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 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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Gotta get up at 5 am so good night all. We'll see what tomorrow brings.
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Quoting stormwatcherCI:
After Ivan the entire island was without power. The central district had power back within a week or so. The outer districts 2 1/2 - 3 months without power. It wasn't nice at all. Hot and the mosquitoes nearly ate us alive.

wow. 3 months. Thats extreme
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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?
Member Since: July 11, 2006 Posts: 14 Comments: 11442
Quoting popartpete:
Who has links to the computer models?

Fsu models site
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1399. Sockets
Quoting Funkadelic:


Looks poorly organized, not as good as this afternoon. I dont know what you guys are seeing....?

its currently going into some dry air to its west...however it looks like it will be able to draw some of the moisture from its south to keep it "healthy"....the system has been creating some new convection
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Quoting weatherwatcher12:

I think here they didn't turn off the national grid in time and it got damaged.
After Ivan the entire island was without power. The central district had power back within a week or so. The outer districts 2 1/2 - 3 months without power. It wasn't nice at all. Hot and the mosquitoes nearly ate us alive.
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GFS and NOGAPS significantly lessen the shear in the Caribbean in 5-7 days, and the environment could become anticyclonic by that time. This is definitely something to watch, since the subtropical ridge to the north of the system is strong enough to the point where the wave will keep moving west, even if it becomes a vertically deep system.
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Who has links to the computer models?
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1395. centex
No mention than yellow circle, changes fast.
Member Since: August 10, 2007 Posts: 0 Comments: 3293
Quoting Funkadelic:


Looks poorly organized, not as good as this afternoon. I dont know what you guys are seeing....?


actually it is better organized than this afternoon, dont let the convection fool you
My bad, posted the wrong link in 1386. It's fixed now.
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Quoting wunderkidcayman:


yep and yep


Hey good call on the yellow circle this afternoon. Too bad you took c^&p for calling it!
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A bit early to compare this to Dean, Ivan, Gilbert, Allen, etc. However, I found this on youtube the other day. This is dedicated to all you "Dean" lovers or haters. Well put together vid IMO.
Link
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After D/MAX i rather suspect that the system will be labelled 95l
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Something!!!!! 1st in weeks
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Quoting stormwatcherCI:
I know. We have friends in Kingston and kept in contact. He was headed to Cayman too but we were spared pretty much. No power for about 24 hours or so.

I think here they didn't turn off the national grid in time and it got damaged.
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lol nrti :~) He might be able to, but doesn't.

Very well could have...I noticed plenty of rotation through out that whole feature before it moved off shore. Almost looked like a tornado tried to form just to the west of me. There was a hook echo, but only for about 2 frames on the radar loop. I just uploaded some lightning pics from system.
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1385. Sockets
Quoting stoormfury:
EATL DISTURBANCE GETTING BETTER ORGANISE

Link


almost looks like it is trying to draw in some of the moisture beneath it...beginning of a feeder band?
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Quoting weatherwatcher12:

Dean was really strong here. No power for weeks.
I know. We have friends in Kingston and kept in contact. He was headed to Cayman too but we were spared pretty much. No power for about 24 hours or so.
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Quoting wunderkidcayman:

some dumb idot was puting up shutters after the storm
Maybe they had damaged windows .
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Quoting weatherwatcher12:

lol. After!

Quoting hunkerdown:
as opposed to a smart idiot ?

yep and yep
Member Since: June 13, 2009 Posts: 2 Comments: 12789
Quoting wunderkidcayman:

some dumb idot was puting up shutters after the storm
as opposed to a smart idiot ?
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Quoting wunderkidcayman:

some dumb idot was puting up shutters after the storm

lol. After!
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1378. Ossqss
Quoting KEEPEROFTHEGATE:


Meteosat 4 km IR4 Floater
(CIRA Personnel Only)



Is this the stage prior to the congealing of the 3 areas referred to earlier ?
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Quoting Acemmett90:

that sucks

yeah
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Quoting weatherwatcher12:

Lol. One time I saw someone here putting up the shutters during the storm.

some dumb idot was puting up shutters after the storm
Member Since: June 13, 2009 Posts: 2 Comments: 12789
Quoting stormwatcherCI:
I hope this is not the case as Dean was heading straight for Grand Cayman and made a jog south at the last minute thankfully. Still had some decent winds and high surf. The road in East End was unpassable for a day or so until they cleared the conch shells etc. out of the way.

