August hurricane outlook

By: Dr. Jeff Masters , 3:02 PM GMT on July 31, 2009

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The Atlantic remains quiet today, with no threat areas to discuss and no models calling for tropical storm formation over the seven days. Not much has changed in the Atlantic since my mid-July Atlantic hurricane outlook posted two weeks ago. However, we are now at the cusp of when hurricane activity begins a steep rise (Figure 1). Early August is typically when wind shear begins a major decline, sea surface temperatures continue to rise, African dust and dry air outbreaks diminish, and the African Monsoon and Intertropical Convergence Zone (ITCZ) become quite active, spawning frequent and powerful tropical waves. These tropical waves serve as the instigators of about 85% of all major hurricanes.

Since the current active hurricane period began in 1995, ten out of fourteen years (71%) have had a named storm form during the first half of August, with an average of 1.4 named storms per year. The last nine years in a row have had a named storm form during the first half of August, but the previous four year stretch (1996 - 1999), did not have any storms form.


Figure 1. The seasonal distribution of Atlantic hurricane activity shows a steep rise at the beginning of August. Image credit: NHC.

Sea Surface Temperatures
Eighty-five percent of all major hurricanes form in the Main Development Region (MDR) of the Atlantic, from the coast of Africa to the coast of Central America, between 10° and 20° latitude. This region also spawns 60% of all weaker hurricanes and tropical storms. Sea Surface Temperature (SST) anomalies in the MDR have slowly but steadily risen during July, and now stand at a respectable 0.5°C (0.9°F) above average (Figure 2). SSTs are well below the record levels observed in 2005 and 2006, when they were up to 2°C above average over large portions of the Main Development Region. Still, there is plenty of heat energy available for strong hurricanes to form this year. The strength of the Azores-Bermuda high has been below average over the past month, driving 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 slightly below average-strength trade winds through mid-August, so SST anomalies should continue to warm during this period.


Figure 2. Sea Surface Temperature (SST) departure from average for July 30, 2009. SSTs were about 0.5°C (0.9°F) above 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 an El Niño episode. Image credit: NOAA/NESDIS

El Niño
El Niño conditions have remained steady over the tropical Eastern Pacific over the past month. Ocean temperatures in the area 5°N - 5°S, 120°W - 170°W, also called the "Niño 3.4 region", remain at 0.8°C above average, which is 0.3°C above the threshold for a weak El Niño, according to the Australian Bureau of Meteorology (Figure 3). An increase of another 0.2°C would push the current El Niño into the "moderate" category. NOAA's Climate Prediction Center issued an El Niño Advisory earlier this month, and predicts that El Niño conditions will intensify over the next few months, and last through the coming winter. The latest set of mid-July runs of the El Niño computer models are almost universally calling for El Niño conditions to remain 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. The NOAA CFS model is calling for continued above-average wind shear over most of the tropical Atlantic for the August-September-October peak part of hurricane season.


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 "Niño 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 31, 2009, SSTs in the Niño 3.4 region had risen to 0.83°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, but in August the jet stream retreats to the north, and wind shear typically falls.

Wind shear over the past month (Figure 4) has mostly been above average over the tropical Atlantic, particularly over the Caribbean. The presence of El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. However, wind shear has been slowly falling over the southern portion of the Caribbean and southern MDR over the past week, and is forecast by the GFS model to fall to near-average levels by mid-August. This should present a more favorable environment for hurricanes to form in by mid-month.


Figure 4. Departure of wind shear from average in m/s for the 1-month period ending July 27, 2009. Higher than average wind shear (blue colors) was observed over the Caribbean. The El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. Image credit: NOAA Climate Prediction Center.

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 diminish in the coming month, allowing a greater chance for African tropical waves to develop.

Steering currents
The steering current pattern has remained virtually the same all summer. A persistent trough of low pressure has remained entrenched over the Eastern U.S., bringing cool and relatively moist weather to the northeastern portion 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. It is often difficult to break a months-long steering current pattern like the current one, and it's reasonable to forecast that the current steering pattern will continue to dominate into September.

