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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Quoting Levi32:


I'm not sure how that's unusual for a TS when it's the weaker systems that have the least uniformity in wind field, which would make such distortion expected from a low-end TS. Mature hurricanes would be strong in all quadrants.


Levi, are you still confident in your forecast posted yesterday that Helene will eventually move into the northern GOM, possibly just offshore toward Tx/La??
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Quoting gugi182:
Here in Puerto Rico Ada said that the models are shifting more to the west , but 94L was expected to turn NW before getting to the Northern Caribbean Islands. Well i hope she is right.


Weak systems have a tendency to follow E but a hurricane tends to move WNW or NW.... That's why the sooner it develops, the highest probability to see it move N of the islands.... We will also get more accurate forecasts from the models....
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Quoting bappit:
Y'all are ogling an ensemble model.
What else is there right now. Its all fun entertainment.
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Quoting SFLWeatherman:

Please stop posting every individual ensemble member...just link us.
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There is Three Options 94L can take...

(1). Become strong at a fast rate, gain some lattitude and go out to sea around 50W

(2). Stay weak a little longer, Then pick up strength and ride the Islands Split 20N/60W, and Either....

- Ride the trough safely out to sea, while threatening Bermuda...

- Get the door shut in it's face by the two rebuilding ridges overhead, and get pushed back west... MAJOR THREAT TO THE UNITED STATES.


(3). Stay weak until reaching the Islands, and head into the caribbean(Ernesto).
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You all are with the quickness on posting maps!
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Y'all are ogling an ensemble model.
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BTW those are the ensembles not the official model run..that SFLweatherman was posting..
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Quoting sunlinepr:
Post 700. Beautiful Storm, yet deadly and destructive.
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0z GFS is running.

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Quoting HurricaneDean07:

There has been No evidence of an El Nino in affect AT ALL.
It's not even officially an El Nino...

Trade winds across the Caribbean and stronger troughs are indicative of an atmospheric-based El Nino.
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NEW one
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Here in Puerto Rico Ada said that the models are shifting more to the west , but 94L was expected to turn NW before getting to the Northern Caribbean Islands. Well i hope she is right.
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Quoting BCastro:


Mostly due to a weak El Niño infusing some wind shear and dry air to disrupt tropical systems.

There has been No evidence of an El Nino in affect AT ALL.
It's not even officially an El Nino...
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Quoting SFLWeatherman:
Thats my Birthday and all over my house!
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Quoting SFLWeatherman:

REALLY?????
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So from NY to LA and me in FL lol maybe a Hurricane soon
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Quoting photonchaser:


Does that that say 932mb?!
972 mb. right click and open in new tab to see a larger image.
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Quoting SFLWeatherman:
NO 972!!


Phew! Guess I need my glasses back on.
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Looking forward to the next GFS run. I'm curious which of the day's runs it will most resemble...
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NO 972!!
Quoting photonchaser:


Does that that say 932mb?!
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Quoting SFLWeatherman:
WOW
My poor house.
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Quoting washingtonian115:
2012..year of the multiple centers.


Mostly due to a weak El Niño infusing some wind shear and dry air to disrupt tropical systems.
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Quoting SFLWeatherman:
bye bye Gulf Coast beaches!
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Quoting kmanislander:


Unusual for a TS, not for the BOC. This one is a farce


I'm not sure how that's unusual for a TS when it's the weaker systems that have the least uniformity in wind field, which would make such distortion expected from a low-end TS. Mature hurricanes would be strong in all quadrants.
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Link
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Quoting SFLWeatherman:
WOW


Does that that say 932mb?!
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Quoting SFLWeatherman:
S FL and GOM!! 18Z

That has 2004 and 2005 written all over it. hehehe
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This is not going to be good for the U.S
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Quoting washingtonian115:
2012..year of the multiple centers.

I've never seen such a thing... you almost have to assume competing llcs coming out of the gate Cape Verde or homegrown Debby types! We've had to develop an axiom about it: if your TC is doing strange things AND the models are in bitter disagreement, start looking around for multiple centers.
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Back tomorrow. Gnite all
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Quoting washingtonian115:
2012..year of the multiple centers.
Nah... that was last year... never forget Emily of the seven centres....
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WOW
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Quoting VAbeachhurricanes:
Besides the MLC diving into mexico, I don't see any rotation here.


I'm honestly looking for this alleged off-shore LLC they keep talking about, and the only candidate I can find on Mexican radar is so poorly defined and their radar sucks so badly anyway, that you can't tell whether it's just a n imbedded thunderstorm or what.

The mid level circulation is most definitely well inland and moving SW, though it may have stalled in the past 30 minutes.


there's almost no rains to speak of off-shore according to the radar, and even the on-land rainfall is scattered and minimal now.
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Quoting VAbeachhurricanes:
Besides the MLC diving into mexico, I don't see any rotation here.
Wait and watch for the refire of convection around the COC at DMAX. The center is NOT on land.
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S FL and GOM!! 18Z

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happy B-day Andrew
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Quoting SFLWeatherman:
One of the 18Z it go out to 180HR now


Could you rewind that please :-(
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When do the next runs come out again?
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Quoting Tropicsweatherpr:


Low in front of 94L dissipates according to the 00z TAFB analysis.

So basically 94L sucks it up...
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One of the 18Z it go out to 180HR now
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IS that two TS??? i see
Quoting sunlinepr:


You said that and the NHC heard it.... and updated it...

If it keeps expanding that will be a Gilbert....

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2012..year of the multiple centers.
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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