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Impact Weather: Your Weather Department

Chris Hebert
Chris Hebert,  ImpactWeather’s lead hurricane forecaster
With a B.S. in Meteorology from Texas A&M University and more than 27 years of forecasting experience, Chris is ImpactWeather’s lead hurricane forecaster. For a detailed bio…

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Updated 2010 Hurricane Season Outlook - 18 Named Storms
June 4th, 2010
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By now, you’ve heard of the National Oceanic and Atmospheric Administration’s (NOAA) outlook calling for between 14-23 named storms this season.  That range encompasses just over 1 standard deviation above and below 18.5 named storms.  What NOAA is saying is that there is a 70% chance that the total number of named storms this season will be in that range.  While I think that a 10 storm range is quite large (an average season has 11-12 named storms), it is significant that even the low end of the range (14) constitutes an above-normal number of named storms.  NOAA appears quite confident that the Atlantic Basin will be quite active in 2010, and so are we.

On April 20th, our outlook was for 15 named storms with 9 of the storms becoming hurricanes, and 4 of those hurricanes reaching major status (Saffir-Simpson Category 3-4-5).   We could tell that all of the seasonal parameters that we monitor were suggesting a very active season, but there was still a good deal of uncertainty.  El Niño was forecast to steadily weaken, but would it really weaken as quickly as some of the models were forecasting?   Sea surface temperatures were at record high levels for April, but sometimes the heating drops off later in the spring.  The European model was forecasting very low surface pressure anomalies in the tropics, but was it right?  There were many questions back in April, questions that have at least partially been answered during the month of May.

There’s no question at all that El Niño is gone.  The El Niño threshold for Pacific sea surface temperature anomalies is +0.5C and higher.  Temperatures in the main El Niño region dropped from about +0.7C in late April to -0.2C.  The La Niña threshold is -0.5C and cooler, so the current value is fast-approaching the La Niña threshold.  All model guidance now forecasts at least a weak La Niña from July-November.  This would be a significant enhancing factor for development, as a La Niña promotes more rising air (thunderstorms) in the Atlantic Basin, as well as decreased upper-level wind shear.  So this uncertainty no longer exists.

In March and April,  the European model’s seasonal projections of surface pressure anomalies in the deep tropics was for lower and lower pressures during peak season (August-October).  That trend continued with their late May update.  In fact, extremely low pressures are forecast for the entire region from the Gulf of Mexico through the Caribbean and all the way to Africa.  This represents a Bermuda High that is significantly weaker than in 2009 and also one that is displaced farther to the east. The net effect is for weaker easterly trade winds across the Tropical Atlantic and the Caribbean Sea.  Weaker trade winds means less low-level wind shear in this region, the “Main Development Region”.    But weaker trade winds also means less dry, dusty air blowing off the west coast of Mexico this season, another enhancing factor.  Finally, the lower pressures in the tropics means more rising air and thunderstorms in 2010.  Last year, the Caribbean islands saw very little rainfall because the Bermuda High was stronger than normal during the summer.  This year looks to be a complete reversal of the 2009 pattern.   Less low level wind shear and more moisture would mean not only an increased likelihood of development, but greater chances for larger and more powerful hurricanes.

The final uncertainty from last April was whether the unseasonably warm ocean temperatures would start to moderate through the month of May into June.  Sometimes, sea surface temperatures are anomalously warm in March and April, only to decrease to near normal levels by the start of the season.  It’s clear that won’t be the case in 2010.  Sea surface temperatures remain quite warm as of today, and there are no signs of a decrease.   Ocean temperatures will not be an inhibiting factor for development this year, even in the Gulf of Mexico where temperatures were below normal back in April.

Based upon all that I’ve discussed above, we have revised our prediction for this season as of June 1st.   We are now forecasting a total of 18 named storms, with 11 of those hurricanes and 5 of the hurricanes reaching major hurricane strength (Category 3-4-5).     Projected numbers are one thing, but most people want to know what the seasonal prediction means to them.  We’ve highlighted the areas that are most likely to be significantly impacted in 2010 on the graphic below.

Risk Areas

We expect a concentration of tracks generally toward the islands of the northern Caribbean Sea and northwest into the eastern Gulf of Mexico. Secondarily, a few storms may track just north of the Caribbean and impact the Bahamas and/or the Carolinas.  But there may be times when a hurricane develops or moves into the Caribbean Sea and high pressure across the Gulf of Mexico is a little stronger.  This could lead to a track into the northern Yucatan Peninsula  with eventual landfall on the coast of Mexico south of Texas.

That’s our June update.  We’ll be watching to see how things materialize over the coming month before issuing our next update on Thursday, July 1st.   You can reserve your seat at our July 1st webinar by visiting the link below.  See you there!


https://www1.gotomeeting.com/register/705317729

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