Tropical Storm Helene arrives; 94L a potential threat to the Lesser Antilles

By: Dr. Jeff Masters , 10:41 PM GMT on August 17, 2012

After a long path across the Atlantic that mirrored the track of Hurricane Ernesto, Tropical Depression Seven finally got its act together enough over the Bay of Campeche tropical storm breeding grounds to earn the name Helene. Helene's formation on August 17 ties 2012 with 1933 for the 2nd earliest appearance of the Atlantic's eighth tropical storm. Only 2005 had an earlier formation date of the season's eighth storm. Most of this year's storms have been weak, though, so the total Accumulated Cyclone Energy (ACE) of 21 is not that much higher than the normal year-to-date ACE of 15, based on a 1981 - 2010 ACE climatology. The normal yearly ACE for the Atlantic is 104. Helene doesn't have much room to work with before landfall, but has the potential to be a prodigious rainmaker for Mexico, with NHC predicting 5 - 10 inches for portions of Northeast Mexico. This part of the coast is not in drought, so will be prone to heavy flooding. Fortunately, Ernesto's main rains fell to the south of where Helene's rains are falling. Helene's rains should remain south of Texas, though we can't rule out a few thunderstorms bringing 1 - 2 inches of rain to extreme South Texas on Saturday and Sunday.


Figure 1. Afternoon satellite image of Helene.

94L a potential threat to the Lesser Antilles
A large tropical wave emerged from the coast of Africa Thursday night, and was designated Invest 94L by NHC this Friday morning. The models have been impressed this system, and develop it into a tropical storm by the middle of next week. The latest set of 12Z (8 am EDT) model runs are divided on how far west 94L will make it before curving more to the northwest. The ECMWF model keeps 94L weak for the next 5 - 6 days, and has progressively been bringing the storm closer to the Lesser Antilles Islands with each successive run. The ECMWF predicts 94L will pass very close to the northern Lesser Antilles August 24 - 25 as a weak tropical storm. The 12Z GFS model predicted recurvature of 94L well to the east of the Lesser Antilles and Bermuda, but the latest 18Z run has the storm plowing through the Lesser Antilles on Thursday, August 23, as a strong tropical storm, then becoming a hurricane in the Eastern Caribbean. Given that our two top models for forecasting hurricane tracks are increasingly showing a threat to the Lesser Antilles, residents of the islands should pay close attention to the progress of 94L. The eventual track of 94L will depend on the strength of the storm over the next seven days, which is difficult to forecast, since 94L will have the usual trouble with dry air to the north from the Saharan Air Layer (SAL.) With the models changing their tune drastically from run to run, its tough to say what land areas might be most at risk from the storm in the long term. In their 2 pm EDT Tropical Weather Outlook, NHC gave 94L a 10% chance of developing into a tropical depression by Sunday afternoon.


Figure 2. MODIS satellite image of Invest 94L taken at 8 am EDT August 17, 2012, off the coast of Africa. Image credit: NASA.

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.

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Okay, so seriously... what did the NHC look at and say "looks 5mph stronger than 530"

Product: Air Force Vortex Message (URNT12 KNHC)
Transmitted: 17th day of the month at 23:31Z
Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 303)
Tropical Depression: Number 7 (flight in the North Atlantic basin)
Mission Number: 2
Observation Number: 16
A. Time of Center Fix: 17th day of the month at 23:20:00Z
B. Center Fix Coordinates: 20°47'N 96°01'W (20.7833N 96.0167W)
B. Center Fix Location: 154 miles (249 km) to the SE (130°) from Tampico, Tamaulipas, México.
C. Minimum Height at Standard Level: Not Available
D. Estimated (by SFMR or visually) Maximum Surface Wind: 8kts (~ 9.2mph)
E. Location of the Estimated Maximum Surface Wind: 4 nautical miles (5 statute miles) to the NW (319°) of center fix
F. Maximum Flight Level Wind Inbound: From 50° at 15kts (From the NE at ~ 17.3mph)
G. Location of Maximum Flight Level Wind Inbound: 4 nautical miles (5 statute miles) to the NW (319°) of center fix
H. Minimum Sea Level Pressure: 1006mb (29.71 inHg) - Extrapolated
I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 24°C (75°F) at a pressure alt. of 303m (994ft)
J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 26°C (79°F) at a pressure alt. of 290m (951ft)
K. Dewpoint Temp (collected at same location as temp inside eye): 24°C (75°F)
K. Sea Surface Temp (collected at same location as temp inside eye): Not Available
L. Eye Character: Not Available
M. Eye Shape: Not Available
N. Fix Determined By: Penetration, Wind, Pressure and Temperature
N. Fix Level: 1,500 feet
O. Navigation Fix Accuracy: 0.02 nautical miles
O. Meteorological Accuracy: 3 nautical miles
Remarks Section:
Maximum Flight Level Wind: 30kts (~ 34.5mph) in the south quadrant at 22:28:00Z
Sea Level Pressure Extrapolation From: Below 1,500 feet
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Quoting Felix2007:


The blue moon is on August 31st and that's when Isaac will supposedly strike according to the 18z GFS. Also it doesn't happen very often.
Yea...once in a blue moon!
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Helene = toast.

