Any pilot who’s thinking of flying more than a few miles from his or her home airport between March and September should look at the Day 1 Convective Outlook from the National Weather Service’s Storm Prediction Center.
The center, in Norman, Oklahoma, is the NWS’s source of outlooks and forecasts for convective weather. Meteorologists use “convection” to describe thunderstorms and the violent weather they sometimes create, such as tornadoes, squall lines, and mesoscale convective systems—which are large, long-lasting, organized collections of thunderstorms.
“Day one” refers to the day you look at the web page. The center also has outlooks for days two and three—if you’ re planning to fly tomorrow or the day after tomorrow, you should take a look at these to help evaluate your plans. And since the exact locations of thunderstorms can’t be forecast in detail until the storms begin forming, you should check to see whether they are possible across or near where you plan to fly. A 15-percent probability for severe thunderstorms means you have a 15-percent chance of such a storm occurring within 25 miles of your location. In general, your chances of getting hit by severe weather are small. Let’s say you have a 1 percent statistical history of severe thunderstorms within 25 miles on this day. A 15-percent probability means 15 times the normal odds of a severe thunderstorm occurring nearby. Fifteen times the normal odds mean a one in 6.66 chance of a severe thunderstorm.
SECURE YOUR AIRPLANE. A storm that occurred on June 29 and 30, 2012, brought the term derecho to the attention of people along its 240-mile path. The derecho blasted from the Midwest to the Mid-Atlantic coast, killing 27 people and leaving millions without power; thousands had to cope with damage from fallen trees.
The NWS defines a derecho as a squall line that produces 57 mph or stronger winds along a path at least 240 miles long over a period of hours.
Almost all derechos hit east of the Rocky Mountains, with a focus around the area where Kansas, Oklahoma, Missouri, and Arkansas meet. This area averages four derechos every three years. The main derecho season is from May through August, but they can occur just about any time of the year. During the summer, derechos are most likely in the Midwest. During the rest of the year, they most
often target the lower Mississippi Valley.
Some of the strongest and longest-lasing derechos, including the June 2012 event, occur on the fringes of major heat waves “It turns out that the meteorological conditions favorable for large-scale heat waves often also are conducive to derechos. In the United States, this is especially true from the upper Mississippi Valley and upper Great Lakes into the Ohio Valley and Northeast,” the Storm Prediction Center said.
HOW DERECHOS OCCUR. Bob Henson, in the University Center for Atmospheric Research (UCAR) newsletter, described how a derecho differs from a hurricane or tornado: “While the high winds of a
tornado or hurricane spin around powerful, circular updrafts, a derecho’s wind consists of rain-cooled air that descends and plows into very warm, unstable air. Most such downbursts only span a few miles and last a few minutes, but sometimes the atmosphere is primed for this process to intensify in a repetitive fashion. In that case, the winds generate new thunderstorm updrafts as they push forward, and in turn, this creates more rain-fueled downdrafts. If there’s a brisk jet stream adding momentum to the successive downdrafts, then a derecho can race forward at interstate speeds along a track that’s almost bullet-straight, often traversing several states in a single day or night.”
Henson also said, “When the SPC issued its first Day 1 Outlook at 2 a.m., the chances of high winds in Maryland and Virginia were less than 5 percent. As the day went on, however, new observations and predictions from computer models based on these were indicating that a derecho was brewing. By 2:30 p.m. the derecho was going strong in the Midwest with 91-mph gusts in Indiana. The SPC raised the risk of high winds in the Washington, D.C., area; local NWS offices and news media forecasters begin issuing alerts.”
