Residents of Hurricane Alley now have even more reason to anticipate an above-normal Atlantic hurricane season for the rest of 2021, said NOAA’s Climate Prediction Center on Wednesday, August 4. In addition, after a long quiet spell, there now are two disturbances in the Atlantic, including one with the potential to become a tropical depression by next week (see below).
In its second seasonal forecast for 2021, NOAA predicted a 65% chance for an above-normal Atlantic hurricane season, up from 60% in its first outlook for the season on May 20. There is a 25% chance for a near-normal season (down from 30% in the prior outlook), and a 10% chance for a below-normal season.
NOAA gave a 70% likelihood of 15-21 named storms, 7-10 hurricanes, 3-5 major hurricanes (category 3 or higher on the Saffir-Simpson Hurricane Wind Scale) and an Accumulated Cyclone Energy (ACE) 110-190% of the median. Taking the midpoint of these ranges, NOAA called for 18 named storms, 8.5 hurricanes, and 4 major hurricanes, compared to 16.5, 8, and 4 in the May outlook. The 1991-2020 seasonal averages are 14 named storms, 7 hurricanes and 3 major hurricanes.
So far in 2021, there have been five named storms, one hurricane (Elsa), and an ACE index of 12.8. All of these quantities are above average for this date, and NOAA’s forecast includes this activity.
NOAA cited four main factors influencing its Atlantic forecast:
1) There’s been an era of high Atlantic hurricane activity for more than 25 years. Since 1995, 18 of 26 (about two-thirds) of seasons have been above average and only 4 (15%) have been below average, based on the 1951-2020 climatology of ACE. Most (10 out of the 18) of the above-normal years were hyperactive (ACE > 165% of median). The continuation of the high-activity era is related to a set of conditions including lower wind shear, weaker trade winds, and stronger than average West African Monsoon.
2) ENSO conditions remain neutral this summer, and there is increasing potential for a La Niña event to develop over the next few months (up to a 55% chance for the September-November period, according to the most recent outlook, issued in early July). (ENSO refers to El Niño/ Southern Oscillation, which has three phases: El Niño, Neutral, and La Niña.) El Niño suppresses hurricane development in the Atlantic by increasing the amount of vertical wind shear and dry, stable air that tends to prevail over the Main Development Region (MDR) for hurricanes, which includes the tropical North Atlantic Ocean and Caribbean Sea between 10°N and 20°N latitude.
3) Near to slightly below-average sea surface temperatures (SSTs) have been measured in portions of the tropical Atlantic during much of June and July, favoring a near-normal season. In addition, surface pressures have been above normal, which does not favor an active season.
4) An early-season named storm (Elsa) formed on July 1 in the eastern Caribbean. Historically, years with early-season activity in this region have a much higher likelihood of being above-normal.
NOAA reiterated these perennial words of wisdom: “It only takes one storm hitting an area to cause a disaster, regardless of the overall activity for the season. Therefore, residents, businesses, and government agencies of coastal and near-coastal regions are urged to prepare every hurricane season regardless of this, or any other, seasonal outlook.”
CSU decreases its forecast for the 2021 season
In contrast to NOAA, the seasonal forecasters from Colorado State University (CSU) have slightly decreased their outlook for Atlantic hurricane activity. However, their new forecast and the new NOAA midrange forecast are virtually identical. As of Thursday, August 5, the CSU group is predicting a 2021 total of 18 named storms, 8 hurricanes, and 4 major hurricanes, with a seasonal ACE of 160. This compares to its July 8 outlook for 20 named storms, 9 hurricanes, 4 major hurricanes, and an ACE of 170.
CSU cited the absence of El Niño and the potential development of La Niña, along with the relatively low wind shear over the tropical Atlantic and Caribbean and the presence of slightly cooler-than-average SSTs over the eastern and central tropical Pacific and warmer-than-average SSTs for the tropical Atlantic. “All of these conditions in combination point to an above-average season in 2021,” they said, “but our statistical model and statistical/dynamical models [now] indicate slightly less activity than was anticipated in early July – hence the slight decrease in the seasonal forecast numbers.”
