An active Atlantic hurricane period coming

By: Dr. Jeff Masters , 1:48 PM GMT on August 17, 2010

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The remnants of Tropical Depression Five are no longer a threat, done in by high wind shear and close proximity to land. However, an active period for Atlantic hurricanes is likely for the remainder of August, as the global atmosphere undergoes a major change to the circulation pattern that has dominated Northern Hemisphere weather during July and August. A large trough of low pressure is gathering strength over Europe, and is expected to push eastward. By Thursday, this trough should be able to push away the blocking ridge of high pressure that has given Russia its worst heat wave in history. The shift in circulation has already weakened the large region of sinking air that has brought dry, stable conditions to the tropical Atlantic over the past month. Vertical instability, which was unusually low since late July, has now returned to near normal levels over the tropical Atlantic (Figure 1), though it remains quite low over the rest of the North Atlantic. Instability is measured as the difference in temperature between the surface and the top of the troposphere (the highest altitude that thunderstorm tops can penetrate to.) If the surface is very warm and the top of the troposphere is cold, an unstable atmosphere results, which helps to enhance thunderstorm updrafts and promotes hurricane development. Since SSTs in the Atlantic were at record highs and upper tropospheric temperatures were several degrees cooler than average in July, enhancing instability, something else must have been going on to reduce instability. Dry air can act to reduce instability, and it appears that an unusually dry atmosphere, due to large-scale sinking over the Atlantic, was responsible for the lack of instability. Now that vertical instability has returned to near normal levels, Atlantic hurricane activity should increase to at least average levels over the next two weeks. This is particularly true since SSTs are at record highs and vertical wind shear is at average to below average levels over the tropical Atlantic.


Figure 1. Vertical instability (in °C) over the Caribbean (left) and tropical Atlantic between the Lesser Antilles Islands and coast of Africa (right) in 2010. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere became much more stable than normal in both regions at the end of July. This lack of instability also extended to the Gulf of Mexico and North Atlantic Ocean between Europe and North America. However, in the past few days, vertical instability has returned to normal, thanks to a major pattern shift in the global atmosphere. Image credit: NOAA/CIRA.


Figure 2. The climatology of Atlantic hurricane activity shows a sharp rise in activity around August 18.

Analysis
August 18 historically marks the point where Atlantic hurricane activity makes a major spike upwards (Figure 2.) On average, we can expect to see two named storms and one hurricane during the last half of August. The last half of August usually sees a moistening of the atmosphere off the coast of Africa, as the the African monsoon kicks into high gear. This year is no exception (Figure 3.) The dry Saharan Air Layer (SAL) has retreated to the north, leaving a moist atmosphere conducive for tropical cyclone development off the coast of Africa.

It would not be a surprise to see atmospheric instability increase to above-average levels by early next week as the major atmospheric pattern shift progresses. Will this usher in a hyperactive period of Atlantic hurricane activity next week, with a parade of three or four simultaneous storms strung out across the Atlantic? Probably not, since the Madden-Julian Oscillation (MJO) only marginally favors upward motion over the tropical Atlantic, and is not forecast to change much over the next ten days. The Madden-Julian oscillation is a pattern of enhanced rainfall that travels along the Equator from west to east. The pattern has a wet phase with large-scale rising air and enhanced thunderstorm activity, followed by a dry phase with large-scale sinking air and suppressed thunderstorm activity. Each cycle lasts approximately 30 - 60 days. When the Madden-Julian oscillation is in its wet phase over a hurricane-prone region, the chances for tropical storm activity are greatly increased. The bottom line: I expect we will see 2 - 3 named storms in the Atlantic by the end of August, including one hurricane. Where these storms might develop and move is difficult to say. It currently appears that the global shift in circulation will bring near-average steering currents to the Atlantic over the next ten days, with a series of troughs of low pressure capable of recurving hurricanes, moving across the Atlantic. The GFS model is indicating, though, that during the few days of August, these troughs may weaken, making recurvature of hurricanes less likely, and increasing the probability of landfalling storms.

The GFS, NOGAPS, and ECMWF currently predict that one or two tropical storms will form between Africa and the Lesser Antilles Islands during the period 4 - 10 days from now. The NOGAPS model is predicting the development of a strong tropical disturbance near the coast of Honduras late this week.


Figure 3. Saharan Air Layer (SAL) analysis shows that the dry air and dust of the SAL (orange colors) lies well to the north of the hurricane breeding grounds off the coast of Africa, near the Cape Verdes Islands. Image credit: University of Wisconsin/NOAA Hurricane Research Division.

