WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

TSR predicts very active hurricane season; Atlantic May MDR SSTs warmest on record

By: Dr. Jeff Masters, 1:51 PM GMT on June 10, 2010

The British private forecasting firm Tropical Storm Risk, Inc. (TSR) has joined the ranks of NOAA and Colorado State University in calling for an exceptionally active 2010 Atlantic hurricane season. The latest TSR forecast issued June 4 calls for 17.7 named storms, 9.5 hurricanes, 4.4 intense hurricanes, and an Accumulated Cyclone Energy (ACE) 181% of average. These numbers are much above the 50-year average of 10 named storms, 6 hurricanes, and 2 intense hurricanes, and are an increase from their April forecast of 16.3 named storms, 8.5 hurricanes, and 4 intense hurricanes. The TSR June forecast numbers are the highest they've ever gone for in the eleven years they've been issuing Atlantic hurricane season forecasts. TSR predicts a 85-90% chance that activity will rank in the top 1/3 of years historically, and a 85% chance that U.S. landfalling activity will be above average. TSR rates their skill level as 20-34% higher than a "no-skill" forecast made using climatology, though an independent assessment by the National Hurricane Center (Figure 1) gives them somewhat lower skill numbers.

TSR projects that 5.7 named storms will hit the U.S., with 2.5 of these being hurricanes. The averages from the 1950-2009 climatology are 3.2 named storms and 1.5 hurricanes. They rate their skill at making these June forecasts for U.S. landfalls at 10 - 17% higher than a "no-skill" forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.8 named storms, 0.8 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR cites two main factors for their forecast of an exceptionally active season:

1) Their model predicts that sea surface temperatures will be 0.6°C warmer than average in August and September over the Main Development Region (MDR) for Atlantic hurricanes. This is the area between 10°N and 20°N, between the coast of Africa and Central America (20°W - 80°W). It is called the Main Development Region because virtually all African waves originate in this region. These African 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.)

2) Their model predicts slower than normal trade winds in August and September over the Main Development Region (MDR). Trade winds are forecast to be 1.2 meters per second (about 2.7 mph) slower than average. This would create more spin for developing storms, and allow the oceans to warm up, due to reduced mixing of cold water from the depths and lower evaporational cooling.

Figure 1. Comparison of the percent improvement over climatology for May and August seasonal hurricane forecasts for the Atlantic from NOAA, CSU and tropicalstormrisk.com (TSR) from 1999-2009 (May) and 1998-2009 (August), using the Mean Squared Error. Image credit: Verification of 12 years of NOAA seasonal hurricane forecasts, National Hurricane Center.

2010 hurricane season forecasts from CSU and NOAA
NOAA's 2010 Atlantic hurricane season forecast, issued May 27, called for 18.5 named storms, 11 hurricanes, 5 major hurricanes, and an ACE index 210% of normal (using the mid-point of their range of numbers.) The 2010 Atlantic hurricane season forecast from Colorado State University (CSU) issued on June 2 called for 18 named storms, 10 hurricanes, and 5 intense hurricanes. So, the consensus forecast from NOAA, CSU, and TSR is 18 named storms, 10 hurricanes, and 5 intense hurricanes. The June forecast numbers from all three groups were the highest they've ever gone for in their history of issuing Atlantic hurricane season forecasts.

May SSTs in the tropical Atlantic set a new record
Sea Surface Temperatures (SSTs) in the Atlantic's Main Development Region for hurricanes had their warmest May on record, according to an analysis of historical SST data from the UK Hadley Center. SST data goes back to 1850, though there is much missing data before 1910 and during WWI and WWII. SSTs in the Main Development Region (10°N to 20°N and 20°W to 80°W) were a remarkable 1.51°C above average during May. This is the fourth straight record warm month, and the warmest anomaly measured for any month. The previous record warmest anomaly for the Atlantic MDR was 1.46°C, set last month. Third place goes to June 2005 and March 2010, with a 1.26°C anomaly. As I explained in detail in a post on record February SSTs in the Atlantic, the Arctic Oscillation (AO) and its close cousin, the North Atlantic Oscillation (NAO), are largely to blame for the record SSTs, though global warming and Atlantic Multidecadal Oscillation (AMO) also play a role. However, trade winds over the tropical Atlantic have increased to near-normal speeds over the past week, since the Bermuda-Azores High has strengthened to near-normal pressures. The Bermuda-Azores High and its associated trade winds are forecast to increase to above average strength during mid-June, according to the latest runs of the GFS model. This means that Atlantic SST anomalies have probably peaked for the year, and we can anticipate that the June SST anomaly in the MDR will not be as great as the May anomaly--and may even fall below the June record set in 2005.

Figure 2. The departure of sea surface temperature (SST) from average for June 10, 2010. Image credit: NOAA/NESDIS.

Oil spill update
Light southeast or south winds of 5 - 15 knots will blow today through Tuesday, according to the latest marine forecast from NOAA. These winds will keep oil near the beaches of Alabama, Mississippi, and the extreme western Florida Panhandle, according to the latest trajectory forecasts from NOAA and the State of Louisiana. The latest ocean current forecasts from the NOAA HYCOM model are not predicting eastward-moving ocean currents along the Florida Panhandle coast this week, and it is unlikely that surface oil will affect areas of Florida east of Pensacola. Long range surface wind forecasts from the GFS model for the period 8 - 14 days from now show a southeasterly wind regime, which would prevent any further progress of the oil eastwards along the Florida Panhandle, and would tend to bring significant amounts of oil back to the shores of eastern Louisiana next week. If you spot oil, send in your report to http://www.gulfcoastspill.com/, whose mission is to help the Gulf Coast recovery by creating a daily record of the oil spill.

Figure 3. The oil spill as imaged on June 9, 2010, by NOAA's Terra satellite. The spill appears highly reflective in the sunglint portion of the image.

Oil spill resources
My post, What a hurricane would do the Deepwater Horizon oil spill
NOAA's fact sheet on Hurricanes and the Oil Spill
My post on the Southwest Florida "Forbidden Zone" where surface oil will rarely go
My post on what oil might do to a hurricane
Oil trajectory forecasts from NOAA
Gulf Oil Blog from the UGA Department of Marine Sciences
Oil Spill Academic Task Force
University of South Florida Ocean Circulation Group oil spill forecasts
Synthetic Aperture Radar (SAR) imagery from the University of Miami

I'll have a new post on Friday. The tropical Atlantic is quiet right now, with no models predicting tropical cyclone development over the next seven days.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.