TD 8 forms; 97L a potential threat to the Caribbean and U.S.

By: Dr. Jeff Masters , 3:30 PM GMT on August 19, 2011

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Tropical Depression Eight formed last night near the coast of Honduras, and is headed westwards towards a landfall in Belize on Saturday. TD 8 is a small storm, so will impact a relatively small area of northern Honduras, northern Guatemala, all of Belize, and southern portions of Mexico's Yucatan Peninsula. TD 8 has just enough room between its center and the coast of Honduras to intensify into a moderate strength tropical storm with 50 - 60 mph winds before landfall. It is very unlikely TD8 has the time or room to intensify into a hurricane; NHC gave the storm just a 7% chance of making it to hurricane strength in their 11am EDT wind probability forecast. Should TD8 make it to tropical storm strength, it would be called Harvey.


Figure 1. Morning satellite image of TD 8.

Invest 97L likely to become a tropical storm next week, could threaten the U.S.
A tropical wave near 14°N 48°W, about 800 miles east of the Lesser Antilles Islands, is moving westward near 20 mph. This wave, designated Invest 97L by NHC yesterday, has seen a marked increase in its heavy thunderstorm activity this morning, but dry air to the north and west is slowing development. An impressive amount of large-scale spin is obvious in visible satellite loops, but the storm is at least a day away from forming a well-defined surface circulation. Ocean temperatures are about 28.5°C, about 2°C above the threshold needed to support a tropical storm, and wind shear is low, 5 - 10 knots.


Figure 2. Morning satellite image of the tropical wave 97L.

The computer models have shown an unusual amount of agreement in developing 97L over the past few days, and all the ingredients seem to be in place for a tropical storm to form by Monday or Tuesday as 97L crosses the Northeast Caribbean. The atmosphere is expected to be moister over the Caribbean, wind shear will remain a low 5 - 10 knots, and sea surface temperatures will increase to near 29°C. The main impediment for development will likely be two-fold: too much dry, stable air, and proximity to land.

As seen in Figure 3, there has been an unusual amount of dry, stable air in the Atlantic this year, due to a combination of dry air from Africa, and upper-atmosphere dynamics creating large areas of sinking air that dry as they warm and approach the surface. This stable air has been largely responsible for the fact that none of our seven tropical storms so far this year has made it to hurricane strength, despite the presence of sea surface temperatures that are the 3rd warmest on record across the tropical Atlantic. Tropical Storm Emily in early August encountered problems with dry air when it crossed the Northeast Caribbean, and 97L may have similar difficulties.


Figure 3. Vertical instability of the atmosphere during 2011 in the Caribbean (left) and tropical Atlantic between the Lesser Antilles Islands and coast of Africa (right.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Observed vertical instability (blue line) has been much lower than the climatological average from previous years (black line), due to an unusual amount of dry air in the atmosphere, inhibiting tropical storm development this year. Image credit: NOAA/CIRA.

Encounters with land will be another potential major problem for 97L. Most of the computer models take 97L near or over Puerto Rico Sunday night, then very close to or over mountainous Hispaniola Monday night through Tuesday. It is unlikely that 97L will be stronger than a 55 mph tropical storm when it encounters these islands, and passage over the islands could severely disrupt the storm. However, if 97L takes a path just south or north of Hispaniola, the potential exists for the storm to intensify into a hurricane.

There will be moderate wind shear of 10 - 20 knots to the north of the islands early next week, so a path just to the south of the Dominican Republic and Haiti would be more likely to let 97L intensify into a hurricane. A west-northwest motion is likely for 97L through Wednesday, which would bring the storm to the vicinity of Jamaica-Central Cuba-the Central Bahamas on Wednesday. On Wednesday and Thursday, the models agree that a trough of low pressure will dip down over the Eastern U.S., which is likely to turn 97L to the north. The exact timing and strength of this trough varies considerably from model to model, and will be critical in determining where and when 97L will turn to the north. The best model for predicting the timing and strength of such troughs over the past two years has been the ECMWF (European Center model), and this model currently brings 97L into the Florida Keys on Thursday night next week. You can view ECMWF forecasts on our wundermap with the model layer turned on. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models.

Remember that a 7-day forecast by even our best model will be off by an average of over 700 miles, so it is too early to tell what part of the U.S. might be most at risk from a strike by 97L. This weekend would be a good time to go over your hurricane preparedness.


Figure 4. Morning satellite image of Invest 98L off the coast of Africa.

