After slamming into Louisiana with 150 mph winds, Ida has weakened below tropical depression status. But even though the winds are gone, the threat for significant flash flooding remains as the system pushes northeast. In fact, that threat will only grow larger as the tropical moisture merges with a jet stream diving south from the Great Lakes.
The expectation for widespread 4 to 8 inch rainfalls across some parts of the Mid-Atlantic and Northeast has prompted the National Oceanic and Atmospheric Administration’s Weather Prediction Center to issue a rare “high risk” for torrential rainfall, warning that this could be a 1-in-100 year event for some areas.
On Tuesday night, the system was located over the eastern Tennessee Valley and Central Appalachians. The rain was falling moderately, but had yet to be supercharged by the impending jet stream. That will happen on Wednesday and Wednesday night.
Weather models are predicting 4 to 6 inches of rain over a widespread area, with isolated spots picking up over 8 inches. The heaviest rain looks to fall from the hills of northern West Virginia and Maryland, northeastward through Pennsylvania, northern New Jersey, southeastern New York and southern New England. This includes cities like Cumberland, Maryland, Harrisburg and Allentown, Pennsylvania, New York City and Hartford, Connecticut.
NOAA mentioned in their discussion that for some spots, this may amount to rainfall so heavy that it would only be expect once every 100 years, thus explaining the need for a high risk outlook. A small number of days every year are designated as high risk, but 90% of all flood damage happens on these extreme days.
At times, the rain will fall at 2 inches per hour, overwhelming the ground’s ability to absorb the water. As a result, the excess will run off down hills and into swelling streams and rivers. Making matters worse, plenty of heavy rain has already fallen recently thanks to tropical storms Fred and Henri. Many areas have seen 3 to 5 times their normal rainfall over the past two weeks.
Pockets of flash flooding are almost guaranteed. Areas that typically flood in this zone can expect to experience streams overflowing, streets becoming impassable and water in some homes.
To the south of the heaviest rain, the combination of leftover spin from the circulation of Ida, the upper level jet stream and very warm, unstable air at the surface will cause severe storms with some tornadoes. The region most likely to be threatened by tornadoes includes the Philadelphia and Washington, D.C., areas later Wednesday into Wednesday evening.
The heaviest rain will fall Wednesday evening into Thursday morning from New York to Boston before exiting New England later Thursday morning.
While it’s not uncommon for the remains of tropical systems to dump flooding rainfall, heavier rain events have become more common since 1960, especially in the Northeast. This is because warming waters along the Northeast coast are supercharging storms and warmer air is able to hold more moisture — both symptoms of human-caused climate change. The frequency of the heaviest rain events has increased by 50% in the Northeast in that time.
But the impacts of climate change on Ida are not limited to heavy rainfall. A warmer climate is also increasing the pace of intensification and intensity of hurricanes — both of which were on full display with Ida.
While Ida would have still been a major hurricane without climate change, it is likely that the already warm waters of the Gulf of Mexico were made hotter because of it. Since 1900, tropical Atlantic ocean temperatures have increased by 2 degrees Fahrenheit.
Ida strengthened by 65 mph in 24 hours — almost double the definition of rapid intensification (RI). RI makes storms like Ida even more dangerous because they leave less time to prepare.
One study found that, over the past 4 decades, RI has increased by 4.4 mph per decade. That means if a storm was able to increase its intensity by 40 mph in a day in 1980, it now has the capacity to increase intensity by 55-60 mph in 24 hours. While that study’s authors said the increase was most likely the result of a natural cycle of warming and cooling water in the Atlantic, a 2019 study found that climate change is also to blame.
In addition, hurricanes are becoming stronger around the globe and especially in the Atlantic Ocean. There has been a steady trend in the past few decades of a greater proportion of major hurricanes — category 3, 4 and 5’s. That’s because for every 2 degree increase in water temperature, the intensity of a hurricane can increase about 20 mph.
In the Atlantic, run-of-the-mill hurricanes now have twice the chance of making the jump to major hurricanes than they did just a few decades ago. This is due to warming waters, not only from heat trapping greenhouse gases, but also the lessening of air pollution across the Atlantic, which just a few decades ago would block more of the Sun and dampen the ability of tropical cyclones to intensify.
These intensity changes make a huge difference because damage does not trend upwards linearly with increases in wind speed. Instead, the damage potential goes up exponentially as the winds strengthen. So, comparing a storm with winds of 75 mph to one with 150 mph winds means the damage does not double. Perhaps shockingly, the damage potential goes up 250 times.
Even though major hurricanes make up a smaller proportion of tropical storms than weaker ones, major hurricanes produce the vast majority of damage — around 85% according to NOAA.
At the halfway point of the 2021 hurricane season, we are already way ahead of normal pace. If the 2020 record-breaking season is any guide, the second half of the 2021 hurricane season will be active, with the potential for more intense storms.