Outlook for the remainder of hurricane season

By: Dr. Jeff Masters , 2:46 PM GMT on October 16, 2007

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A low pressure system (99L) over the Gulf of Mexico, about 500 miles south of the Texas/Louisiana border, is headed to the north at about 10-15 mph. QuikSCAT showed top winds of about 30 mph near this low. Satellite loops show that the thunderstorm activity has increased slightly this morning near the low, but remains disorganized. Wind shear is about 15 knots, and is expected to increase to 30 knots by the time 99L crosses the Louisiana coast Thursday morning. The shear should be high enough to prevent 99L from forming into a tropical depression. However, the Hurricane Hunters are on call to fly the storm Wednesday afternoon, if necessary.

The storm and an associated trough of low pressure extending northward to the Gulf Coast will bring heavy rains to the northern Gulf Coast today through Thursday, and should bring welcome rains to the drought-parched Southeast U.S. on Thursday and Friday.

Outlook for the remainder of hurricane season
Atlantic tropical cyclone activity finishes its peak phase in mid-October, and takes a major downturn during the final half of October (Figure 1). Since the current active hurricane period began in 1995, the last half of October through the end of hurricane season has given birth to an average of 1.9 named storms, 0.8 hurricanes, and 0.3 intense hurricanes. These numbers are about double the long-term climatological averages for the past 100 years. So, we're probably not done with the season yet, since this is not an El Nino year--El Nino years typically bring higher wind shear to the Atlantic, and an early end to hurricane season.

Climatology of major hurricanes
Let's examine the possibilities of getting a major hurricane this year, since those are the storms we care most about. Since 1960, there have been ten hurricanes that have existed as major Category 3 or higher storms after October 15. Six of these have occurred since 1995: Wilma of 2005 (Cat 4, Mexico; Cat 3, SW Florida), Beta of 2005 (Cat 3, Nicaragua), Michelle of 2001 (Cat 4, Cuba), Lenny of 1999 (Cat 4, northern Lesser Antilles), Mitch of 1998 (Cat 5, Honduras), and Lili of 1996 (Bahamas, Category 3). The other four were Joan of 1988 (Cat 4, Nicaragua), Kate of 1985 (Cat 3, Gulf of Mexico), Ella of 1962 (Cat 3, west of Bermuda), and Hattie of 1961 (Cat 4, Belize). Wilma of 2005 was the only major hurricane since 1960 to hit the U.S. after October 15. The highest risk region for late season major hurricanes is the Western Caribbean, along the coasts of Nicaragua, Honduras, Belize, Mexico, and Cuba. So, we can say with high confidence that most of the U.S. coast can relax. Only the west coast of Florida, Florida Keys, and South Florida need to still be concerned about the possibility of a major hurricane. The Lesser Antilles Islands, Puerto Rico, and Hispaniola are also at low risk for a major hurricane the remainder of the season.


Figure 1. Atlantic hurricane season activity over the past 100 years.

October storms tend to form both from tropical waves that come off the coast of Africa, and from the remains of old fronts that push off the coast of the U.S. As we can see from the track plot of all last half of October storms (Figure 2), there is a lot of activity during the period, but relatively few storms form out near the African coast. The water temperatures off the coast of Africa are starting to cool and be marginal for hurricane formation, and wind shear is starting to pick up in its normal fall cycle.

The jet stream is now more active and extends further south, which brings higher levels of wind shear to the Atlantic. The more active jet stream also acts to recurve storms more quickly. Any system penetrating north of about 20 degrees north latitude we can expect to recurve quickly to the north and northeast this late in the season.


Figure 2. Tracks of all tropical storms and hurricanes since 1851 that formed October 16-31.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) have been about 0.5 ° C above average over the Caribbean the past few weeks (Figure 3). This is the primary formation area for October storms. Note also the tongue of colder than average SSTs extending out into the Pacific Ocean from the coast of South America. This is the signature of a moderate strength La Nina event.


Figure 3. Sea Surface Temperature (SST) departure from average for the first portion of October. Image credit: NOAA.

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 by tearing a storm apart. Wind shear 10 knots and lower is very conducive for tropical storm formation.

Despite the presence of a La Nina event the past month--which is supposed to bring lower than average wind shear to the tropical Atlantic--wind shear the past month has been near average (Figure 4). The latest two-week wind shear forecast from the GFS model predicts near-average wind shear for the last half of October.


Figure 4. Wind shear departure from average for the 31 days ending October 13. Near average levels of wind shear were observed over the primary hurricane formation regions of the tropical Atlantic, Caribbean, and Gulf of Mexico. 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).

