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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977. NEwxguy
1:20 PM GMT on October 17, 2007
morn,floodman
Member Since: September 6, 2007 Posts: 862 Comments: 15079
976. catastropheadjuster
1:20 PM GMT on October 17, 2007
Baja he's a really nice guy and don't mind answering questions at all.
Sheri
DrM has a NEW BLOG
Member Since: August 24, 2006 Posts: 21 Comments: 3648
975. BajaALemt
1:20 PM GMT on October 17, 2007
MJO

974. BajaALemt
1:18 PM GMT on October 17, 2007
Sheri, I've really been learning alot through his site (and some of the blogs and conversations here)

973. Floodman
1:18 PM GMT on October 17, 2007
971. TampaSpin 1:14 PM GMT on October 17, 2007
Floodman
without of over talk, what is MJO....sorry just trying to learn.



The Madden-Julian Oscillation

It's a band of convection/moisture that moves between the tropivs; when the oscillation is up, more moisture, when it's down, there's less...harder for storms to form in a decreased MJO environment: not enough moisture to fuel convection...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
972. catastropheadjuster
1:16 PM GMT on October 17, 2007
New Blog
Member Since: August 24, 2006 Posts: 21 Comments: 3648
971. TampaSpin
1:14 PM GMT on October 17, 2007
Floodman
without alot of over talk, what is MJO....sorry just trying to learn.
Member Since: September 2, 2007 Posts: 177 Comments: 20430
970. catastropheadjuster
1:14 PM GMT on October 17, 2007
Baja yeah that's what I am going to do. And yes he has his own sight to very imformative.
Sheri
Member Since: August 24, 2006 Posts: 21 Comments: 3648
969. TampaSpin
1:14 PM GMT on October 17, 2007
Hey Floodman.
Member Since: September 2, 2007 Posts: 177 Comments: 20430
968. BajaALemt
1:13 PM GMT on October 17, 2007
Mornin flood.

Well, guess we COULD talk midwest. Be interesting to watch that area this afternoon
967. Floodman
1:12 PM GMT on October 17, 2007
For an heightened MJO, there isn't a great deal of moisture to go around; more than yesterday, certainly, but I would have thought we'd be seeing more convective activity...
Member Since: August 2, 2006 Posts: 10 Comments: 9922
966. BajaALemt
1:11 PM GMT on October 17, 2007
Neat Sheri. Have you tried leaving him a message? Maybe he's checking them. Also, I'm not sure if he admin's his own site or not...but there's an link to contact the admin there...might try that?
965. catastropheadjuster
1:09 PM GMT on October 17, 2007
Baja Thanks. I Stormjunkie is ok cause the last time I really got to talk to him he had been sick. He's a nice person along with alot of others on here.
I love taking pictures to. Our shop is on the creek and I take alot of them.
Sheri
Member Since: August 24, 2006 Posts: 21 Comments: 3648
964. Floodman
1:08 PM GMT on October 17, 2007
Morning, folks!

So let's see, other than the erstwhile 99L, we have a ULL spinning at 16N79W which may or may not be trying to reach the surface, and a feature at 12N41W in the ITCZ that is struggling under about 30knts of shear...

The CMC is trying to develop something in the BOC that will apparently reach the Louisiana coast in 144 hours, the GFDL develops a feature to near TS strength in the same area but doesn't move it out of the BOC to 126 hours, the GFS and the HWRF don't develop anything at all, and the NOGAPS shows a trof and little else in the BOC until high pressure builds back in at the end of the forecast period...

Hmmm...another slow day in the tropics? Or shall we talk Pacific systems?
Member Since: August 2, 2006 Posts: 10 Comments: 9922
963. TheCaneWhisperer
1:03 PM GMT on October 17, 2007
Morning Baha
962. BajaALemt
1:03 PM GMT on October 17, 2007
Morning adjuster. I saw Taz last nite. Haven't seen SJ in 3 or 4 days
961. BajaALemt
1:02 PM GMT on October 17, 2007
Absolutely, ic!! I picked up an HP Coolpix for $149 that's actually produced better pics than my more expensive camera. The added bonus with this one..is that it uses SD cards and walmart often has 2 gig SD cards for $19.98 (a GREAT bargain). I'll go out and shoot maybe...300 pics in an hour *laffs*
960. catastropheadjuster
1:01 PM GMT on October 17, 2007
Morning all. I was just wondering where StormJunkie & TAZ is? Haven't seen them lately.
Member Since: August 24, 2006 Posts: 21 Comments: 3648
959. icmoore
12:52 PM GMT on October 17, 2007
Emt, it is just a kodak digital. I am getting a new one in the mail that I ordered soon. A little step up. I love being able to come right in from my garden and transfer them to my computer. I have just learned to upload to the site.
Member Since: July 18, 2005 Posts: 9 Comments: 4141
958. BajaALemt
12:48 PM GMT on October 17, 2007
Meeeee toooooo, NE!! I like to call them...the creator's canvas
957. BajaALemt
12:46 PM GMT on October 17, 2007
LOL, ic. You sound like ME!! Is your camera digital or slr?
956. icmoore
12:44 PM GMT on October 17, 2007
Good morning NEwxguy and yes I sure am hoping for more butterflies and more sunshine than yesterday EMT. Have been taking alot of pictures and having fun with that.
Member Since: July 18, 2005 Posts: 9 Comments: 4141
955. BajaALemt
12:44 PM GMT on October 17, 2007
Mornin whisperer
954. NEwxguy
12:42 PM GMT on October 17, 2007
952. BajaALemt 12:41 PM GMT on October 17, 2007
Oh gosh! I took my break about 615ish last nite, and I WISH I would have had my camera with me! The sunset over the gulf last nite was just STUNNING. Not often it's SO amazing that it brings a lump in my throat. One of the wonderful things .....after the storm.

