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 NRAamy:
I'm like a momma bear when people mess with WS....other than that, he's just a good kid trying to learn, and I appreciate that...

:)
*buzz sound* wrong answer
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I'm like a momma bear when people mess with WS....other than that, he's just a good kid trying to learn, and I appreciate that...

:)
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JFV and NRA have bonded! It's good to have friends and supporters on here!
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Quoting GeoffreyWPB:
WS%u2026%u2026Why do you always make a statement%u2026then follow it up by asking a question of affirmation?
Insecurity, don't you think, sir chief ?
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whats see if it gets 95L 1st be for we say will it survives
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Well theres a whole lot of nuttin goin own.
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943. WeatherStudent 3:39 PM PDT on July 16, 2009
Quoting GeoffreyWPB:
WS……Why do you always make a statement…then follow it up by asking a question of affirmation?


Well, that's the nature of this beast, what can I say. :) Good evening, Ikster!




rock on, WS!!!!!!!!!

:)
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Hate to put you on the spot 456..but % wise, how do see the TW in question actually crossing the Atlantic unscathed? And do you agree with the current models it will be nothing more than it is now, if it survives?

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Quoting SLU:
good day folks

456/stormfury .. this 30W wave looks great man. You think we may have 95L soon? What are your thoughts on the future track of this disturbance? The GFS has it reaching the Windwards on Monday, Tuesday.


I think an invest is possible if the feature persists over the upcoming days. It seems to head west, most likely location, Central-Northern Antilles. Otherwise, the axis of the wave could extend along the island chain.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting WeatherStudent:


That's a massive ridge, right Ikster?




why not find that out on your own then haveing some one ask that for you all you this have too do is look at the map
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Negative NAO continuing into the final weeks of July. If we have a system out there I don't see much recurvature.

1 week



Member Since: July 24, 2005 Posts: 407 Comments: 19076
941. SLU
good day folks

456/stormfury .. this 30W wave looks great man. You think we may have 95L soon? What are your thoughts on the future track of this disturbance? The GFS has it reaching the Windwards on Monday, Tuesday.
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North, go North and eat some dust!
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Quoting IKE:


Trough comes down and a ULL splits off of it....look east of where it splits...maybe something forms...


Makes more sense. thanks alot
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938. IKE
Quoting LPStormspotter:


Ok Ike for those of us not real hip on what we are seeing.. Can you get a little more in depth?


Trough comes down and a ULL splits off of it....look east of where it splits...maybe something forms...
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Quoting GeoffreyWPB:
WS……Why do you always make a statement…then follow it up by asking a question of affirmation?


lmao
Member Since: July 17, 2008 Posts: 0 Comments: 385
WS……Why do you always make a statement…then follow it up by asking a question of affirmation?
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CMC shows development of the wave later next week.

L
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting IKE:
18Z GFS through 150 hours @ 200 mb's...trough split in the GOM...


Ok Ike for those of us not real hip on what we are seeing.. Can you get a little more in depth?
Member Since: July 17, 2008 Posts: 0 Comments: 385
932. IKE
Quoting NRAamy:
hey IKE.....hey, what happened to Jerry Garcia? haven't seen him around....


Haven't seen him around in the last few weeks. He was on here in June.
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Quoting stoormfury:
if the EATL disturbance were to develop then i cannot see it heading for the bahamas. the strong BH/AZORES high will keep it on a west track thru the central and northern islands

That is exactly what the CMC predicts.
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Quoting stoormfury:
if the EATL disturbance were to develop then i cannot see it heading for the bahamas. the strong BH/AZORES high will keep it on a west track thru the central and northern islands


true, but it seems is pull north by some kind of weakeness near 70-80W. I would not discount a Fay-like track if it develops.
Member Since: July 24, 2005 Posts: 407 Comments: 19076


Isn't that something like all of the human-generated electricity worldwide for a year?
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Quoting Weather456:


Evening, that the system should be watched.

The CMC is bringing it into the Caribbean.
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hey IKE.....hey, what happened to Jerry Garcia? haven't seen him around....
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if the EATL disturbance were to develop then i cannot see it heading for the bahamas. the strong BH/AZORES high will keep it on a west track thru the central and northern islands
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Quoting WeatherStudent:
What might this intail, 456? Good evening by the way.


Evening, that the system should be watched.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
hey WeatherStudent....

:)
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Also if you are going to try and tame hurricanes, there wouldnt be as much need for this website now would there?
Hummm, some fish can walk on land :) L8R

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The energy that hurricanes transfer to the mid-latitudes are unquantified but is spread over a large area and thus taming tropical cyclones could bear consequences. However, these consequences are questionable since much of the heat transfer is done by the Hadley Cell and ocean currents. Nevertheless, leave the damn systems alone. Humans must learn to live in harmony with nature and not try to always seek to control things.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting sebastianflorida:
That my friend is not under consideration at this time; if the circle happens at two 8 tonight I would be shocked. Eventually something will develop in the tropics, but not on our wishcasting, these things takes time to develop, and mostly do not!


I understand. But some on here enjoy the bashing. It’s a blog not the NHC. I personally enjoy reading everyone's opinion and just wait and see. Last year there were many that just knew Ike was going here and going there. A few had crow. Then there are the ones who actually put a lot of thought and knowledge into it and came up with at the time was a guess cause no of really knows. But they did a good job.
Member Since: July 17, 2008 Posts: 0 Comments: 385
Quoting taistelutipu:
801. / 804.

I would be very much in favour of hurricane tanning experiments. *lol*

How much higher SPF would a cat.5 need in comparison to a cat. 1?

Perhaps Bill is working on a filter that he can use in order to "tan" the MS logo on the top of a hurricane so that you could see it from a satellite...insidious!
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12Z ECMWF has the system near the Bahamas next Wednesday and seems to interact with some energy and the based of a frontal trough.

Member Since: July 24, 2005 Posts: 407 Comments: 19076
901. / 904.

I would be very much in favour of hurricane tanning experiments. *lol*

How much higher SPF would a cat.5 need in comparison to a cat. 1?
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Member Since: July 24, 2005 Posts: 407 Comments: 19076
It is not preposterous to control hurricanes WS. However, it is less convenient to control mesoscale cyclones such as tornadoes.
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Quoting LPStormspotter:


Thanks for your insight
That my friend is not under consideration at this time; if the circle happens at two 8 tonight I would be shocked. Eventually something will develop in the tropics, but not on our wishcasting, these things takes time to develop, and mostly do not!
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Go North Blob babby, go North. Be a fish!
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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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