Summer Weather Acc. To El Nino

El Niño Brings Wet Summer to Plains; Western Drought Continues

Atlantic Hurricane Potential

As we stand on the cusp of the peak part of hurricane season, all of the major groups that perform long-range seasonal hurricane forecasts are still calling for an active 2013 Atlantic hurricane season. NOAA forecasts an above-normal and possibly very active Atlantic hurricane season in 2013, in their August 8 outlook. They give a 70% chance of an above-normal season, a 25% chance of an near-normal season, and 5% chance of a below-normal season. They predict a 70% chance that there will be 13 – 19 named storms, 6 – 9 hurricanes, and 3 – 5 major hurricanes, with an Accumulated Cyclone Energy (ACE) 120% – 190% of the median. If we take the midpoint of these numbers, NOAA is calling for 16 named storms, 7.5 hurricanes, 4 major hurricanes, and an ACE index 155% of normal. This is well above the 1981 – 2010 average of 12 named storms, 6 hurricanes, and 3 major hurricanes. Hurricane seasons during the active hurricane period 1995 – 2012 have averaged 15 named storms, 8 hurricanes, and 4 major hurricanes, with an ACE index 151% of the median.

Figure 1. Tropical Storm Dorian on July 25, 2013, when the storm reached peak intensity–sustained winds of 60 mph. Formation of early-season tropical storms like Chantal and Dorian in June and July in the deep tropics is usually a harbinger of an active Atlantic hurricane season. Image credit: NASA.

NOAA cites five main reasons to expect an active remainder of hurricane season:

1) Sea Surface Temperatures (SSTs) are above average in the Main Development Region (MDR) for hurricanes, from the coast of Africa to the Caribbean. As of August 9, SST were 0.4°C (0.8°F) above average.
2) Trade winds are weaker than average across the MDR, which has caused the African Monsoon to grow wetter and stronger, the amount of spin over the MDR to increase, and the amount of vertical wind shear to decrease.
3) No El Niño event is present or expected this fall.
4) There have been two early-season tropical storms in the deep tropics (Tropical Storms Chantal and Dorian), which is generally a harbinger of an above-normal season.
5) We are in an active hurricane period that began in 1995.

Colorado State predicts a much above-average hurricane season
A much above-average Atlantic hurricane season is on tap for 2013, according to the seasonal hurricane forecast issued August 2 by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). The CSU team is calling for 18 named storms, 8 hurricanes, and 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 142. The forecast calls for an above-average chance of a major hurricane hitting the U.S., both along the East Coast (40% chance, 31% chance is average) and the Gulf Coast (40% chance, 30% chance is average). The risk of a major hurricane in the Caribbean is also above average, at 53% (42% is average.)

Analogue years
The CSU team picked five previous years when atmospheric and oceanic conditions were similar to what we are seeing this year: cool neutral ENSO conditions and slightly above-average tropical Atlantic sea surface temperatures. Those five years were 2008, a very active year with 16 named storms and 4 major hurricanes–Gustav, Ike, Paloma, and Omar; 2007, an active year with 15 named storms and two Category 5 storms–Dean and Felix; 1996, an above average year with 13 named storms and 6 major hurricanes–Edouard, Hortense, Fran, Bertha, Isidore, and Lili; 1966, an average year with 11 named storms and 3 major hurricanes–Inez, Alma, and Faith; and 1952, a below average year with 7 named storms and 3 major hurricanes. The average activity during these five analogue years was 12.4 named storms, 7.2 hurricanes, and 3.8 major hurricanes.

TSR predicts an above-average hurricane season: 14.8 named storms
The August 6 forecast for the 2013 Atlantic hurricane season made by British private forecasting firm Tropical Storm Risk, Inc. (TSR) calls for an active season with 14.8 named storms, 6.9 hurricanes, 3 intense hurricanes, and an Accumulated Cyclone Energy (ACE) of 121. 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 good for these August forecasts–47% – 59% higher than a “no-skill” forecast made using climatology. TSR predicts a 58% chance that U.S. land falling activity will be above average, a 26% chance it will be near average, and a 16% chance it will be below average. They project that 4 named storms will hit the U.S., with 1.8 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 August forecasts for U.S. landfalls just 9% – 18% higher than a “no-skill” forecast made using climatology. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.4 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 near average trade winds during these periods, and both of these factors should act to increase hurricane and tropical storm activity.