Official center fix is probably wrong.

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18z HWRF

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Im still going with freeport for Helene
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Quoting TomTaylor:
People have already pointed this out, but here's another look at the shift from the 12z ensemble run to the 18z ensemble run. We shouldn't be surprised to see such a shift, however, since this is 12 days out. Expect another model shift at the 0z run.

12z Ensemble vs 18z Ensemble at 276 Hours



Tom in what coordinate you think 94l will recurve
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I think the gfs is only good for hinting upcoming storms and possible tracks, but when it comes to predicting strength before anything has even formed, it always predicts cat 7 monsters lol.
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Helene appears to be drifting to the N....think its feeling the trough?
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217. JLPR2
Quoting Relix:


Georges. Heh. I don't think the setup for 94L is like this, I believe its closer to a Hugo or Earl.


Eh... If the models continue to trend west and south I wont discard any possibilities. Originally Georges was supposed to pass to our south, around 16N. Ah, the craziness. XD

94L is still too far away and too weak for the models to get a good handle on it. Still, keeping a very close eye on it.
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Hector still has a few clouds :P
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18z GFS Precip--I will be back later..finally able to watch Cowboys and Aliens playing on Cinemax and I want to see it
240 hours


through 360 hours
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back later
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Quoting HimacaneBrees:
HELENE HAS BEEN DRIFTING
NORTHWARD DURING THE PAST COUPLE OF HOURS...BUT IS EXPECTED TO
RESUME A TRACK TOWARD THE NORTHWEST NEAR 7 MPH...11 KM/H LATER
TONIGHT. THIS MOTION IS EXPECTED TO CONTINUE FOR THE NEXT DAY OR
TWO...AND ON THE FORECAST TRACK...THE CENTER OF HELENE WILL MAKE
LANDFALL WITHIN THE WARNING AREA ON SATURDAY.



'Within the warning area' is a good distance either way for such a small storm.
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To the NHC: and if it doesn't turn back then your in trouble again just like debby!
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People have already pointed this out, but here's another look at the shift from the 12z ensemble run to the 18z ensemble run. We shouldn't be surprised to see such a shift, however, since this is 12 days out. Expect another model shift at the 0z run.

12z Ensemble vs 18z Ensemble at 276 Hours

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Sigh. Did the NHC hire someone from this website? They keep naming storms left and right...
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Quoting PalmBeachWeatherBoy:
A little confused, so...cape-verde storms tend to recurve due to the lack of a strong high which I sometimes notice building/spreading over the NE USA region? if so, what dictates the high being weaker this year?


Not necessarily. A CV storm may work its way all the way to the Western periphery of the high and once it reaches there it will turn to the N and then NNE and eventually out to sea around the north of the clockwise steering of the high. Alternatively the storm will get picked up by a trough with Westerlies and that will also lift it out once it reaches the Western periphery of the high. Several variables to this scenario.
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Interesting indeed.



I think the mid-level is decoupling from the LLC, because on satellite it looks like the mid-level circulation and much of the convection is still trucking off to the west...

Unless the GoMex RGB loop is screwed up or something...




Oh well. Time will tell.
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HELENE HAS BEEN DRIFTING
NORTHWARD DURING THE PAST COUPLE OF HOURS...BUT IS EXPECTED TO
RESUME A TRACK TOWARD THE NORTHWEST NEAR 7 MPH...11 KM/H LATER
TONIGHT. THIS MOTION IS EXPECTED TO CONTINUE FOR THE NEXT DAY OR
TWO...AND ON THE FORECAST TRACK...THE CENTER OF HELENE WILL MAKE
LANDFALL WITHIN THE WARNING AREA ON SATURDAY.



From the NHC
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Quoting BahaHurican:
It's not like there hasn't been a "coastal strafer" before...



The blue moon is on August 31st and that's when Isaac will supposedly strike according to the 18z GFS. Also it doesn't happen very often.
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Link
TS Helene's wind field
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201. Relix
Quoting stormchaser19:




This is a example of no-recurve cyclone


Georges. Heh. I don't think the setup for 94L is like this, I believe its closer to a Hugo or Earl.
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Thanks all...still catching up. Back to watching and lurking.
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A little confused, so...cape-verde storms tend to recurve due to the lack of a strong high which I sometimes notice building/spreading over the NE USA region? if so, what dictates the high being weaker this year?
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Quoting PlazaRed:


I'm sure you are being optimistic there!
I'm just stating what the model showed.I'm skeptical right now though on future track/intensity..
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Quoting floridaboy14:
how confident are you 94L recurves? any chance this takes the ECMWF track?




