Hurricane season begins today; normal June activity expected

By: Dr. Jeff Masters , 1:38 PM GMT on June 01, 2009

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Hurricane season is upon us, and it's time to take a look at the prevailing conditions and 2-week forecast for tropical cyclone activity in the Atlantic. 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 fourteen years since the current active hurricane period began in 1995, there have been eleven June named storms (if we include last year's Tropical Storm Arthur, which really formed on May 31). Five tropical storms have formed in the first half of June in that 14-year period, giving a historical 36% chance of a first-half-of-June named storm.


Figure 1. Tracks of all June tropical storms and hurricanes, 1851 - 2007.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are close to average over the tropical Atlantic between Africa and Central America this year (Figure 2). These temperatures are some of the coolest we've seen since 1995, when the current active hurricane period began. This year's cool SSTs should prevent a repeat of the unforgettable Hurricane Season of 2005, which had the highest SSTs on record in the tropical Atlantic. Note also that SSTs along the Equatorial Pacific off the coast of South America are quite a bit above average, signaling the possible start of an El Niño episode. As I discussed in Friday's post, odds are increasing for a weak El Niño to form in time for hurricane season, and this should cut down on the number and intensity of Atlantic tropical storms and hurricanes this year. However, if an El Niño is developing, it shouldn't start affecting Atlantic hurricane activity until August.

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 - 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. 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 of 2008 (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). 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 2. Sea Surface Temperature (SST) departure from average for June 1, 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.

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 considerably since 2005, when the highest SSTs ever measured in the tropical Atlantic occurred. TCHP this year is similar to last year's levels, which were high enough to support five major hurricanes.


Figure 3. Tropical Cyclone Heat Potential (TCHP) for May 31 2005 (top), May 31 of last year (middle) and May 30 2009 (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, and similar to last year. 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. 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 leaves a "hole" of low shear between the two branches off the coast of North Carolina, which is where Tropical Depression One formed. The low shear "hole" has dipped down into the northern Gulf of Mexico a few times. Disturbance 90L, which almost developed into a tropical storm before it came ashore in Mississippi/Alabama on May 23, took advantage of one of these low-shear areas.

The jet stream is forecast to maintain this two-branch pattern over the coming ten days. This means that the waters offshore of the Carolinas are the most likely place for a tropical storm to form during this period, though the northern Gulf of Mexico will at times have shear low enough to allow tropical storm formation. The latest 16-day forecast by the GFS model (Figure 4) predicts that the subtropical jet will weaken and retreat northwards by the middle of June, creating low-shear conditions over the Caribbean. This is a typical occurrence for mid-June, and we need to start watching the Western Caribbean for tropical storm formation by the middle of the month.


Figure 4. Wind shear forecast from the 00Z GMT June 1, 2009 run of the GFS model for June 1 (left panel) and June 17 (right panel). Currently, the polar jet stream is bringing high wind shear to the waters offshore New England, and the subtropical jet is bringing high wind shear to the Caribbean. This leaves the waters off the coast of North Carolina under low shear, making this area the most favored region for tropical storm formation over the next 7 - 10 days. By June 17, the subtropical jet is expected to weaken and move northwards, leaving the Caribbean under low shear, and favoring that region for tropical storm formation. 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.

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 2007's Subtropical Storm Andrea never became a tropical storm. Dr. Amato Evan of the University of Wisconsin will issue his dust forecast for the coming hurricane season later this week, and I'll be discussing his forecast in an upcoming post.

Steering currents
The steering current pattern over the past few weeks has been 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 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 36% chance of a named storm occurring in the first half of June. The current conditions in the atmosphere and ocean are near average, so expect about a 1/3 chance of a named storm between now and June 15. The computer models are currently not forecasting development of any tropical storms over the next seven days.

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

My next analysis and 2-week outlook for hurricane season is scheduled for June 13.

Jeff Masters

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Quoting Cazatormentas:
Another case of an INVEST far away from NHC jurisdiction.........?


NHC declares all tropical and subtropical systems that form in the Atlantic Ocean north of the equator.
Member Since: July 31, 2006 Posts: 56 Comments: 8112
Quoting Stormchaser2007:


Where are you getting this from?


He clearly hasn't looked at quikscat- Link
Member Since: July 31, 2006 Posts: 56 Comments: 8112
Hi everyone,
Did anyone see anything unusual weather wise around the general area of where the plane went down or might have gone down in the Atlantic, yesterday?
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Quoting Ossqss:
Is there any equipment on the islands in that area that could tell us what the pressure is?