Figure 2. Comparison of the percent improvement over climatology for May and August seasonal hurricane forecasts for the Atlantic from NOAA, CSU and 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.

Figure 3. Comparison of the percent improvement in mean square error over climatology for seasonal hurricane forecasts for the Atlantic from NOAA, CSU and TSR from 2003-2012, using the Mean Square Skill Score (MSSS). The figure shows the results using two different climatologies: a fixed 50-year (1950 – 1999) climatology, and a 2003 – 2012 climatology. Skill is poor for forecasts issued in December and April, moderate for June forecasts, and good for August forecasts. Image credit: Tropical Storm Risk, Inc.

FSU predicts an above-average hurricane season: 15 named storms
The Florida State University (FSU) Center for Ocean-Atmospheric Prediction Studies (COAPS) issued their fifth annual Atlantic hurricane season forecast on May 30, calling for a 70% probability of 12 – 17 named storms and 5 – 10 hurricanes. The mid-point forecast is for 15 named storms, 8 hurricanes, and an accumulated cyclone energy (ACE) of 135. The scientists use a numerical atmospheric model developed at COAPS to understand seasonal predictability of hurricane activity. The model is one of only a handful of numerical models in the world being used to study seasonal hurricane activity and is different from the statistical methods used by other seasonal hurricane forecasters such as Colorado State, TSR, and PSU (NOAA uses a hybrid statistical-dynamical model technique.) The FSU forecast has been one of the best ones over the past four years:

2009 prediction: 8 named storms, 4 hurricanes. Actual: 9 named storms, 3 hurricanes
2010 prediction: 17 named storms, 10 hurricanes. Actual: 19 named storms, 12 hurricanes
2011 prediction: 17 named storms, 9 hurricanes. Actual: 19 named storms, 7 hurricanes
2012 prediction: 13 named storms, 7 hurricanes. Actual: 19 named storms, 10 hurricanes

Penn State predicts an above-average hurricane season: 16 named storms
A statistical model by Penn State’s Michael Mann and alumnus Michael Kozar is calling for an active Atlantic hurricane season with 16 named storms, plus or minus 4 storms. Their prediction was made using statistics of how past hurricane seasons have behaved in response to sea surface temperatures (SSTs), the El Niño/La Niña oscillation, the North Atlantic Oscillation (NAO), and other factors. The statistic model assumes that in 2013 the May 0.87°C above average temperatures in the MDR will persist throughout hurricane season, the El Niño phase will be neutral to slightly warm, and the North Atlantic Oscillation (NAO) will be near average.

The PSU team has been making Atlantic hurricane season forecasts since 2007, and these predictions have done pretty well, except for in 2012, when an expected El Niño did not materialize:

2007 prediction: 15 named storms, Actual: 15
2009 prediction: 12.5, named storms, Actual: 9
2010 prediction: 23 named storms, Actual: 19
2011 prediction: 16 named storms, Actual: 19
2012 prediction: 10.5 named storms, Actual: 19

UK Met Office predicts a slightly above-average hurricane season: 14 named storms
The UKMET office forecast for the 2013 Atlantic hurricane season, issued May 13, calls for slightly above normal activity, with 14 named storms, 9 hurricanes, and an ACE index of 130. In contrast to the statistical models relied upon by CSU, TSR, and NOAA, the UKMET model is done strictly using two dynamical global seasonal prediction systems: the Met Office GloSea5 system and ECMWF system 4. In 2012, the Met Office forecast was for 10 tropical storms and an ACE index of 90. The actual numbers were 19 named storms and an ACE index of 123.

Figure 4. Total 2013 Atlantic hurricane season activity as predicted by twelve different groups.

