You may have seen on the news that an onda tropical is approaching Costa Rica. What is that anyway?
Without getting too technical an onda tropical, or tropical wave in English, is an area of unsettled weather that moves across the Atlantic Ocean from east to west.
During Costa Rica’s rainy season, tropical waves can bring heavier rainfall and thunderstorms to the country. These waves can range in intensity from a few isolated thunderstorms to major downpours with localized flooding.
There are around 60 tropical waves tracked per year in the Atlantic Ocean between May and November. On average, about 40 of those impact Costa Rica.
Tropical waves form as a disturbance off Africa’s west coast. Usually they are associated with a trough (elongated area) of relatively low pressure which then starts collecting moisture through upward air movements.
As tropical waves travel westward, they sometimes develop rotation and can even evolve into tropical cyclones (or hurricanes) when conditions are right.
When this happens, the systems usually curve northward away from Costa Rica due to the Coriolis effect and prevailing steering currents in the atmosphere. This is why Costa Rica rarely experiences direct hits from hurricanes, though it can receive heavy rainfall from their outer bands.
If the pattern doesn’t have a significant rotation or the circulation is weaker (among other factors), the waves either break up or keep moving west along the equator, eventually passing over Central America and Costa Rica. Costa Rica sits at about 10 degrees north of the equator.
While it sounds scary, a tropical wave is much like a weather pattern where you are from. It’s like a front moving through that may bring some showers or thunderstorms. And just like those events, they can really vary in severity.
When a tropical wave hits Costa Rica, the impact usually lasts from 6-12 hours. It can mean rain but it might not impact the entire country. Just like those storms where you are from, one town or region may get hit while another is spared.
For example, sometimes the central mountains “soak up” the rain and Costa Rica’s west coast stays relatively dry. Or often the tropical wave’s impact will be mostly at night.
That being said it’s not uncommon for a tropical wave to dump 1-3 inches (25-75mm) of rain in a day. The most intense events can even pour 4-5 inches (100-125mm).
While that may sound like a lot, it’s just a drop in the bucket for the tropics. The sandy tropical soils can percolate that rain in no time. The area of flooding in the photo above, for example, was back to normal the next day. In fact, some parts of Costa Rica see more than 12 feet (3.7 meters) of rain per year. That’s what makes the rainforest such a lush environment after all!
Tropical waves are a common weather pattern for Costa Rica. These systems do bring rain but luckily aren’t usually associated with damaging winds like cyclones or hurricanes. As with any storm, use extra caution, be prepared, and stay updated (we can help with that).
So grab an umbrella, some rubber boots, and enjoy the (tropical) waves!
