WunderBlog Archive » Category 6™

Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.

A 65% chance of an above-average 2008 hurricane season : NOAA

By: Dr. Jeff Masters, 6:25 PM GMT on May 22, 2008

There is a 65% chance that year's Atlantic hurricane season will have above average activity, and only a 10% chance it will be below average, according to today's seasonal hurricane forecast issued by the National Oceanic and Atmospheric Administration (NOAA). In an effort to emphasize the uncertainties in their forecast, NOAA for the first time included a probability with their "numbers" forecast. The numbers NOAA predicts are 12-16 named storms, 6-9 hurricanes, 2-5 intense hurricanes, and an Accumulated Cyclone Energy (ACE) index 100%-210% of the median. They put the odds of experiencing those numbers at 60-70%. This probability is based on the numbers seen in past hurricane seasons that had May climate patterns similar to those of 2008. An average hurricane season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. The forecasters also call for a 70% chance of a below normal season in the eastern Pacific off the coast of Mexico, and a below normal season in the Central Pacific near Hawaii.

The forecasters considered two main climate signals to make their forecast:

1) Continuation of conditions that have been conducive to above-normal Atlantic hurricane activity since 1995, including above-average sea-surface temperatures in the eastern tropical Atlantic Ocean (Figure 1). Between 1995-2007, the Atlantic has averaged 14.5 named storms, 8 hurricanes, and 4 major hurricanes, with an average ACE index of 167% of the median. NOAA classifies nine of the thirteen seasons since 1995 as above normal, with seven being hyperactive (ACE > 175% of median). Only four seasons since 1995 have not been above normal. These include the three El Niño years (1997, 2002, and 2006) and the 2007 season.

2) La Niña or neutral conditions during the peak months (August-October) of hurricane season. The current weak La Niña episode has substantially weakened the upper-level west-to-east winds over the tropical Atlantic. These winds create much of the wind shear that inhibits hurricane development. La Niña is expected to persist (or weaken slightly to neutral) by hurricane season, and these weak westerly winds are predicted to persist over the Atlantic, bringing low levels of wind shear.



Figure 1. SST image from NOAA's May 22, 2008 seasonal hurricane forecast.

Where will this year's storms hit?
NOAA does not present any forecasts of where the steering currents might take this year's hurricanes. In general, steering currents are not predictable more than 5-7 days in advance, although sometimes one can stretch this to two weeks. As far as U.S. landfalls go, I believe that the climatology of landfalls since 1995 offers the best forecast. Between 1995-2007, the U.S. experienced an average of 4.7 named storms making landfall, of which 1.9 were hurricanes and 0.8 were intense hurricanes.

How believable is the NOAA forecast?
Mathematical analysis of forecasts made in late May and early June by two groups--Tropical Storm Risk Inc., and the Colorado State University group led by Phil Klotzbach and Dr. Bill Gray--have shown that these forecasts had reasonable skill over the past decade or so. Unfortunately, NOAA has never performed a mathematical analysis of their May forecasts, so we don't know whether their forecasts have any skill. Their forecast did poorly last year, and the head of NOAA's forecast team, Dr. Gerry Bell, provides an interesting analysis of why last year's forecast failed.

Commentary
NOAA's forecast is a reasonable one--but then, since 1995, it's always been smart to forecast an above-average hurricane season, barring an obvious El Niño event. I like that NOAA is looking to insert more uncertainty into their forecast by providing a percent chance of their expected numbers to verify. However, they really need to provide some verification numbers of how much skill these forecasts have, if they want to make them more useful. In addition, they should consider dropping the "numbers" forecast of named storms, since the number of named storms often does not give a true picture of how active a hurricane season is. For example, recall the 15 named storms from 2007, which was about 50% above average! Yet the ACE index, a more true measure of the destructive power of the storms, was 31% below average. ACE index is a better measure because it uses the square of each storm's peak winds, summed up over the lifetime of the storm. Wind damage from a hurricane is proportional to the square (and possible the cube) of the peak winds, and total damage increases by a factor of 5-10 for each Saffir-Simpson Category between Cat 1 and Cat 4. According to Pielke and Landsea (1998), a doubling of wind speed--from a 74 mph Category 1 hurricane to a 148 mph Category 4 hurricane--has historically resulted in 250 times more damage for U.S. landfalling hurricanes. This figure includes storm surge, flooding, and wind damage.