NOAA predicts a below-average Eastern Pacific hurricane season
NOAA’s pre-season prediction for the Eastern Pacific hurricane season, issued on May 23, calls for a below-average season, with 11 – 16 named storms, 5 – 8 hurricanes, 1 – 4 major hurricanes, and an ACE index 60% – 105% of the median. The mid-point of these ranges gives us a forecast for 13.5 named storms, 6.5 hurricanes, and 2.5 major hurricanes, with an ACE index 82% of average. The 1981 – 2010 averages for the Eastern Pacific hurricane season are 15 named storms, 8 hurricanes, and 4 major hurricanes.

NOAA predicts a below-average Central Pacific hurricane season
NOAA’s pre-season prediction for the Central Pacific hurricane season, issued on May 22, calls for a below-average season, with 1 – 3 tropical cyclones. An average season has 4 – 5 tropical cyclones, which include tropical depressions, tropical storms, and hurricanes. Hawaii is the primary land area affected by Central Pacific tropical cyclones.

West Pacific typhoon season forecast not available this year
Dr. Johnny Chan of the City University of Hong Kong usually issues a seasonal forecast of typhoon season in the Western Pacific, but did not do so in 2012 or 2013. An average typhoon season has 27 named storms and 17 typhoons. Typhoon seasons immediately following a La Niña year typically see higher levels of activity in the South China Sea, especially between months of May and July. Also, the jet stream tends to dip farther south than usual to the south of Japan, helping steer more tropical cyclones towards Japan and Korea.

Quiet in the Atlantic this weekend
There are no Atlantic threat areas to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming seven days. However, there are some indications that the atmosphere over the tropical Atlantic will become more conducive for tropical storm formation beginning around August 15. The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 – 60 days, may move into the Atlantic then, increasing tropical storm formation odds. At the same time, the computer models are indicating an increase in moisture over the tropical Atlantic, due to a series of tropical waves expected to push off of the coast of Africa. There will also be several eastward-moving Convectively-Coupled Kelvin Waves (CCKWs) traversing the Atlantic during that period. These atmospheric disturbances have a great deal of upward-moving air, which helps strengthen the updrafts of tropical disturbances. Formation of the Eastern Pacific’s Hurricane Gil and Henriette were aided by CCKWs. These same CCKWs will cross into the Atlantic and increase the odds of tropical storm formation during the period August 15 – 20.

Have a great weekend, everyone!

Jeff Masters


August Hurricane Potential

A massive dust storm that formed over the Sahara Desert early this week has now pushed out over the tropical Atlantic, and will sharply reduce the odds of tropical storm formation during the first week of August. The dust is accompanied by a large amount of dry air, which is making the Saharan Air Layer (SAL) much drier than usual this week. June and July are the peak months for dust storms in the Southwest Sahara, and this week’s dust storm is a typical one for this time of year. Due in large part to all the dry and dusty air predicted to dominate the tropical Atlantic over the next seven days, none of the reliable computer models is predicting Atlantic tropical cyclone formation during the first week of August.

Figure 1. A massive dust storm moves off the coast of Africa in this MODIS image taken at 1:40 UTC July 30, 2013. Image credit: NASA.

Video 1. The predicted movement through August 3 of this week’s Africam dust storm, using the NOAA NGAC aerosol model. Image credit: NOAA Visualization Laboratory.

How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:

1) Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.

2) The Saharan Air Layer (SAL) is a layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion–a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this “inversion” acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm’s updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane.

3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as “condensation nuclei”–centers around which raindrops can form and grow–the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)

Figure 2. Map of the mean summer dust optical thickness derived from satellite measurements between 1979 and 2000. Maximum dust amounts originate in the northern Sahel (15° to 18° N) and the Sahara (18° to 22° N). The Bodele depression in Chad is also an active dust source. Image credit: Evidence of the control of summer atmospheric transport of African dust over the Atlantic by Sahel sources from TOMS satellites (1979-2000), by C. Moulin and I. Chiapello, published in January 2004 in Geophysical Research Letters.

