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Still-Disorganized 99L Remains a Threat to Southeast U.S. Coast; Gaston Growing

By: Bob Henson 9:54 PM GMT on August 23, 2016

A large and strong tropical wave about 250 miles east of the Lesser Antilles, Invest 99L, continues to move west-northwest at about 20 mph on a course that would bring it within striking distance of the southeast U.S. coast by late in the weekend or early next week, quite possibly at hurricane strength. Venturing into 99L on Tuesday, Air Force Hurricane Hunters found that the wave had an poorly defined circulation, elongated from north to south. Often it will take one or more days for a symmetric, circular center to emerge inside a tropical wave this large. After that point, convection (showers and thunderstorms) can consolidate and the system can grow more rapidly if conditions are favorable. A substantial growth in convection may occur overnight Tuesday, as tropical systems often experience convective bursts during the nighttime hours. Late Tuesday afternoon, convection was scattered over a broad area around 99L (see Figure 1). Increasing upper-level outflow was evident, especially on the wave’s north side.

Two of the latest Tuesday (12Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, and UKMET models--showed development of 99L into a tropical storm over the next five days. In its 2 pm EDT Tropical Weather Outlook, the National Hurricane Center maintained 40 percent odds that 99L will develop at least to tropical depression strength by Thursday and 60 percent odds by Sunday. Given the scenarios being painted by models, and given the location and time of year, the 60% NHC odds that we will have at least a tropical depression by Sunday are on the conservative side, as noted by Jeff Masters in this morning’s post.


Figure 1. Enhanced infrared image of Invest 99 as of 1945Z (3:45 pm EDT) Tuesday, August 23, 2016. Image credit: CIRA/RAMMB/CSU.

Track forecast: 99L likely to affect The Bahamas and Southeast U.S.
One big question for 99L where a center of circulation might develop within the north-south swath of elongation. In its Tropical Weather Discussion issued at 2 pm EDT Tuesday, NHC placed the elongation between 12°N and 20°N, with a center of low pressure near 17°N, 56°W. Late Tuesday afternoon, satellite imagery suggested that a center of circulation may be trying to form around 15-16°N, roughly 70 to 140 miles further south. If this apparent trend holds, it could raise the odds of 99L passing near Puerto Rico on Wednesday and just to the north of Hispanola on Thursday, potentially slowing any rapid short-term development of 99L. Regardless of where the center consolidates, heavy rains from 99L’s large circulation could easily pose flooding problems for islands as far west as Hispaniola. A flash flood watch was already in effect Wednesday for Puerto Rico and the U.S. Virgin Islands, where widespread 2” - 4” totals and locally higher amounts are expected.

Models are in strong agreement on 99L’s general west-northwest motion around a strong upper-level ridge in the Atlantic subtropics over the next several days, which would bring 99L into The Bahamas by Friday into the weekend. There could be a slight northwestward bend around Friday due to a potential weakness in the ridge produced by the remains of now-deceased Tropical Storm Fiona. Beyond that point, models generally agree that the ridge will restrengthen and send 99L back toward the west-northwest or west. The 12Z runs of the ECMWF and UKMET models take 99L on quite similar tracks across southern Florida on Sunday or Monday, then onward into the Gulf of Mexico, which would open the possibility of a landfall somewhere along the Gulf Coast next week. The other of the three most reliable models for tropical cyclone genesis, the GFS, has failed to develop 99L for its last several runs, keeping it as an open wave. The 12Z run of the HWRF model also failed to develop 99L--a major shift from its previous few runs.

Intensity forecast for 99L: a potentially dangerous storm for the Southeast U.S. (and perhaps the Gulf)
It may take some time for 99L to develop into what could be a formidable storm. The 2 pm EDT Tuesday run of the SHIPS model continued to show moderately favorable conditions for development through Friday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 28.5°C (83°F) to 30°C (86°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air from the Saharan air layer (SAL), and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO).

Assuming that it consolidates and develops into a tropical cyclone, as projected by the majority of reliable models, 99L has the potential to strengthen significantly toward the weekend. The region surrounding The Bahamas is a notorious breeding ground for hurricane development, especially in late August and September, when the fairly shallow waters are at their warmest. The track forecast suggests that 99L could be moving for 2-3 days across near-record warm waters around 30°C (86°F). There will be ample amounts of oceanic heat content, and upper-level wind shear is expected to remain light. In short, we have the potential for 99L--which has a large circulation--to rapidly intensify into a large major hurricane that would strike the southeast U.S. coast on Sunday or Monday, potentially moving into the Gulf of Mexico. At present, Florida appears to be at greater risk than states further north. The uncertainties are high, though, and we will have to wait for the storm to develop into a tropical depression before we can have more confidence in what the models are saying.


