Remainder of July hurricane outlook

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

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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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Quoting OSUWXGUY:


I'm not disagreeing with the gist of your post...southwest shear is impinging on Dolores.

But I did want to say that satellite data (admittedly in many forms) is the ONLY data we have for such systems!


Oh, I am not denying that, since recon does not typically fly into Eastern Pacific tropical cyclones, unless they are close to land, and even they don't seem to unless they are major hurricanes.

But even Dvorak estimates have been known to be wrong.
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Quoting OSUWXGUY:


I'm not disagreeing with the gist of your post...southwest shear is impinging on Dolores.

But I did want to say that satellite data (admittedly in many forms) is the ONLY data we have for such systems!


Well that is true but you can see such a huge difference in observations. Based on satellite we had people here saying Dolores was a hurricane. NHC looks at the satellite and says its a 40mph TS lol.

Big difference there. I just think its important to know what you are looking at if you are going to comment on it.
Quiet
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.
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Good late morning

Tropical Update
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Quoting TheCaneWhisperer:


Can you elaborate on that?
Quoting BurnedAfterPosting:


Thank you for posting that, as I said looks can be deceiving.


Yeah, I learned a long time ago not too give much credence to a satellite image. Last year, Norbert looked like a Category 2 or 3 shortly after it had begun to weaken after peaking as a Category 4, and yet recon only discovered Category 1 surface winds.
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Thanks Dr. M; 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. The strength of the El Nino will up for speculative debate, until conditions actually unfold in August and September, but, a good season is a slow season so I do hope that the season shuts down early.....Looks like we will need to watch closer to home (Carribean and US waters) for possible development in August.
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Quoting KoritheMan:


Looking at visible satellite imagery and the shortwave infrared satellite imagery, Dolores's circulation appears slightly exposed northwest of the deep convection. And though I have not looked at any microwave images, I am fairly certain that the low, mid, and upper-level circulations are largely decoupled.

Oh, and just for kicks, look at how asymmetrical the associated wind field is:



Certainly not a well organized system by any means. It's a lot like Hanna was last year when it interacted with an upper-level low, just like this storm is doing. Has a similar satellite appearance, too.


Thank you for posting that, as I said looks can be deceiving.
Quoting btwntx08:

still an 85 mph hurricane but its starting to guadually weaken


It looks horrible!
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Morning All! Thanks for the update Dr. Masters!
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Quoting btwntx08:

it doesn't look i that at all its perffectly organized


Looking at visible satellite imagery and the shortwave infrared satellite imagery, Dolores's circulation appears slightly exposed northwest of the deep convection. And though I have not looked at any microwave images, I am fairly certain that the low, mid, and upper-level circulations are largely decoupled.

Oh, and just for kicks, look at how asymmetrical the associated wind field is:



Certainly not a well organized system by any means. It's a lot like Hanna was last year when it interacted with an upper-level low, just like this storm is doing. Has a similar satellite appearance, too.
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Quoting BurnedAfterPosting:


because that storm diffuses the argument that July has few high impact storms lol, so it was left out lmao
It would only be one more. That would make, what, 3 on the map? I think it has to be something else.
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Quoting BurnedAfterPosting:


again its not, the NHC tells how assymetric the system is, 40mph TS for now is a good call

certainly not a Cat 2 lol


It's probably strengthening to some degree with that amount of deep convection...and may have restacked pulling the LLC under the deep convection.

Fact is we don't KNOW how strong it is...though it probably is mid range TS.

My only point in posting that satellite is to show how much more impressive looking a TS can be...while a diminutive hurricane can look so innocuous.
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Quoting Chucktown:
We don't know what happenned 5000 years ago - and I don't want to hear about ice core samples. How do you determine what happenned in the sub tropical climates where there was no ice.



That's not how it works.

First off, Antartica *was* tropical. That might give you an idea of just how bad it can get.

Secondly, Antartic ice (And greenland. It isnt from just one place) is used to measure the isotope of water. Heavier isotopes of the water vapor condense the fastest, and you measure out the concentration of those isotopes at any given point in time to determine the average temperature.

Basically, its like a thermometer in the past based on air pressure. Heavier particles sink faster, so we can determine the temperature based on the number of heavy isotopes versus normal isotopes.

Now, you also can gain insight into trace gasses in the atmosphere by studying the frozen air bubbles within the ice.

Why does this show us what the planet was like? Why, Entropy, my dear watson. Well, its not actually entropy.. but it is basically caused by it. The same air you breathe here in the united states is basically the same air you'll breathe in Antarctica.

This dosen't preclude that there are going to be pockets of warmer or colder environments because of localized anomalies, but it does show us an average.
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Still staying with my thought from last night. I agree that the 3 waves look good and all but no named tropical system for the rest of July.
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From The NWS in Miami.

