Elsa churning through the Caribbean Sea
By Skip Rigney
Last year’s Atlantic hurricane season set a record as the busiest in history. While it’s too early to know whether 2021 will make a run at breaking that record, this season has already put its mark in the weather history books.
Elsa, the fifth named tropical storm of 2021, formed Thursday, July 1st, over the central Atlantic Ocean. That’s the earliest date ever to have an “E” storm in the Atlantic, breaking the record set last year when Tropical Storm Eduordo developed on July 6, 2020.
Thirty-year statistics of the number of tropical cyclones only go back as far as the 1960s when the advent of weather satellites made it much easier to detect storms. Each decade since then, the 30-year average date for the fifth named storm has crept earlier and earlier. For 1961-1990, it was early September. For 1991-2020, it was August 23rd. Elsa will help nudge the average even earlier when the next set of statistics is computed.
On Friday morning, Elsa’s maximum winds reached 75 miles per hour, making it the first hurricane of 2021. That’s over a month earlier than the 1991-2020 average date of the first hurricane on August 14.
Where will Elsa go? The two best medium-range computer weather models, the U.S. National Weather Service’s Global Forecast System (GFS) and the model run by the European Center for Medium-Range Weather Forecasting (ECMWF), are very consistent in their predictions through Sunday morning with both models predicting that Elsa will continue racing west-northwest through the Caribbean Sea.
Beginning Sunday, the forecast gets murkier as Elsa slows down. A strong trough of low pressure will move off the east coast of the United States into the North Atlantic. If the trough is strong enough and digs far enough south, Elsa will feel its tug, turn northward, pass east of Florida and out to sea over the Atlantic.
As I write this on Friday morning, that early northward turn is the scenario favored by several of the ensemble members of the ECMWF model. Ensemble members are the outputs from a single weather model that is run multiple times with slightly different initial atmospheric conditions. That’s done to account for the fact that there are always errors in our observations of the atmosphere and ocean, and even the smallest errors eventually result in different model forecasts.
However, other ECMWF ensemble members, and almost all of the GFS ensemble members, bring Elsa into the southeastern Gulf before curving the storm sharply northward on Tuesday.
The experts at making sense out of these kinds of model variations are the forecasters at the National Hurricane Center (NHC). Check their latest five-day forecast for Elsa online at www.nhc.noaa.gov. Don’t focus on the centerline of the forecast track three to five days out. Instead, concentrate on whether we are in the “cone” of average forecast error that is shown on the NHC’s forecast map.
Another useful NHC forecast map displays a swath of colors corresponding to the probability of tropical storm force winds. Over the next couple of days, the probabilities for coastal Mississippi will likely increase as Elsa moves closer. Hopefully by Monday or Tuesday, the probabilities in our area will begin to decrease as Elsa sets a heading for someplace to our east.