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

By: Dr. Jeff Masters , 03:30 PM GMT em 19 de Agosto de 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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4492. 7544
Quoting USAFwxguy:
GFS looks like it will be very similar to 06Z


hmm agrees with the nam so far and show a stronger system
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4491. Patrap
Gulf and Tropics (Updated every ~1/2 hour)

Member Since: Julho 3, 2005 Posts: 427 Comments: 129439
Quoting WeatherNerdPR:

Surface or flight level?
surface winds, but it was flagged.
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4489. aquak9
looks like we do have recon today.

I. ATLANTIC REQUIREMENTS
1. SUSPECT AREA (APPROACHING LESSER ANTILLES)
FLIGHT ONE -- TEAL 71
A. 20/1800Z
B. AFXXX 01GGA INVEST
C. 20/1615Z
D. 15.3N 57.5W
E. 20/1730Z TO 20/2130Z
F. SFC TO 10,000 FT

modified- already posted?
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4401 TropicalAnalystwx13 "I hope the NHC issues a Specical Update, they usually do in situations like this, especially if the system makes landfall before the Intermediate/Complete advisories."

Unless TS.Harvey's travel-speed has increased noticeably, there's still ~1&1/2 hours left until projected landfall.
And 15minutes after then, the next intermediate would normally be posted (15minutes before the nominally scheduled-hour)
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Quoting TropicalAnalystwx13:


O rly?

13:23:00Z 17.233N 87.883W 842.7 mb
(~ 24.88 inHg) 1,561 meters
(~ 5,121 feet) 1008.7 mb
(~ 29.79 inHg) - From 43� at 21 knots
(From the NE at ~ 24.1 mph) 17.0�C
(~ 62.6�F) 12.9�C
(~ 55.2�F) 22 knots
(~ 25.3 mph) 95 knots*
(~ 109.2 mph*) 1 mm/hr*
(~ 0.04 in/hr*) 90.7 knots* (~ 104.3 mph*)
Category Two Hurricane*
431.8%*
Where did you get that from ? I see nothing like that.

VDM
RNT12 KNHC 201529
VORTEX DATA MESSAGE AL082011
A. 20/15:10:00Z
B. 16 deg 49 min N
087 deg 46 min W
C. 850 mb 1437 m
D. 49 kt
E. 048 deg 9 nm
F. 141 deg 63 kt
G. 049 deg 7 nm
H. 1000 mb
I. 19 C / 1525 m
J. 20 C / 1531 m
K. 14 C / NA
L. open nw
M. E32/16/10
N. 12345 / 8
O. 0.02 / 0 nm
P. AF309 0508A HARVEY OB 19
MAX FL WIND 63 KT NE QUAD 15:07:40Z
MAX FL TEMP 20 C 049 / 7 NM FROM FL CNTR
;
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4486. emcf30
Compairson between 06Z and 12z run. Yea 97L is just a tad further North but not by much.



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Quoting cat5hurricane:

Perhaps for the next 12 hours, that is a distinct possibility. However, latest guidance continues to take it over a good portion of Hispaniola, and then over possibly a very good chunk of Cuba. If this is the case, I do not expect anything more than a TS if 97L shall reach Florida.


That's what I was thinking. The GFS keeps it pretty strong, even after crossing Hispaniola, and Cuba. I know it takes that into account, but I just don't see it maintaining that strength depicted. Now, if it tracks just a little further west (or east) then heads north, a different ball game.
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4484. angiest
Quoting TropicalAnalystwx13:




Rather surprising a hurricane watch/warning was never raised, given the forecast did call for hurricane strength.
Member Since: Agosto 26, 2006 Posts: 16 Comments: 4766
This is an interesting development in the GFS to be sure. Seems to support northcasters for a while, then jumps south to what the southerly casters are seeing....I give up..

Quoting dfwstormwatch:

51 hours out gaining strength between p.r and d.r but farther south
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57 hours out passing south of d.r
Member Since: Julho 31, 2011 Posts: 5 Comments: 891
Quoting Seflhurricane:
Is recon heading to 97l


at 2pm, yes.
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Member Since: Junho 29, 2009 Posts: 13 Comments: 5315
Quoting TropicalAnalystwx13:



Hallelujah.
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4476. Patrap
Member Since: Julho 3, 2005 Posts: 427 Comments: 129439
Quoting WeatherNerdPR:

Surface or flight level?


Member Since: Julho 6, 2010 Posts: 113 Comments: 32711
Quoting TropicalAnalystwx13:
Finding 61 kts.

Surface or flight level?
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4473. WxLogic
Quoting Seflhurricane:
Is recon heading to 97l


If it hasn't been cancelled it should be departing in ~18min.
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Finding 61 kts.
Member Since: Julho 6, 2010 Posts: 113 Comments: 32711
Parameter - CMC TS
23.08.2011 18 GMT Santo Domingo 986.5 hPa
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51 hours out gaining strength between p.r and d.r but farther south
Member Since: Julho 31, 2011 Posts: 5 Comments: 891
Is recon heading to 97l
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4465. aquak9
Quoting cloudburst2011:


if they did it was a gust in a thunderstorm not sustained...