TUESDAY...TIME TO LOOK TO THE EAST!!! GLOBAL MODELS (GFS, ECMWF)
HAVE BEEN PERSISTENT IN DEVELOPING A RATHER HEALTHY TROPICAL WAVE
OVER THE WESTERN ATLANTIC JUST OFF AFRICA. PREVIOUS RUN OF BOTH, GFS
AND ECMWF, DEVELOPED A CYCLONIC CIRCULATION WITH THIS SYSTEM BY THE
TIME IT REACHES THE SOUTHEASTERN BAHAMAS, THEN RECURVED IT NORTHWARD
AROUND THE PERIPHERY OF THE ATLANTIC RIDGE. HOWEVER, LATEST RUNS
OF THESE MODELS PUSH THE SYSTEM FURTHER WESTWARD AND BRING IT
RATHER CLOSE TO SOUTH FLORIDA (TOO CLOSE FOR COMFORT) BEFORE
RECURVING. WHILE THIS IS STILL TOO FAR IN THE FUTURE AND THE
MODELS ARE BOUND TO CHANGE MANY TIMES, IT IS NOT OUT OF THE REALM
OF POSSIBILITIES AS WE ENTER THE ACTIVE PORTION OF THE ATLANTIC
HURRICANE SEASON, AND IT BEARS WATCHING.

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Dry air and shear are still going to be problems for our features in the central Atlantic, but the GFS and NOGAPS relax the shear a bit by 120-168 hours across the eastern Caribbean and north of the islands.

Since those waves are weak, they'll likely continue westward with the low-level flow, and encounter destructive shear in the Caribbean (even though I mentioned the models easing the shear to the north and in the extreme eastern Caribbean, even if that does verify, shear will still only be marginally favorable at best, and will be largely westerly, precluding any significant development of these waves).

I'm still not sold on tropical cyclogenesis, at least not in the next 5-7 days. When they get closer to home, who knows what could happen. They are certainly some very vigorous features.
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Quoting BahaHurican:
Morning everybody.

Doc, why doesn't your historical map show this hurricane?



I would have thought it fit the parameters u outlined in the caption. This hurricane is not as well known as its later companion, which struck Miami, but it did extensive damage to Nassau, the capital of The Bahamas, and as a result is relatively well remembered here.


because that storm diffuses the argument that July has few high impact storms lol, so it was left out lmao
is carlos still a cane?
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Quoting btwntx08:

it doesn't look i that at all its perffectly organized


again its not, the NHC tells how assymetric the system is, 40mph TS for now is a good call

certainly not a Cat 2 lol
Morning everybody.

Doc, why doesn't your historical map show this hurricane?



I would have thought it fit the parameters u outlined in the caption. This hurricane is not as well known as its later companion, which struck Miami, but it did extensive damage to Nassau, the capital of The Bahamas, and as a result is relatively well remembered here.
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Quoting NEwxguy:
month long steering pattern? The east coast trough has been around almost 2 months,hard to believe its going to stay this way through the entire summer.I use the word summer loosely,at least here in the northeast.


It's been cool here in the Midwest too! I can't remember having a 41 Dewpoint like we did yesterday here in Columbus in the middle of July...
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He said "...Carolina waters..."

Ggggrrrr....
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month long steering pattern? The east coast trough has been around almost 2 months,hard to believe its going to stay this way through the entire summer.I use the word summer loosely,at least here in the northeast.
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Quoting btwntx08:
does this look like a 40 mph storm
look like a cat 2
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thanks doc for update

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Quoting btwntx08:
they revised the quake from 7.9 to 7.6 still pretty strong quake in new zeland

Main Quake
Magnitude 7.6 - OFF WEST COAST OF THE SOUTH ISLAND, N.Z.
Location: 45.725S, 166.681E
Depth: 35 km

Aftershock

Magnitude 5.8 - OFF WEST COAST OF THE SOUTH ISLAND, N.Z.
Location: 45.991S, 166.238E
Depth: 35 km

Aftershock

Magnitude 5.3 - OFF WEST COAST OF THE SOUTH ISLAND, N.Z.
Location: 45.380S, 166.803E
Depth 35 km
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Quoting btwntx08:
does this look like a 40 mph storm


Can you make out a tiny dot in the center.
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Quoting btwntx08:
does this look like a 40 mph storm


yes it does, center is displaced from the convection for the most part

looks can be deceiving
Thanks for the update.
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Let's play a little game called find Carlos the hurricane... What...you mean that's the minimal tropical storm???

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15. IKE
12Z NAM...troughs in the east.
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14. WAHA
Quoting chays:
yeah, and 2004 wasn't exactly a walk in the park for us Floridians....

I am a floridian, and I got to say I was pretty freaked out on charley, could hace hit us as cat 4
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Blog Update
Reflector site for those at work, which includes Weather456, daily updates


AOI

AOI

Member Since: October 1, 2007 Posts: 81 Comments: 26516
Thank You for the update Dr. Masters
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11. IKE
He gives a 30% chance of a July system. I'll go with the 70% and say zilch.
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yeah, and 2004 wasn't exactly a walk in the park for us Floridians....
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From Dr. Masters... 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.


Great news...and with El Nino establishing itself....2009 may be a safer one along the gulf coast.

138 days left and it's over.
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Thanks Doc.
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Thank you for the update!
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Link

http://earthquake.usgs.gov/eqcenter/recenteqsww/Quakes/us2009jcap.html

So we can relax until the end of July...Thanks, Dr. Masters!
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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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