Dust in Africa’s Sahel region and Atlantic hurricane activity
The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18° – 22° N) and the northwestern Sahel (15° – 18° N) (Figure 3.) The dust from the Southwest Sahara stays relatively constant from year to year, but the dust from the Northwest Sahel varies significantly, so understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. In 2012 (Figure 3), precipitation across the northwestern Sahel was much above average, which should result in less dust than usual over the Atlantic this fall, increasing the odds of a busy 2013 hurricane season.

Figure 3. Rainfall over the Northwest Sahel region of Africa was about 200% of average during the 2012 rainy season. The heavy rains promoted vigorous vegetation growth in 2013, resulting in less bare ground capable of generating dust. Image credit: NOAA/Climate Prediction Center.

Saharan Air Layer Analysis from the University of Wisconsin

Atlantic dust forecast from the Tel-Aviv University Weather Research Center

Dr. Amato Evan published a study in Science magazine March 2009 showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.

Kaufman, Y. J., I. Koren, L. A. Remer, D. Rosenfeld, and Y. Rudich, 2005a: The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc. Natl. Acad. Sci. USA, 102, 11 207–11 212.

Wang, Chunzai, Shenfu Dong, Amato T. Evan, Gregory R. Foltz, Sang-Ki Lee, 2012, Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes, J. Climate, 25, 5404–5415.


Rains on the East Coast

Tropical Storm Andrea is rapidly losing its tropical characteristics as it barrels northeastwards at 27 mph up the U.S. East Coast, but it still has plenty of tropical moisture that is feeding very heavy rains. Rains of 2 – 4″ are expected along a swath from South Carolina to New England from Andrea over the next two days. Pine Ridge, NC has received 6.5″ of rain from Andrea, and New River MCAS, North Carolina picked up 2″ of rain as of 9 am EDT this morning, along with a wind gust of 47 mph at 3:18 am. The same band of heavy thunderstorms spawned a possible tornado near Hubert, North Carolina at 4:45 am EDT. Andrea has spawned a preliminary count of eleven tornadoes, which is a respectable number for a landfalling June tropical storm, but not a record. According to TWC’s severe weather expert Dr. Greg Forbes, there have been two other June tropical storms since the year 2000 that spawned far more tornadoes–Tropical Storm Bill during June 29 – July 3, 2003 (32 tornadoes in FL, GA, LA, AL, MS, SC, NC, NJ), and Tropical Storm Allison of June 7 – 17 2001 (28 tornadoes in FL, AL, GA, LA, MS, SC, VA, MA, ME.) Only one of Andrea’s tornadoes caused an injury, a tornado that hit The Acreage in Palm Beach County at 6:45 am EDT. The highest storm surge from Andrea was 4.55′ at Cedar Key, Florida.

Figure 1. Yummy’s cafe in Gulfport, Florida was hit Thursday morning by a waterspout that moved ashore and became a tornado.(LAUREN CARROLL/Tampa Bay Times)

Figure 2. Predicted rainfall for the 48-hour period from 8 am EDT Friday, June 7, to 8 am EDT Sunday, June 8, 2013. Image credit: NOAA.

Video 1. NASA animation of Andrea satellite images. More cool NASA images of Andrea are here.

The Atlantic hurricane season is getting longer
Andrea’s formation in June continues a pattern of an unusually large number of early-season Atlantic named storms we’ve seen in recent years. Climatologically, June is the second quietest month of the Atlantic hurricane season, behind November. During the period 1870 – 2012, we averaged one named storm every two years in June, and 0.7 named storms per year during May and June. In the nineteen years since the current active hurricane period began in 1995, there have been fifteen June named storms (if we include 2013’s Tropical Storm Andrea.) June activity has nearly doubled since 1995, and May activity has more than doubled (there were seventeen May storms in the 75-year period 1870 – 1994, compared to 6 in the 19-year period 1995 – 2013.) Some of this difference can be attributed to observation gaps, due to the lack of satellite data before 1966. However, even during the satellite era, we have seen an increase in both early season (May – June) and late season (November – December) Atlantic tropical storms. Dr. Jim Kossin of the University of Wisconsin looked at the reasons for this in a 2008 paper titled, “Is the North Atlantic hurricane season getting longer?” He concluded that there is a “apparent tendency toward more common early- and late-season storms that correlates with warming Sea Surface Temperature but the uncertainty in these relationships is high.” He found that hurricane season for both the period 1950-2007 and 1980-2007 got longer by 5 to 10 days per decade (see my blog post on the paper.)

