Fred fading; halfway point of hurricane season reached

By: Dr. Jeff Masters , 1:40 PM GMT on September 10, 2009

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Hurricane Fred peaked in intensity yesterday afternoon, attaining Category 3 strength with 120 mph winds. It is quite unusual to have such a powerful system so far east in the Atlantic, and Fred is only the third major hurricane to exist east of 35W. Fred is also the strongest hurricane so far south and east in the data record. However, this type of system would have been difficult to document before satellite pictures began in the 1960s.

Fred's glory is past, and the storm is on a downslide now, thanks to moderate wind shear of 15 - 20 knots and dry air eating into the hurricane's southwest side. The shear and the dry air will increase over the next few days, with the shear rising above 40 knots by Monday morning. Sea surface temperatures (SSTs) will also cool to near the 26.5°C threshold needed to sustain a tropical cyclone. The combination of high shear, dry air, and cool SSTs will likely kill Fred by Tuesday.


Figure 1. Hurricane Fred at peak strength, 8:55am EDT UTC 9/9/09. At the time, Fred was a Category 3 hurricane with 120 mph winds. Image credit: NASA.

Elsewhere in the tropics
An upper-level low pressure system has moved over Texas and is expected to spawn a surface low pressure system along the Texas Gulf of Mexico coast on Friday. This low will probably have characteristics of both a tropical and extratropical storm. The surface low is likely to move northeastward and move ashore near the Texas/Louisiana border region on Saturday or Sunday. There will be some high wind shear to the west of the low (shear is currently a high 25 knots), so it is uncertain whether this low will be capable of developing into a tropical cyclone. Regardless, this storm will bring heavy rain capable of causing flooding--and help alleviate the exceptional drought conditions over Southeast Texas.

Early next week, we should be alert for tropical storm development over the waters between the Bahamas and North Carolina, along an old frontal zone. None of the reliable models are forecasting tropical storm development in this area or in the Gulf of Mexico, though.


Figure 2. The climatological halfway point of the Atlantic hurricane season is today, September 10.

Halfway point of hurricane season
September 10 marks the halfway point of the Atlantic hurricane season. Despite a late start (Tropical Storm Ana did not form until August 15, the latest start to a hurricane season since 1992), our number of storms has been near average. An average Atlantic hurricane season has 5 named storms, 3 hurricanes, and 1 intense hurricane by the midpoint of the season. So far this year, we've had 6 named storms, 2 hurricanes, and 2 intense hurricanes. A better measure of hurricane activity that takes into account their destructive power is the Accumulated Cyclone Energy (ACE) index. ACE for an individual storm is computed by squaring the maximum sustained winds of the storm at each 6-hourly advisory, and summing up over the entire lifetime of the storm. As of 5am EDT this morning, the seasonal ACE tally was 37.5. This number should rise to around 40 by the end of the day, thanks to the presence of Hurricane Fred. Over the period 1950 - 2005, the average ACE index for a half-season was 51, so 2009 ranks about 20% below average for the halfway point of the season. But when compared to the hurricane seasons we've been having since the current active hurricane period began in 1995, this year has been quite inactive. Between 1995 and 2008, the average ACE index for the halfway point of the season was 72. Thus, 2009 is about 45% less active than what we've been accustomed to over the past 14 years.

We've been lucky this year that the steering currents have aligned to keep our two major hurricanes, Bill and Fred, out to sea. What will the rest of the season have in store for us? I'll present an analysis on Friday.

Twenty years ago on this date
On September 10, 1989, the strong tropical wave that had moved off the coast of Africa the previous day acquired an organized circulation at the surface and began building a concentrated area of heavy thunderstorms near its center. A new tropical depression, the 12th of the season, was born. Moving westward at 20 mph, the depression brought strong, gusty winds and heavy rain showers to the Cape Verdes Islands as it passed to the south. Forecasters at the National Hurricane Center predicted that the steadily organizing tropical depression would strengthen into a tropical storm within the next day or two. The next name on the list of Atlantic tropical storm names for 1989: Hugo.


Figure 3. AVHRR visible satellite image of Tropical Depression Twelve taken on September 10, 1989. Image credit: Google Earth rendition of the NOAA HURSAT data base.

Jeff Masters

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Quoting btwntx08:

shear is decreasing


he's taking about about the mid-upper winds blowing the mid-level vort east which is evident through vapor winds



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Hey Storm!
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1211. IKE
Vorticity...SE GOM....

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Quoting Dakster:
I see a little rotate on the radar..and it's heading into the Gulf.

But yes, I trust the NHC and some key ppl here.
any storm can a rotation on Radar the question is is the spin on Sattelite? so far no
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1208. WxLogic
Quoting tornadodude:


hello there


Hehe... hello.
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Quoting RitaEvac:
That's why its moving east, its the mid/upper level shear blowing it that way into the Gulf

shear is decreasing
Member Since: July 13, 2008 Posts: 6 Comments: 10796
1206. WxLogic
GOM and W/SW ATL look quite active (at least debri wise)...

