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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What everyone needs to remember about South Florida is this.

Since we have had no tropical threats since 05, people here have become complacent and most of them are not ready when it comes to a storm. So if anything does come through here this year, its going to be devastating. On top of that look at all the new construction projects up and going, plus there are still blue roofs here as well. So if a storm does threaten us or come near South Florida, Its going to get nuts in a quick minute.
Member Since: Posts: Comments:
What will hit South Florida in about 2 weeks W.S.and why? I am still trying to learn. Thank you in advance.
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Quoting nrtiwlnvragn:


Go to your blog page, on the right hand side "Modify My Profile", set the default filter level.


Thank you!
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Quoting Funkadelic:


yes sir I did so what does that mean for florida? It seem as if it develops it will move south of the sunshine state fortunately!


Hey now..lol. as long as it miss the great state of texas
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1196. JRRP
if the wave is moving faster then development will be slow
Member Since: August 16, 2007 Posts: 0 Comments: 6823
So...we have Code Yaller?
bbl
Member Since: July 11, 2006 Posts: 14 Comments: 11501
Quoting GeoffreyWPB:
My filter is acting up tonight also. What was the fix?


Go to your blog page, on the right hand side "Modify My Profile", set the default filter level.
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Hey chuck any this near the moss would i be alright to go get dinner.
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Where is anyone getting any idea that their specific area is safe from a hit this season?
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My filter is acting up tonight also. What was the fix?
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Quoting Funkadelic:
At least florida is protected with that steering out there...

See post 1152
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1187. IKE
Shear is rather low if it continues near 270 degrees movement...

Member Since: June 9, 2005 Posts: 23 Comments: 37890
The most I can see right now is an invest heading west. For now, I do agree with the NHC, slow development. But development and intensity are not as easy as track.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Whats the thoughts on the wave that just rolled off of Africa being able to enhance itself based on the moisture field that the other two waves have created? Should we focus on that wave instead, as the dry air and shear will be effective against the two but possibly not the newest wave?
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Hey Chuck! yea...they've gotten hammered...
Member Since: August 13, 2007 Posts: 0 Comments: 10533
Quoting wunderkidcayman:

I just hope its not another Ivan or Charly
Don't even think it and don't start worrying already. Don't know if it will become anything yet.
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Quoting WeatherStudent:


Chill man, I didn't mean it that way. I was just wondering why you had mentioned my name in one of your previously written posts, that's all. :)


It's cool. Thought you were tripping..
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Quoting stormpetrol:
Funny but we know whats on our minds, please don't come here to hurricanes. 1146. Weather456 , Thanks.
You said it. Since Sept 2004 I think we have had our fair share of hits and near hits. I could do without anything stronger than normal thunderstorms.
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The speed of the tropical wave is indicative of the ridge's strength.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Evening everyone.

Just watching the blob...
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Quoting stormwatcherCI:
This is what I am trying to find out. The way this looks I think Cayman and all things Caribbean need to watch closely including 456.

I just hope its not another Ivan or Charly
Member Since: June 13, 2009 Posts: 2 Comments: 14972
Quoting StormW:
PSU-eWALL STEERING FCST MAPS


Lol, that was my next choice.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
1173. IKE
Quoting Chucktown:
C'mon people, settle down. It's a tropical wave. I know there are a lot of new people on the blog this year, but its got over a week before it even possibly effects anyone. Cape Verde storms rarely make it across the Atlantic in July and I really don't think this one is going to be an exception.


It's moving 360 miles a day...it's about 1700 miles east of the islands. It could reach them by Tuesday at the rate it's moving now.

I'm not saying it will be a TD or TS though.
Member Since: June 9, 2005 Posts: 23 Comments: 37890
1172. WxLogic
Quoting Weather456:
Let's say it's maximum potential is a cat 1 hurricane, even at that intensity it is bound for west



If it stays the way it is current... as a strong open wave or if it even acquires INVEST status it could be troublesome for the Carib due to the hi octane found in the area as you guys already know. Also looking at the latest TPW Mimic... this way is paving the way for any subsequent Tropical waves... All that will prevent this system from being upgraded any further would be shear and any unforeseen entrainment of dry air.
Member Since: August 14, 2008 Posts: 4 Comments: 5170
Quoting stormpetrol:
Funny but we know whats on our minds, please don't come here to hurricanes. 1146. Weather456 , Thanks.
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1169. STXpat
re:1150

A.E. Housman,"The Oracle" a shropshire lad
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1167. IKE
Quoting presslord:
IKE

#7...What is dmax?


or dmin.

NHC has the system moving WESTWARD ABOUT
15 MPH.


I wouldn't be surprised to see it speed up more...18-20 mph...

Looks like it's moving a little faster then it was earlier.
Member Since: June 9, 2005 Posts: 23 Comments: 37890
also to watch would be the next wave off the coast of Africa. Shows some signs of feeding off of the moisture trail from the other two.
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Hey there Press. Some good storms over in Berkeley county tonight. We may still see some here later.
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Quoting wunderkidcayman:


GUYS YOU NEED TO LOOK AT THIS
Link
This is what I am trying to find out. The way this looks I think Cayman and all things Caribbean need to watch closely including 456.
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Quoting WeatherStudent:


Me what? Excuse me? :)


Why dont you calm down. I was saying you already told that guy good job... GEEZZZ!
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Quoting stormwatcherCI:
W456 if this does develop can you please tell me the most likely track it will take.
Funny but we know whats on our minds, please don't come here to hurricanes. 1146. Weather456 , Thanks.
Member Since: April 29, 2007 Posts: 0 Comments: 8535
Another interesting obs is that despite the models not forecasting development, all forecast the wave to reach the islands, with little recuvature. Just covering all angles - bound for west regardless of development that is.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
C'mon people, settle down. It's a tropical wave. I know there are a lot of new people on the blog this year, but its got over a week before it even possibly effects anyone. Cape Verde storms rarely make it across the Atlantic in July and I really don't think this one is going to be an exception.
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RGB seems to hit on the second system, not the first one. If thats true, it might be feeding off the remnants of the first.
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Quoting presslord:
IKE

#7...What is dmax?


If it comes in pill form...I'll take it! LOL
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IKE

#7...What is dmax?
Member Since: August 13, 2007 Posts: 0 Comments: 10533

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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