Summary
We've gotten very lucky this hurricane season since the departure of Hurricane Felix in early September. We had a record eight named storms form in September, yet we had only four hurricane days that month. Wind shear has been strategically high at the right time and right place, and storms have tended to form too close to land to develop. A good case in point is the current storm 99L. Had that system formed just 200 miles further east, it would have spent five days meandering over the Western Caribbean instead of over the Yucatan Peninsula, and could have easily grown into a Category 4 or 5 hurricane. There are still several low-shear periods ahead for the Western Caribbean this hurricane season, and I expect our luck may not hold for one of these periods. There is still one hurricane likely to form this season, possibly a major hurricane. The GFS model is predicting a large area of low shear will develop over the Caribbean around October 30, and I expect further low shear periods may occur through the first half of November.

I thank Margie Kieper for helping out the major hurricane seasonal stats.

Jeff Masters


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527. KEEPEROFTHEGATE (Mod)
9:44 PM GMT on October 16, 2007
swirl in cen carb. movin too fast
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526. Floodman
9:44 PM GMT on October 16, 2007
Amy, you're welcome...kinda catchy, isn't it?

Fahren dieses Zug
Höhe auf Kokain
Casey Jones passen Sie besser
Ihre Benzedrin auf!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
525. KEEPEROFTHEGATE (Mod)
9:41 PM GMT on October 16, 2007
refire 99l detected
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524. NRAamy
2:41 PM PDT on October 16, 2007
454. Floodman 1:42 PM PDT on October 16, 2007

And futher, Amy:

Höhe auf Kokain
Casey Jones passen Sie besser Ihre Geschwindigkeit auf


oh man, that's the funniest thing I've heard all day! Thanks Jerry!!

:)
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522. Floodman
9:38 PM GMT on October 16, 2007
StormW, can you have one without the other? Sounds like feedback to me...one feeds the other, yes?
Member Since: August 2, 2006 Posts: 10 Comments: 9922
521. sammo
9:37 PM GMT on October 16, 2007
Yeah, melly, I remember your story. Those images are burned into my head. I used to walk to school too when I was a kid and I can't imagine being hit by a tornado and pelted with gravel while hiding in a ditch. Must have been traumatic.
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519. sporteguy03
9:35 PM GMT on October 16, 2007
Ike thanks for the NAM update as always your the best!
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518. aubiesgirl
9:35 PM GMT on October 16, 2007
517. beell 9:35 PM GMT on October 16, 2007
aubiesgirl, ok, but i'm taking all the credit

lol well there's noone else here to claim it.lol
517. beell
9:34 PM GMT on October 16, 2007
aubiesgirl, ok, but i'm taking all the credit
Member Since: September 11, 2007 Posts: 148 Comments: 17205
516. Floodman
9:32 PM GMT on October 16, 2007
512. taistelutipu 9:32 PM GMT on October 16, 2007
496. Floodman 12:23 AM EEST on October 17, 2007
Anyone notice the increase in convection around 10N39W?

Which satellite shows it best? Do you possibly have a loop?



EUMETSAT shows it but in "profile"; try this
Member Since: August 2, 2006 Posts: 10 Comments: 9922
515. aubiesgirl
9:33 PM GMT on October 16, 2007
IKe told me a week ago to expect rain well..right about today!!!!!
514. beell
9:32 PM GMT on October 16, 2007
mlc said it would rain a week ago and he got a professional drummer
Member Since: September 11, 2007 Posts: 148 Comments: 17205
513. Floodman
9:31 PM GMT on October 16, 2007
All hail beell, rainmaker!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
512. taistelutipu
12:25 AM EEST on October 17, 2007
496. Floodman 12:23 AM EEST on October 17, 2007
Anyone notice the increase in convection around 10N39W?


Which satellite shows it best? Do you possibly have a loop?
Member Since: August 20, 2007 Posts: 12 Comments: 640
511. CaneAddict
9:30 PM GMT on October 16, 2007
485. CaneAddict 5:12 PM EDT on October 16, 2007
StormW whats your take on that ULL in the central Caribbean? I have gained a little concern for that area as thunderstorm activity it currently firing around it...


I think it would take a while to work down to the surface. I'm on it!


