Coastal Flooding: A really big deal and a really big challenge.
If you’ve gotten this far, I’m guessing you’re one of two types: either you’re the bottom-line, cut-to-the-chase type or the detail-oriented, I-want-to-know-everything type.
Before I lose my bottom-line, cut-to-the-chase types, here’s your takeaway:
I’m expecting a moderate to major coastal flood event for parts of the eastern seaboard from the Delmarva through coastal Connecticut this weekend (including coastal New York) during the morning and evening high tides. This means the possibility of extensive flooding of low lying homes and businesses near the coast and flood-prone roadways that could require localized evacuations. Additionally, substantial beach erosion is possible along exposed coastlines. Although this is the type of flooding you might have a 5–10% chance of experiencing in any given year, it’s not the rare catastrophic flooding wrought by Superstorm Sandy or the powerful nor’easter of 1992. We’re still 3–4 days away from the main event, which means a lot can still change so check back in the coming days.
And now for my detail-oriented friends…
Forecasting storm surge beyond three days is laced with uncertainties. It’s like trying to make a 7-day weather forecast. We might get the flavor right (cold and wet in the east), but the details and timing will likely be off. So take any specifics three days out with a grain of salt.
That said, if you hunt around online you’ll find several National Weather Service websites with storm surge forecasts out to a week or more. So what gives?
What you’re seeing is actually guidance from an individual storm surge modeling system rather than a National Weather Service forecast. It’s an important distinction since this is just one piece of information forecasters use to help make a forecast. But like our weather models, each of these storm surge models has its strengths and limitations. Let’s quickly review two of the more widely available models used in wintertime storm surge forecasts.
Extratropical Surge and Tide Operational Forecast System (ESTOFS)
Let’s first define storm surge. Storm surge is seawater flooding produced by the strong onshore winds of a coastal storm. It’s usually attributed to hurricanes but occasionally a powerful winter storm can bring significant coastal flooding. So don’t get caught up with the whole tropical/extratropical thing. Extratropical simply refers to a low pressure system that isn’t tropical (so not a tropical storm or hurricane).
The Extratropical Surge and Tide Operational Forecast System (otherwise known as ESTOFS) is a storm surge modeling system run by NOAA, the parent organization of the National Weather Service. It uses the Advanced Circulation (ADCIRC) storm surge model, which is widely regarded as one of the premiere storm surge models out there. So at first glance this seems like the go-to modeling system for forecasters because it has the most up-to-date physics.
But here’s the rub. With large winter storms the model is having to cover a large stretch of coastline and more area means more time to crunch the numbers. To keep our number-crunching to hours rather than days or weeks (we need to use it to make a forecast after all), we sacrifice some details in the model. So small rivers and coastal water features might get missed. For example, in the weekend forecast above, the highest storm surge is tucked away in western Long Island Sound. This is a known problem with ESTOFS since the East River isn’t included, so with east-to-west blowing winds storm surge tends to get trapped in the western reaches of the sound and is artificially high.
Another gotcha with ESTOFS is that it doesn’t actually model flooding of land areas. The forecast that you’re seeing is only valid over open water, so buyer beware when extrapolating that guidance over land. Oh and the system doesn’t consider the additional water rise from waves which can add substantially to coastal flooding in the northeast.
Extra-Tropical Storm Surge Model (ET-SURGE or ETSS)
The other widely available storm surge guidance for wintertime storms is the National Weather Service Extra-Tropical Storm Surge model, otherwise known as ET-SURGE or ETSS.
It uses the Sea, Lakes, and Overland Surges from Hurricanes (SLOSH) model, the same model we use to forecast hurricane storm surges. SLOSH has a simpler physics scheme than ADCIRC, but because the physics are less complicated we can include more detail in the ocean and still crunch the numbers quickly. This is key for operational storm surge forecasters. Smaller water bodies like the East River are included and the model considers surges over land so what you see is what you could get.
Like any forecast model, ET-SURGE isn’t without its limitations. It does consider tides but like ESTOFS, it doesn’t include waves.
What about the forecast?
I could go on about these models and a multitude of others ad naseum but regardless of which one you consider, the guidance is only as good as the meteorology going into the model. Subtle shifts in the path of the storm, magnitude of the storm, speed of the storm, etc., often have huge implications on the storm surge produced which is why a good forecast considers multiple scenarios (we look at thousands of scenarios to make a single hurricane storm surge forecast). And in the northeast, it’s all about when the surge comes in relative to the times of high tide, another something that is darn near impossible to forecast, even a few hours out. Then there are the myriad of issues users face when using inconsistent storm surge baselines (something called datums).
Ultimately, it’s easiest to consult an expert rather than try to WebMD or cobble together a coastal flood forecast. It’s far less scary and is the best option for keeping you and your family safe. Finally, don’t assume your run-of-the-mill meteorologist knows about storm surge forecasting. Consulting someone for a storm surge forecast with little understanding of these hydrodynamic models is like visiting an ophthalmologist for heart palpitations. Yes an ophthalmologist knows the basics of medicine, but I’d put my stock in a cardiologist.
In terms of Winter Storm Jonas, keep an eye and ear to the forecast as we fine tune things over the next few days. Your local National Weather Service forecast office has trained forecasters that will be issuing important coastal flood statements over the next few days. I’ll be working closely with our team here to update the coastal flood outlook both on-air and online. Finally, in the event that you are asked to evacuate, have a safe place that’s up and inland where you can take refuge from the rising seas.
Stay safe everyone. I’ll see you on-air.