Figure X. Enhanced infrared image of Tropical Storm Gaston as of 1745Z (1:45 pm EDT) Tuesday, August 23, 2016. Image credit: NOAA/NESDIS.

Gaston is on its way to hurricane status
After an impressive growth spurt earlier on Tuesday, Tropical Storm Gaston continued to pack top sustained winds of 65 mph as of the 5 pm EDT update from NHC. An eye-like feature has intermittently appeared within Gaston during the day Tuesday. Located far out in the eastern Atlantic at 14.2°N, 35.8°W, Gaston is now projected to become a strong Category 1 hurricane before it encounters several days of strong wind shear (above 20 knots). Beyond that point, it could strengthen even further as it passes over subtropical SSTs that will be 1-2°C above average for this time of year. Fortunately, Gaston’s northwestward path will keep it many hundreds of miles from any land area for at least the next several days, and it is likely to recurve long before approaching North America or even Bermuda.

We’ll have our next update on Wednesday morning.

Bob Henson

Hurricane

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

1501. LargoFl
NWS Key West Florida...........................There remains significant uncertainty in the extended portion of the
forecast due to a strong tropical wave moving west northwestward
through the Lesser Antilles. The environment is expected to become
increasingly conducive for tropical cyclone formation. Regardless of
development, it is becoming increasingly likely that some sort of
tropical feature will be near South Florida and the Keys sometimes
early next week. There is large run to run and model to model
variation, and as a result, confidence in the long range forecast is
low. Have nudged forecast variables with this in mind.

&&
1502. Sandcat
Quoting 1495. ReiredNavyGuy:


What is concerning is the storm/whatever it becomes is going into the gulf with a high pressure bearing down and continues to show movement towards Louisiana/Texas, already started the supplies just to be on the safe side, we'll find more out 3 to 4 days. Be safe all

A tropical system would be devastating for Louisiana right now.
1503. MahFL
Quoting 1493. LargoFl:

GFS now in line with the Euro for a south florida crossing.............................


Cept it's not really a crossing, it rides up the whole peninsular...
1504. LargoFl
Quoting 1498. Bucsboltsfan:



Beginning to look more and more that this is a south Fl threat. Not set in stone though.
yes sure looks like it so far.
Yesterday Levi said 99L was decoupled. Cantore just said the same thing. Said the llc is to the NE of the convection.
GFS run looks like another Fay scenario.
Quoting 1384. NunoLava1998:


I see a circulation forming. Chances should be raised to around 90%.


I'll be interested in what they say at 8. If it is designated anything at 11, I wonder how the 12z GFS handles things (if it initializes that, I never remember what time it comes out, 12z utc is 7 AM). It keeps it open still, making a Donna-like track as a TD near the bend of FL.
1508. LargoFl
Quoting 1503. MahFL:



Cept it's not really a crossing, it rides up the whole peninsular...
you know..im sure watching out for that..no track is set just yet..suppose it goes into say..Melbourne and crosses over to the gulf etc..right now anything is possible huh
Good morning

Sitting down with my first cup of coffee, looking around to see what's in store for us in the next day or two.

Lindy
1510. MahFL
99L has to be a TD now :


Quoting 1433. StormJunkie:



Like the mountains of PR/DR, after all...that is a lot of land. Too many times the past few years have systems like this not been able to get N of the islands.

Morning all

Well, look what the cat dragged in ...
LOL.


Not much so far. :/
According to the Guadeloupe Doppler radar this invest is too far for being closed. The radar shows some showers over the convection now over Guadeloupe and nearby islands
Quoting 1504. LargoFl:

yes sure looks like it so far.

But don't those models have a weaker system? Wouldnt a stronger system go.more north?
Quoting 1510. MahFL:

99L has to be a TD now :




NO, don't be impressed by the convection blob
Up to 4.5" in Guadeloupe
Quoting 1515. wunderkidwx:


But don't those models have a weaker system? Wouldnt a stronger system go.more north?


Not with uniform zonal flow over the eastern US.
1520. MahFL
Warmest morning of the year here in Orange Park, 81F at 06:44 am, and 63% humidity, ridiculous !
So much for the forecast of a low of 77F.
Quoting 1519. KoritheMan:



Not with uniform zonal flow over the eastern US.

Wouldn't Fiona disrupt that?
Quoting 1521. wunderkidwx:


Wouldn't Fiona disrupt that?


There could be some binary interaction, of course, but unless Hispaniola really screws with it (a distinct possibility), it's more likely that 99L will just eat Fiona. Any northward deviation would not result in a track out to sea, imo.
Quoting 1472. HaoleboySurfEC:

Best it has ever looked but looks like it is going to come very close to Hispaniola.




I still believe the MLC/LLC are misaligned. The LLC is probably north of that convective ball, whereas the MLC is tucked underneath it, giving 99L a more organized look than it actually is. This illusion has always tricked bloggers. Appears to be more consolidated looking though as of 09:00 UTC.

