Puerto Rico Faces Potential Hurricane Fiona Soon

It is Sunday morning here on the East Coast of the United States. As I rise from my slumber, my morning “meteorological check-in” reveals that Tropical Storm Fiona is getting better organized. By the time you read this, it may already have been upgraded to a hurricane. Here are my 3 big concerns for the next 48 hours about the storm.

The storm is intensifying

It is starting the have that “look” that we meteorologists worry about. For much of the past week, Fiona has been a disorganized mess with the low-level circulation racing ahead of the mid-level circulation and thunderstorms to the east of it. The storm now appears more vertically-stacked and has an exhaust system above it. The radar loop above clearly indicates an eye and eyewall. As such, the National Hurricane Center wrote on Sunday morning, “….very warm sea-surface temperatures and sufficient mid-level moisture are still expected to allow for some intensification as Fiona approaches Puerto Rico and the Dominican Republic….NHC forecast calls for Fiona to become a hurricane later today before it moves near or over Puerto Rico.”

Rain, Lots of Rain

I have lamented about this for years. The rainfall is often “undervalued” as a risk in these types of storms. What do I mean? For many people, “tropical storm” or a Category 1 hurricane evokes, “Oh, it’s just a ______.” My own scholarly studies have shown that rainfall amount and flooding (sustained or flash) can be disastrous with storms on the “weaker” end of the spectrum. We published a study in Geophysical Research Letters (2007) entitled, “Quantifying the contribution of tropical cyclones to extreme rainfall along the coastal southeastern United States.” The study debuted a metric called a “wet millimeter day” to characterize extremely rainy days. It is simply a tool comparing daily rainfall at locations to one of the wettest places on Earth, Mt. Waialeale, Hawaii. We found that tropical storms and weaker hurricanes (like Fiona) produce the most wet millimeter days in the mainland U.S in a given year. Most of the flooding associated with Harvey (2017) was after it had been downgraded from a hurricane as you may recall.

The National Weather Service in San Juan is already sounding the alarm for flooding. It tweeted this warning Sunday morning, “8AM AST Sep 18: The streamflow has increased along Río Blanco. Communities along this river, especially along flood-prone areas, should remain vigilant or consider moving to a higher location #PRwx.” Weather Channel hurricane expert Dr. Rick Knabb tweeted, “Devastating flooding in #Guadeloupe from #Fiona overnight reminds us of what Erika did to Dominica in 2015, also as a tropical storm… and highlights even further the need to prepare and avoid the most flood-prone areas in Puerto Rico and Virgin Islands.” The Dominican Republic could also face dangerous flooding on Monday. Referring back to the radar loop, there is worrisome evidence for significant rainfall potential for Puerto Rico in the next 24 hours.

San Juan, the mountains, and the power grid

The geography is my other concern. Much of the elevated terrain in Puerto Rico is on the eastern side of the island, which in the sweet spot of some of Fiona’s worst impacts. With up to 20 inches of rainfall expected from Fiona and the mountainous terrain, this is a recipe for landslides. Additionally, urban flooding will likely be a problem in places like San Juan, which have lots of impervious surfaces (parking lots and roads) that speed up rainfall runoff into nearby streams or rivers and hinders seepage into the soil (infiltration). Another looming infrastructure challenge is the power grid. Reports of power outages are already starting to come in and the storm has not even reached Puerto Rico yet. It’s worth noting that the infrastructure is likely still somewhat vulnerable from sustained damage associated with Hurricane Maria.

This is not Hurricane Maria-scale event, but Fiona is still a significant threat to people in this region and should be monitored with appropriate concern. Beyond Puerto Rico and Hispaniola, our best model guidance and the official National Weather Service forecasts call for a turn out to see (with some additional strengthening) but not before skirting perilously close the some of the islands in the Bahamas. If you are in those regions, continue to pay attention to the forecasts.

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