Only 10 miles separate Paradise Valley from Phoenix Sky Harbor International Airport. Despite their proximity, the amount of rainfall they received Thursday morning wasn’t even close: Paradise Valley — 2.52 inches; the airport — 0.01 inch.
QHow can there be such variations in rainfall between adjacent areas?
Summer thunderstorms in the Valley typically are small and short-lived, unlike the squalls that can rock the Midwest during tornado season. The explanation is upper-level winds; the Midwest has the jet stream and fronts to boost the power of storms. But Arizona lacks those winds, so storms here burst like “popcorn,” drop rain and fade fast.
Q Why aren’t forecasts of storms more specific — how much rain will fall where and at what time?
Weather prediction is an inexact science. Doug Green, science operations officer at the National Weather Service’s office in Phoenix, said it’s understood why thunderstorms start, but determining location, duration and intensity still remains a matter of educated guesses.
Q Is it possible to predict if a storm will pack dangerous lightning?
Meteorologists can engage in informed speculation. Radar and rainfall provide clues, Green said, as does a knowledge of the atmosphere. At the Kennedy Space Center in Florida, NASA is a pioneer in forecasting trends in electric field potential and the locations of clouds capable of supporting lightning.
Q Don’t summer thunderstorms usually strike the Valley in the afternoon and continue into the evening?
Yes, which is what makes Thursday morning’s tempest different. Meteorologists had predicted a chance of storms for late Wednesday, but nothing happened. Yet with an atmosphere still waterlogged, all that was needed for a storm was a trigger — and then came an upper-level disturbance forecasters had missed.