Smoke bedeviling Moscow again
Light easterly winds over the past few hours have brought smoke from wildfires back into Moscow today. Temperatures at Moscow's Domodedovo airport hit 29°C (84°F) today, which is 11°C (20°F) above average. The latest forecast for Moscow predicts that just one more day remains for Russia's greatest heat wave in recorded history. On Thursday, a strong trough of low pressure will move through European Russia, finally bringing below average temperatures.

Jeff Masters

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Yet another shot of PGI27L, which is seeing slightly increasing odds of survival (click image for full-size version):

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1299. IKE
Quoting clwstmchasr:
Turning north again. Depression starts to set in.


Hankies available...PM me.

$$
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TAFB experimental graphical forecast has a strengthening system in the CATL.

Member Since: June 9, 2007 Posts: 4 Comments: 15950
Link

check out the radar link for PR yikes is that comeing my way?
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1295. IKE
Quoting DestinJeff:
Why are long-range solutions no longer "too far out" to pay attention to?


I don't think they ever were.
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Quoting MiamiHurricanes09:
That's what I'm thinking. But then, what happens with the vigorous tropical wave currently over western Africa? Does that one just dissipate or does it also tie-in with PGI30L and PGI31L to form a tropical cyclone? Very confusing.


It's really confusing, but after staring at the 850 vort loop I think it's a standalone system which'll combine with yet another wave behind it, leading to the GFS's second storm.
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Quoting MiamiHurricanes09:
Nah, the wave is over water...PGI31L. PGI31L will then interact with PGO30L to form a tropical cyclone.
the wave at 10W is not over water, what is currently over water is not likely to amount to much, except maybe helping to moisten things up. As Kman said, I am in agreement with him the the atmosphere will not moisten up enough "over night" to make these waves pop the way the models are showing. Could I be wrong, yes, but...
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Quoting TropicalAnalystwx13:


I thought the one over Western Africa was 31L...?
I thought that too.

Member Since: September 2, 2009 Posts: 130 Comments: 21194
1291. robj144
I'm not trying to be a "downcaster" or down play the season (which is I believe the more appropriate term) as I believe the season will most likely pick up soon. That being said, I know that most here just look at the models as being interesting, but I think some take them too seriously. Didn't the models have xTD5 regenerating? I mean it missed that and that was only a couple days out, so how accurate could they possibly at all at around ten days from now? The storms could be anywhere by then.
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Mainly focus on intensity not track...this track this far out is useless...i think a 2 year old can take a crayon and draw a line and it will have the same signifiance...Remember guys, Ike was forecasted to be a recurve storm and it hit the Western GOMEX and that was when it was already near the Bahamas...
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Quoting MiamiHurricanes09:
Yup, this is how the run ends. A hurricane north of the Lesser Antilles with part 3 developing. Once it gets going it won't stop for a couple months. All I've gotta' say is, "enjoy the peace while ya' still have it".

384 hours:



In addition, long range GFS isn't good at picking up other systems that may form besides those Cape Verde ones.
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Quoting MiamiHurricanes09:
That's what I'm thinking. But then, what happens with the vigorous tropical wave currently over western Africa? Does that one just dissipate or does it also tie-in with PGI30L and PGI31L to form a tropical cyclone? Very confusing.



I thought the one over Western Africa was 31L...?
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PGI30L/31L and the vigorous African wave.

Member Since: June 9, 2007 Posts: 4 Comments: 15950
Does anyone have a link to Dr. Master's radio show today? Tks.
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Quoting hunkerdown:
come on people, get a grip...you're boarding up over a model 4 months out for a wave this is currently over land. The odds are still not that great that it will sustain as the models make it out to be when it rolls off the coast. Atmosphere still needs to be moistened up AND who knows how weak/how strong, when and where, the troughs and ridges will be at that time if anything close to that scenario plays out. Just sayin....
Nah, the wave is over water...PGI31L. PGI31L will then interact with PGI30L to form a tropical cyclone.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting hunkerdown:
come on people, get a grip...you're boarding up over a model 4 months out for a wave this is currently over land. The odds are still not that great that it will sustain as the models make it out to be when it rolls off the coast. Atmosphere still needs to be moistened up AND who know how weak/how strong, when and where, the troughs and ridges will be at that time if anything close to that scenario plays out. Just sayin....


Unlike you, we dont take over 12 consistent runs of this becoming a Major Hurricane likely. The odds of us getting our next named storm late this week into next week are really REALLY high.
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Quoting hunkerdown:
come on people, get a grip...you're boarding up over a model 4 months out for a wave this is currently over land. The odds are still not that great that it will sustain as the models make it out to be when it rolls off the coast. Atmosphere still needs to be moistened up AND who know how weak/how strong, when and where, the troughs and ridges will be at that time if anything close to that scenario plays out. Just sayin....