The Colorado State University group led by Phil Klotzbach and Dr. Bill Gray issues their seasonal hurricane forecast on June 3 this year, and I'll be sure to provide additional commentary then.

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

500. StormW 4:02 AM GMT on May 23, 2008
496. JFV 11:54 PM EDT on May 22, 2008
Hey StormW, are you leaning towards the potential development of the Carib storm next week or not sir?

Going to have to see a few more runs on different parameters, and model consensus...but I am not totally ruling it out. I want to see what the wind shear forecast looks like further out. We should still have some upward motion left from the MJO as well.


Do you use the PSU site on the GFS? They have the complete wind shear up to 384hrs.
501. StormW 4:04 AM GMT on May 23, 2008
498. Drakoen 12:02 AM EDT on May 23, 2008
StormW i'm surprised your still up. Isn't it pass your bedtime (as Bob would say) lol...


Yeah...it is. In fact, I'm getting ready to sign off. Just been waiting for the Mrs. to get home from work...She went in for the 7-11 shift.

oh ok. Its nice to you have around and back since I don't get to see you around here too often.
@StormW: http://tc.met.psu.edu/
Back from watering..it's really dry here.

MJO doesn't come out as strong during strong La Nina/El Nino events. Supposedly it's stronger during neutral conditions

Exactly...another one of the pieces that made 2005 so bad.

ENSO I could see possibly making it to El Nino by fall but not as sure about it as something has happened now & the move to neutral is here or about here.

The ACE seems to move up or down in steps each year. We are so close to the bottom I don't see how we can get anywhere near as high as like '05.
No problem
Have a good sleep, StormW!
I'm out too its late here. See you guys tomorrow!
Have a good sleep, Drak.
pretty late for me as well... But when does 00z GFS come out?
BULLETIN - EAS ACTIVATION REQUESTED
SEVERE THUNDERSTORM WARNING
NATIONAL WEATHER SERVICE NEW ORLEANS LA
1231 AM CDT FRI MAY 23 2008

THE NATIONAL WEATHER SERVICE IN NEW ORLEANS HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...
NORTHERN JEFFERSON PARISH IN SOUTHEAST LOUISIANA...
THIS INCLUDES THE CITIES OF...TIMBERLANE...METAIRIE...MARRERO...
KENNER...HARVEY...AVONDALE...
SOUTHWESTERN ORLEANS PARISH IN SOUTHEAST LOUISIANA...
THIS INCLUDES THE CITY OF NEW ORLEANS...
NORTHWESTERN PLAQUEMINES PARISH IN SOUTHEAST LOUISIANA...
THIS INCLUDES THE CITY OF BELLE CHASSE...
WEST CENTRAL ST. BERNARD PARISH IN SOUTHEAST LOUISIANA...
THIS INCLUDES THE CITY OF CHALMETTE...
EASTERN ST. CHARLES PARISH IN SOUTHEAST LOUISIANA...

* UNTIL 115 AM CDT

* AT 1227 AM CDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
SEVERE THUNDERSTORM CAPABLE OF PRODUCING PENNY SIZE HAIL...AND
DAMAGING WINDS IN EXCESS OF 60 MPH. THIS STORM WAS LOCATED NEAR
WAGGAMAN...OR NEAR AVONDALE...AND MOVING EAST AT 21 MPH.

* THE SEVERE THUNDERSTORM WILL BE NEAR...
WESTWEGO AND BRIDGE CITY BY 1240 AM CDT...
MARRERO AND NEW ORLEANS BY 1250 AM CDT...
GRETNA AND HARVEY BY 1255 AM CDT...
TERRYTOWN...GRETNA AND TIMBERLANE BY 100 AM CDT...
BELLE CHASSE BY 105 AM CDT...

THIS IS A DANGEROUS STORM. IF YOU ARE IN ITS PATH...PREPARE
IMMEDIATELY FOR DAMAGING WINDS...DESTRUCTIVE HAIL...AND DEADLY CLOUD
TO GROUND LIGHTNING. PEOPLE OUTSIDE SHOULD MOVE TO A SHELTER...
PREFERABLY INSIDE A STRONG BUILDING BUT AWAY FROM WINDOWS.


