Tropical trouble is brewing off the South Carolina coastline as thunderstorms organize around a developing low pressure. The system could produce torrential rainfall for the East Coast, with particular focus over the Northeast.
Invest 98L, the low pressure system that the National Hurricane Center has given a 70% chance of becoming a tropical depression by Friday, only just crossed into the ocean Wednesday morning. With balmy sea-surface temperatures (SSTs) in the low-mid 80s°F (upper 20s°C) off the Carolina coastline, the developing low has plenty of fuel to intensify into a Tropical Depression or possibly, Tropical Storm. However, it will only have a brief window to reach Tropical Storm intensity. The low is expected to reach the Delmarva coast Thursday evening, where SSTs are in the mid-70s°F (mid 20s°C). These temperatures are too cool to sustain tropical cyclone growth.
If Invest 98L intensifies into a Tropical Cyclone it would become Tropical Storm Fay — the earliest “F” storm on record. Tropical Storm Eduord in the open Atlantic most recently broke the record for the earliest named “E” storm on July 6. Regardless of whether this system becomes Tropical Storm Fay or remains a Tropical Depression, the East Coast will be confronted with strong rip tides, gusty coastal winds, and heavy rainfall with particular focus over the Northeast.
The developing low is expected to hug the coast as it slowly increments northeastward through the weekend. The bulk of its precipitation is expected to remain to the north and east of the low until it loses tropical characteristics and begins getting absorbed by an upper-level system on Friday. As a result, only coastal areas of the Carolinas, Virginia, and Maryland should experience impacts until the low reaches coastal waters of the Northeast, by which point its precipitation field will have expanded north and northwest of the low’s center.
Heavy rain could advance into Atlantic City and Philadelphia as early as Friday morning, working its way northward into New York City and Long Island by Friday afternoon, and Boston and Cape Cod by early Friday night. Rain is expected to be heaviest near the coast, closest to the low’s center. Easterly winds wrapping around the low could also produce a secondary area of heavy rain over the Taconics, Berkshires, Adirondacks, and Green Mountains Friday night through Saturday.
The shield of organized showers and thunderstorms Friday and Saturday is expected to be limited to areas near the low’s center. But with help from the advancing upper-level system and an increasing low-level jet stream, scattered heavy showers and thunderstorms are expected to pop up across eastern Pennsylvania, Delaware, New Jersey, eastern New York, and New England further away from the low’s center. Record quantities of tropical moisture being drawn into the Northeast will make the downpours spectacularly efficient rainfall producers, having the potential to deluge afflicted areas with rainfall anywhere from 2″ to as much as 6″ over the course of six hours or less.
It is difficult to pinpoint the specific areas favored to be hit by the heaviest downpours but locations along/east of I-95 and eastern facing mountain slopes are most at risk. Rainfall of this caliber is not currently expected to be widespread, but flash flooding will be inevitable wherever it does occur. For some areas across southeast Pennsylvania, New Jersey, and Southeast New York recently doused with up to 6″ of rain from the previous Monday’s thunderstorms, it may not take much rain to provoke flash flooding.
While guidance remains uncertain on the exact placement of the storms center of circulation, and thus the heaviest rain-bands, it’s important to note that there is general agreement on the intensity of the rainfall. Our proprietary Flood Index, which calculates the likelihood and severity of flooding, is currently favoring southeastern Pennsylvania for highest flood impacts. That is likely a result of the hi-resolution NAM showing a track further west than some of the other deterministic and ensemble guidance, so the solution is far from set in stone. The takeaway here is that a small region will likely see high rainfall totals over a short-period of time, resulting in a flood index between 5 and 8 (out of 10). Historically this level of flooding means creeks/rivers reach moderate to major flood stage, urban flooding occurs, and flooding on roadways could lead to closures. We’ll be monitoring the potential impacts closely.
Gusty winds are not expected to be too intense, but could be high enough locally to result in some minor property damage (broken branches, trash-cans and umbrellas blown over). Furthermore there will likely be some scattered power outages, but as you can see from our Power Outage Index valid Friday evening, we are not expecting widespread outages for any location. The areas that will be most susceptible to power going out will be along the coastline from the Delmarva through New Jersey and southern New York. Heavier bands of rain will allow for some of the stronger winds to mix down towards the surface, so it’s possible even locations further inland see some impact from wind.
The tropical depression or its remnants should clear the Northeast Saturday afternoon or Saturday evening, but that will not be the end of the rain this weekend. The upper-level system absorbing the weakening tropical depression will still be present, generating additional showers and thunderstorms throughout the weekend with the help of two back-to-back cold fronts. Details regarding the passage of these fronts and the showers and thunderstorms they produce will ultimately depend on the evolution of the yet-to-form tropical depression. However, additional rainfall can be expected throughout the weekend, including over interior areas largely spared by the tropical system.