Remainder of July hurricane outlook

By: Dr. Jeff Masters , 2:31 PM GMT on July 15, 2009

Not much has changed in the Atlantic since my early July Atlantic hurricane outlook posted two weeks ago. Tropical cyclone activity typically picks up a bit during the last half of July, but we are still a month away from when hurricane season really gets going. Since the current active hurricane period began in 1995, nine of 14 years (63%) have had a named storm form during the last half of July. We had two last-half-of-July named storms last year--Christobal and Dolly. As seen in Figure 1, most of the late July activity occurs in the Gulf of Mexico, Western Caribbean, and Carolina waters. However, a few long-track "Cape Verdes" hurricanes begin to occur. These are spawned by tropical waves that come off the coast of Africa. Tropical waves serve as the instigators of about 85% of all major hurricanes.


Figure 1. Tracks of all tropical storms and hurricanes 1851 - 2006 that formed July 16 - 31.The Gulf of Mexico coast is the preferred strike location. There are still very few major Cape Verdes-type hurricanes forming in the last half of July.

Sea Surface Temperatures
Sea Surface Temperature (SST) anomalies have warmed slightly over the past two weeks, and are about 0.3°C (0.5°F) above average over the tropical Atlantic between Africa and Central America (Figure 2). These are some of the coolest SST anomalies for this time of year that we've seen since 1994. The strength of the Azores-Bermuda high has been near or slightly below average over the past two weeks, driving slightly below average trade winds. Weaker trade winds don't mix up as much cold water from the depths, and cause less evaporative cooling. The latest 2-week run of the GFS model predicts continued near-average or slightly below average-strength trade winds through the end of July, so SSTs should remain slightly above average during this period.


Figure 2. Sea Surface Temperature (SST) departure from average for July 13, 2009. SSTs were about 0.3°C (0.5°F) average over the tropical Atlantic's Main Development region for hurricanes, from Africa to Central America between 10° and 20° North Latitude. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, the hallmark of a developing El Niño episode. Image credit: NOAA/NESDIS

El Niño
El Niño conditions continue to amplify over the tropical Eastern Pacific. Ocean temperatures in the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region", are now 0.4°C above the threshold for a weak El Niño, according to the Australian Bureau of Meteorology (Figure 3). An increase of another 0.1°C will push the current El Niño into the "moderate" category. NOAA's Climate Prediction Center issued an El Niño Advisory earlier this month. The latest set of mid-June runs of the El Niño computer models are almost universally calling for El Niño conditions to become well-established for the peak months of hurricane season, August - October. It is likely that Atlantic hurricane activity will be suppressed in 2009 due to the strong upper-level winds and resulting wind shear an El Niño event usually brings to the tropical Atlantic.


Figure 3. Sea Surface Temperature (SST) departure from average for the the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 8, 2009, SSTs in the Niño 3.4 region had risen to 0.9°C above average. To be considered an "El Niño episode", El Niño conditions must occur for five consecutive months, using 3-month averages. Image credit: Australian Bureau of Meteorology.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream's band of strong high-altitude winds is the main source of wind shear in July over the Atlantic hurricane breeding grounds, since the jet is very active and located quite far south this time of year.

The jet stream over the past three months has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England.

The jet stream is forecast to maintain this two-branch pattern for the next week. However, during the final week of July, the subtropical jet is forecast to weaken. This will leave regions of low wind shear over the Caribbean and Gulf of Mexico for the final week of July (Figure 4), increasing the chances of hurricane development.


Figure 4. Wind shear in m/s between 200 mb and 850 mb on July 31, 2009, as forecast by the 00Z July 15, 2009 run of the GFS model. The subtropical jet is forecast to weaken by this time, leaving regions of low wind shear over the Caribbean and Gulf of Mexico for the final week of July. Wind speeds are given in m/s; multiply by two to get a rough conversion to knots. Thus, the red regions of low shear range from 0 - 16 knots.

Dry air and African dust
June and July are the peak months for dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past month. Expect dust from Africa to be a major deterrent to any storms that try to form between Africa and the Lesser Antilles Islands in July. Several well-developed African waves have been done in by dry air from Africa over the past few weeks.