Hey... Is it just me, or does the area around 4N 87W look interesting to anyone else? The QuikSCAT descending pass shows some winds flowing in opposite directions right next to each other. Nothing major yet, but it's in an area of fairly low shear and warm water. It's hard to see the low level clouds on the visible beneath the upper-level (darn sun, cowardly hiding behind earth and ruining contrast), but there sure is a lot of convection. Of course, that is pretty far south for cyclogenesis.
(also, the steering currents from there are headed south, so it's mostly just a curiosity... it's just the only remotely interesting thing I'm seeing on the QuikSCAT right now, so I thought I'd mention it).

Also, looking at some shear forecasts, and it looks like for the first time this season, shear is going to take a nosedive in the western gulf this weekend:



Warm enough waters there now, although the air is dry as a bone. Will have to add that to my "to watch" list if that dry air moves out and the shear stays low.
Alrighty- morning update for you guys. The newest run of the GFS keeps the storm but moves the track even further west. It now swings well into the central gulf then back up and hits north of tampa. We are seeing a trend. With every run it move further west...goodbye fl rains?
We are so busy with the GFS model not noticing that a tropical wave is closing off a circulation near 5N. I am updating my blog.
The GFS run has a LOW forming in the souhtern CAR. around 150 hr and then going up into the GOM and hitting Florida and skirting up the coast to North Carolina.
Morning everyone.

Any model concensus that may have appeared to be forming yesterday with the potential SW Caribbean has disappeared. UKMET, ECMWF, CMC, and NOGAPS all develop the EPAC storm and have it drowning our SW Caribbean storm. The GFS is still at it though, but it has pushed the storm's development back to 104 hours. It's possible some of the other models may still catch on with the GFS.
456
you are so correct . recent QuickScat shows the wave in the eastern atlantic with almost a closed low except for the south west. wind shear is low in the area and sst are is warm enough to sustain cyclogenesis.
456 is always on the ball
"The GOmex blob is marching east."

Hopefuly southeast.

thanks to 456 the blog will be interesting today. the emphasis will shift from the gfs and the waestern carib to the far eastern atl. it is also puzzling how none of the models latched on to this system. it is rather early and against climatology to see development so far out in the atl.
Development in the E-Pac usually sucks the life out of anything developing in the W-Caribbean. That is why although the upper atmosphere favors the W.Caribbean both early in the season and late in the season, development usually occurs in the W-Caribbean late in the season when the Pacific season is nearing it's end.

As for the GOMex blob, while it is marching east, the sun is rising over the peninsula which will become unstable very quickly. Look for storms to fire to the south of the current storms over N Florida and be spreading southward by mid-day. If you live on the eastern half of the peninsula, watch for some strong storms later today.
I just noticed that the NAM has a low in the SW Caribbean at around 78 hours. However, I think the EPAC scenario is far more likely at this point, both due to proximity issues and model concensus.

recent QuickScat ot the area . surface winds are about 25knots





Link
532.With the low in the W.Caribbean that far south, what is going to dig that far south and pull the low northward? That low will move slowly westward and emerge over the E-Pac or anything developing over the E-Pac will absorb it. What may end up being of more interest is the stationary front in the Florida Straights. If it sits and festers down there long enough, that could be a source of development.
534. Don't know if this matters or not but 850mb - 700mb steering currents are pointing northward.

CIMSS Layer Mean Steering Analyses


...appears to be building a cloud top, but will have to hold on at that latitude. The storm to the right, just coming off the coast looks nice, but will likey dissipate out over water.
536. While you are correct, the models are all pointing toward E-Pac development. Climatology agrees. Once development begins in the E-Pac, outflow and sinking air surrounding any such system would affect the surrounding steering currents. I've seen too many early season times like this when the upper levels favor both the W.Caribbean and the E-Pac and the E-Pac wins this time of year almost every time. That is why the E-Pac season begins in Mid May and the Atlantic Season waits till June. If this were October and we were watching a similar situation I'd bet on the W.Caribbean but this is May.
Morning all :~)

So, I see the GFS long range has gone form Tampa to the Tx/Mx border! lol, only a minor discrepancy!

