Posted by: coastlinesproject | June 9, 2015

Bill Ryan’ analysis of upcoming hurricane season forecast.

Attachments: hurricane-shear.tiff
You may have seen the various hurricane season forecasts already. They are consistent in forecasting a light hurricane season. There are usually 12 named storms and 6.5 hurricanes per Atlantic hurricane season. Most of the forecasts to date are in the 6-10 named storms and 3-5 hurricanes range. Of course, it only takes one.

The low frequency forecasts are driven primarily by the expected strength of the warm phase of the El Nino (ENSO) cycle. There is already a weak/moderate El Nino in place at this time and 22 of 24 of the standard ENSO forecast models expect this to continue through the remainder of the year and likely strengthen. Bear in mind, these forecasts are from mid-May and the spring (MAM) forecasts for El Nino are less reliable than those from other months. If the June forecasts are consistent with those from April and May, we will have more confidence in the forecast of the continuation of the El Nino and, therefore, the likelihood of a light hurricane season in the Atlantic.

The presence of an El Nino suppresses hurricanes because it increases vertical wind shear in the Atlantic tropical atmosphere. That is, winds aloft become very strong and act to shear the top off the developing storms. Hurricanes need a relatively constant wind with height in order to become well established. A nice figure showing this effect from Bill Gray’s forecast article is attached.


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