It’s Sunday morning here on the east coast of the United States. As I wake up 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 top 3 concerns for the next 48 hours about the storm.
The storm is getting stronger
We’re starting to get that “look” that we meteorologists are so concerned about. For most of the past week, Fiona has been in disorganized chaos with a low-level spin race before mid-level circulation and thunderstorms to its east. The Storm now looks more vertically stacked and has an exhaust system on top of it. The radar loop above clearly indicates the eye and the eye wall. As such, the National Hurricane Center wrote on Sunday morning, “….Very warm sea surface temperatures and sufficient mid-level humidity are still expected to allow for some intensification as Fiona approaches Puerto Rico and the Dominican Republic….is expected.” NHC expects Fiona to become a hurricane later today before it moves near or over Puerto Rico.”
a lot of rain
I mourned this for years. Precipitation is often ‘underrated’ as a hazard in these types of storms. What do you mean? For many people, a “tropical storm,” or Category 1 hurricane, evokes, “Oh, it’s just ______.” My own scientific studies have shown that the amount of rain and flooding (continuous or rapid) can be disastrous with storms on the “weaker” end of the spectrum. We published a study in Geophysical Research Letters (2007) entitled “Determining the Contribution of Tropical Cyclones to Heavy Precipitation Along the Southeast Coast of the United States.” The study revealed for the first time a metric called “wet rainy day” to describe heavily rainy days. It is simply a tool that compares the sites’ daily precipitation to one of the wettest places on Earth, Mount Wayal, Hawaii. We found that weaker tropical storms and hurricanes (such as Fiona) produce the wettest millimeter days in the mainland United States in a given year. Most of the flooding associated with Harvey (2017) was after it was downgraded from a hurricane as you may remember.
The National Weather Service in San Juan has already sounded the alarm for flooding. The warning tweeted Sunday morning, “8 AM AST September 18: Downstream flow increased along the Rio Blanco. Communities along this river, especially along flood-prone areas, should remain vigilant or consider relocating to a higher location #PRwx” . Weather Channel hurricane expert Dr. Rick Knapp chirp, “The devastating floods in #Guadeloupe from #Fiona overnight remind us of what Erika did to Dominica in 2015, also as a tropical storm… and also highlight the need to prepare and avoid the most flood-prone areas of Puerto Rico and the Virgin Islands.” Dominican severe flooding on Monday. Going back to the radar loop, there is worrying evidence that heavy rain may fall for Puerto Rico in the next 24 hours.
San Juan, mountains and the power grid
Geography is my other concern. Much of Puerto Rico’s high terrain lies on the eastern side of the island, which is in the sweet spot of some of Fiona’s worst ruins. With up to 20 inches of rain expected from Fiona and the mountainous terrain, this is a recipe for landslides. Additionally, urban flooding is likely to be a problem in places like San Juan, which have a lot of impermeable surfaces (parking and roads) that speed runoff into nearby streams or rivers and impede infiltration into the soil (infiltration). Another challenge looming is the power grid. Reports of power outages are already starting to emerge and the storm has yet to reach Puerto Rico. It should be noted that infrastructure is still likely to be somewhat weak from the ongoing damage associated with Hurricane Maria.
This is not an event on the scale of Hurricane Maria, but Fiona still represents a significant threat to people in this area and should be monitored with appropriate care. Away from Puerto Rico and Hispaniola, our best model guidance and official National Weather Service forecast calls for getting out to see (with some extra reinforcement) but not before dangerously rounding off some island closures in the Bahamas. If you’re in those areas, keep paying attention to the forecast.