More rain is coming tonight, and where it lands matters more than how much falls. Tropical Wave (a stretch of unsettled air that moves west across the tropics) 36 reaches Costa Rica overnight and will push rainfall up while it passes. Because of that, here is how the day shapes up before it gets here. The Caribbean and northern regions stay cloudy, with on-and-off rain and showers. The heavier spells come early in the morning and again at night. Afternoon rain scatters and thins, though it may push into the northern and eastern edges of the Central Valley.
The Pacific side gets the better end of it, with skies from partly to mostly clear. Expect afternoon showers and thunderstorms in the Central and South Pacific and across parts of the Nicoya Peninsula. Then the wave arrives after dark and the rain picks up again. On its own, that is an ordinary forecast for late August. The problem is what it is falling on.
The ground across the Caribbean and northern regions has barely stopped absorbing water since July 20. Eight tropical waves have crossed the country since then. Between August 16 and 17, parts of the southern Caribbean and Turrialba took 360 millimetres of rain, about 14 inches, in 48 hours.
Soil that saturated stops soaking anything up and new rain runs straight off into rivers and creeks instead, which is why streams there rise faster now than the rainfall totals alone would suggest. It is also why slopes slide. Nine areas remain under a national emergency declaration because of that rainfall — Guatuso, Turrialba, SarapiquÃ, and Limón, PococÃ, Siquirres, Talamanca, Matina and Guácimo. Tonight’s rain lands on all of them.
There is a second reason to be careful this week, and it has nothing to do with weather. Yesterday’s earthquakes near Coronado shook loose material along the mountain route between San José and the Caribbean. Rock and soil came down on Route 32 around the 30-kilometre mark, near mile 19, closing the road for part of the morning. Roughly 80 aftershocks followed through the day.
Slopes that held through the last storm are weaker than they were a month ago. Rain on freshly disturbed ground is the combination that puts material on the road. So, if you are driving to or from the Caribbean coast this week, check conditions before you leave rather than after you are already in the mountains. Route 32 has closed repeatedly this month, sometimes with only minutes of notice, and it can reopen and close again the same day.
When it does close, there are other ways across. Route 10 runs between Siquirres and Turrialba. The Bajos de Chilamate–Vuelta de Kooper road offers a northern alternative. Vara Blanca is passable but has been restricted to light vehicles during past closures. All three take longer, and all three are subject to the same weather.
If you meet water across the road, turn around. This is the advice that gets ignored most and causes the most trouble. Moving water hides how deep it is and how fast it is going, and flooding regularly scours away the roadbed underneath, so a surface that looks intact may not be there at all.
The same applies on foot near swollen rivers and creeks, which can rise sharply with little warning when the ground upstream is already full. Costa Rica’s weather service updates its bulletins through the day, and the emergency commission publishes alert levels by region. Both are worth a look before you commit to a drive this week rather than after.
The wave will move on. The saturated ground it leaves behind will take considerably longer.
