The 2015 Atlantic hurricane season is officially underway. We've already had one early season storm, Tropical Storm Ana; will we have an early June Tropical Storm Bill? There are indications that the second tropical depression of the year has a chance to form late this week in the waters near South Florida or the Bahama Islands on Friday or Saturday. We have warmer than average SSTs in these waters, and the Madden-Julian Oscillation (MJO) is expected to be active in the Western Hemisphere late this week, and both of these factors argue for higher than usual odds of an early June tropical depression forming in the Atlantic.
Over the past few days, the GFS model has been consistently advertising the possibility that an area of low pressure capable of developing into a tropical depression will form in this region, although the European model (so far) has not gone along with this idea. If we do get something developing, it would potentially be a heavy rain threat for South Florida and the Northern Bahamas over the weekend, but then move northeastwards out to sea without troubling any more land areas.
Summary of the 2015 Atlantic hurricane season forecasts
The major hurricane forecasting groups are not impressed with this season's potential to be an active one, and are universally calling for 2015 to be a below average to way below-average year for the Atlantic. The highest forecast numbers were from Weather Underground Community Hurricane Forecast, which called for 11 named storms, 5 hurricanes, and 2 major hurricanes. The lowest numbers were from North Carolina State University: 5 named storms, 2 hurricanes, and 1 intense hurricane. The long-term averages for the past 65 years are 11 named storms, 6 hurricanes, 3 intense hurricanes. Here are the forecasts:
--NOAA: 8.5 named storms, 4.5 hurricanes, 1 major hurricane, ACE index 62.5% of normal.
--Colorado State University (CSU): 8 named storms, 3 hurricanes, 1 major hurricane, ACE index 44% of normal.
--Tropical Storm Risk, Inc. (TSR): 10 named storms, 4 hurricanes, 1 intense hurricane, ACE index 36% of normal.
--UKMET office: 8 named storms, 5 hurricanes, ACE index 74% of normal (June - November.)
--FSU Center for Ocean-Atmospheric Prediction Studies (COAPS):
--Weather Underground Community Hurricane Forecast: 11 named storms, 5 hurricanes, 2 major hurricanes
--WSI: 9 named storms, 5 hurricanes, 1 major hurricane
--Penn State: 8 named storms.
--North Carolina State University: 5 named storms, 2 hurricanes, 1 intense hurricane.
--Coastal Carolina University: 8 named storms, 4 hurricanes, 2 intense hurricanes.
--The Cuban Meteorological Service, INSMET: 9 named storms, 3 hurricanes.
The main reason for the quiet forecasts is the likely intensification of the current moderate strength El Niño. Every 3 - 7 years, variations in tropical winds and pressure shift warm ocean waters eastwards from the Western Pacific to the South American coast, causing an El Niño event.
The unusually warm water tends to drive an atmospheric circulation that brings strong upper-level winds to the tropical Atlantic, creating high levels of wind shear that tend to tear hurricanes apart. Another factor leading to lower forecast numbers than in previous years is the fact that sea surface temperatures in the tropical Atlantic are near average to below average this year--quite a bit cooler than we've seen during the typical year during our active hurricane period that began in 1995.