Alright, off to work. See y'all in a little while.
537. Morning MLC. They just keep on coming off the coast of Africa dont they. If it were mid July, that would be one to watch. With the warming of the waters off the coast of Africa, those waves may not tend to dissipate as the peak of CV season hits. That would ultimately be a good thing as they would recurve very early.


Not really any dry air to speak of in the immediate wave area, but there is some further out near 39w according to wv.
Ivan, agreed.
MLC, Casper is running for Mccain's soon to be vacated senate seat.
Morning StormW! What's your opinion concerning the GFS storm as well as the low level circulation out in the east?
StormW, if the wave at 27w continues to run low as it is now, couldn't it slip in under the TUTT (looks to lifting out to the ne) as it moves west?
If it were July, that E.Atl system would be a real concern for down the road. It may well develop briefly but would then tend to recurve and SSTs would not support it long. If the E ATL stays as active as it has been it will likely be a busy season.
Morning all.
I hear ya, Ivan!

Photobucket
550, it all fell on the East Coast. Today and tomorrow look more favorable for both east and west coasts to get some rain.
Morning All....I see you guys are already on the ball in terms of the semi-closed wave in the eastern Altlantic.......For me, at home in the Tallahassee area today, I was awoken this early am to the sound of rain on skylight; Ahhhh, slept in a little bit and we have been getting some nice steady rain for the past several hours........The plants and lawn very happy this morning.............
Hey StormW how goes it. How is your Father?
W/V image of eastern ATL



W/Vimage of eastern Link
NOt much working 10-500 today. Gonna be kinda boring my girl fly to Jersey so no fun at work.
see you all later got to get ready.
good morning my friends! Happy Friday to you. I see we have stuff to watch out there already. Always love it when it becomes interesting.

Man, when I got home last night noticed the streets wet and some water on the grass, etc... I just knew we got a little rain out of those storms that crossed South Fla yesterday but my rainguage was empty! Nothing measurable fell at my house. Bah! But some areas got as much as an inch of rain yesterday all around me.. it was sort of "hit or miss" type of storms...
hope we have better luck today! LOL
does anybody think that they will name it an invest?(the tropical wave)
GM, all,those videos from colorado were pretty intense. People out in that area,will have to watch the skies again today,atomosphere is primed.
Good morning everyone! TGIF! The rain missed my portion of the panhandle too.
Gamma, Most of Palm Beach Co. got well over an inch. I got almost three. Local news claimed areas close by got more than 4. With the calmer winds, a weak seabreeze and the likely interaction those will have with the mid level system over N Florida, today and tomorrow are the best bets for rain we have had in months. Yesterdays rains will help fuel todays storms too.
does anybody think that they will name it an invest?(the tropical wave)

It's possible; but it will need to sustain itself for most of today and into tomorrow before they start thinking of declaring 90L. I'd like to see it sustain itself and look at what the 12z model runs have to say about this before I make my opinion...satellite presentation is fairly impressive and there is obviously a LLC in the making. Dry air will likely become a problem for this somewhere down the line.

It's not in a favorable position, either.
Good morning all...

Anything new and exciting I should know about?
569. smmcdavid 9:19 AM EDT on May 23, 2008 Hide this comment.
Good morning all...

Anything new and exciting I should know about?


There is a well-defined tropical wave approaching the central atl and the GFS is still forming a system in the SW carib and moving into the gulf later next week.
Anything new and exciting I should know about?

Morning! There's a wave with a low level spin the Eastern Atlantic but thats really not looking like anything special in terms of development. The GFS is still up to it's SW Caribbean storm.
572. CJ5
I appears there is a potential 90L at 27, as you guys have been commenting on. Things are begining to take shape for the 08 season.
g'morning everyone, looks like we will soon start seeing some activity..also (head's up) my mom is visiting florida in 3 weeks, last time she came down from Kansas City to visit, she brought her friend Wilma with her..lol, Charlie came with her the previous trip..so beware, she just might be a better cane predictor than NOAA.
This has alittle spin going maybe some good rain coming to florida.
It appears that a trough has a low that is getting alittle better organized in the GOM. Some great rain could be coming to the SE regions.
Morning all.

Unfortunately, all that lovely precipitation missed my part of the Bahamas . . . But I admit I wasn't expecting anything until next week. Perhaps the GFS forecast people have been talking about will pan out in that it may bring some tropical rain to the area . . .