Invest 92L in the Central Atlantic no threat to develop
Satellite images show that disorganized tropical wave is in the Central Atlantic, about a two-day journey from the Lesser Antilles Islands. NHC designated this system 92L Thursday afternoon. High wind shear of 30 – 40 knots is ripping up the thunderstorms in 92L as they form, and wind shear is predicted to remain 30 – 40 knots for the next five days, making development unlikely. The wave will likely bring heavy rain showers and gusty winds to the northern Lesser Antilles Islands beginning on Sunday night. None of the reliable computer models is showing development of a tropical cyclone in the Atlantic over the next seven days.


Overview of the 2012 Hurricane Season fr/Jeff Masters

The bizarrely active hurricane season of 2012 draws to a close

Published: 4:50 PM GMT on November 30, 2012
The long and highly destructive hurricane season of 2012 has finally drawn to a close. The hurricane season of 2012 will long be remembered for spawning Hurricane Sandy–a freakish storm that was the largest, most powerful, and second most destructive Atlantic hurricane on record. But this year’s hurricane season had a number of unique attributes, making it one of the most bizarre seasons I’ve witnessed. Despite featuring a remarkable nineteen named storms–tied for the third highest total since record keeping began in 1851–this year’s hurricane season had just one major hurricane. That storm was Hurricane Michael, which stayed at Category 3 strength for a scant six hours. This is the least number of major hurricanes in a season since the El Niño year of 1997, which had only Category 3 Hurricane Erika. There were no Category 4 or 5 hurricanes in 2012, for just the 3rd time since the active hurricane period we are in began in 1995. The only two other years since 1995 without a Category 4 or stronger hurricane were the El Niño years of 2006 and 1997. Both of those seasons had around half the number of named storms of 2012–nine in 2006, and eight in 1997. The relative lack of strong storms in 2012 helped keep the Accumulated Cyclone Energy (ACE) down to 128, about 30% above average.

Figure 1. Hurricane Sandy at 10:10 am EDT October 28, 2012. Image credit: NASA/GSFC.

A near-average year for number of tropical cyclones hitting the U.S.
Since the active hurricane period we’ve been in began in 1995, the U.S. has averaged getting hit by 4 named storms per year, with an average of 1.7 of these being hurricanes, and 0.6 being major Category 3 and stronger hurricanes. This year, we were hit by 3 named storms (Beryl, Debby, and Isaac). One of these was a hurricane (Isaac). Sandy didn’t count as a hurricane strike on the U.S., since it transitioned to an extratropical cyclone a few hours before landfall. No major hurricanes hit the U.S., making 2012 the 7th consecutive year without a major hurricane strike. The only other time we’ve had a streak that long occurred between 1861 – 1868, during the decade of the Civil War.

Figure 2. Vertical instability over the tropical Atlantic in 2004 – 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere (note that the same scale is not used in all the plots, making the black climatological line appear different, when it is really the same for each plot.) Thunderstorms grow much more readily when vertical instability is high. Instability was near average during the August – October peak of hurricane season in 2004 – 2009, but was much lower than average during the hurricane seasons of 2010 – 2012. There was an unusual amount of dry, sinking air in the tropical Atlantic during 2010 – 2012, and the resulting low atmospheric instability reduced the proportion of tropical storms that have intensified into hurricanes. Vertical instability from 2004 – 2011 is taken from NOAA/RAMMB and for 2012 from NOAA/SSD.

