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Posted on June 1, 2017
Courtesy of Surfline.com
Originally published on May 31,2017
By Mike Watson
As the Atlantic hurricane season opens on June 1st, the buzz generated around the start of the season reverberates through East and Gulf Coast surfers. While tropical swells can be fleeting, the season creates a certain aura that is missing from typical frontal low pressure systems that deliver swells during the other seasons.
Maybe it’s the slow buildup from a puff of clouds forming a tropical storm until the swell reaches the shore, or the flat summer spells that tropical swells offer a reprieve from. Whatever the reason, the allure of the tropical season is palpable on the East Coast.
Each year various university, government and private entities publish season hurricane forecasts for the upcoming tropical season in the Atlantic. While there are obvious limitations on these forecasts, they have shown skill and are useful. While we cannot forecast formation locations and tracks on a seasonal time scale which is one limitation, these forecasts give us a general idea of what to expect, especially if we have a clear climate drive like El Nino or La Nina.
Mid-may El Nino predictions from the International Research Institute for Climate and Society (IRI), a cooperative between NOAA’s Climate Program Office and Columbia University show a 46% chance of El Nino developing and a 47% chance of a neutral season. (Image IRI)
Our main predictor of expected tropical cyclone activity over the Atlantic waters is the El Nino-Southern Oscillation or ENSO. El Nino, or the warm phase of ENSO, typically results in enhanced vertical wind shear over the Atlantic waters, which is unfavorable for tropical cyclone formation. La Nina, or the cold phase of ENSO, typically results in lower vertical wind shear, which is more conducive for tropical development over the Atlantic waters. Last year’s La Nina almost did not produce as the Atlantic was seeing below average activity through much of September, but along came Matthew. This storm alone accounted for around 40% of the Atlantic tropical activity as measured by ACE or Accumulated Cyclone Energy. ACE is a simple measure that quantifies the activity at the storm level and at the seasonal level. At the storm level ACE is a measure of the strength and duration of storm. The summation of each storms' individual ACE gives up a seasonal ACE.
During the most active part of the Atlantic tropical season, August through October, the average of the statistical/dynamical models currently points to a neutral ENSO to very slight warm event. Latest official forecast probabilities from the International Research Institute for Climate and Society (IRI), a cooperative between NOAA’s Climate Program Office and Columbia University show a 46% chance of El Nino developing and a 47% chance of a neutral season. The latest NOAA forecast puts the chances of El Nino development a little higher at 50%. Most forecasts are expecting the ENSO phase to be near neutral, so at this point we are not expecting ENSO to have too much influence on the upcoming Atlantic tropic season.
Most predictions for this season are forecasting normal to slightly above normal levels of activity. The recently released NOAA forecast is one of the more aggressive forecasts calling for a 45% chance of above average tropical cyclone activity over the Atlantic basin, a 35% chance of near normal activity and only a 20% chance of below normal activity. The Colorado State University forecast (issued in April) is the most bearish, forecasting normal to below normal activity, however, a new forecast is due out on June 1st.
Chart showing the seasonal forecasts from the various universities, government agencies and private weather companies. (Image BSC center)
Some of the early season forecasts likely called for lower activity due to the expected return of El Nino this fall. EL Niño forecasts from early in the year are known to be unreliable, known as the ‘spring barrier’, and may have skewed these early hurricane season forecasts. Given the trend of a decreasing chance of El Nino, updated CSU numbers and other early season forecasts are likely to nudge upward.
It is well known in the meteorological community that an aggregate or consensus of the forecast can be even better. In other words, an aggregate or consensus of the forecasts from various experts will produce a more accurate forecast compared to a single forecast. In 2016, the consensus of the seasonal forecasts from the various groups provided an almost exact prediction of the actual Atlantic tropical season. Averaging the forecasts from the various groups for the upcoming season yields 12-13 named systems, 5-6 hurricanes and 2-3 major hurricanes which would make the 2017 tropical season activity average to slightly above average.
Another important piece of the Atlantic tropic season prediction puzzle are the formation locations and tracks of these storms. While we cannot point to exact formation dates or tracks weeks to months in advance, there are studies on general expected impacts based on the phase of ENSO. Obviously, the warm phase of ENSO (El Nino) has the lowest probability of hurricane landfall along the Gulf, Florida or East Coast since less storms typically form. The cold phase of ENSO (La Nina) produces the highest chance of landfalling hurricanes along the East Coast. Enso-neutral year and El Nino year see this chance decrease by nearly 40%. Neutral ENSO years lead to higher chances of a landfalling hurricane for Florida and along the Gulf Coast, especially for storms that form to the west of 50W.
One thing we can look at in the Atlantic tropical cyclone prediction puzzle is the Madden-Julian Oscillation or MJO. This oscillation circles the tropics with a period of 40-60 days and has been shown to modulate tropical cyclone activity. This oscillation is predictable in the 2-3 week time frame and can give us indication on whether or not we will see period of enhanced tropical activity or periods of suppressed tropical activity.
No matter what the season ahead may hold, remember to be prepared for the worst and stay tuned to Surfline for all the storm and surf information as we move through the tropical season.
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