Tenacious Tropical Storm Elsa, retaining its identity as it traversed the Southeast U.S. on Thursday, was on track to hug the Mid-Atlantic and Northeast coastlines as it accelerates into Friday, dropping torrential rains along much of the Interstate 95 corridor. The New York City metroplex, in particular, faced the potential for very heavy rain from late Thursday night into Friday. Tropical storm warnings were in place at midday Thursday along the coast from North Carolina to Massachusetts, and flash flood watches extended well inland as far north as southern Maine.

At 2 p.m. EDT Thursday, the National Hurricane Center placed the center of Elsa about 25 miles southwest of Raleigh, North Carolina. Top sustained winds were 45 mph, and Elsa was heading northeast at about 20 mph. A shield of heavy rain extended mainly north and east of Elsa’s center across North Carolina and southeast Virginia. Widespread 24-hour totals of 5 to 7 inches were reported on Thursday morning by CoCoRaHS stations across coastal South Carolina, with a maximum of 7.15 inches near Beaufort. Heavy rains also fell along Elsa’s southern flank in northern Florida, with a total of 7.15 inches near Bronson. (By far the standout rains of the period were not associated with Elsa; instead, they fell in south Texas, where the Rockport area saw widespread flooding and more than 10 inches of rain in association with a stalled upper-level low. More heavy rain is expected along and near the central Texas coast into Friday.)

Most of Elsa’s tropical-storm winds at midday Thursday were in strong bands of thunderstorms near and just off the coast of North Carolina and Virginia. Minimal tropical storm-force winds, sustained at 40 mph with gusts to 58 mph, were reported from a buoy near Sunset Beach, North Carolina.

Forecast for Elsa

A strong steering current at upper levels will take Elsa on a straightforward, high-confidence northeast track near the coastline of the northeast U.S. through Friday and southeast Canada into Saturday.

Once Elsa gets past the Outer Banks of North Carolina, sea surface temperatures along its path will drop below the threshold of 26 degrees Celsius (79 degrees Fahrenheit) typically needed to sustain a tropical cyclone. Even so, it will take a few hours for Elsa to transition into a post-tropical storm, so it is possible that Elsa will make another U.S. landfall or two as a tropical storm, perhaps over the Delmarva Peninsula of Maryland and Virginia and/or over Long Island, New York. In NOAA’s hurricane database going back to 1851, only a handful of tropical storms (and no hurricanes) have made landfall during July from New York City across Long Island and southern New England.

The main threat from Elsa’s parting shot at the U.S. is a corridor of very heavy rain over or near the heavily populated I-95 corridor. High-resolution models suggested on Thursday that a small pocket of particularly heavy rain in the 4-6 inch range could develop over or just inland from New York City, most of it falling between midnight Thursday night and noon Friday. In southern and eastern New England, the heaviest rains should occur during the day on Friday, tapering off by evening.

Elsa’s rapid motion will help limit total rainfall, but some historic totals are still possible. In records at New York’s Central Park going back to 1869, the heaviest calendar-day rainfall for any July day is surprisingly low: just 3.75 inches on July 24, 1997. At Newark, the July record is 3.54 inches on the same date, and at Kennedy Airport, the record is 3.51 inches on July 31, 1996. Output from the 12Z Thursday run of the HRRR regional model implies that these numbers could be challenged if not exceeded on Friday if the projected pocket of heaviest rain happens to fall across the metro area.

Excellent forecasts were made for Elsa

The National Hurricane Center and several of the top hurricane models made excellent forecasts for Hurricane Elsa. Using a graph provided by Brian Tang from the SUNY Albany, the approximate track errors for Elsa for forecasts out to 24, 48, 72, 96, and 120 hours were approximately 40, 45, 55, 60, and 95 nautical miles, respectively. The National Hurricane Center’s five-year average errors for all Atlantic tropical cyclones have been 37, 50, 65, 96, and 133 nautical miles, respectively. Three computer models – the UKMET, GFS, and HWRF – made superb forecasts at all time periods. The European model, usually a top performer, lagged considerably in accuracy.

Elsa is likely a harbinger of an above-average Atlantic hurricane season

An above-average Atlantic hurricane season is likely for the remainder of 2021, the Colorado State University hurricane forecasting team says in its latest seasonal forecast issued July 8.

Led by Dr. Phil Klotzbach, with coauthors Dr. Michael Bell and Jhordanne Jones, the Colorado team is calling for an Atlantic hurricane season with 20 named storms, 9 hurricanes, 4 major hurricanes, and an accumulated cyclone energy of 160. In comparison, the long-term averages for the period 1991-2020 were 14.4 named storms, 7.2 hurricanes, 3.2 major hurricanes, and an accumulated cyclone energy of 123. 

The numbers in the July 8 outlook are an increase from their previous June 3 forecast, which predicted 18 named storms, 8 hurricanes, 4 major hurricanes, and an accumulated cyclone energy of 150. The authors cited several factors for their July 8 forecast of high activity:

• Early-season Atlantic hurricane activity (like was observed in 2021 before Hurricane Elsa arrived) usually has very little correlation with overall Atlantic hurricane activity. However, when an early-season storm occurs in the tropics (south of 23.5°N), that is typically a harbinger of a very active season. Hurricane Elsa formed in the tropical Atlantic and then tracked into the eastern Caribbean (10-20°N, 75-60°W) at hurricane strength. Since 1900, only six other years have had eastern Caribbean hurricanes prior to August 1: 1926, 1933, 1961, 1996, 2005, and 2020. All six of those years were classified as hyperactive Atlantic hurricane seasons using the NOAA Atlantic hurricane season definition (>=160 accumulated cyclone energy). 

• Sea surface temperatures in the Caribbean and subtropical Atlantic are warmer than normal, but near-average in the rest of the tropical Atlantic. When this pattern is observed in July, active hurricane seasons usually follow.

• The African monsoon has been very active so far this year, spawning an above-average number of strong tropical waves.

• The odds of an El Niño event this fall are low. In fact, in NOAA’s just-released July 8 monthly discussion of the state of the El Niño/Southern Oscillation, or ENSO, the agency issued a La Niña Watch, which means it sees La Niña likely emerging (~55%) during the September-November period and lasting through winter. El Niño events tend to create strong upper-level winds over the Atlantic, which creates high wind shear that interferes with development of tropical cyclones. La Niña events, on the other hand, tend to have upper-level winds more favorable for development of Atlantic tropical cyclones.

The Colorado State University outlook predicts the odds of a major hurricane hitting the U.S. to be 68% this year (the average yearly odds for 1991-2020 were 52%). It gives a 43% chance for a major hurricane to hit the East Coast or Florida Peninsula (long-term average: 31%), and a 43% chance for the Gulf Coast (long-term average: 30%). The Caribbean is forecast to have a 57% chance of having at least one major hurricane pass through (long-term average: 42%).

The Colorado team’s forecast uses a statistical model honed from 39 years of past Atlantic hurricane statistics, plus output from the ECMWF (European) model and UK Met Office GloSea6 model to augment the statistical technique. Their next forecast will be issued August 5.

The Atlantic looks to be quiet after Elsa

Although the Colorado team may be forecasting an active remainder of hurricane season, it appears that the next 10 days will be on the quiet side. The latest long-range runs of the operational GFS and European models and their ensembles are showing little activity in the Atlantic through mid-July. This is likely partially due to sinking air over the Atlantic expected to occur due to the passage of the suppressed part of an atmospheric disturbance called a convectively-coupled Kelvin wave (CCKW, see Tweet by Eric Webb).

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