Thanks- I was beginning to think I had really missed something. Which is often the case.
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4464. wxhatt
I think the GFS is seeing this now.

Notice how the L (Low) is to the SW of the convective X initially. as opposed to Harvey and the low just off Africa.



Then on the 12Hr frame the low begins to migrate more under the convetion, just to its south.



by 24Hrs it is just about colocated with the initial convective ball to the north (at that higher latitude)



Now look how close to PR it should be



Member Since: Outubro 5, 2005 Posts: 2 Comments: 930
Quoting cloudburst2011:



lol he tends to go overboard aqua..


O rly?

13:23:00Z 17.233N 87.883W 842.7 mb
(~ 24.88 inHg) 1,561 meters
(~ 5,121 feet) 1008.7 mb
(~ 29.79 inHg) - From 43� at 21 knots
(From the NE at ~ 24.1 mph) 17.0�C
(~ 62.6�F) 12.9�C
(~ 55.2�F) 22 knots
(~ 25.3 mph) 95 knots*
(~ 109.2 mph*) 1 mm/hr*
(~ 0.04 in/hr*) 90.7 knots* (~ 104.3 mph*)
Category Two Hurricane*
431.8%*
Member Since: Julho 6, 2010 Posts: 113 Comments: 32711
Quoting USAFwxguy:
The NNW jump in the L center is suspect from 18 to 24




Probably a relocation or something.
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45 hours out the system either moved s.w or reformed there...
Member Since: Julho 31, 2011 Posts: 5 Comments: 891
4458. angiest
Quoting USAFwxguy:
The NW jump in the L center is suspect from 18 to 24





Heh, what is that, a motion of about 50mph?
Member Since: Agosto 26, 2006 Posts: 16 Comments: 4766
Quoting CybrTeddy:


It was flagged.

Even if it was flagged, seems unusual...
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4456. hotrods
It looks like a LLC around 16/57 i could be wrong.
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4454. Patrap
12z Early Cycle NHC model tracks
Invest98
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)




Dynamic Models (More sophisticated models)




Member Since: Julho 3, 2005 Posts: 427 Comments: 129439
Quoting TropicalAnalystwx13:


They found 104 mph winds earlier.


It was flagged.
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4451. aquak9
Quoting TropicalAnalystwx13:


They found 104 mph winds earlier.

o-kay...I stand corrected. :/
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Maybe old news, but the 00z EURO (the one Dr. M. wrote we should watch) is basically on the same track as 00z gfs. Only difference is that it has 97 a tad west, skirting W Fl coast, as opposed to running up the spine. An even worse case scenario, should that pan out. imo

Now I see the 12z gfs is running, but I think it will be pretty much as the last several runs; we'll see.

Now that the storm is really forming, recon will check it out, we may have some model shift.

But I can't recall a storm, predicted 13 days out having so much model support AND consistency in each run, coupled with the fact that many other model agree.
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Santo Domingo
Date Minimum Mean Maximum Std. Derivation
20.08.2011 00 GMT 1012.1 hPa ± 0.4 hPa
21.08.2011 00 GMT 1013.2 hPa ± 0.5 hPa
22.08.2011 00 GMT 1012.4 hPa ± 0.6 hPa
23.08.2011 00 GMT 1009.3 hPa ± 2.4 hPa
24.08.2011 00 GMT 1009.9 hPa ± 2.5 hPa
25.08.2011 00 GMT 1011.9 hPa ± 1.9 hPa
26.08.2011 00 GMT 1013.4 hPa ± 1.5 hPa
27.08.2011 00 GMT 1013.7 hPa ± 1.2 hPa
28.08.2011 00 GMT 1012.7 hPa ± 1.0 hPa
29.08.2011 00 GMT 1013.3 hPa ± 1.0 hPa
30.08.2011 00 GMT 1014.0 hPa ± 0.9 hPa
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Quoting TropicalAnalystwx13:
I can't believe the HH's found a 104 mph wind in Harvey...Good thing they didn't find anymore.


That was very unexpected. I thought only hurricanes got winds that high.
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Quoting aquak9:


???? there was a pressure of 1004 mb, but no winds of what you speak...


They found 104 mph winds earlier.
Member Since: Julho 6, 2010 Posts: 113 Comments: 32711
36 hours, further north on this run.. Makes sense due to the potential LLC developing a bit more to the north.

Member Since: Julho 1, 2008 Posts: 13 Comments: 7436

39 hours out p.r gets a direct hit
Member Since: Julho 31, 2011 Posts: 5 Comments: 891

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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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