Dean was really strong here. No power for weeks.
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Quoting Acemmett90:

let me know when
the next three week will be iffy but will figure it out
To do it right, it should be a hurricane party. Shouldn't have to worry about the excess crowd if there a storm looming.
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Quoting stormwatcherCI:
In East End they normally rush around pulling up boats first and then shutter their homes.

Lol. One time I saw someone here putting up the shutters during the storm.
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Trough split in GOM at 72 hours on 00z Nam.
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Quoting wunderkidcayman:

some people like me put up shutters 36-30 hours before the storm
In East End they normally rush around pulling up boats first and then shutter their homes.
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1368. WxLogic
Quoting StormW:
Good night all!



Hi Storm...
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Quoting KEEPEROFTHEGATE:
not at this time
Quoting stoormfury:
the area of disturbed weather is getting better organised. convection is beginning to wrap around the circulation near 12N 31W. Looking at theinhibiting factors at this moment, the system will take a few days to organise should the system get to near 50W where conditions will be very conducive for development, then the islands will have a problem on their hands. T he track forcast by some of the Models put it near the track of Hurricane Dean in aug of 2007
I hope this is not the case as Dean was heading straight for Grand Cayman and made a jog south at the last minute thankfully. Still had some decent winds and high surf. The road in East End was unpassable for a day or so until they cleared the conch shells etc. out of the way.
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Quoting stormwatcherCI:
In Grand Cayman they don't normally shutter until it is about 12-24 hours away. They always wait until the last minute.

some people like me put up shutters 36-30 hours before the storm
Member Since: June 13, 2009 Posts: 2 Comments: 12789
1364. KEEPEROFTHEGATE (Mod)


Meteosat 4 km IR4 Floater
(CIRA Personnel Only)
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Quoting hunkerdown:
You mean you don't have yours up already...you don't have much time left before the first feeder comes through.


Really LMAO
Member Since: September 10, 2007 Posts: 0 Comments: 11619
Quoting GeoffreyWPB:


As we type...convection is down from earlier today.
Quoting GeoffreyWPB:


So I assume no shutters for the Islands or So. Fla. as of yet?
In Grand Cayman they don't normally shutter until it is about 12-24 hours away. They always wait until the last minute.
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1360. KEEPEROFTHEGATE (Mod)
Quoting GeoffreyWPB:


So I assume no shutters for the Islands or So. Fla. as of yet?
not at this time
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EATL DISTURBANCE GETTING BETTER ORGANISE

Link
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Quoting GeoffreyWPB:


So I assume no shutters for the Islands or So. Fla. as of yet?
You mean you don't have yours up already...you don't have much time left before the first feeder comes through.
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conditions are currently very unfavorable around the islands, but that could change in a few days
Quoting stoormfury:
the area of disturbed weather is getting better organised. convection is beginning to wrap around the circulation near 12N 31W. Looking at theinhibiting factors at this moment, the system will take a few days to organise should the system get to near 50W where conditions will be very conducive for development, then the islands will have a problem on their hands. T he track forcast by some of the Models put it near the track of Hurricane Dean in aug of 2007


As we type...convection is down from earlier today.
Member Since: September 10, 2007 Posts: 0 Comments: 11619
Quoting KEEPEROFTHEGATE:
1348. GeoffreyWPB 2:28 AM GMT on July 17, 2009
how many miles
is it away from the Islands?


almost 1500


So I assume no shutters for the Islands or So. Fla. as of yet?
Member Since: September 10, 2007 Posts: 0 Comments: 11619

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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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