Do you agree that it seems to be trying to?
Member Since: October 5, 2007 Posts: 3 Comments: 2151
510. aubiesgirl
9:29 PM GMT on October 16, 2007
LOL Beell someone did that's for sure
508. Floodman
9:26 PM GMT on October 16, 2007
500. taistelutipu 9:25 PM GMT on October 16, 2007

You have to love the germanic languages! By the way, my compliments on your english, it's outstanding!
Member Since: August 2, 2006 Posts: 10 Comments: 9922
507. CaneAddict
9:22 PM GMT on October 16, 2007
That Central Carib. ULL is really trying to work its way to the surface....I see 90L coming soon. Well im off to update my blog! Ill be back.
Member Since: October 5, 2007 Posts: 3 Comments: 2151
506. beell
9:27 PM GMT on October 16, 2007
ok, it's almost unanimous-I made it rain. Thank you, Thank you one and all
Member Since: September 11, 2007 Posts: 148 Comments: 17205
505. sporteguy03
9:25 PM GMT on October 16, 2007
Hi Stormjunkie,
For everyone here at WU we miss your blogging, right up there with Patrap. Hope SC is fun and sunny!
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504. TEXASYANKEE43
9:24 PM GMT on October 16, 2007
495. melly 9:23 PM GMT on October 16, 2007
So what did my family do??? We all moved to Florida, and I again experienced severe weather. Francis & Jeane in 2004, Then Wilma in 2005....I now live in Palm Beach county.All I can say is ."DON'T FOLLOW ME"


You need to put a live weather cam outside your house and post a link to WU. hehehe
503. aubiesgirl
9:26 PM GMT on October 16, 2007
501. melly 9:26 PM GMT on October 16, 2007
aubies.Loved the yellow Springs area

,lol so did my husband
501. melly
9:26 PM GMT on October 16, 2007
aubies.Loved the yellow Springs area
500. taistelutipu
9:25 PM GMT on October 16, 2007
taistelutipu, thanks! I'll do that, though the Germans (I'm actually naturlized an American citizen...geboren im Deutschland) tend to just glue words together to form new descriptors, you know, like "earschplittenloudenboomer" (thanks to John Kay and Steppenwolf)...makes it hard to keep up with the language LOL

Oh, then we have something in common, bin auch in Deutschland geboren :) though living in Finland now.
Finnish is also a language with extremely long words and tends to have own words for everything. My favourites in the weather sector are:
hirmumyrsky for hurricane (where German e.g. also has Hurrikan)
pyörretuuli for a tornado (same in German)
ilmastotilasto for climate statistics
lumimyräkkä for blizzard
sääennuste for weather forecast and so on...
Of course you can have then compounds with those like lumimyräkkävaroitus or pyörretuulihavaintolomake etc.
Member Since: August 20, 2007 Posts: 12 Comments: 640
499. Floodman
9:25 PM GMT on October 16, 2007
It does seem to be following the path of least resistance up the dry air in front of it, but it has made a few inroads over the course of the day...there was a slight spin in the area this morning, but I don't see it now...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
498. aubiesgirl
9:24 PM GMT on October 16, 2007
beell We had plenty of rain on the coast and we really needed it!
497. Bobbyweather
9:23 PM GMT on October 16, 2007
Posted by: JeffMasters, 10:45 AM EDT on October 16, 2007

Had that system formed just 200 miles further east, it would have spent five days meandering over the Western Caribbean instead of over the Yucatan Peninsula, and could have easily grown into a Category 4 or 5 hurricane.

I love that part.
Member Since: September 7, 2006 Posts: 91 Comments: 2679
496. Floodman
9:23 PM GMT on October 16, 2007
Anyone notice the increase in convection around 10N39W?
Member Since: August 2, 2006 Posts: 10 Comments: 9922
495. melly
9:23 PM GMT on October 16, 2007
So what did my family do??? We all moved to Florida, and I again experienced severe weather. Francis & Jeane in 2004, Then Wilma in 2005....I now live in Palm Beach county.All I can say is ."DON'T FOLLOW ME"
494. aubiesgirl
9:23 PM GMT on October 16, 2007
493. melly 9:21 PM GMT on October 16, 2007
I spoke of this before, but it was early A.M. and many posters were not here. I grew up in Xenia Ohio, and experienced the F5 tornado that hit Xenia , April 3rd, 1974, at exactly 3:40 pm, while i was walking home from school
Action: | Ignore User
my husband grew up in Enon..his family went thru that storm
493. melly
9:21 PM GMT on October 16, 2007
I spoke of this before, but it was early A.M. and many posters were not here. I grew up in Xenia Ohio, and experienced the F5 tornado that hit Xenia , April 3rd, 1974, at exactly 3:40 pm, while i was walking home from school
492. beell
9:20 PM GMT on October 16, 2007
i really thought the rain would make its way farther inland-front stalled early i guess
Member Since: September 11, 2007 Posts: 148 Comments: 17205
491. aubiesgirl
9:18 PM GMT on October 16, 2007
We have had lots of rain today!!!!yea!!!
490. beell
9:18 PM GMT on October 16, 2007
gcd, mlc did not mention any-you need to share.
Member Since: September 11, 2007 Posts: 148 Comments: 17205
489. Floodman
9:17 PM GMT on October 16, 2007
488. TEXASYANKEE43 9:16 PM GMT on October 16, 2007
482. Floodman 9:10 PM GMT on October 16, 2007
(I'm actually naturlized an American citizen...geboren im Deutschland)


I'm jus guessin'"American born in Germany"?