For those asking if this can go north turn on the weather channel and listen to Jim cantore. The way the high will be set up over the mid Atlantic there is no way whatever 99l turns into can go north. Exactly what Jim said literally 30 min ago lol.
1525. LargoFl
in about 2 weeks we Could..be going thru all this Once again,peak of season is here now huh.......
Quoting 1523. CybrTeddy:



I still believe the MLC/LLC are misaligned. The LLC is probably north of that convective ball, whereas the MLC is tucked underneath it, giving 99L a more organized look than it actually is. This illusion has always tricked bloggers.




Sometimes including me. :)
1527. red0
99L on Radar





Link
Quoting 1524. Austin72893:

For those asking if this can go north turn on the weather channel and listen to Jim cantore. The way the high will be set up over the mid Atlantic there is no way whatever 99l turns into can go north. Exactly what Jim said literally 30 min ago lol.


Anyone that watches a water vapor loop knows that. Those who deny it are submitting to wishful thinking.
Quoting 1522. KoritheMan:



There could be some binary interaction, of course, but unless Hispaniola really screws with it (a distinct possibility), it's more likely that 99L will just eat Fiona. Any northward deviation would not result in a track out to sea, imo.

Thanks. I was asking because I have family in S Fl and I live in Central Fl and want to know who i should worry more for.
I don't expect invest 99L be a big rain maker in PR, unless it "explodes" later in terms of convection, something I doubt so much. It will remain weak or weaker by the time it approaches PR. (IMO)
Quoting 1509. LindyVirginIslander:

Good morning

Sitting down with my first cup of coffee, looking around to see what's in store for us in the next day or two.

Lindy
Morning, Lindy. You should get some rain out of this, if nothing else.
Quoting 1530. wunderkidwx:


Thanks. I was asking because I have family in S Fl and I live in Central Fl and want to know who i should worry more for.


...Both of them? lol
Exactly and I don't think we'll see anything develop substantially until it gets in the Bahamas area. The water heat content there is off the charts even Jim cantore said he's worried about rapid intensification in that area.

Quoting 1529. KoritheMan:



Anyone that watches a water vapor loop knows that. Those who deny it are submitting to wishful thinking.
Quoting 1533. KoritheMan:



...Both of them? lol

Touche' haha
Quoting 1525. LargoFl:

in about 2 weeks we Could..be going thru all this Once again,peak of season is here now huh.......


Yea, that's pretty far into nonsense land but it does illustrate that even if 99L doesn't develop the threat for further storms may exist over the next 2-3 weeks.
cyber your a denier too. 99=going to move right over s. florida.
Quoting 1537. islander101010:

cyber your a denier too. going to move right over s. florida.


What? XD
Uh... oh crap. XD


Good Morning All
Well I was hoping this thing had changed it's mind and not come towards the Gulf
But I see I'm wrong.... Still not stacked but sure looks like it's a TD anyway....

Good to see some of the older folks back on here....


Taco :o)
Appears the HWRF wants to be unreliable garbage and is dropping 99L on the 06z.
Quoting 1542. CybrTeddy:

Appears the HWRF wants to be unreliable garbage and is dropping 99L on the 06z.


ugh
*Topic: *GOES-13 (GOES-East) Standard CONUS RSO

*Details/Specifics of Change:* Tropical systems being monitored near
Puerto Rico
Link



Visible Loop still filling in, appears surface circulation on northern end of blob.
Quoting 1537. islander101010:

cyber your a denier too. 99=going to move right over s. florida.


Huh? A LLC further south would make 99L trend towards Hispaniola, which would almost certainly kill it. A LLC about where the NHC has it would be more of a threat to Florida.
1546. A4Guy
You can see from this wind map that there is a closed circulation...just elongated. Can't tell about how well stacked it is/is not.
Link
Quoting 1543. KoritheMan:



ugh


Yea, absolutely zero consistency from it. 18z tropical storm, 00z major hurricane, 06z nada.
I'm holding my tongue on 99L for many reasons at this current moment. *tries not to laugh a little* (6Z hwrf -_______-) My thought is, if the Euro this afternoon shows a carbon copy of the SFL track it has been showing then it will likely come to pass. 99L Looks impressive right now but in reality at 8am I don't see the development odds going over 60% for 2 days.
1549. hydrus
1550. luigi18

Quoting 1548. George1938:

I'm holding my tongue on 99L for many reasons at this current moment. *tries not to laugh a little* (6Z hwrf -_______-) My thought is, if the Euro this afternoon shows a carbon copy of the SFL track it has been showing then it will likely come to pass. 99L Looks impressive right now but in reality at 8am I don't see the development odds going over 60% for 2 days.


Yeah if the Euro sticks to its guns again at 12z, I'm completely disregarding the GFS.
1552. joHS
good morning, and to you mlc, stormjunkie and sandcrab, good to see you.

