Is That Rain Just a Shower or a Nasty Squall?
By Carolyn Shearlock
Not all rain is the same. A steady shower might just mean getting wet, but a squall is a sudden, intense burst of wind and rain that can change conditions fast. Winds can jump 20 knots or more, waves build quickly, and visibility may drop to near zero. Squalls don't always come with lightning, so they aren’t technically thunderstorms, but they can be just as dangerous.
The tricky part is figuring out what kind of rain might be coming. A 40% chance in the forecast could mean a light passing shower—or something much more intense. Radar isn’t much help. The colors show how much rain is falling, but not how strong the wind might get or whether a squall is buried in that cell.
Even the Rain/Thunder layer on Windy doesn’t give the full picture. It shows where rain is likely and may highlight lightning activity, but it won’t tell you how strong a storm could become. Forecast gusts aren’t much help either, since they only reflect wind from large-scale systems, not storm-driven bursts.
That’s where a lesser-known but useful tool comes in: the CAPE Index.
CAPE stands for Convective Available Potential Energy. It measures how unstable the atmosphere is, or how much energy is waiting to fuel a thunderstorm. The more energy available, the more likely it is that any rain could quickly turn into a wind and wave event.
Think of CAPE as a fuel gauge for the sky. A high CAPE value means conditions are primed. All it takes is a trigger—like a weak front or local heating—and that energy can be released as a strong storm.
Fortunately, you don’t have to memorize the numbers. When you view the CAPE layer on a weather map, the color scale does the work:
• Yellow to orange: Moderate instability. Gusty storms are possible.
• Red to deep purple: High instability. Squalls or strong thunderstorms are much more likely.
Even if the radar looks quiet, a patch of red or purple on the CAPE map should get your attention. That area might be calm now, but if a storm starts forming, it could ramp up fast.
If you use Windy (the red version from Windy.com), checking CAPE is easy. Open the “Clouds/Airports” section in the menu, then select “CAPE Index.” Windy lets you view both GFS and ECMWF forecast models. I like to compare them to see if they agree.
Use the time slider to look a few hours ahead. CAPE is most useful for short-term planning and can show whether trouble might be brewing—even if skies look fine.
For the clearest picture, I also check the Rain and Thunder layers alongside CAPE. Together, they show not just where storms might form, but how strong they could be.
CAPE isn’t the only thing I check before heading out, but it’s one of nine key weather factors I rely on. It’s a quick way to get a better read on whether those afternoon clouds might just bring rain—or bring trouble.
This article is based on material from Carolyn Shearlock’s new book, Weather Basics for Boaters: The Details That Matter. Available in The Boat Galley Store (products.theboatgalley.com/wx) and on Amazon (paperback and Kindle).