The closest station found so far and based on the wind reports the low pass near them from the west.

Member Since: July 24, 2005 Posts: 407 Comments: 19076
From Wiki:

There are two definitions currently used for subtropical cyclones. Across the north Atlantic and southwest Indian ocean, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere. Across the eastern half of the northern Pacific, they require a mid-tropospheric cyclone to cut off from the main belt of the westerlies and only a weak surface circulation.

Our Invest meets both those criteria. It has a well-defined surface circulation with consolidated thunderstorms, with warm-core characteristics in the low to mid-levels.
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It has an eye, is that not enough evidence of a circulation? lol
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Quoting cg2916:
Guys, there's a reason the NHC hasn't named it Ana. It lacks a real low pressure center and low-level circulation.


Where are you getting this from?
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Quoting Cazatormentas:
Another case of an INVEST far away from NHC jurisdiction.........?


Matters to the Navy. They monitor, regardless of where it is.
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Quoting CatastrophicDL:
What type of naming criteria is there for a subtropical storm?


Same as a tropical storm, 40 mph. Only they form under hostile conditions.
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Guys, there's a reason the NHC hasn't named it Ana. It lacks a real low pressure center and low-level circulation.
Member Since: December 21, 2007 Posts: 13 Comments: 3037
Ponta Delgada, south of the low center
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Another case of an INVEST far away from NHC jurisdiction.........?
Member Since: October 18, 2007 Posts: 0 Comments: 149
What type of naming criteria is there for a subtropical storm?
Member Since: September 3, 2007 Posts: 3 Comments: 1519
Quikscat shows winds in the 30-35 mph range.
Member Since: July 31, 2006 Posts: 56 Comments: 8112
Is there any equipment on the islands in that area that could tell us what the pressure is?
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Quoting Cazatormentas:
Wooooooooooooooow!! Here is a High Resolution image from MODIS! Cyclone phase diagram (GFS) suggest a shallow warm core. Subtropical cyclone perhaps?





It does look impressive

Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting Cazatormentas:
Wooooooooooooooow!! Here is a High Resolution image from MODIS! Cyclone phase diagram (GFS) suggest a shallow warm core. Subtropical cyclone perhaps?





Very impressive shot.
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Quoting Cazatormentas:
Wooooooooooooooow!! Here is a High Resolution image from MODIS! Cyclone phase diagram (GFS) suggest a shallow warm core. Subtropical cyclone perhaps?





WOW! That looks like a hurricane. I think it's kind of irresponsible for the NHC not to even name this, let alone label it an invest.
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Quoting Cazatormentas:
Wooooooooooooooow!! Here is a High Resolution image from MODIS! Cyclone phase diagram (GFS) suggest a shallow warm core. Subtropical cyclone perhaps?





Yup.....don't care about what it will be.....we care about what it is NOW, and it is a subtropical cyclone that deserves a name.
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Wooooooooooooooow!! Here is a High Resolution image from MODIS! Cyclone phase diagram (GFS) suggest a shallow warm core. Subtropical cyclone perhaps?



Member Since: October 18, 2007 Posts: 0 Comments: 149
Quoting cg2916:
For those of you who don't think it's an invest, here: http://www.ssd.noaa.gov/PS/TROP/tdpositions.html.
It's named Invest, but not 92L. Right before 91L was 91L, it was just Invest.


That has been the procedure for years now...lol
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Quoting Ossqss:


Do you really think so? I just don't see it or any discussion points or data that would lend itself to such an upgrade. Just my take :)


Well now that you mention it, the chance of it becoming an invest tonight is probably lower than I originally thought. It looks like if it were to become an invest that would probably happen tomorrow or possibly Wednesday. It all depends on how it does tonight and tomorrow.
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Quoting Ossqss:


Do you really think so? I just don't see it or any discussion points or data that would lend itself to such an upgrade. Just my take :)


Could be for instructional purposes...
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For those of you who don't think it's an invest, here: http://www.ssd.noaa.gov/PS/TROP/tdpositions.html.
It's named Invest, but not 92L. Right before 91L was 91L, it was just Invest.
Member Since: December 21, 2007 Posts: 13 Comments: 3037
Quoting cg2916:

Do you think this might form something. I think it's too early and too far north. Even if it was September 10th, I wouldn't excpect that invest to form anything.


Uncertain, I can tell you that the structure has change from 24 hrs ago with a tightening of the wind field and the area remains embedded in a text-book style subtropical enviroment ( area of low pressure emebbedded or enveloped in a larger upper level circulation) with it forecast to become non frontal but it lays over waters below 20C, too cold for even subtropical development.

Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting Stormchaser2007:


No. SSD labels them as an invest so that they can have a floater on it. NRL will probably designate this as an official invest (92L) either late tonight or tomorrow.


Do you really think so? I just don't see it or any discussion points or data that would lend itself to such an upgrade. Just my take :)
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Quoting Weather456:


No not that


Sorry about that.
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Quoting cg2916:
I'm back. Seems that we have an invest, but it's not 92L. Wierd. Maybe that's how the NHC treats "subtropical" invests.


No. SSD labels them as an invest so that they can have a floater on it. NRL will probably designate this as an official invest (92L) either late tonight or tomorrow.
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Quoting Weather456:
I was looking at the low over the Azores over the NE and I said in mind, nar not that. It did possses a shallow warm type but was frecast to become entirely cold core.

Do you think this might form something. I think it's too early and too far north. Even if it was September 10th, I wouldn't excpect that invest to form anything.
Member Since: December 21, 2007 Posts: 13 Comments: 3037
Quoting Stormchaser2007:


Didnt quite catch that...


I said in my mind, No not that
Member Since: July 24, 2005 Posts: 407 Comments: 19076
I'm back. Seems that we have an invest, but it's not 92L. Wierd. Maybe that's how the NHC treats "subtropical" invests.
Member Since: December 21, 2007 Posts: 13 Comments: 3037
Quoting Weather456:
I was looking at the low over the Azores over the NE and I said in mind, nar not that. It did possses a shallow warm type but was frecast to become entirely cold core.


Didnt quite catch that...
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I was looking at the low over the Azores over the NE and I said in mind, nar not that. It did possses a shallow warm type but was frecast to become entirely cold core.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Discussion from earlier on the Atlantic.

Activity in the tropical Atlantic this morning is quiet with no areas of concern. The various model guidance is forecasting that things will remain quiet throughout this week. Looking way ahead to next week, the GFS and the European model is forecasting that convection may substantially increase across the western Caribbean, in response to the MJO wet pulse pushing eastward. So, conditions may become favorable for development at some point next week in the western Caribbean. It’s something that I’m going to keep an eye on and will keep you all updated.

The next tropical weather discussion will be issued by 6 am EDT Wednesday morning. No tropical weather discussions will be issued on Tuesday.
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Quoting jeffs713:
While I appreciate the humor.. y'all need to lighten up a bit. What difference does it make if it is named vs. not named? The storm doesn't magically become super-incredible powerful once it gets a name. The only thing changing is it goes from an invest/area of concern to a named system.

Relax.


Thats not the problem. He did this with 90L and a few other storms last year. Its a reoccurring thing and It might have been funny the first time but now its just down right annoying.
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Quoting StSimonsIslandGAGuy:
RitaEvac that was not cool. And you weren't joking. You were flat out lying. Reported that comment to admin.


I did the same. If I remember correctly this isn't the first time a false post has been posted by this blogger.
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Quoting Stormchaser2007:


Well its not funny. People come here for real information not stupid jokes.


Agreed.
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While I appreciate the humor.. y'all need to lighten up a bit. What difference does it make if it is named vs. not named? The storm doesn't magically become super-incredible powerful once it gets a name. The only thing changing is it goes from an invest/area of concern to a named system.

Relax.
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Update on Galveston and Hurricane Ike. Link
Member Since: September 3, 2007 Posts: 3 Comments: 1519
Quoting RitaEvac:
Because there's nothing going on and when theres nothing going on I like to joke and not be all serious on here.


Well its not funny. People come here for real information not stupid jokes.
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Quoting CatastrophicDL:
Would they name a storm that forms that far north?


Its a hazard to shipping and its in the NHC's zone of responsibility.
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New cumulonimbus are developing in out "invest".

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Quoting RitaEvac:
Canewarning....it was a joke


Real mature RitaEvac !
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.
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Sad they can't find the crash for survivors.Must be scary to go through a rough thunderstorm and crash in the middle of the Atlantic somewhere.Perhaps it's the tropical wave that just came off Africa which is now halfway between Africa and South America is what caused the plane to go down or something.R.I.P. for those who lost their loved ones if they don' make it.Is is possible to survive a plane crash diving right into the water nose first.
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Would they name a storm that forms that far north?
Member Since: September 3, 2007 Posts: 3 Comments: 1519
.
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I'm calling Bill Read
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2 points for RitaEvac for punking the blog!
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.