Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 2:47 PM GMT on April 05, 2012
Expect one of the quietest Atlantic hurricane seasons since 1995 this year, say the hurricane forecasting team of Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU) in their latest seasonal forecast issued April 4. They call for an Atlantic hurricane season with below-average activity: 10 named storms, 4 hurricanes, and 2 intense hurricanes. An average season has 10 - 11 named storms, 6 hurricanes, and 2 intense hurricanes. The 2012 forecast calls for a below-average chance of a major hurricane hitting the U.S., both along the East Coast (24% chance, 31% chance is average) and the Gulf Coast (24% chance, 30% chance is average). The Caribbean is forecast to have a 34% chance of seeing at least one major hurricane (42% is average.) Four years with similar pre-season March atmospheric and oceanic conditions were selected as "analogue" years that the 2012 hurricane season may resemble: 2009, 2001, 1965, and 1957. These years all had neutral to El Niño conditions during hurricane season. The average activity for these years was 9.5 named storms, 4.8 hurricanes, and 2.3 major hurricanes.
Figure 1. Departure of sea surface temperature (SST) from average for April 5, 2012, as computed by NOAA's NESDIS branch. SSTs in the hurricane Main Development Region (red box) were near average to below-average.
Why the forecast of a quiet season?
The CSU team cited two main reasons why this may be a quieter than average hurricane season:
1) La Niña has weakened rapidly over the tropical Eastern Pacific over the past month, and is expected to be gone by the end of April. In its wake, El Niño conditions may develop in time for the August - September - October peak of hurricane season. 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. The CSU team is leaning towards putting their trust in the ECMWF model, which is predicting that a weak El Niño event will be in place by fall.
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 to below average in March 2012. Virtually all African waves originate in the MDR, and 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.) Conversely, when MDR SSTs are cooler than average, a below-average Atlantic hurricane season is more likely. This year's SSTs in the MDR are among the coolest we've seen since our current active hurricane period began in 1995. The cool temperatures are largely due to strong surface winds that blew during the winter over the tropical Atlantic in response to the positive phase of the North Atlantic Oscillation (NAO.) The strong winds stirred up the water, bringing up cooler waters from the depths.
How good are the April forecasts?
The forecasters are using a new statistical model developed last year 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 correctly predicted a more active than average season for last year, though called for more activity than was actually observed. However, 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.
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 by June 1, is the one worth paying attention to. Their early June forecasts have shown considerable skill over the years.
Preliminary NWS survey of the April 3rd, 2012 Dallas, Texas tornadoes
The Fort Worth Weather Service office began surveying tornado damage yesterday from three tornadoes that ripped through the Dallas metro area on Tuesday afternoon. Official storm surveys will be released in the next few days. The Arlington/Kennendale tornado has a preliminary rating of EF-2. They suspect wind speeds peaked around 135mph, a path length of 4.6 miles, and a maximum width of 400 yards (1/4 mile). The Lancaster/Hutchins tornado has a preliminary rating of EF-2, and they suspect it had a maximum width of 200 yards (1/8 mile). The Forney tornado has a preliminary rating of EF-3, with suspected winds up to 150 mph. Surveys are ongoing--there's a lot of damage to see along the tornado paths. These ratings reflect the most severe damage the teams have seen so far. Eighteen tornado warnings were issued by the National Weather Service in Fort Worth on Tuesday, which saved hundreds of lives. There were no fatalities Tuesday, which is welcome news in the wake of 2011's deadly tornado season.
Figure 2. This photo was taken by a NWS Storm Survey team in Lancaster TX on April 4, 2012. It shows EF-2 tornado damage that occurred in parts of Lancaster on April 3, 2012.
Figure 3. From the Weather Service: This is an aerial photograph of a tornado damaged area in Arlington TX. The damage from the tornado that affected Kennedale and Arlington on April 3, 2012 has been given a preliminary rating of EF-2. The photo was taken on Wednesday, April 4, looking to the east.
Portlight disaster relief charity responds to this year's tornadoes
Disaster relief charity portlight.org sent Thomas Hudson to the DFW area yesterday to do damage assessment and determine whether there is a need for Portlight's services in the wake of the tornadoes. Check out the Portlight blog to see the latest updates, and catch up the great work they've been doing in Harrisburg, Illinois in the wake of the devastating EF-4 tornado that hit the town on Leap Day, 2012.
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