Mississippi Gulf Coast weren't enough in 1969, her remnants spawned truly epic flooding in central Virginia, with Nelson County's record 27 inches (unofficially up to 36 inches) spawning geography-changing flooding and mudslides.

Could we have a true hurricane here in Roanoke? If an intense hurricane were to hit the coast of the Carolinas and then beeline rapidly for our area, it's possible that we could get hurricane-force winds of 75 mph out of such a storm. But it's not likely. Tropical storm force winds that can topple trees are very possible, though.

So for the next few months here in Weather Journal, we'll peer out to sea from time to time and see what might be churning out there.

.May 26, 2004

WHEN IT'S BOTH BAD AND BEAUTIFUL

"It is really a strange feeling to see something so beautiful, but so incredibly destructive."

These words Dave Carroll wrote after a close encounter with a tornado during a storm chase in Nebraska Saturday so perfectly encapsulate the dilemma posed to every true weather enthusiast.

To reconcile one's love of big weather with the inherent destruction and human tragedy they cause poses moral difficulties for anyone who follows weather. Let's face it: I'd have nothing to write about if every day were sunny and mild, if there were no snowstorms, thunderstorms, hail, flooding, tornadoes, hurricanes or heat waves.

One way of reconciling this dilemma is to point out that all weather has benefits, and all weather is deadly.

Snowstorms, for instance, replenish water tables. Go ask people in the Western states about this. Their ongoing drought is mostly the result of a lack of the big snowstorms in recent winters, the same snowstorms that strand travelers and lead to avalanches.

Thunderstorms and hurricanes remove latent heat energy from the atmosphere and convert it to kinetic energy in the form of wind, precipitation and lightning. Sure, the hurricane is destructive for whatever stretch of coastline it crashes into, but what would happen if there were no hurricanes and all that heat remained in the atmosphere indefinitely? I don't think we want to find out.

Lots of folks pine for sunny days, but if the combined toll of worldwide heat deaths and sun-related skin cancer could be totalled, it's quite possible that sunshine is the most deadly kind of weather.

But even when one considers that all weather is potentially deadly, it still makes one swallow hard when the tornado you're excited to witness also sweeps people's houses, livelihoods and possibly their lives away.

Carroll knows this firsthand. His gang of 10 chasers, nine of whom hail from Pulaski and Blacksburg, saw numbing scenes of destruction. In one case, they offered help to a couple who had ridden out the storm in the basement of a home that was half destroyed.

I've been there, too. As a news reporter, I once wrote a story about a group of teenagers struck by lightning as they attempted to wait out a thunderstorm during a canoe trip. A 17-year-old boy was killed; his girlfriend and four other teens were injured. Days later I sat on the swing with the boy's dad, a Church of Christ minister, as he pondered the broader meaning of life and loss. I shot baskets with the girl, a state most valuable player in basketball who eventually returned to lead a state championship team, as she came to grips with a bolt out of the blue.

Another time, I walked with a group of relatives at the scene of a home swept off its foundation by a tornado. A retired man and his wife had been killed there the day before. Amid the debris, I picked up a cast-iron model of a tractor with a plaque on it noting the man's retirement from John Deere. A memento of somebody not unlike your father or mine, taken by the tempest.

But these true tales of woe have done nothing to faze my interest in weather. Perhaps it is the both the beauty and the danger of it that attracts us. It is also true that the research and study done by weather enthusiasts, professional and amateur, has led to new knowledge and technology that save untold thousands of lives each year.

It's inevitable that every one of us will in some way be affected in a negative way by weather sooner or later. As the Bible states: "It rains on the just and on the unjust."

WEATHER JOURNAL