The years cited by CSU as analogs for the 2021 season are 1966, 2001, 2008, 2011, 2016, and 2017. All of these had above-average activity, with ACE values ranging from 110 to 225.
While there is little to no skill in seasonal hurricane forecasts issued early in the calendar year, CSU’s August forecasts have demonstrated considerable skill.
Tropical Storm Risk calls for a near-average season
The forecast group at TropicalStormRisk.com has also reduced its numbers in its outlook issued Thursday, August 5, compared to its July 6 outlook. “TSR has lowered its outlook for basin tropical cyclone activity because the July 2021 Atlantic meridional mode, the July Atlantic multidecadal oscillation and, crucially, the July trade winds and sea surface temperatures over the tropical North Atlantic and Caribbean Sea all point to hurricane activity that will be close to the 10-year norm rather than to well-above norm,” the group said.
The new TSR outlook is for 18 named storms, 7 hurricanes, 3 major hurricanes, and an ACE index of 122. This compares to the July outlook numbers of 20, 9, 4, and 141.
On a more ominous note, the group is calling for an above-average risk of U.S. landfalls. This judgment is based on the landfall-favorable steering winds that were present in July, and also the increasing chance of La Niña development, which tends to be associated with tracks closer to the United States, according to TSR.
The Atlantic is finally waking up
After a long pause in tropical cyclone activity, the Atlantic Ocean is finally waking up as the traditional peak period of the season approaches, usually beginning around August 20. The seasonal peak in SSTs in the Northern Hemisphere is nearing, and ocean temperatures all across the tropical and subtropical Atlantic are near or above the 26.5 degrees Celsius (80°F) threshold typically needed to sustain a hurricane. Ocean temperatures will continue to warm until the first week of September, when SSTs typically peak for the year. SSTs in the Main Development Region (MDR) for hurricanes – the tropical Atlantic from the coast of Africa to the coast of Central America – are currently near average. This situation is favorable for development of hurricanes, though not greatly so.
Tropical disturbance 92L could become Fred
A strong tropical wave near the coast of Africa on Thursday afternoon has been designated 92L by the National Hurricane Center. 92L was headed west to west-northwest at about 15 mph on Thursday, and was positioned at a low latitude, near 11.5°, as seen on satellite images. This position relatively close to the equator will keep development of 92L slow to occur. Also slowing development through Friday morning will be high wind shear of 20-30 knots. However, 97L had favorable sea surface temperatures near 28 degrees Celsius (82°F), and a moist atmosphere with a mid-level relative humidity of 70%.
Forecast for 92L
As 92L progresses west to west-northwest at about 15 mph Friday through Saturday, it will be south of the dry air from the Saharan Air Layer. Conditions will be favorable for development, according to the 18Z Thursday run of the SHIPS model, with moderate wind shear of 10-20 knots and sea surface temperatures near 27-28 degrees Celsius (81-82°F).
On Sunday, 92L is expected to move over cooler waters of 26 degrees Celsius (79°F), and it will likely be positioned far enough north to begin wrapping dry air from the Saharan Air Layer into its core. Despite these marginal conditions, 92L does have good model support for development, and the National Hurricane Center expects it to become a tropical depression by this weekend. In a 2 p.m. EDT Thursday Tropical Weather Outlook, NHC gave 92L two-day and five-day odds of formation of 40% and 70%, respectively. The next name on the Atlantic list of storms is Fred. Coincidentally, 92L is expected to become a tropical depression very close to the location where the last incarnation of Fred occurred, in 2015 (see Tweet above).
Tropical disturbance approaching the Lesser Antilles remains disorganized
In the central tropical Atlantic, a tropical wave near 12°N, 46°W on Thursday afternoon was headed west-northwest at about 10-15 mph. The wave had marginal conditions for development, with sea surface temperatures near 27 degrees Celsius (81°F), moderate wind shear, and plenty of dry air from the Saharan Air Layer to contend with. Satellite images showed the wave had a modest amount of spin, but few heavy thunderstorms. In a 2 p.m. EDT Thursday Tropical Weather Outlook, National Hurricane Center gave the wave two-day and five-day odds of formation of near 0% and 20%, respectively.
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