Steering currents
The steering current pattern over the past few weeks has not changed much. A persistent trough of low pressure has remained entrenched over the Eastern U.S. all summer, bringing cool and relatively moist weather to the eastern half of the country. This trough is strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. At present, it appears that the coming two weeks will maintain the strong trough over the Eastern U.S., which decreases the hurricane risk to the U.S. Gulf Coast. There is no telling what might happen to the steering current pattern during the peak months of August, September, and October, but it is often difficult to break a months-long steering current pattern like the current one.

Summary
Recent history suggests a 63% chance of a named storm occurring in the last half of July. Given that none of the computer models are forecasting tropical storm formation in the coming seven days, and SST and wind shear patterns look pretty average, I'll go with a 30% chance of a named storm forming this month. Such a storm would most likely form near the end of the month, when wind shear is expected to decline due to a weakening of the subtropical jet stream. The last time we went this long in the season without a named storm forming was in 2004, when the first storm (Alex) formed on August 1.

I'll have a new post on Friday.

Jeff Masters

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The 12Z CMC develops it after it crosses into the GOM, hits Florida, go figure.
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I tend to agree it won't amount to much SSIG...It may have a chance at some sort of slow development before it hits the shear. Seems that will tear it up pretty good though...And if it is weak when it enters the shear then I doubt it will have much shot of making the E coast waters, or the Gulf.

just my guess though.
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NC Man Calls 911 To Report He's Robbing Bank



Link
Member Since: May 21, 2006 Posts: 5101 Comments: 118560
Quoting BurnedAfterPosting:


yup reed it is expected to decrease drastically in a few days, this wave is not moving that fast

I find it odd, yesterday you were all over this saying it would form, now the NHC actually is paying attention to it and you have made a complete 180 lol.

Indications are shear will decrease drastically by the time this wave gets to the Caribbean.


Yep, shear is forescast to be marginaly favorable for TC development thats is the main the reason that NHC is paying atention to this wave, looks like is trying to consolidate the convection, even tought there still a broad low at this moment, organization continues...
Member Since: July 15, 2008 Posts: 0 Comments: 799
The 12Z EURO brings the wave towards the USA then makes a weak low out of it and heads to Long Island, NY. Interesting, this I think is the only model doing anything with it. I just checked the 18Z NOGAPS and it shows moisture, but nothing significant. maybe the models will jump back on it now that the shear is forecast to weaken.
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Quoting StSimonsIslandGAGuy:
This wave probably won't amount to anything--but it is the first to have a chance at developing.


well the NHC is paying attention to it, so it must have a chance to develop
Quoting reedzone:
Guys wind shear is "devastating" in the Caribbean, it needs to go north of the Carribean to possibly do something. 60 knots!!!! That's not good for development lol. A stretch of 50 knots from the Carribean to near the Islands. Yeah, it'll get impressive tonight because wind shear is low, but they may not even tag it due to the future of the wave.


GFS and NOGAPS develop anticyclonic flow aloft, in particular the NOGAPS, by 5-7 days in the western and central Caribbean.
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Quoting BurnedAfterPosting:


The shear is expected to lessen drastically by all the forecast models reed, you cant say this doesnt have a chance by looking at shear that is over 1000 miles to its west

shear changes like everything else


Well haha, ok.. Lets wait and see I guess. can't let one map fool ya. Shear is 5-10 knots over the Carribean, now the High is very strong, so if it does form and shear drastically weakens, we really need to watch this. It does look good though, some convection firing.
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Quoting StSimonsIslandGAGuy:
The formula for finding the distance between longitude lines on the earth is you take the cosine of the latitude where the disturbance is and multiply it by 69 miles which is the length of a degree of longitude at the equator. At 11 north, that comes to 68 miles.


so its 680 miles/15 mph

comes out to 45 hours
I thought it was expected to gain latitude.
Member Since: July 11, 2006 Posts: 14 Comments: 11944
Quoting StormJunkie:
Thanks Chicklit, and sorry about that. I fixed it and here it is again.

