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Seasonal Tropical Outlook- El Nino strengthens

3:16 PM

The key driver this year for the Tropical Season this year (June 1 through November 30) is going to be the developing El Nino signature.  This is the latest sea surface anomaly across the Pacific and Atlantic Oceans.  There are several key factors that stand out in the SSTA map.  For one, warmer than normal sea surface temperatures are developing off of the northern coast of South America.  The strengthening of this feature the past few weeks have been rather impressive and suggests that a moderate El Nino will be developing towards the end of the season.  Meanwhile, sea surface temperatures are cooler than normal over large parts of the Tropical Atlantic, especially off the African coast.  If these factors do not change, than the development of disturbances off the African coast will be limited.  

The development of El Nino has several keys effects to the upper level pattern.  In fact, we are already seeing the influence of a weak El Nino pattern right now over the eastern United States.  When an El Nino develops, the upper level

westerly winds become stronger, which means wind shear will be stronger and thus inhibit development of tropical systems.  Meanwhile, a strong ridge establishes itself over the Gulf Coast and eastern United States and a trough develops off the Northwest coast and over eastern Canada.  Sounds familiar, doesn’t it?  That’s exactly the same time of pattern that is developing as of right now over the eastern United States.  

 

The National Hurricane Center, a part of the National Weather Service and NOAA, recently issued a forecast stating that a 9 to 14 named storms are expected, 4 to 7 hurricanes, and 1 to 3 major hurricanes.  This forecast would illustrate a near normal hurricane season.    However, lets push a little bit beyond this forecast and look at how this pattern will likely influence the tracks of tropical systems.

Naturally, I am not going to try to forecast exact tracks, however I think this pattern does hold some important clues on where the most likely areas are going to be impacted.  If we consider that a ridge will be present to the north of the Caribbean Sea and that a weakness will be present over the southern Plains and over New England, we can gain an understanding of the overall upper level wind patterns.  The upper level structure would suggest a pattern where tropical systems would likely move towards the Yucatan Peninsula and the central/western Gulf Coast.  These tropical systems would then recurve and become entrained along cold fronts that will slowly move through or stall over the Northeast and Mid Atlantic.  I believe there is also a threat for tropical disturbances to develop along old cold front boundaries over northern Gulf of Mexico.  

The development of El Nino will continue through the summer and will become a major determining factor on the “Cape Verde” season.  The Cape Verde season is generally from late July through late October over the eastern Atlantic.  The season is characterized by disturbances moving off the African coast that move across the Atlantic and rapidly develop.  These storms can become very strong hurricanes, reaching major status for periods of time.  Now, the Cape Verde season will be in full swing as El Nino continues to strength.  A strengthening El Nino will mean that westerly winds will strengthen, which will produce stronger shear.  As a result, I think the Cape Verde season will have a limited impact on the United States.  This does not mean a storm can not develop and track through the Atlantic, however this is not the season to expect an active tropical season from this sector of the Atlantic.

Overall, this hurricane season will be influence by the developments of El Nino.  The faster the El Nino develops, the more likely the Atlantic tropical season will be quiet.  Look for the potential for several tropical disturbances enhancing cold fronts over the Mississippi Valley, Ohio Valley, northern Mid Atlantic, and Northeast.

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