NOAA experts release updated hurricane outlook

NOAA experts release updated hurricane outlook

MIAMI, FL (WAFB) - The National Oceanic and Atmospheric Administration (NOAA) hurricane forecasting team, led by Dr. Gerry Bell of the NWS Climate Prediction Center, issued the NOAA updated hurricane forecast Thursday morning for the current season.

NOAA's original seasonal forecast was issued in late May: NOAA forecasters issue an August update each season in preparation for the climatologically most-active part of the tropical season for the Atlantic Basin, which runs from mid-August into early October.

Based on current and forecasted ocean and atmosphere conditions, the NOAA forecast team is calling for:

  • 6-10 ‘Named’ Storms (including the three ‘named’ storms already this season)
  • 1-4 Hurricanes
  • 0-1 ‘Major’ Hurricanes (Category 3 or stronger)
  • Accumulated Cyclone Energy (ACE) range of 25%-70% of the median

The NOAA forecast includes a 70 percent probability that the season's activity will fall within these stated ranges; forecasters are now 90 percent confident that the season totals will be below the long-term normals of 12 'named' storms, six hurricanes, and three 'major' hurricanes. (The normals are based on average seasonal activity for the period 1981-2010.) The ACE is a composite index derived from the peak winds and lifespan of individual storms over the course of the season.

Compared to the pre-season May outlook, the prediction for each of the ranges has been lowered. In the May forecast, the outlook called for six to 11 'named' storms, three to six hurricanes, and zero to two 'major' hurricanes. The forecasted seasonal ACE has also been reduced from a May forecast range of 40 percent to 80 percent of the median.

Three factors are identified as key contributors to the below-normal seasonal forecast:

  • Expectations for a ‘moderate to strong’ El Niño during the next several months
  • A non-conducive atmospheric environment over the tropical Atlantic
  • Below-normal sea-surface temperatures (SSTs) across most of the tropical Atlantic

The presence of El Niño - signaled by unusually-warm waters over the eastern equatorial Pacific Ocean - is linked to increased mid- and upper-level shearing winds over the tropical Atlantic. Wind shear inhibits the vertical development of tropical cyclones.

In addition to wind shear over the Atlantic Basin, enhanced sinking of air from aloft (subsidence) is expected to be a persistent feature over the tropical Atlantic through the 2015 Hurricane Season. Sinking air "warms and dries" as it descends, reducing the potential for thunderstorm development needed for tropical cyclone organization.

While wind shear and subsidence will result in unfavorable atmospheric conditions for much of the remainder of the hurricane season, cooler-than-normal SSTs over much of the tropical Atlantic will offer less energy for cyclone development. Heat energy stored in warm tropical waters serves as the fuel for tropical systems: a lower heat content in the ocean waters means less energy available for storm development and strengthening.

Bell emphasized, however, that these three conditions do not mean that the 2015 season will be "dead." What's more, the NOAA forecast offers no direct insights as to the threat of landfalls over the remainder of the season. Indeed, even with these three storm-inhibiting factors already in place, we still have had three 'named' storms this season, including two US landfalls.

Tropical Storm Ana struck South Carolina in May, before the official onset of the hurricane season. And in June, TS Bill formed over the Gulf of Mexico and made landfall along the Texas coast.

The bottom line is that Louisiana residents need to take the usual precautions and complete their hurricane season preparations regardless of the forecasted storm numbers. Based on the state's tropical history, Louisiana has a 50/50 chance of being impacted by a 'named' storm in any given year.

In addition, some of Louisiana's most notorious landfalls occurred during seasons with below-normal storm counts across the basin, including Audrey in 1957, Betsy in 1965 and Andrew in 1992.

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