Invest 98L near the coast of Africa
A tropical wave near the coast of Africa, a few hundred miles southeast of the Cape Verde Islands, is moving west to west-northwestward at 10 - 15 mph. This wave, designated Invest 98L by NHC yesterday, is large and well-organized, with a modest amount of spin and heavy thunderstorm activity. 98L will bring strong, gusty winds and heavy rains to the Cape Verde Islands today and Saturday as the storm skirts to the south. So far this morning, top sustained winds measured in the islands were 24 mph at Mindelo. Water temperatures are warm, near 27 - 28°C, and wind shear is low, 5 - 10 knots, so 98L should continue to organize today. NHC gave the storm a 50% chance of developing into a tropical depression in their 8am advisory. Once 98L passes to the west of the Cape Verde Islands, it has a long stretch of ocean to cross before it could affect any other land areas. Approximately 70 - 80% of all tropical cyclones that pass this close to the Cape Verde Islands end up curving out to sea and not affecting any other land areas, according to Dr. Bob Hart's excellent historical probability of landfall charts. The latest set of long-range model runs go along with this idea, but it is too early to be confident of this.

Jeff Masters

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Quoting cycleranger:


Oh is it time for our daily radical Westward/Eastward shift of outlier models?

At least it's not plotting a DOOM scenario for Houston this go-around.


Nope. We can't even get any out lying model love! Lol. But this could go anywhere. Spahettis to the north. CMC to the west. This run anyway. :)
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Quoting TropicalWeatherGrl88:


I agree with you it may very well not develop, but what if it does? It has been showing this for almost 12 days now. Very close to the same scenerio. How shocked would we all be if it actually ended up happening


It needs to get some giddyup in it's gitalong for sure.
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3490. JLPR2
Quoting Gearsts:
:O Why the shift north?


The relocation of the LLC, well it's more a formation of a new LLC a little farther north, hence the shift.
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Quoting cycleranger:


Mixed blessing this time of year as always.

With a Drought prompting water restrictions we'd love for something Humberto or Allison like to come our way.

On the other hand. This is the heart o the season. Monsters lurk in these warm waters.


Yep. A double edged sword to be sure. If we don't get some serious rain from somewhere we'll be in pretty bad shape and soon. Some towns already out of water. Unbelievable! And a little scary. ;-)
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Quoting AtHomeInTX:
Hmmmm.



Oh is it time for our daily radical Westward/Eastward shift of outlier models?

At least it's not plotting a DOOM scenario for Houston this go-around.
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Quoting Stormchaser2007:
00z CMC

Well that's not good.



I doubt this happens. For one thing, the GFS and Euro don't agree. Secondly, the weakness should be a little farther east...
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Quoting NJcat3cane:
if anyone hasent seen the nam it spit out a stronger storm a tad more north then the gfs that will track north of cuba most likely

Link

also whens the 06z runs comeout again


NAM is usually done 30 minutes or so prior to the GFS' initialization.
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3485. JLPR2
Meanwhile Harvey refired nicely.
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3484. Gearsts
Quoting JLPR2:


That shift now has PR under the gun. I wish we had visible already to see if it's spinning at the surface. XD

Another view:
:O Why the shift north?
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Quoting MiamiHurricanes09:
Worst case scenario is that it misses Hispañola and Cuba towards the north and still goes on to affect the Bahamas and Florida.

Again though, still a lot of variables in play that could considerably change the track before anyone starts going crazy lol.


Definitely. There could still be a considerable shift in track over the next few days.

Still a wide variety of possibilities.

Member Since: June 9, 2007 Posts: 4 Comments: 15945
Quoting Stormchaser2007:



Actually,
there was a pretty decent shift east. Not too much interaction with Cuba.

Worst case scenario is that it misses Hispañola and Cuba towards the north and still goes on to affect the Bahamas and Florida.

Again though, still a lot of variables in play that could considerably change the track before anyone starts going crazy lol.
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Quoting Stormchaser2007:


00z=8pm
06z=2am
12z=8am
18Z=2pm


gotcha thanks. I knew it was something to do with time but wasn't sure exactly how to look at it.
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Quoting AtHomeInTX:


Yep. That's just about it. :)


Mixed blessing this time of year as always.

With a Drought prompting water restrictions we'd love for something Humberto or Allison like to come our way.

On the other hand. This is the heart o the season. Monsters lurk in these warm waters.
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Quoting HimacaneBrees:


what does 00z, 18z, mean?


00z=8pm
06z=2am
12z=8am
18Z=2pm
Member Since: June 9, 2007 Posts: 4 Comments: 15945
Quoting Stormchaser2007:
00z CMC

Well that's not good.



Nope!
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3477. JLPR2
Quoting Stormchaser2007:


Actually, more than half of those are 00z.

There was a pretty decent shift east. Not too much interaction with Cuba.



That shift now has PR under the gun. I wish we had visible already to see if it's spinning at the surface. XD

Another view:
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3476. Gearsts
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Quoting Stormchaser2007:


Actually, more than half of those are 00z.

There was a pretty decent shift east. Not too much interaction with Cuba.



what does 00z, 18z, mean?
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Hmmmm.

