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Severe Atlantic hurricane season expected; tropical disturbance in the Western Caribbean

By: Dr. Jeff Masters, 4:09 PM GMT on May 31, 2007

A severe Atlantic hurricane season is on tap for 2007, according to the May 31 seasonal forecast issued by Dr. Bill Gray and Phil Klotzbach of Colorado State University (CSU) today. The Gray/Klotzbach team is calling for 17 named storms, 9 hurricanes, and 5 intense hurricanes--unchanged from their April forecast. An average season has 10-11 named storms, 6 hurricanes, and 2 intense hurricanes. The forecast calls for a much above normal chance of a major hurricane hitting the U.S., both along the East Coast (50% chance, 31% chance is normal) and the Gulf Coast (49% chance, 30% chance is average). The Caribbean is also forecast to have an above normal risk of a major hurricane.

The forecasters cite the expected lack of an El Nio event, the continuation of above average sea surface temperatures in the Atlantic, and slower trade winds (which result in reduced evaporative cooling of the ocean), as the justification for their forecast of a much above average hurricane season.

Figure 1. Top: Tropical Atlantic Ocean temperatures in the Main Development Region for hurricanes (green box) were 0.6 C above average during March and April 2007. This anomalous warmth is expected to persist though hurricane season. Bottom: The 0.6 C above average temperatures are consistent with the exceptionally warm temperatures seen since 2003. Image credit: NOAA.

How good are the CSU forecasts?
The CSU forecast team has been making seasonal hurricane forecasts since 1984. If one grades their late May forecasts based on predictions of a below average, average, or above average season, they have done pretty well over the past eight seasons. Seven of their past eight forecasts have been correct. Their only failure occurred last year, when they called for a very active season, and it was a normal year with 10 named storms, 5 hurricanes, and 2 intense hurricanes. A more rigorous way of determining forecast skill is to compute the mathematical correlation coefficient. A correlation coefficient of 1.0 is a perfect forecast, and 0.0 is a no-skill forecast. The late May CSU forecasts have a respectable correlation coefficient of 0.57 for predicting the number of named storms (1984-2005). This decreases a bit to 0.46 and 0.42 for number of hurricanes and intense hurricanes, respectively. These are respectable correlation coefficients, and the late May CSU forecasts are worth paying attention to. This is in contrast to the December and April CSU forecasts, which have had a correlation coefficient near zero (and thus no skill).

Last year, the CSU team made their first steering current forecast. They predicted that a ridge of high pressure over the Eastern U.S. would steer more storms than average towards the Gulf Coast. However, the opposite happened--a trough of low pressure set up over the Eastern U.S.--and the 2006 steering current forecast was a bust. They've given up on trying to predict what this year's steering currents might be, citing the need to perform more research on this issue. In theory, such a forecast is possible. Gray and Klotzbach published a 2004 paper showing a statistical relationship between variations in Atlantic sea surface temperature and whether hurricanes are more likely to hit the U.S. Gulf Coast or East Coast. These SST variations influence the steering patterns, and help determine whether a persistent trough of low pressure will settle over the East Coast and recurve hurricanes out to sea--as happened in 2006--or whether a ridge of high pressure will settle in, pushing more storms towards the Gulf Coast--as happened in 2004 and 2005. The problem with all of these statistical Atlantic seasonal forecasts is that the atmosphere/ocean system is always changing in new ways that have not occurred in the past. Thus, a statistical scheme that works for forecasting past activity is much worse at predicting the coming year's activity. There is hope that the global dynamical computer models used to forecast the weather will soon be able to surpass the statistical methods used by the CSU team. Indeed, recent papers have shown the the European model (ECMWF) and GFDL model both make seasonal hurricane forecasts that rival the CSU forecasts in skill. No word yet on when these new computer model seasonal forecasts will be available to the public though--more research is needed to develop them.