Here's the words in my hood, am off to Pancake House :).

Area Forecast Discussion
National Weather Service Melbourne FL
418 AM EDT WED AUG 24 2016

.DISCUSSION...
.....

Weekend/Early Next Week...A tropical wave located just east of the
Leeward Islands, Invest 99L, will keep a high level of uncertainty
in the extended range forecast. Global models suggest a favorable
environment for intensification as the system moves closer to the
Bahamas; however, dry air and its proximity to the hilly terrain
of Puerto Rico, the Dominican Republic, and Haiti will remain
limiting factors over the next 24-48 hours. Latest guidance
continues to support a strong enough ridge over the eastern CONUS
that would prevent a developing cyclone from recurving into the
open Atlantic east of the Peninsula.

The bottom line is: with a continued west-northwest motion expected
for the next several days, it is recommended that both residents and
visitors to Florida remain abreast of the latest forecasts from
local National Weather Service Forecast Offices and the National
Hurricane Center.

FWIW, Cantore favors the ECMWF track.
Quoting 1553. KoritheMan:

FWIW, Cantore favors the ECMWF track.
Yeah he loves the euro
Quoting 1539. KoritheMan:

Uh... oh crap. XD




Where did I put those keys for shields to activate on Texas border?
Good morning, all. I'm looking forward to some great information/discussion with the forecasting. However, at this point it appears that SFL will be "eyes wide open" this weekend as the system approaches and there goes "your regular scheduled program!" Our weather peeps will be all over this by Friday and likely non-stop into the weekend. Ugh.

This site, Dr Masters, Bob and the bloggers will be my BFFs, for sure! Thank you one and all.
as 96hr on hwrf comes out I might be corrected on my whale face. Okay I'm over analyzing. bye XD
@hurrtrackerapp: Invest #99L became more organized overnight and is closer to becoming a depress/tropical storm. https://twitter.com/hurrtrackerapp/status/76838773 5035998208/photo/1
JeffMasters has created a new entry.
From the 4:30 AM Forecast Discussion from NWS Miami:

"The National Hurricane Center is monitoring a strong tropical
wave with the potential to become a tropical depression near
Guadeloupe, that is forecast to to track northwest the next
several days. Conditions are expected to be more favorable for
development as it is forecasted to move near the Southeast and
Central Bahamas late this week. Currently the National Hurricane
Center gives this tropical wave a 50 percent chance of tropical
cyclone formation over the next two days and a 70 percent over the
next 5 days.

Residents and visitors to South Florida should continue to closely
monitor the products of the National Hurricane Center over the
next few days to stay alert for any potential impacts to South
Florida with this developing system.

Forecast for the weekend...There is obvious considerable
uncertainty due to the potential for tropical cyclone development
or lack thereof over the weekend. However even in the absence of
development there will likely be deep tropical moisture,
widespread showers and thunderstorms and windy conditions through
the weeekend likely continuing into at least Monday
."
Here is PBC the weather& local news has been all over this since Monday. It was the lead story at 11 last night including preparing your hurricane supplies (although I think they are jumping the gun a little bit)

Quoting 1556. SFLstevez1:

Good morning, all. I'm looking forward to some great information/discussion with the forecasting. However, at this point it appears that SFL will be "eyes wide open" this weekend as the system approaches and there goes "your regular scheduled program!" Our weather peeps will be all over this by Friday and likely non-stop into the weekend. Ugh.

This site, Dr Masters, Bob and the bloggers will be my BFFs, for sure! Thank you one and all.
1562. JRRP7
60% good
Quoting 1542. CybrTeddy:

Appears the HWRF wants to be unreliable garbage and is dropping 99L on the 06z.
Yeah, HWRF!!!

:o)
BTW, what are the chances the blog hits 2000 before it turns over? :o)

AL, 99, 2016082318, , BEST, 0, 159N, 583W, 30, 1009, LO
AL, 99, 2016082400, , BEST, 0, 162N, 598W, 30, 1009, LO
AL, 99, 2016082406, , BEST, 0, 167N, 613W, 30, 1009, LO
AL, 99, 2016082412, , BEST, 0, 172N, 628W, 30, 1009, LO
Quoting 1545. CybrTeddy:



Huh? A LLC further south would make 99L trend towards Hispaniola, which would almost certainly kill it. A LLC about where the NHC has it would be more of a threat to Florida.




Going over Hispaniola is not a guarantee of anything. Just ask anyone in Houston.
Quoting 1565. BahaHurican:

BTW, what are the chances the blog hits 2000 before it turns over? :o)




Near 0 :)
1569. beell
I don't know. Perhaps some subsidence/dry descending air in the gap between mid-level 99L and the clockwise band of convection to the north of the system is being entrained into the mid-levels of the convective mass.