too bad about the camera,I love sunsets.
Member Since: September 6, 2007 Posts: 862 Comments: 15079
952. BajaALemt
12:41 PM GMT on October 17, 2007
Oh gosh! I took my break about 615ish last nite, and I WISH I would have had my camera with me! The sunset over the gulf last nite was just STUNNING. Not often it's SO amazing that it brings a lump in my throat. One of the wonderful things .....after the storm.
951. BajaALemt
12:38 PM GMT on October 17, 2007
G'mornin Mr NE
950. BajaALemt
12:37 PM GMT on October 17, 2007
lol...enjoying the butterflies again today?
949. NEwxguy
12:37 PM GMT on October 17, 2007
good morning,all,
another beautiful day here in new england,could use some rain though,but still not as bad as you people in the south
Member Since: September 6, 2007 Posts: 862 Comments: 15079
948. icmoore
12:32 PM GMT on October 17, 2007
Good Morning to you StormW, emt, and everyone else! Smiling back at you emt.
Member Since: July 18, 2005 Posts: 9 Comments: 4141
946. BajaALemt
12:30 PM GMT on October 17, 2007
mornin wannabe
945. BajaALemt
12:29 PM GMT on October 17, 2007
weathers4, I was wondering the same thing
944. weathermanwannabe
12:28 PM GMT on October 17, 2007
Morning Folks......Looking forward to some rain the Panhandle (love it when the rain happens during the week and clears out nicely for the weekend)......Tropics are quiet...Guess we'll have to wait to see, in accordance with Dr, M's analysis, what happens at the end of this month if the shear drops down significantly.........Will be around lurking but cetainly do not expect anything significant to form through the end of the week...See Yall Later...
Member Since: August 8, 2006 Posts: 0 Comments: 8278
943. PensacolaDoug
12:25 PM GMT on October 17, 2007
G'morn'n all. I see the tropics are still behaving nicely for us. Its a wonderful thing.
Member Since: July 25, 2006 Posts: 0 Comments: 519
942. BajaALemt
12:24 PM GMT on October 17, 2007
Thanks Storm...and g'morning, btw

Mornin ic(less) lol *smiles*

...emt
940. icmoore
12:09 PM GMT on October 17, 2007
Good morning everyone. You must have all gone for a collective coffee break.
Member Since: July 18, 2005 Posts: 9 Comments: 4141
938. weatherg8r
11:51 AM GMT on October 17, 2007
Morning Storm
937. weathers4me
11:50 AM GMT on October 17, 2007
Is it the ULL that is supposed to bring the mositure to the SE or something else? I do not see a lot of mositure moving N into FL.
Member Since: May 24, 2007 Posts: 0 Comments: 118
935. Cavin Rawlins
11:29 AM GMT on October 17, 2007
93P - Just to the north of the islands that make up Vanuatu.

Member Since: July 24, 2005 Posts: 407 Comments: 19076
934. Cavin Rawlins
11:25 AM GMT on October 17, 2007
sporteguy03,

Ur welcome..have a great day ur self.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
933. sporteguy03
11:22 AM GMT on October 17, 2007
Thanks for the update Weather456 have a great day!
Member Since: July 7, 2005 Posts: 0 Comments: 5120
932. sporteguy03
11:21 AM GMT on October 17, 2007
Where's Stormjunkie did he leave the blog too?
Member Since: July 7, 2005 Posts: 0 Comments: 5120
931. Cavin Rawlins
11:20 AM GMT on October 17, 2007
Area of Interest

In Its Favor

Shear has decrease somehwat over the past 24 hrs from near 70 knots to 30 knots.

Associated with a low level disturbance - tropical wave.

Has associated 25+ knot winds

Against It

Despite wind shear decreasing...it is still well above the favorable threshold.

Climatologically not a favored area.

No surface circulation



Member Since: July 24, 2005 Posts: 407 Comments: 19076
928. ShenValleyFlyFish
9:55 AM GMT on October 17, 2007
Good-morning all

885. cattlebaroness 1:42 AM EDT on October 17, 2007
Moonlight, I love it. You would be amazed how much poop a small herd of cattle can make. I step in it enough! No sandals, boots only.

Been there done that, Actually don't feel fully dressed without boots on. How many critters you have?
Member Since: September 9, 2007 Posts: 36 Comments: 4687
927. franck
8:03 AM GMT on October 17, 2007
Good for you Flamboy inga.
Member Since: August 30, 2005 Posts: 0 Comments: 1150

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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.