The bottom line: expect a very active Atlantic hurricane season
The CSU forecast matches up well with the TSR, Inc. forecast (16 named storms, 9 hurricanes, and 4 intense hurricanes) and the NOAA forecast (13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes). Also, keep in mind that the active hurricane period that began in 1995 has never seen two consecutive years with average or below-average hurricane activity. Given these factors, I am confident that the coming season will be a very active one. The two most recent years that had patterns of El Nio/La Nia events and SSTs similar to what are expected this year were 1995 and 2003. Note that 1995 was the third busiest hurricane season on record, with 19 named storms. However, the great majority of these storms recurved out to sea, since a trough of low pressure settled over the Eastern U.S. I have a similar hope for this season--if the steering currents are your friend, even a top-five hurricane season can have an ordinary number of landfalls. Let's hope the steering currents are our friend this year!

The Tropical Storm Risk, Inc. 2007 Atlantic hurricane season forecast
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issues monthly 2007 Atlantic hurricane season forecasts. Their May 3 forecast has almost the same forecast as the CSU team--16 named storms, 9 hurricanes, and 4 intense hurricanes. I like how they put their skill level right next to their forecast numbers: 30% better than chance skill at forecasting the number of named storms, 34% skill for hurricanes, and 30% skill for intense hurricanes. TSR projects that five named storms will hit the U.S., with 2.3 of these being hurricanes. Their skill in predicting the number of named storms hitting the U.S. is only 8% above a no-skill forecast, but the skill rises to 30% for hurricanes hitting the U.S. In the Lesser Antilles Islands of the Caribbean, TSR projects 2.4 named storms, and 0.7 hurricanes. TSR cites two main factors for their forecast of an active season: above normal Sea Surface Temperatures (SSTs) are expected in August-September 2007 across the tropical Atlantic, as well as slower than normal trade winds. Trade winds are forecast to be 0.78 meters per second (about 1.6 mph) slower than average, which would create greater spin for developing storms, and allow the oceans to heat up due to reduced evaporative cooling. SSTs are forecast to be about 0.2 degrees C above normal. TSR gives an 84% chance that the 2007 Atlantic hurricane season will rank in the top third of active seasons observed since 1957. Their next forecast will be issued June 4.

The NOAA 2007 Atlantic hurricane season forecast
NOAA is also predicting a very active 2007 hurricane season in the Atlantic NOAA's season hurricane forecast issued May 22 predicts a very high (75% chance) of an above-normal hurricane season, a 20% chance of a near-normal season, and only a 5% chance of a below-normal season. They expect 13-17 named storms, 7-10 hurricanes, and 3-5 major hurricanes (a normal season has 10-11 named storm, 6 hurricanes, and 2 intense hurricanes). Most of these storms are expected during the usual August-October peak of hurricane season, but NOAA does not give any breakdown of which portions of the coast are more likely to be affected. They give two reasons for predicting an above-normal hurricane season:

1) A continuation of conditions since 1995 that have put us in an active hurricane period (in particular, the fact that seas surface temperatures in the Atlantic Main Development Region for hurricanes are currently about 0.6 C above normal, Figure 1).

2) The strong likelihood of either neutral or La Nia conditions in the tropical Pacific Ocean.

Tropical disturbance in the Western Caribbean
A large area of disturbed weather developed over the Western Caribbean last night. This disturbance is bringing winds of up to 55 mph over the ocean, according to the 7:07am EDT pass of the QuikSCAT satellite. The NOAA Buoy off the coast of the Yucatan Peninsula recorded winds this morning at 30mph, gusting to 35mph. There is no circulation evident on QuikSCAT or satellite loops, but the disturbance does have the potential to develop into a tropical depression by Saturday as it moves to the northeast over Western Cuba and South Florida. A Hurricane Hunter aircraft is scheduled to investigate the system at 2pm EDT on Friday.

