Tropical Storm Helene arrives; 94L a potential threat to the Lesser Antilles
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
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Still disorganized at the surface based upon that partial pass but the 850 mb signature is looking good.
I would give it another 24 hrs to TD status, possibly 36. These CV systems usually take their time developing closer to 40W.
Notice during the run it shows it slightly weaken near the DR due to land interaction and the trough is quite flat over Canada. Thus setting up a classic trap for 94L as ridging will likely build over the system and force a WNW to NW motion. However the GFS underestimate the unually strong subtropical high and the system shoots up the slight weakness in the ridge rather than it building it over Issac. Stay tune for the 12z run.............................
Maybe he is waiting the 12z gfs
NATIONAL WEATHER SERVICE NEW ORLEANS LA
1012 AM CDT SAT AUG 18 2012
MSC005-157-181715-
/O.NEW.KLIX.FA.Y.0086.120818T1512Z-120818T1715Z/
/00000.N.ER.000000T0000Z.000000T0000Z.000000T0000 Z.OO/
AMITE MS-WILKINSON MS-
1012 AM CDT SAT AUG 18 2012
THE NATIONAL WEATHER SERVICE IN NEW ORLEANS HAS ISSUED AN
* URBAN AND SMALL STREAM FLOOD ADVISORY FOR MINOR FLOODING OF POOR
DRAINAGE AREAS IN...
WESTERN AMITE COUNTY IN SOUTHERN MISSISSIPPI...
EASTERN WILKINSON COUNTY IN SOUTHERN MISSISSIPPI...
* UNTIL 1215 PM CDT
* AT 1012 AM CDT DOPPLER RADAR SHOWED HEAVY THUNDERSTORMS OVER
EASTERN WILKINSON AND WESTERN AMITE COUNTIES IN MISSISSIPPI.
RAINFALL AMOUNTS OF 2 TO 4 INCHES HAVE OCCURRED OVER THE LAST 3
HOURS. ADDITIONAL RAINFALL AMOUNTS OF 1 TO 2 INCHES ARE LIKELY
THROUGH NOON.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
EXCESSIVE RUNOFF FROM HEAVY RAINFALL WILL CAUSE ELEVATED LEVELS ON
SMALL CREEKS AND STREAMS...AND PONDING OF WATER ON COUNTRY ROADS AND
FARMLAND ALONG THE BANKS OF CREEKS AND STREAMS.
&&
Unlikely, as the ridge remains adamant. In fact the system might take a slight dip wsw then a west to WNW track ensues.
That last ascat SLU posted depicted the western half of a confused center.. Skyepony should get us the goods as soon as they come in. No need to send the HHs in until one storm gets absorbed by the other.
http://www.miamiherald.com/2012/08/16/2956859/eve nts-commemorate-hurricane-andrew.html
I don't really like the word 'commemorating' as I usually associate that word with a positive event - I don't have positive memories of Hurricane Andrew.
IIRC, NHC never expected more than slow development before tomorrow anyway... it's why they didn't go right to high. Like H23 and others have said, we have plenty of time to watch this.
I can see the CAR part of this. I think it's way too early to call anything else right now. There are too many complex relationships out there to resolve, especially if the Twave behind 94L does attempt its own, separate, cyclogenesis. What we might end up with is 94L catapulted off to the NW, while the system behind it meanders into the CAR...
Lots of possibilities out there.
Invest 94L has been designated for a robust tropical wave that moved off the African coast a few days ago. Models are latching onto development of this system and conditions appear to be ripe in the central and western Atlantic as 94L makes its trek westward. All indications are that this system could become Tropical Storm Isaac in 24 to 36 hrs. This system will need to be watched closely over the next few days and will likely become the leading weather story as most models predict 94L developing into a powerful system. The first landmass to feel future Isaac's effects will be the Lesser Antilles Islands and Puerto Rico in approximately 5 days or so. Thereafter the models diverge somewhat on a track into the Gulf of Mexico by a weaker system, or recurve just off the East Coast by a strong hurricane. A recurve near Bermuda is also not out of the question. Future Isaacs eventual track will depend on the orientation of the A/B high, and timing of troughs of low pressure over the eastern seaboard. This area needs to be watched carefully over the next 10 days or so.
0600z GFDL still takes Gordon to 87kts and 973mb on the 20th...
And the 1200z takes it to 95kts...
...but the other models keep it in the 75kts to 80kts range.
What do they have for building code in the Azores?
If GFDL or HWRF is right, they are about to get a category 2 hurricane, or a very close call from one anyway...
Yeah it's great to see it returning, it looks like there will be plenty more over the next several days, wet pattern setting up that's for sure. There will be periods of concentrated heavy rain as upper disturbances move through occasionally as well.
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