Latest Quickscat

Thank-you.
..mellow yellow this evening
Member Since: July 11, 2006 Posts: 14 Comments: 11944
Quoting Chicklit:
So 2 days at 15 mph to go ten degrees at what latitude range?


That is if it goes due west basically, it wont gain much latitude, so about 36-48 hour is a good bet for it to get to 42W
Quoting reedzone:


Funny you did mention that, it looked really good 2 days ago when it came off and it still had a ways to go before reaching the Carribean, so I gave it a chance. Now that I've seen the wind shear and how it has gotten worse, I changed my mind, notice the GFS backed off it to, due to the changes in the wind shear. Though, I guess we'll see, maybe, just MAYBE INVEST 95L in the morning.


The shear is expected to lessen drastically by all the forecast models reed, you cant say this doesnt have a chance by looking at shear that is over 1000 miles to its west

shear changes like everything else
So 2 days at 15 mph to go ten degrees at what latitude range?
Member Since: July 11, 2006 Posts: 14 Comments: 11944
1436. Melagoo
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Quoting BurnedAfterPosting:


yup reed it is expected to decrease drastically in a few days, this wave is not moving that fast

I find it odd, yesterday you were all over this saying it would form, now the NHC actually is paying attention to it and you have made a complete 180 lol.

Indications are shear will decrease drastically by the time this wave gets to the Caribbean.


Funny you did mention that, it looked really good 2 days ago when it came off and it still had a ways to go before reaching the Carribean, so I gave it a chance. Now that I've seen the wind shear and how it has gotten worse, I changed my mind, notice the GFS backed off it to, due to the changes in the wind shear. Though, I guess we'll see, maybe, just MAYBE INVEST 95L in the morning.
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Thanks Chicklit, and sorry about that. I fixed it and here it is again.

Latest Quickscat
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quikscat
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the NHC wouldnt have even bothered with the yellow circle if they thought that shear would kill it
Quoting StormJunkie:


Computer models, and then some...

Latest Quickscat shows a strong wind shift, but no west ind yet.

SJ. You posted wrong link for Quickscat. We got your link site twice.
Member Since: July 11, 2006 Posts: 14 Comments: 11944
Quoting reedzone:
Guys wind shear is "devastating" in the Caribbean, it needs to go north of the Carribean to possibly do something. 60 knots!!!! That's not good for development lol. A stretch of 50 knots from the Carribean to near the Islands. Yeah, it'll get impressive tonight because wind shear is low, but they may not even tag it due to the future of the wave.


yup reed it is expected to decrease drastically in a few days, this wave is not moving that fast

I find it odd, yesterday you were all over this saying it would form, now the NHC actually is paying attention to it and you have made a complete 180 lol.

Indications are shear will decrease drastically by the time this wave gets to the Caribbean.
Quoting weatherwatcher12:

It is predicted to lessen significantly.

Look at Dr.Master's post
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Quoting reedzone:
Guys wind shear is "devastating" in the Caribbean, it needs to go north of the Carribean to possibly do something. 60 knots!!!! That's not good for development lol. A stretch of 50 knots from the Carribean to near the Islands. Yeah, it'll get impressive tonight because wind shear is low, but they may not even tag it due to the future of the wave.

It is predicted to lessen significantly.
Member Since: Posts: Comments:
Guys wind shear is "devastating" in the Caribbean, it needs to go north of the Carribean to possibly do something. 60 knots!!!! That's not good for development lol. A stretch of 50 knots from the Carribean to near the Islands. Yeah, it'll get impressive tonight because wind shear is low, but they may not even tag it due to the future of the wave.
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Quoting KoritheMan:


Hmm, I always thought it was a tad earlier in that part of the Atlantic. Perhaps a misconception on my part.

Thanks for the correction.


Well it is in relation to here, D-Min always occurs as the sun is setting, since its the East Atlantic, the sun goes down there several hours before it does here, so it does seem earlier
Looks like it could be trying to put some convection back on.

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Quoting BurnedAfterPosting:


D-min passed a few hours ago when the sun went down, its 3am over there

D-Max is in a few hours


Hmm, I always thought it was a tad earlier in that part of the Atlantic. Perhaps a misconception on my part.

