Extended range forecast of Atlantic seasonal hurricane activity and landfall strike probability for 2013 from CSU

By Philip J. Klotzbach and William M. Gray

From Department of Atmospheric Science, Colorado State University (CSU)

(as of 10 April 2013)

We anticipate that the 2013 Atlantic basin hurricane season will have enhanced activity compared with the 1981-2010 climatology. The tropical Atlantic has anomalously warmed over the past several months, and it appears that the chances of an El Niño event this summer and fall are unlikely. We anticipate an above average probability for major hurricanes making landfall along the United States coastline and in the Caribbean. Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much or how little activity is predicted.

ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR 2013

Forecast Parameter and 1981-2010                      Issue Date

Median (in parentheses)                                         10 April 2013

Named Storms (NS) (12.0)                                      18

Named Storm Days (NSD) (60.1)                           95

Hurricanes (H) (6.5)                                                    9

Hurricane Days (HD) (21.3)                                    40

Major Hurricanes (MH) (2.0)                                  4

Major Hurricane Days (MHD) (3.9)                        9

Accumulated Cyclone Energy (ACE) (92)              165

Net Tropical Cyclone Activity (NTC) (103%)        175

PROBABILITIES FOR AT LEAST ONE MAJOR (CATEGORY 3-4-5) HURRICANE

LANDFALL ON EACH OF THE FOLLOWING COASTAL AREAS:

1)        Entire U.S. coastline – 72% (average for last century is 52%)

2)        U.S. East Coast Including Peninsula Florida – 48% (average for last century is 31%)

3)        Gulf Coast from the Florida Panhandle westward to Brownsville – 47% (average for last century is 30%)

PROBABILITY FOR AT LEAST ONE MAJOR (CATEGORY 3-4-5) HURRICANE

TRACKING INTO THE CARIBBEAN (10-20°N, 60-88°W)

1)   61% (average for last century is 42%)

ABSTRACT

Information obtained through March 2013 indicates that the 2013 Atlantic hurricane season will have more activity than the median 1981-2010 season. We estimate that 2013 will have about 9 hurricanes (median is 6.5), 18 named storms (median is 12.0), 95 named storm days (median is 60.1), 40 hurricane days (median is 21.3), 4 major (Category 3-4-5) hurricanes (median is 2.0) and 9 major hurricane days (median is 3.9). The probability of U.S. major hurricane landfall is estimated to be about 140 percent of the long-period average. We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2013 to be approximately 175 percent of the long-term average

This forecast is based on a new extended-range early April statistical prediction scheme that was developed utilizing 29 years of past data. Analog predictors are also utilized. We anticipate an above-average Atlantic basin hurricane season due to the combination of an anomalously warm tropical Atlantic and a relatively low likelihood of El Niño. Coastal residents are reminded that it only takes one hurricane making landfall to make it an active season for them, and they need to prepare the same for every season, regardless of how much activity is predicted.

To view the whole report it can be downloaded at:

http://typhoon.atmos.colostate.edu/forecasts/2013/apr2013/apr2013.pdf

Dr Jeff Masters of Weather Underground says in his Wunderblog

“The CSU team cited two main reasons why this may be an active hurricane season:

1) Neutral El Niño conditions are expected during the August – September – October peak of hurricane season. Since the active hurricane period we are in began in 1995, neutral years have seen much above-average activity (remember the neutral El Niño year of 2005?) If El Niño conditions are present this fall, this will likely bring about a quiet Atlantic hurricane season due to increased upper-level winds over the tropical Atlantic creating wind shear that will tend to tear storms apart.

2)   Sea surface temperatures (SSTs) in the Main Development Region (MDR) for hurricanes from the Caribbean to the coast of Africa between 10°N and 20°N were near-average in the western tropical Atlantic, but unusually warm in the eastern tropical Atlantic, in March 2013. Much of this unusual warming was due to a persistent negative phase of the North Atlantic Oscillation (NAO) since mid-February (which also brought an unusually cold March to the Eastern U.S. and Western Europe.) A negative phase of the NAO is associated with a weakened Bermuda-Azores High and slower trade winds across the tropical Atlantic. The slower winds allow the ocean to heat up more, due to less mixing of cool water to the surface. Virtually all African tropical waves originate in the MDR, and these tropical waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. When SSTs in the MDR are much above average during hurricane season, a very active season typically results (if there is no El Niño event present.) Conversely, when MDR SSTs are cooler than average, a below-average Atlantic hurricane season is more likely.

How good are the April forecasts?

The forecasters are using a statistical model developed in 2011 for making April forecasts, so we don’t have a long enough track record to judge how good the new model is. The new model predicted a below-average year for 2012, with 10 named storms, 4 hurricanes, and 2 intense hurricanes. The actual tally was much higher, with 19 named storms, 10 hurricanes, and 2 intense hurricanes. April forecasts of hurricane season activity are low-skill, since they must deal with the so-called “predictability barrier.” April is the time of year when the El Niño/La Niña phenomenon commonly undergoes a rapid change from one state to another, making it difficult to predict whether we will have El Niño, La Niña, or neutral conditions in place for the coming hurricane season. Correctly predicting this is key, since if El Niño conditions are present this fall, this will likely bring about a quiet Atlantic hurricane season due to increased upper-level winds over the tropical Atlantic creating wind shear that will tend to tear storms apart. Currently, ocean temperatures are very close to average in the Eastern Pacific, and the large majority of the El Niño models are predicting a continuation of these neutral conditions for the coming hurricane season.

CSU maintains an Excel spreadsheet of their forecast errors (expressed as a mathematical correlation coefficient, where positive means a skilled forecast, and negative means they did worse than climatology) for their their April forecasts. For now, these April forecasts should simply be viewed as an interesting research effort that has the potential to make skillful forecasts. The next CSU forecast, due on Monday, June 3, is the one worth paying attention to. Their early June forecasts have shown considerable skill over the years.

TSR predicts an active hurricane season

The April 5 forecast for the 2013 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for an active season with 15 named storms, 8 hurricanes, 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 131. The long-term averages for the past 63 years are 11 named storms, 6 hurricanes, 3 intense hurricanes, and an ACE of 103. TSR rates their skill level as low for these April forecasts–just 6 – 15% higher than a “no-skill” forecast made using climatology. TSR predicts a 63% chance that U.S. land falling activity will be above average, a 21% chance it will be near average, and a 16% chance it will be below average. They project that 4.4 named storms will hit the U.S., with 1.9 of these being hurricanes. The averages from the 1950-2012 climatology are 3.1 named storms and 1.4 hurricanes. They rate their skill at making these April forecasts for U.S. landfalls just 7 – 8% higher than a “no-skill” forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.5 named storms, 0.6 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR’s two predictors for their statistical model are the forecast July – September trade wind speed over the Caribbean and tropical North Atlantic, and the forecast August – September 2013 sea surface temperatures in the tropical North Atlantic. Their model is calling for warmer than average SSTs and slower than average trade winds during these periods, and both of these factors should act to increase hurricane and tropical storm activity.

WSI predicts an active hurricane season

The April 8 forecast from the private weather firm WSI (part of The Weather Company, along with The Weather Channel, Weather Central, and The Weather Underground), is calling for an active season with 16 named storms, 9 hurricanes, and 5 intense hurricanes.

Please see attached for all the forecasts.

For more on this story go to:

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2381