June hurricane season outlook and Arthur/Alma post-mortem

The season's first tropical storm, Arthur, has come and gone. Arthur formed Saturday afternoon--one day before the official start of hurricane season--and immediately made landfall in northern Belize on the Yucatan Peninsula. Arthur brought heavy rain to Belize, southeast Mexico, Guatemala, and Honduras. Resulting flooding in Belize has killed at least two people, left five missing, and flooded many homes and businesses. Rainfall totaled 171mm (about 7 inches) at Tikal International Airport in Northern Guatemala, and satellite rainfall estimates suggest as much as 10 inches of rain may have fallen in some isolated areas of southeast Mexico. Arthur's remains will continue to soak the region with up to six more inches of rain in the coming two days. No computer models are suggesting that Arthur's center will drift over any ocean areas and rise from the dead again. However, a new low pressure area (91E) has developed in the Eastern Pacific off the southeast coast of Mexico, just south of Arthur's remains. Moisture from Arthur/Alma may fuel the development of a third tropical storm--Boris--which could form Tuesday or Wednesday, and move northwestward into Mexico.

Before Arthur was a he, he was a she--Tropical Storm Alma in the Eastern Pacific. Alma soaked Costa Rica and Nicaragua with up to ten inches of rain, damaging or blocking 117 roads and destroying a number homes in Costa Rica, where an estimated 1,500 people are homeless. In Nicaragua, three people died, ten are missing, and 25,000 people are homeless in the wake of the storm.

June Atlantic hurricane season outlook
June is typically the quietest month of the Atlantic hurricane season. On average, we see only one named storm every two years in June. Only one major hurricane has made landfall in June--Category 4 Hurricane Audrey of 1957, which struck the Texas/Louisiana border area on June 27 of that year, killing 550. The highest number of named storms for the month is three, which occurred in 1936 and 1968. In the 13 years since the current active hurricane period began in 1995, there have been ten June named storms (not including this year's Arthur). Five tropical storms have formed in the first half of June in that 13-year period, giving a historical 38% chance of a first-half-of-June named storm.


Figure 1. Tracks of all June tropical storms and hurricanes since 1851.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are still quite cool in June, which limits the regions where tropical storm formation can occur. SSTs are typically too cold to allow storms to develop between the coast of Africa and the Lesser Antilles Islands, and there has only been once such development in the historical record (Figure 1). This year (Figure 2), SSTs are about 2°C above average off the coast of Africa, which has led to some unusually vigorous tropical waves for this time of year. SSTs near the Cape Verde Islands are about 25°C, and this will need to increase to at least 26°C before we need to be concerned about African tropical waves developing.

Typically, June storms only form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest. SSTs are 26°C-28°C, which is 0.5°C above average over most of this region. June storms typically form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. African tropical waves, which serve as the instigators of about 85% of all major hurricanes, are usually too far south in June to trigger tropical storm formation. Every so often, a tropical wave coming off the coast of Africa moves far enough north to act as a seed for a June tropical storm. This was the case for Arthur this year (which also had major help from the spinning remnants of the Eastern Pacific's Tropical Storm Alma). Another way to get Atlantic June storms is for a disturbed weather area in the Eastern Pacific Intertropical Convergence Zone (ITCZ) to push north into the Western Caribbean and spawn a storm there. This was the case for Tropical Storm Alberto of 2006 (which may have also had help from an African wave).


Figure 2. Sea Surface Temperature (SST) departure from average for May 29, 2008.

Tropical Cyclone Heat Potential
It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential (TCHP, Figure 3) is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification. As we can see, the heat energy available in the tropical Atlantic has declined steadily since 2005, when the highest SSTs ever measured in the tropical Atlantic occurred. I expect that the TCHP will continue to remain well below 2005 levels this year, so we should not see any intense hurricanes in July, like we saw that year.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for May 31 2005 (top), May 31 of last year (middle) and May 31 2008 (bottom). TCHP is a measure of the total heat energy available in the ocean. Record high values of TCHP were observed in 2005. TCHP this year is much lower. Image credit: NOAA/AOML.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart.

Wind shear over the past 11 days (Figure 4) has been unusually low over the Gulf of Mexico, Caribbean, and Central America. Th shear was 4-8 m/s (8-16 knots) below average last week, aiding the formation of Tropical Storm Alma/Arthur. The jet stream is usually very active and quite far south in June, bringing plenty of shear. The jet stream looped unusually far northwards during late May, but is forecast to return to a more normal position over the coming two weeks, increasing the shear over the June breeding grounds for tropical storms. The jet stream will gradually weaken and retreat northwards as summer progresses, bringing lower wind shear and greater chances for tropical storm formation.


Figure 4. Top: Average wind shear over the 11 days ending on May 30. Wind shear is the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude) in meters per second (multiply by two to get the approximate wind shear in knots). In most circumstances, wind shear above 20 knots (10 m/s, the blue colors in the top image) will act to inhibit tropical storm formation. Wind shear below 12 knots (6 m/s, the orange colors) is very conducive for tropical storm formation. Bottom: Departure of wind shear from average for the past 11 days in meters per second. Note the unusually low wind shear area near Central America where Alma/Arthur developed. Image credit: NOAA/CPC.

Dry air and African dust
It's too early to concern ourselves with dry air and dust coming off the coast of Africa, since these dust outbreaks don't make it all the way to the June tropical cyclone breeding grounds in the Western Caribbean and the Gulf of Mexico. Developing storms do have to contend with dry air from Canada moving off the U.S. coast; this was a key reason why last year's Subtropical Storm Andrea never became a tropical storm.

Steering currents
The steering current pattern over the past few weeks has typical for June, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are well-predictable only about 3-5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. At present, it appears that the coming two weeks will maintain the typical June pattern, bringing many troughs of low pressure off the East Coast capable of recurving any June storms that might form. There is no telling what might happen during the peak months of August, September, and October--we might be in for a repeat of the favorable 2006 steering current pattern that recurved every storm out to sea--or the unfavorable 2005 pattern, that steered so many hurricanes into the U.S.

Summary
Recent history suggests a 38% chance of a named storm occurring in the first half of June. Given the current two-week wind shear forecast, the odds are that Arthur will be the only tropical storm we'll see during the first half of June. Still, there will be "holes" opening up from time to time in the shear pattern, so we need to keep our eye on the Gulf of Mexico and Western Caribbean. None of the computer models are forecasting tropical storm development in the coming seven days.

I'll have an update Tuesday afternoon, when the latest Colorado State University Atlantic Hurricane season forecast by Phil Klotzbach and Dr. Bill Gray will be available.

Jeff Masters