Thanks for the correction.
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Quoting KoritheMan:


Not in that part of the Atlantic.


D-min passed a few hours ago when the sun went down, its 3am over there

D-Max is in a few hours
D-Min is already passed, D-Max is in a few hours

Not in that part of the Atlantic.
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Quoting Tazmanian:



dont jump the gune


Very true. Despite all the wave has going for it, I'm still inclined to only believe slow development will occur before the wave reaches at least 75W, at which point some greater development may begin to occur as the shear relaxes. When it reaches the western Caribbean, the flow aloft may be anticyclonic, favoring tropical cyclogenesis.

But as for Cape Verde development from this, I highly doubt it.
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Quoting KoritheMan:
Also, another positive thing for the wave is that it is not moving nearly as fast as the other waves have been thus far, and the SAL is easing up, and may continue to do so, particularly if the wave moistens the environment ahead of it.

The SAL is horrible in the Caribbean


But is very light at current location.
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oh you said 10 degrees lol

well then about 600/15 would equal 40 hours or just under 2 days to go 10 degrees.
Quoting popartpete:


Thanks for that link, but I have trouble reading those maps. I was looking for the "spaghetti" maps that are usually on wunderground tropical as soon as an invest is made. Do you know where to find that? It would be super big help.


Here are the spagetti models

on top just click the storm you want to go to once an invest is named
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Quoting popartpete:


Thanks for that link, but I have trouble reading those maps. I was looking for the "spaghetti" maps that are usually on wunderground tropical as soon as an invest is made. Do you know where to find that? It would be super big help.


They are only there when an invest has been declared...And then not until a few hours later. Best bet is to learn to use the other model pages. There are some short videos here that give you a real basic look at how to navigate and use some of the better model sites, like the FSU site.
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Quoting Funkadelic:
Well if it looks like this at D-Min, then once in the middle of D-MAX Anna may be born. Stay tuned......


D-Min is already passed, D-Max is in a few hours
Also, another positive thing for the wave is that it is not moving nearly as fast as the other waves have been thus far, and the SAL is easing up, and may continue to do so, particularly if the wave moistens the environment ahead of it.
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Quoting popartpete:


Thanks for that link, but I have trouble reading those maps. I was looking for the "spaghetti" maps that are usually on wunderground tropical as soon as an invest is made. Do you know where to find that? It would be super big help.

when an invest is declare you can look at www.tropicalatlantic.com
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Quoting Funkadelic:
Well if it looks like this at D-Min, then once in the middle of D-MAX Anna may be born. Stay tuned......



dont jump the gune
Member Since: May 21, 2006 Posts: 5101 Comments: 118560
1411. msphar
Dean was a bad Jose. Its outer bands sweeped the Eastern end of PR with 40 kt winds for a day.
That was 200 miles from the track. No fun.
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Quoting weatherwatcher12:

Fsu models site


Thanks for that link, but I have trouble reading those maps. I was looking for the "spaghetti" maps that are usually on wunderground tropical as soon as an invest is made. Do you know where to find that? It would be super big help.
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Quoting Chicklit:
Evenin' Wunderfolk,
Question: If the wave at 32 longitude is traveling 15-20 nautical miles per hour, then how long will it take to travel ten degrees (within five degrees latitude) on about its present course?


Hmmmm a math problem

well its usually about 300 miles for every 5 degrees of longitude, so the wave is about 1800 miles east of the islands. 1800/15 gives you 120 hours or 5 days
Quoting popartpete:
Who has links to the computer models?


Computer models, and then some...

Latest Quickscat shows a strong wind shift, but no west ind yet.
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Quoting weatherwatcher12:

wow. 3 months. Thats extreme
Tell me about it. I felt like crying sometimes. I work at the post office and as you know the mail has to go out. We had no power, no generator and no windows. Unbearable.. The post office I work at is right next to a pond which flooded the post office so we also had dead fish and mildew inside. Not nice at all.
Member Since: October 9, 2008 Posts: 0 Comments: 8599
Quoting Sockets:


almost looks like it is trying to draw in some of the moisture beneath it...beginning of a feeder band?


That big blob going into the center does look like a feeder band.
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