Unusually stable air over the Tropical Atlantic in 2012
For the third consecutive hurricane season, 2012 featured an unusual amount of dry, sinking air over the tropical Atlantic and Caribbean Sea. Due to warmer-than-average sea surface temperatures and an active African Monsoon that generated plenty of African waves, a remarkably high number of tropical storms managed to form, but the unusually stable air in the hurricane genesis regions made it difficult for the storms to become strong. When we did see storms undergo significant intensification, it tended to occur outside of the tropics, north of 25°N, where there was not as much dry, sinking air (Sandy’s intensification as it approached landfall in Cuba was an exception to this rule.) If we look at the last nine hurricane seasons (Figure 2), we can see that the hurricane seasons of 2010, 2011, and 2012 all featured similar levels of highly stable air over the tropical Atlantic. This is in marked contrast to what occurred the previous six years. The past three seasons all featured a near-record number of named storms (nineteen each year), but an unusually low ratio of strong hurricanes. Steering patterns the past three years also acted to keep most of the storms out to sea. Is this strange pattern something we’ll see more of, due to climate change? Or is it mostly due to natural cycles in hurricane activity? I don’t have any answers at this point, but the past three hurricane seasons have definitely been highly unusual in a historical context. I expect the steering currents to shift and bring more landfalling hurricanes to the U.S. at some point this decade, though.

Figure 3. Sea water floods the Ground Zero construction site at the World Trade Center, Monday, Oct. 29, 2012, in New York City. Image credit: AP.

Most notable events of the Hurricane Season of 2012
Hurricane Sandy was truly astounding in its size and power. At its peak size, twenty hours before landfall, Sandy had tropical storm-force winds that covered an area nearly one-fifth the area of the contiguous United States. Since detailed records of hurricane size began in 1988, only one tropical storm (Olga of 2001) has had a larger area of tropical storm-force winds, and no hurricanes has. Sandy’s area of ocean with twelve-foot seas peaked at 1.4 million square miles–nearly one-half the area of the contiguous United States, or 1% of Earth’s total ocean area. Most incredibly, ten hours before landfall (9:30 am EDT October 30), the total energy of Sandy’s winds of tropical storm-force and higher peaked at 329 terajoules–the highest value for any Atlantic hurricane since at least 1969. This is 2.7 times higher than Katrina’s peak energy, and is equivalent to five Hiroshima-sized atomic bombs. At landfall, Sandy’s tropical storm-force winds spanned 943 miles of the the U.S. coast. No hurricane on record has been wider; the previous record holder was Hurricane Igor of 2010, which was 863 miles in diameter. Sandy’s huge size prompted high wind warnings to be posted from Chicago to Eastern Maine, and from Michigan’s Upper Peninsula to Florida’s Lake Okeechobee–an area home to 120 million people. Sandy’s winds simultaneously caused damage to buildings on the shores of Lake Michigan at Indiana Dunes National Lake Shore, and toppled power lines in Nova Scotia, Canada–locations 1200 miles apart!

Figure 4. Hurricane Isaac lit up by moonlight as it spins towards the city of New Orleans, LA, on August 26, 2012. The Suomi National Polar-orbiting Partnership (NPP) satellite captured these images with its Visible Infrared Imaging Radiometer Suite (VIIRS). The “day-night band” of VIIRS detects light in a range of wavelengths from green to near-infrared and uses light intensification to enable the detection of dim signals. Image Credit: NASA/NOAA, Earth Observatory.

Hurricane Isaac hit Louisiana as a Category 1 hurricane with 80 mph winds on August 28, but the storm’s massive wind field brought a storm surge characteristic of a Category 2 hurricane to the coast. A storm surge of 11.1 feet was measured at Shell Beach, LA and higher surges were reported in portions of Louisiana. Fortunately, the new $14.5 billion upgrade to the New Orleans levee system kept the city dry. Isaac killed 9 people in the U.S., and 29 in the Caribbean.

Hurricane Ernesto hit Mexico’s Yucatan Peninsula as a Category 1 hurricane with 85 mph winds on August 7. The storm killed 12 and did at least $250 million in damage.