Wind shear is a not-too-unfriendly 10-20 knots, and the shear is expected to remain at these levels through Saturday. Thereafter, most of the models are indicating that the disturbance will get caught up by a strong trough of low pressure with high shear that should stop further development and sweep the system northeastward out to sea. I doubt this system has enough time to get organized into a tropical depression before wind shear rips it up, but the disturbance should bring welcome heavy rains to South Florida over the weekend. Lake Okeechobee recorded its record lowest water level yesterday--8.97 feet (about 4 feet below normal). This was the lowest level since record keeping began in 1931, according to a Miami Herald article this morning. The lake has been dropping about 1/2 inch per rainless day. I expect 1-3 inches of rain over the area this weekend, which should temporarily stabilize the lake water level.

NHC issued this statement at noon today:

Special tropical disturbance statement
1150 am EDT Thu May 31 2007

Showers and thunderstorms in the northwestern Caribbean Sea... southeastern Gulf of Mexico and adjacent land areas are associated with a broad area of low pressure centered about 75 miles southeast of Cozumel Mexico. Although this system has some potential for tropical development over the next day or so...the low is expected to move slowly northward into the southern Gulf of Mexico where environmental conditions would likely favor further development as a non-tropical low. Regardless of development...this system should bring heavy rains across western Cuba and southern Florida over the next couple of days. Please monitor products issued by your local Weather Service office for more details.

Figure 2. Visible satellite image of the Western Caribbean tropical disturbance.

June outlook and the Barometer Bob show
I'll be posting my forecast for the first two weeks of June tomorrow (June 1). I plan to offer 2-week hurricane activity forecasts on the 1st and 16th of each month (except August 1, when I'll be on vacation). These forecasts will have the probability of hurricane formation for the coming two weeks, where the hurricanes will go if they form due to the prevailing steering currents, plus a look at how sea surface temperatures, wind shear, the trade winds, and dry air coming off of Africa are affecting hurricane formation in the Atlantic.

Tonight, I'll be a guest on the Barometer Bob Show, if you want to hear a sneak preview of my outlook for the first two weeks of June and hear about the tropical disturbance in the Western Caribbean. You can listen at barometerbobshow.com, or dial in via their toll-free number 1-866-931-8437 (1-866-WE1THER).

Jeff Masters

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

look's like the strongest winds will stay off shore, but 985mb and 60knots forecasted by the gfdl is interesting indeed.....
1502. Drakoen
that swirl north of Cuba looks more defined and maybe trying to "wrap" something.
Looks like it will be extratropical.
1504. Drakoen
If there is a swirl south of Cuba then this system has a better chance of developing into something as it will be in an area more favorable for tropical cycone development. Anything that forms now need to avoid the GOM to sustain itself.
Winds at my house are coming from the SE at 5 MPH, pressure is at 1014.8 and dropping.
1506. Drakoen
buoy 42056 has winds coming out of the SSW at 18 mph near the "secondary" vortex.

Hows that Kman?? LOL
If it's tropical or not, it looks like florida will be getting some rain! Here is a WRF 48 hour rainfall prediction.

Because the measurements are in millimeters, some people (such as me) may have some difficulty interperating the measurements. I recommend This Page.
thelmores, it appears that models are coming into better agreement and have shifted slightly left
Does anyone know when the new model runs will be coming out?
...then what was Tammy would have been Wilma...

You forgot Vince!

: )

Don't worry. You got your general point across.
WoW jp, so I survived a Hurricane named Alpha. Gosh, I like saying I survived Wilma better.

I knew I could count on you lol
Let me know if you can see any rain on this
anyway have to get some work done. Will BBL
1518. FLBoy
This system has sped up a bit. Check where the models have this centered.
1519. Drakoen
Heres graphic i made. something possibe.... look hard to see the cirle i placed and the outflow lines.
Posted By: MisterPerfect at 9:48 AM EDT on June 01, 2007.
Let me know if you can see any rain on this

Most of that is clutter because it sure isn't raining at my house.
1521. Drakoen
Posted By: FLBoy at 1:48 PM GMT on June 01, 2007.