1. It's over water
2. Nobody's boarding up
3. GFS only goes out to 384 hours
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Look at the SST's. This won't help at all.
Member Since: May 1, 2010 Posts: 0 Comments: 352
1278. NJ2S
Quoting TerraNova:
I remember GFS having Dean hitting NYC on lots of runs, which were all immediately followed by a swing over to NOLA.


Bill also was poised in the begining to hit NYC on a few early runs.....actually had it hitting seaside heights NJ which would put NYC in the same relative position as biloxi, ms during katrina
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Quoting TerraNova:

Yeah, PGI31L after undergoing an interaction with PGI30L.
That's what I'm thinking. But then, what happens with the vigorous tropical wave currently over western Africa? Does that one just dissipate or does it also tie-in with PGI30L and PGI31L to form a tropical cyclone? Very confusing.

Member Since: September 2, 2009 Posts: 130 Comments: 21194
1275. KEEPEROFTHEGATE (Mod)
warfighter 18z 144hrs
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Evening Storm!
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Quoting MiamiHurricanes09:
Now the question is, which is the wave turned into a major? It seems to me like PGI31L is the one.

Yeah, PGI31L after undergoing an interaction with PGI30L.
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100 knots at 180 hours

Member Since: June 9, 2007 Posts: 4 Comments: 15950
Quoting StormW:
Good evening!
Good evening Storm!
Member Since: September 2, 2009 Posts: 130 Comments: 21194
PREDICT Summary for August 17 2009

The target today was PGI27L. This wave has been tracked for the
past four days. Originally there was no deep convection
associated with it. Late on 14 August, isolated convective cells
developed within a localized maximum of precipitable water. The
progression of convection organization within PGI27L is shown in
Image 1, Image 2 and Image 3. The pouch of PGI27L was actually
not the original pouch assigned to the African wave whose pouch
was designated PGI27. As shown in Image 4, at 12 UTC 14 August,
two pouches have formed. The one farther west became the pouch
that was flown today. Deep convection developed within 12 h of
formation of this pouch, in association with a locally enhanced
TPW (Image 5) that persists, roughly within the pouch (Image 6,
Image 7). A factor of unknown significance in initiating the
strong convection observed earlier this morning was an
upper-tropospheric shear line (Image 8, 200 mb winds at 00 UTC
17 August). Convection within the pouch intensified on the
downshear side of this line.

Plans are to fly PGI27L tomorrow to see if its development
continues and to document the pouch structure and the
environment.
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Quoting StormW:
Good evening!


Evening storm! What ya think of the 18z GFS grazing cape cod as a strong hurricane?
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1267. NJ2S
Quoting AstroHurricane001:
GFS 18z, August 24 00z:


GFS 12z, August 24 00z


The new run is farther north and east, the storm is stronger, and the Bermuda high is more elongated.

12z run storm near landfall:


CONCLUSION: The storm could travel in a track that would allow it to threaten New York City.


NYC? track similar to long island express or more of an inland after effect.....once storms come ashore in the carolinas NYC is safe its the ones that potentially stay off shore or start to deflect back after recurving a bit thats the ones NYC has to watch for. this system does or doesnt have this potetial? any answers?
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Five track possibilities:

West:



Middle-west:



Middle:



Middle-east:



East:

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Now the question is, which is the wave turned into a major? It seems to me like PGI31L is the one.
Member Since: September 2, 2009 Posts: 130 Comments: 21194
Ex-PGI29L (Ex-Td #5)
[say that name quickly three times in a row]

Ex-PGI29L is currently located over southeast LA and is tracking to the NW
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Quoting extreme236:


The pattern clearly favors continued Cape Verde activity into September.
Yup, this is how the run ends. A hurricane north of the Lesser Antilles with part 3 developing. Once it gets going it won't stop for a couple months. All I've gotta' say is, "enjoy the peace while ya' still have it".

384 hours:

Member Since: September 2, 2009 Posts: 130 Comments: 21194
Quoting MiamiHurricanes09:
Yup, up to 192 hours is where I pay attention to the GFS, after it gets iffy. We're talking cat 4 here, maybe stronger.

lucky ..it will be on ocean crusher. no land threat. seems the followers will track the same???
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1258. KEEPEROFTHEGATE (Mod)
warfighter 18z 120hrs
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BBL, folk.
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Quoting MiamiHurricanes09:
Just for grins, part 2 ensues.

348 hours:



The pattern clearly favors continued Cape Verde activity into September.
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looks like thing are picking up
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Quoting MiamiHurricanes09:
I know someone was going to say that! LOL!

Oh god, lol.
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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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