Yep...born in Neustadt an der Aisch and adopted by an American national and his German wife...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
488. TEXASYANKEE43
9:16 PM GMT on October 16, 2007
482. Floodman 9:10 PM GMT on October 16, 2007
(I'm actually naturlized an American citizen...geboren im Deutschland)


I'm jus guessin'"American born in Germany"?
487. beell
9:15 PM GMT on October 16, 2007
weatherg8r,
jp and i will move heaven and earth to bring it to you lol
Member Since: September 11, 2007 Posts: 148 Comments: 17205
486. Floodman
9:14 PM GMT on October 16, 2007
StormW, I know that divergent winds create, for want of a better term, a void, causing, well, suction...so convergent winds create a "push"? Can you explain these two items?
Member Since: August 2, 2006 Posts: 10 Comments: 9922
485. CaneAddict
9:12 PM GMT on October 16, 2007
StormW whats your take on that ULL in the central Caribbean? I have gained a little concern for that area as thunderstorm activity it currently firing around it...
Member Since: October 5, 2007 Posts: 3 Comments: 2151
484. Bobbyweather
9:12 PM GMT on October 16, 2007
Good afternoon! I am giving up the chance for 99L becoming a TD if it doesn't form by Thursday. I saw the chart of October 16~31 tropical storms and hurricanes and there are quite a lot of them!

I expect between 3~5 named storms, 1 or 2 hurricanes, and maybe one or none major hurricane for the remainder of the hurricane season.
Member Since: September 7, 2006 Posts: 91 Comments: 2679
483. beell
9:11 PM GMT on October 16, 2007
Good TUTT Post MichaelSTL
(i may have said that twice)
Member Since: September 11, 2007 Posts: 148 Comments: 17205
482. Floodman
9:10 PM GMT on October 16, 2007
477. taistelutipu 9:06 PM GMT on October 16, 2007
456. Floodman 11:48 PM EEST on October 16, 2007
Wow, got really quiet in here all of a sudden...

Floodman, you astonished us all with your German skills :)

Speed is the word for amphetamine in the drug scene also in Germany. I didn't know its correct term is benzedrin though. Thanks for telling me.

If you want to practice your German you can do so on the German wikipedia about Kyrill. Compared to the English site it contains a lot more details about the storm and the preparations before it struck. Kyrill 2007



taistelutipu, thanks! I'll do that, though the Germans (I'm actually naturlized an American citizen...geboren im Deutschland) tend to just glue words together to form new descriptors, you know, like "earschplittenloudenboomer" (thanks to John Kay and Steppenwolf)...makes it hard to keep up with the language LOL
Member Since: August 2, 2006 Posts: 10 Comments: 9922
481. beell
9:09 PM GMT on October 16, 2007
Good Post MichaelSTL
(It was better the second time)
Member Since: September 11, 2007 Posts: 148 Comments: 17205
480. myrc60
9:08 PM GMT on October 16, 2007
Please bring some rain to Atlanta and Upstate SC, we are very dry and Atlanta wants to "borrow" our water here in the Upstate. How do you "borrow" water?
479. weatherg8r
9:07 PM GMT on October 16, 2007
gulfcoastdweller 3:59 PM EST on October 16, 2007
I now have rain!! Thanks for beating the drum Beell


Beell...please keep it up...no rain here YET
477. taistelutipu
9:06 PM GMT on October 16, 2007
456. Floodman 11:48 PM EEST on October 16, 2007
Wow, got really quiet in here all of a sudden...


Floodman, you astonished us all with your German skills :)

Speed is the word for amphetamine in the drug scene also in Germany. I didn't know its correct term is benzedrin though. Thanks for telling me.

If you want to practice your German you can do so on the German wikipedia about Kyrill. Compared to the English site it contains a lot more details about the storm and the preparations before it struck. Kyrill 2007
Member Since: August 20, 2007 Posts: 12 Comments: 640

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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.

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