This system has sped up a bit. Check where the models have this centered.

hmm even if thats the case there might be a secondary low which would be more interesting to watch.
I have seen, heard, felt, smelled and tasted 3 Hurricanes: Andrew 1992, Charley 2004, Katrina 2005.

Those are some whopper storms, aren't they?...

bbl, gotta get some work done!
Posted By: WPBHurricane05 at 1:50 PM GMT on June 01, 2007.

Most of that is clutter because it sure isn't raining at my house.

The big stuff is south of the middle/lower keys right now. By 5 o'clock we'll be swimmin' in MIAMI.
Lets see I went through some hurricanes before the 90's but don't remember there names. In 1999 I went through Irene, in 2004 it was Frances, and Jeanne, and in 2005 I went through Wilma (or Alpha) which went right over me.
1526. Drakoen
everyone go to this link and focus your eyes south of Cuba whether this is a mid level or low level swirl i don't know but there is one there. You can see the counter clockwise rotation as it ascends to the ENE.
1527. IKE
Posted By: MisterPerfect at 8:51 AM CDT on June 01, 2007.
I have seen, heard, felt, smelled and tasted 3 Hurricanes:

You "smelled" a hurricane? What exactly did it smell like?
1528. C2News
I've been through Charley, Frances, Jeanne, Tammy (05), Ophelia (05), and Andrea (if you can call that a storm)
I lived in a coastal area all my life, yet I've never been through a hurricane...
1530. C2News
hurricanes i think smell like rain and...i don't know what else...i guess whatever the wind blows in
1531. PBG00
I did the frances, jeanne and wilma thing too..Wilma was kinda cool, loved the weather after..I really hope we don't have another season like Frances and Jeanne..not fun!
1532. IKE
I've been through Dianne, Tammy....weren't hurricanes, just ex-wives!
What area of Texas do you live in 1900?
1534. Drakoen
lets see if the NHC say anything about it...
1535. C2News
Houston/Galveston area.
Link to the image.
Explains 1900hurricane...
1541. PBG00
Loved it down here in Jupiter..We had the fire going..no power and sweaters on..Thats the way to do a hurricane!With a trailing cold front
And we bought an air conditioner too for Wilma!
that cold front was a blessing for us down here after the storm went through......there was a breeze as we tried to sleep without power.
1544. Drakoen
Sure does!
1546. C2News
new blog!!!!
1547. Drakoen
Posted By: PBG00 at 2:01 PM GMT on June 01, 2007.

Loved it down here in Jupiter..We had the fire going..no power and sweaters on..Thats the way to do a hurricane!With a trailing cold front

lol yea.
Does everyone still think the plane will go in today? I think it is too close to the mainland not to investigate
1549. FLBoy
953 AM EDT FRI JUN 01 2007

VALID 12Z TUE JUN 05 2007 - 12Z FRI JUN 08 2007



There's the latest on the east coast situation.
Charley visited us first, then Frances, Jeanne, Ivan, Wilma....pretty weird for a coastal area that hadn't seen a cane in 40 yrs...although Frances, Jeanne and Ivan just brushed us... do you think this one might acutally have the chance of becoming a TD or TS...I'm thinking that its running out of time......
Well, here in Broward:

1992-Andrew: Category 1 Winds
1999-Irene: Moderate Tropical Storm winds
2004-Frances: Strong/Moderate Tropical Storm winds
2004-Jeanna: Strong Tropical Storm Winds
2005-Katrina: Weak Category 1 winds/Strong Tropical Storm winds
2005-Rita: Mild Tropical Storm winds
2005-Wilma: Weak Category 2 winds (a lot of damage..gusts to 110/115 mph)
2006-Ernesto: TD winds (didn't streghten like forecasted)

And then in the past three years South Florida had threats from Charley, Gamma, and Chris...some that I can think off of the top of my head. And those were a big preperation deal for us too (but of course they either dissipated or didn't hit).

Most of those were in the past three years. So..Florida could be under the gun again this year.