Tropical Storm Debby formed on June 23, the earliest formation date on record for the season’s 4th storm. The previous record was Dennis, on July 5, 2005. Debby killed seven and did over $300 million in damage, but helped relieve drought conditions over Northern Florida and Southern Georgia.

Tropical Storm Beryl, which made landfall on May 28 near Jacksonville Beach, FL with 70 mph winds, was the strongest tropical storm to make landfall in the U.S. prior to June 1. Beryl killed two but did minimal damage.

Nadine lasted for 21.75 days as a named storm, the 5th longest-lasting tropical storm in the Atlantic basin.

It was the 3rd year in a row with 19 named storms.

No named storms existed during the month of July and November, but we still managed big numbers.

Only 7 seasons have had more hurricanes than 2012.

The season had two named storm before the official June 1 start of hurricane season, only the 3rd time that has occurred.

Eight named storms formed in August, which tied 2004 for the most to form in that month.

Typhoon Bopha a threat to the Philippines
In the Western Pacific, where typhoon season commonly brings several storms in December, we have impressive Typhoon Bopha. Bopha is expected to head west-northwest and intensify over the weekend, potentially arriving in the Philippines on Tuesday as a powerful Category 3 typhoon. Bopha formed at an unusually low latitude for a tropical cyclone–near 4°N. Storms forming that close to the Equator don’t get much help from the Earth’s spin to get spinning, and it is rare to see a tropical cyclone forming southwards of 5°N.

The Colorado State University hurricane forecast team, led by Phil Klotzbach and Bill Gray, has a more in-depth summary of the 2012 hurricane season.

Jeff Masters


Quiet Hurricane Season Expected

fr/Dr. Jeff Masters:

CSU: expect a quiet 2012 Atlantic hurricane season; EF-3 tornado confirmed in Texas
Posted by: JeffMasters, 2:47 PM GMT on April 05, 2012 +29
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: 200920011965, 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 NESDISbranch. 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.

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Dr. Jeff Masters Forecast for Rest of Hurricane Season

Invest 98L spinning up; outlook for remainder of hurricane season
Posted by: JeffMasters, 2:09 PM GMT on September 19, 2011 +6
A tropical wave midway between Africa and the Lesser Antilles (Invest 98L) continues to look well-organized on satellite imagery, with a modest amount of heavy thunderstorm activity and spin. An ASCAT pass from 8:08 pm EDT last night showed 98L was close to closing off a well-defined surface circulation. Wind shear as diagnosed by the SHIPS model is light, less than 10 knots, and is predicted to stay light to moderate through Friday. Ocean temperatures are 28 – 28.5°C, well above the threshold typically needed for a tropical storm to spin up. Water vapor satellite images show 98L is embedded in a moist environment, but there is dry air to the system’s northwest. However, given the light wind shear, this dry air may not pose a hindrance to development at this time. An analysis of upper level winds from the University of Wisconsin CIMMS group shows a pattern favorable for development, with an outflow channel open to both the north and south available to ventilate the storm and allow 98L to efficiently lift plenty of moisture to high levels.

Figure 1. Morning satellite image of 98L.

The models are not very aggressive about developing 98L into a tropical depression, but most of them do show some weak development. NHC gave the disturbance a 60% chance of developing into a tropical depression by Wednesday in their 8 am Tropical Weather Outlook. Given the recent increase in spin on visible satellite images and favorable environment for development, I’d bump these odds up to 70%. 98L is currently moving little, but is expected to begin a westward motion at 10 mph today. This motion would take 98L into the Lesser Antilles Islands by Friday or Saturday. The northern Lesser Antilles would be most likely to see the core of the storm, as has been the case for all of this year’s disturbances. However, a more southerly path across Barbados, as predicted by the GFS model, cannot be ruled out. Once 98L does reach the Lesser Antilles, all of the models indicate the storm will see a sharp increase in vertical wind shear due to strong upper-level winds out of the west. This shear should make it difficult for 98L to intensify as it moves though the islands.