Shear appears to be low enough... yet high enough to promote sporadic intense vortices... on TSTM clusters. If one of these manages to keep its strength specially in the S GOM, NW Carib SW and SW ATL... then a short lived system won't be a out of the possibility.
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Quoting btwntx08:

remember claudette formed on the tampa radar this looks like it wants to do the same


at that time a srfc low was already evident. Not saying that this wont develop but you gotta take things a step at a time.
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1203. KEEPEROFTHEGATE (Mod)


AOI/XX/XX
MARK
24.1N/95.4W
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Quoting Weather456:


correct. Its not a surface low.

However I dont know if this is the primary low or not.


There was a hint of one earlier today SE of Brownsville trying to spin up but has fizzled out along with the convection, really have to wait till tomm morning to see what happens
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1201. OneDay
I was just kidding...radar is very unreliable and not used to determine whether a low is deepening. Often as we lose daytime heating and the storms start to dissipate you'll see little "eyes" form. in a couple of hours the convection off the brownsville coast will have all but dissipated. Wait to see if the bouys in the area start indicating a fall in pressure. (Of course, quikscat and sat would be nice, too...but the buoys' hourly observations would be way ahead of those.)
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Quoting IKE:


GOM...looks like a spin on that radar.


Oh, well that's not good at all, so what would you say are the chances of it being a tropical cyclone by morning, because i see that the sheer is weakening a bit and the GOM is like a hot tub!
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Quoting WxLogic:
Good evening...


hello there
Member Since: June 28, 2006 Posts: 25 Comments: 8360
Well shoot I got laundry to fold. Ugh. BBL. :(
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Quoting IKE:


GOM...looks like a spin on that radar.


your right
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Quoting RitaEvac:
That's why its moving east, its the mid/upper level shear blowing it that way into the Gulf


that is a reasonable solution per upper level vapor winds.
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Quoting Weather456:


ur getting ahead of yourself

remember claudette formed on the tampa radar this looks like it wants to do the same
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1194. Dakster
Looks like a mess from the Satellite view... That's why I want to see a QuickScat of the area...

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1193. WxLogic
Good evening...
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That's why its moving east, its the mid/upper level shear blowing it that way into the Gulf
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Quoting iceman55:
homelesswanderer heyyyyyyyyy


Hey Ice. :)
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Quoting RitaEvac:
weather456 isn't that just a mid level vort that spun off from todays convection? I would't think that is going to be the primary low center. That's why its heading east into the Gulf, it can't be the real deal yet.


correct. Its not a surface low.

However I dont know if this is the primary low or not.
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Quoting laflastormtracker:
homeless, looks interesting :)


Yeah. A little too interesting.
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weather456 isn't that just a mid level vort that spun off from todays convection? I would't think that is going to be the primary low center. That's why its heading east into the Gulf, it can't be the real deal yet.
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Quoting fuzzy3456:



i'm usually just a reader of your guys comments i find them very informing, but i would just like to know are you talking about Fred or AOI in the gulf?


AOI in gulf
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Quoting Dakster:
Who's the guest? I can't listen to Bob right now?


Me either the Steelers vs. Tennessee in the back ground lol
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1182. IKE
Quoting fuzzy3456:



i'm usually just a reader of your guys comments i find them very informing, but i would just like to know are you talking about Fred or AOI in the gulf?


GOM...looks like a spin on that radar.
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Quoting btwntx08:
claudette part 2 lol
Hope your not talking about Tropical Storm Claudette from 1979.
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1179. Dakster
DARN IT! I like listening to the Master Chief... I'll have to listen to it later...

Would be nice if Dr. Masters would go on again, too!
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homeless, looks interesting :)
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Quoting IKE:


Is that a pinhole eye on radar?



i'm usually just a reader of your guys comments i find them very informing, but i would just like to know are you talking about Fred or AOI in the gulf?
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Quoting tharpgomex:
Good evening everybody!


Good evening Tharp
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1174. KEEPEROFTHEGATE (Mod)
Quoting Dakster:
Who's the guest? I can't listen to Bob right now?
thomas walsh just over now a break on to next part of the show
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Quoting tornadodude:
Good evening everyone


evening
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1172. Dakster
BTWN - Hopefully the worst is that it is another "Claudette". I would like to see the "mets" comment on the forming radar signature though.
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Quoting tornadodude:
Good evening everyone


Hey Matt. :)
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Quoting btwntx08:
claudette part 2 lol


ur getting ahead of yourself
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Good evening everybody!
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1167. Dakster
Who's the guest? I can't listen to Bob right now?
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claudette part 2 lol
Member Since: July 13, 2008 Posts: 6 Comments: 10796
Good evening everyone
Member Since: June 28, 2006 Posts: 25 Comments: 8360
1163. Dakster
When is the next QuickScat of the Gulf?
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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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