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if anyone hasent seen the nam it spit out a stronger storm a tad more north then the gfs that will track north of cuba most likely

Link

also whens the 06z runs comeout again
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3471. Gearsts
Quoting Stormchaser2007:
00z NAM4

Can i have the link?
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00z CMC

Well that's not good.

Member Since: June 9, 2007 Posts: 4 Comments: 15945
Quoting JLPR2:


Well the 8am TWO should be pretty interesting. But I wont wait for it, going to bed in a little while. XD
LOL, I'll probably take an hour nap and then wake up for the TWO.
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00z NAM4

Member Since: June 9, 2007 Posts: 4 Comments: 15945
3466. JLPR2
Quoting MiamiHurricanes09:
Probably why the percentage was upped at 2a.m. Telling by the ATCF coordinates, the NHC are probably confident that 97L is forming a surface circulation under the recent convective blow-up.


Well the 8am TWO should be pretty interesting. But I wont wait for it, going to bed in a little while. XD
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3465. JLPR2
Quoting Stormchaser2007:


It might.

I'd watch to see if the models shift after recon investigates later today.


Well, you bet I'll be watching it closely. I just hope it doesn't pull a Harvey and strengthens quickly.

Quoting wxhatt:


Actually, from what I've read, a stronger storm earlier on, means a more northward bias (will move more northward). That could be good if it scoots north of you.

That would be rather neat.
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3464. wxhatt
Quoting JLPR2:


That would make the system track closer to me, right?


Actually, from what I've read, a stronger storm earlier on, means a more northward bias (will move more northward). That could be good if it scoots north of you.
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Quoting JLPR2:


That would make the system track closer to me, right?


It might.

I'd watch to see if the models shift after recon investigates later today.
Member Since: June 9, 2007 Posts: 4 Comments: 15945
Quoting JLPR2:


That would make the system track closer to me, right?
cloudburst warned you yesterday
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Quoting GTcooliebai:
Winds also expand:



Classic example of extratropical transition.
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3460. JLPR2
Quoting Stormchaser2007:


This might be able to form a day sooner than most expected.


That would make the system track closer to me, right?
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Quoting JLPR2:


All we need is proof of the existence of a LLC and bingo! A td.
Probably why the percentage was upped at 2a.m. Telling by the ATCF coordinates, the NHC are probably confident that 97L is forming a surface circulation under the recent convective blow-up.
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Quoting cycleranger:


Texas continues to bake.

Dome of Death may shift West a bit but only enough to allow systems tropical in nature to cause havoc to our friends out East.



Yep. That's just about it. :)
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Quoting JLPR2:


All we need is proof of the existence of a LLC and bingo! A td.


This might be able to form a day sooner than most expected.
Member Since: June 9, 2007 Posts: 4 Comments: 15945
3456. wxhatt
Quoting GTcooliebai:
216hrs. out over GA/SC border pressure continues to fall 988mb.



but how can a storm intensify over land?? Just doesn't make sense. I think the models are out to lunch...
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Winds also expand:

Member Since: August 31, 2009 Posts: 0 Comments: 5628
240hrs. out over the DC area, starting to get absorbed by the frontal boundary...end of run:

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3453. 7544
97l looks like a td at this hour
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3452. JLPR2
Quoting Stormchaser2007:
Right under the convection.

Winds up to 35 mph

AL, 97, 2011082006, , BEST, 0, 143N, 530W, 30, 1007, DB



All we need is proof of the existence of a LLC and bingo! A td.
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Member Since: August 31, 2009 Posts: 0 Comments: 5628
intensity models=too many variables butterfly island going to get the brunt of it
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216hrs. out over GA/SC border pressure continues to fall 988mb.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Right under the convection.

Winds up to 35 mph

AL, 97, 2011082006, , BEST, 0, 143N, 530W, 30, 1007, DB

Member Since: June 9, 2007 Posts: 4 Comments: 15945
Pressure down to 990mb.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
192hrs. out over north central FLA (the carpet is rolled out leaving the opening wide open to the North):

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Quoting 7544:


thanks how strong is that gt
Here's another view

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting PrivateIdaho:


If little kids are up now we need to beat their parents....btw....isn't it about 6 hours past your bedtime?


Shhh, my parents think I'm asleep.
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19/1745 UTC 13.2N 49.0W TOO WEAK 97L
19/2345 UTC 13.6N 50.6W TOO WEAK 97L
20/0545 UTC 13.8N 52.5W TOO WEAK 97L


A bit of a Northerly component to our Invest-That-Would.
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Quoting smuldy:
usually dont post here much about to write up my synopsis for the other place, but being in miami it is troubling that not only have the euro and gfs come into agreement on track generally for the last few runs, but now the timing, which was a big gap, has also closed to a small one
problem with s.e. flor. its surrounded by 85-90 water right now need a fill up
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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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