This is a example of no-recurve cyclone
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Quoting tropicfreak:
I would put the center of Helene right around 21N 97W.

its not at 97w lol
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18z GFS ensembles..180 hours

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Quoting RkptMom:
Latest from NHC...TROPICAL STORM HELENE DRIFTING NORTHWARD IN THE SOUTHWESTERN GULF OF MEXICO...WITH LITTLE CHANGE IN STRENGTH...

So, what constitutes a drift vs. movement?



"Drifting" usually means about 1 to 3mph of movement, or else non-determined movement BELIEVED to be in that direction.

In other words, "drifting" can be substituted for, "Nearly stationary, but we think it's moving in direction 'X," if at all".
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Quoting Levi32:


"Recurve" is defined as the storm moving poleward and then into the mid-latitude westerlies. For some reason the blog defines "recurve" as missing the United States. Following the true definition, the storm is very likely to recurve in this pattern, and the vast majority of Cape Verde storms do recurve. My thought so far has been that the recurve will take place between 80W and 65W.
I respect your opinion on the track. Why does the ECMWF model bend it back west though? it shows it moving NW and then just gets shoved back west
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Quoting TropicalAnalystwx13:
I love major hurricanes.

Yeah..out to sea though...Surprise the ship doesn't run this to cat 5.Lol.
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Meanwhile...
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Quoting RkptMom:
Latest from NHC...TROPICAL STORM HELENE DRIFTING NORTHWARD IN THE SOUTHWESTERN GULF OF MEXICO...WITH LITTLE CHANGE IN STRENGTH...

So, what constitutes a drift vs. movement?

A drift is very slow.
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Quoting washingtonian115:
Helene has formed in the BOC.The latest run of the GFS shows future Isaac as a strong storm plowing through the Antilies and being pulled northward out of the caribbean around PR.Then making a run at the east coast and wiping the north east off the map with N.C likely being affected first.


I'm sure you are being optimistic there!
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Quoting RkptMom:
Latest from NHC...TROPICAL STORM HELENE DRIFTING NORTHWARD IN THE SOUTHWESTERN GULF OF MEXICO...WITH LITTLE CHANGE IN STRENGTH...

So, what constitutes a drift vs. movement?


Speed
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I love major hurricanes.

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Latest from NHC...TROPICAL STORM HELENE DRIFTING NORTHWARD IN THE SOUTHWESTERN GULF OF MEXICO...WITH LITTLE CHANGE IN STRENGTH...

So, what constitutes a drift vs. movement?
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Quoting tropicfreak:
I would put the center of Helene right around 21N 97W.

I put the new center (if it happens) at 23N 97W.
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Quoting ylibrach:


Thank you, that's good advice. Where can I find NWS / NHC discussions? Do they also provide any sort of forecasts in terms of direction?


Here:

http://www.nhc.noaa.gov/

Active Link to same NHC address


Click on the active storm or invest, and click again, a page will pop up and among other things like maps and such, there is a link titled "discussion," which should be useful.

For storms not yet classified, there isn't much information relevant to vacation or business plans...
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Quoting ylibrach:


Thank you, that's good advice. Where can I find NWS / NHC discussions? Do they also provide any sort of forecasts in terms of direction?

http://www.nhc.noaa.gov
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Just Curious. who think we are in el nino RIGHT NOW meaning the atmosphere is behaving like an el nino.?
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Quoting MiamiHurricanes09:
I haven't been on my laptop for a few days and I'm currently on my phone, but does anyone care to inform me on what's going on with the whole NNE-moving Helene/Issac thing that the blog's going crazy over please?
Helene has formed in the BOC.The latest run of the GFS shows future Isaac as a strong storm plowing through the Antilies and being pulled northward out of the caribbean around PR.Then making a run at the east coast and wiping the north east off the map with N.C likely being affected first.
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Quoting floridaboy14:
can anyone answer my question. why does EVERY cape verde storm ALWAYS recurve? in 2004 they went right into the coast. ever since then everything either goes out to sea or curves into the coast like irene. now a days, there are so many troughs that nothing can hit the coast DIRECTLY. thoughts and comments?
Not every CV storm recurves. Irene is just one example of hundreds that made it to the US coast. Hurricanes Andrew, Hugo were CV hurricanes that caused enormous U.S. damage. Here's a list...
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Quoting bappit:

The CPC says we aren't feeling it. Waters warmed but the atmosphere has not responded yet.


The SOI is a direct measure of the atmosphere's response to the Walker Circulation. The atmosphere is very clearly feeling the effects of the El Nino and stair-stepping in negative territory. Fluctuations of this nature are very common.

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I would put the center of Helene right around 21N 97W.

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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather