Atlantic hurricane outlook for the remainder of June

The last half of June is usually one of the quietest portions of hurricane season. In the 14 years since the current active hurricane period began in 1995, only four tropical storms formed in the last half of June. Thus, recent history gives us a 29% chance of a last-half-of-June named storm. None of those four storms since 1995 became a hurricane, and hurricanes are quite rare in June.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have remained close to average over the tropical Atlantic between Africa and Central America this month (Figure 1). These are the are the coolest SST anomalies we've seen since 1994. The strength of the Azores-Bermuda high has been near average over the past two weeks, driving near-average trade winds. Stronger-than-average trade winds were observed through most of the period November 2008 - May 2009, which helped cool the tropical Atlantic substantially. Strong winds mix up colder water from the depths and cause greater evaporative cooling. The latest 2-week run of the GFS model predicts continued average trade winds over the tropical Atlantic for the remainder of June, so expect the near-average SSTs to continue over the tropical Atlantic as we head into July.

Typically, June tropical storms form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest. SSTs are 26 - 28°C in these regions, which is about 0.5°C above average for this time of year. 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. SSTs are too cold in June to allow storms to develop between the coast of Africa and the Lesser Antilles Islands--there has only been once such development in the historical record--Ana of 1979, which coincidentally will be the name given to this year's first storm.


Figure 1. Sea Surface Temperature (SST) departure from average for June 11, 2009. SSTs were near average over the tropical Atlantic. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, signaling the possible start of an El Niño episode. Image credit: NOAA/NESDIS

El Niño
NOAA's Climate Prediction Center issued an El Niño Watch last week, saying "that conditions are favorable for a transition from neutral to El Niño conditions during June - August 2009". The pattern of changes in surface winds, upper-level winds, sea surface temperatures, and deeper water heat content are all consistent with what has been observed during previous developing El Niños. We are currently experiencing neutral conditions, with ocean temperatures in the Equatorial Eastern Pacific just 0.2°C below the threshold for El Niño. In the week since the El Niño watch was issued, ocean temperatures have remained nearly steady in the Eastern Pacific, so we are not rushing into an El Niño just yet. As I discussed in detail in an earlier post, most of our more advanced El Niño computer models are predicting a weak El Niño event for the coming Atlantic hurricane season. If this indeed occurs, it is likely that Atlantic hurricane activity will be suppressed due to the strong upper-level winds an El Niño usually brings to the tropical Atlantic, creating high wind shear that tears hurricanes apart.

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. The jet stream's band of strong high-altitude winds is the main source of wind shear in June over the Atlantic hurricane breeding grounds, since the jet is very active and located quite far south this time of year.

The jet stream over the past few weeks has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England. This often leaves a "hole" of low shear between the two branches off the coast of North Carolina, which is where Tropical Depression One formed at the end of May.

The jet stream is forecast (Figure 2) to maintain this two-branch pattern over the coming two weeks. This means that the waters offshore of the Carolinas are the most likely place for a tropical storm to form during this period.


Figure 2. Wind shear in m/s between 200 mb and 850 mb, as forecast by the 00Z June 12, 2009 run of the GFS model. The position of the subtropical jet stream is forecast to change little over the next two weeks, and this jet will bring high wind shear to the Caribbean and Gulf of Mexico for most of the remainder of June. There will at times be a region of low shear between the polar jet (northern set of arrows on the plots) and the subtropical jet, allowing for possible tropical development off the coast of North Carolina. Wind speeds are given in m/s; multiply by two to get a rough conversion to knots. Thus, the red regions of low shear range from 0 - 16 knots.

Steering currents
The steering current pattern over the past few weeks has not changed much, and is typical for June. We have 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 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 2008 pattern, that steered Ike and Gustav into the Gulf of Mexico.

Summary
Recent history suggests a 29% chance of a named storm occurring in the second half of June. Given that the current SST pattern and two-week wind shear forecast look fairly typical for June, I'll go with a 20% chance of a named storm forming during the last half of June. There's currently nothing out there of note, but we should start watching the region off the North Carolina coast 4 - 7 days from now.

Other stuff
Saturday, June 13 marks the last day of the Vortex2 tornado research project. The team of University of Michigan students writing our Vortex2 blog has posted some great photos and accounts of the storms they caught up to this week.

The Portlight.org charity is hard at work helping victims of the Volusia County, Florida floods.

Today's post will likely be my final "live" post until June 29, as I am headed to London, England, and Kefalonia Island, Greece for my first-ever European vacation. My fellow wunderground meteorologists will be posting to my blog if any tropical weather of note develops. I also recommend following the blog of wunderblogger Weather456, who works as a forecaster on St. Kitts Island in the Lesser Antilles. If the tropics remain quiet, I've prepared some "canned" blogs that will be posted on my blog. The topics include:

--The Atlantic Meridional Mode: implications for the 2009 hurricane season
--African dust forecast for the 2009 Atlantic hurricane season
--U.S. vulnerability to sea level rise: the Coastal Vulnerability Index (CVI)
--The six Hurricane and Typhoon Hunter flights that never came back
--Sea level rise: the forecast
--Sea level rise in the Northeast U.S. from ocean current changes

Jeff Masters