Atlantic hurricane outlook for the rest of September
Ocean temperatures are starting to decline in the North Atlantic, though remain much above average in the Main Development Region (MDR) for hurricanes, from the coast of Africa to the coast of Central America, between 10°N and 20°N latitude. The latest departure of sea surface temperature (SST) from average plot (Figure 2) shows a large area of ocean temperatures near 1°C above average. The water temperatures were 0.8°C above average in this region during August, which is the 4th highest such reading on record. These warm waters will allow for an above-average chance of African tropical waves developing through early October. By early October, the African Monsoon typically begins to wane, spawning fewer tropical waves that tend to be weaker, and we should stop seeing development of newly-emerged tropical waves off the coast of Africa.

Figure 2. Departure of sea surface temperature (SST) from average for September 19, 2011. Ocean temperatures were about 1°C above average over much of the Main Development Region (MDR) for hurricanes, from the coast of Africa to the coast of Central America, between 10°N and 20°N latitude. In the Pacific off the coast of South America, we can see the tell-tale signature of a La Niña event, with cooler than average waters along the Equator. Also note the cooler than average waters between Bermuda and Puerto Rico, due to the passage of Hurricane Maria and Hurricane Katia. Image credit:NOAA/NESDIS.

Wind shear has been near average over the tropical Atlantic this hurricane season, and is currently at its climatological low point, which occurs in mid-September. The latest 2-week run of the GFS model shows wind shear will remain at the sort of typical low levels we usually see this time of year. With ocean temperatures at near-record warm levels, this combination would tend to favor formation of at least two tropical storms between now and the beginning of October. One inhibiting factor, though, may be the continued presence of dry, stable air over the tropical Atlantic. Hurricanes like to have an unstable atmosphere, with moist, warm air near the surface, and cold, dryer air aloft. This situation helps the updrafts in the storm grow stronger. This year, we’ve had unusually stable air (Figure 3.) This has really put the brakes on intensification of most of the tropical storms that have formed. The current ratio of 14 named storms but only 3 hurricanes is unprecedented in the historical record, going back to 1851. Usually, just over half of all Atlantic tropical storms intensify into hurricanes. One other factor to consider, the 30-60 day pattern of increased thunderstorm activity known as the Madden-Jullian Oscillation (MJO), looks like it will have little influence over the coming week. The MJO has been weak all month, and is predicted to stay weak for the remainder of this week.

Figure 3. Vertical instability, as measured by the difference in temperature near the surface to the bottom of the stratosphere. The atmosphere in the Caribbean (left) and tropical Atlantic between Africa and the Lesser Antilles Islands (right) has been much more stable than average this year (average is the thick black line). Image credit: NOAA/CIRA.

Forecast of the rest of hurricane season
We are past the half-way point of the Atlantic hurricane season, which typically peaks on September 10. On average, about 60% of the activity has occurred by this point in the season. Since we’ve already had 14 named storms and 3 hurricanes, at the current rate, we would expect to see another 8 or 9 named storms, with 1 or 2 of them reaching hurricane strength. It’s pretty tough to maintain the sort of activity levels we’ve seen so far this year, so I am forecasting we’ll see 7 more named storms during the remainder of this season, taking us all the way to “W” in the alphabet. With the unusually stable air over the Atlantic showing no signs of abating, I predict that we’ll see just 2 of these storms reach hurricane strength. As far as steering currents go, the latest 2-week forecast from the GFS model doesn’t show any significant changes to the jet stream pattern we’ve seen all summer. There will continue to be a parade of troughs of low pressure moving off the U.S. East Coast that will tend to curve any storms northwards and then northeastwards out to sea, once they penetrate north of the Lesser Antilles Islands. This pattern favors strikes on North Carolina and New England, and discourages strikes on Texas. I doubt Texas will see a tropical storm this year given this steering pattern, and considering that Texas’ tropical cyclone season tends to peak in late August and early September. It is quite unusual for Texas to have a tropical storm or hurricane this late in the season, so they will probably have to look elsewhere for drought-busting rains.

Jeff Masters