Summary
Recent history suggests a 71% chance of a named storm occurring in the first half of August. However, this is not a typical year. The ITCZ has been remarkably inactive, and there have been an unusually low number of tropical waves coming off the coast of Africa. Although SST anomalies should continue to rise and wind shear should slowly fall over the next few weeks, the computer models suggest no significant changes to the current inactive weather pattern. I'll go with a 30% chance of a named storm forming in the first half of August.

I'll have a new post on Monday.

Jeff Masters

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Quoting SQUAWK:


That depends on where you are sitting!!!!!



That would be me...helicity is pretty high in the DC area today - twisty afternoon, even with low topped t-storms...it feels like the right-front quadrant of a decaying hurricane today...
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Maybe the east coast pattern will break now?
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you mean JFVster....
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Quoting CycloneOz:
You folks are gonna freak when you see it!

All those strong, good-lookin' waves that came off of Africa in July...they detonate. It's one of the coolest things I've seen in a long, long time.

It's also really cool that they detonate in different areas...all up and down the MDR / AWDZ.

Just when you think one of them is going to get through...BOOM! Detonated! It's so wicked! :)


I think the culprit has been that semi-stationary clone forming trough off the NW African coast. It's doesn't look to be replaced this time.
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Hey StormW,
Just got done listening to the show...good work, very professional and well spoken. Kudos from a fellow mustanger!

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2004, Aug. 1: 15 named storms, 9 hurricanes.


Need I say more?

On Nov 31 we'll know what the 2009 season had in store for us.
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Quoting mikatnight:
Howdy Guys and Gals.

Looks like the doc is getting famous. Better watch out, before you know it he'll be moving to Hollywood...


Doc is already famous, and, he used to fly into Hurricanes for NHC...Probably one of the best in the business and we should be honored that he takes the time to do this and give us a forum to discuss the issues.
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Quoting quasigeostropic:


MDR is climatological. It was defined as such because of looking at a long history of development there. The issue about something not forming there right now is not on a climatological time scale.


Like he doesn't know that???? Get real.
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Quoting CycloneOz:
I'm calling it the AWDZ instead of the MDR at this time, but that's just me.

The problem I have with MDR at this time is the "key word" DEVELOPMENT.

In this region where development is supposed to occur, the exact opposite is happening. So the term MDR is misleading.

Question: Should MDR still be used as a term when development is being severly hampered?Shouldn't MDR be canned and a new term that accurately describes a region where both development and inhibiting can exist?


Good Afternoon Oz. I have enough trouble remembering the terms we have now.
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Hi Kid!!! Saw you were zinging em earlier. Good job for a sick girl.
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Quoting CycloneOz:
I'm calling it the AWDZ instead of the MDR at this time, but that's just me.

The problem I have with MDR is the "key word" DEVELOPMENT.

In this region where development is supposed to occur, the exact opposite is happening. So the term MDR is misleading.

Question: Should MDR still be used as a term when development is being severly hampered?Shouldn't MDR be canned and a new term that accurately describes a region where both development and inhibiting can exist?


MDR is climatological. It was defined as such because of looking at a long history of development there. The issue about something not forming there right now is not on a climatological time scale.
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SQUAWK!
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Howdy Guys and Gals.

Looks like the doc is getting famous. Better watch out, before you know it he'll be moving to Hollywood...
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Patience folks; from the Sun Sentinnel yesterday morning and looks like we're not the only ones reading this Blog.

By this time, next month, we will probably be talking about how many more "to go" before the season slows down again.


Hurricane season off to a late start
Posted by Ken Kaye on July 30, 2009 05:00 AM

Not that anyone is complaining, but this hurricane season is clearly off to a slow start. It’s the end of July and the first named storm isn’t even on the horizon.

By this time last year, four named storms had emerged, including two hurricanes. By this time in chaotic 2005, seven named storms had formed, including three hurricanes.

On average, the first named storm arises on July 10 and the second on Aug. 6. The first hurricane: Aug. 14.

This year's late start isn't that unexpected.

Government and private forecasters predicted this would be a slower than normal season thanks to cooler sea surface temperatures as well as El Nino, the large-scale atmospheric condition that hampers hurricanes.

And, in general, late starts aren't that unusual.

In his blog on Wednesday, Jeff Masters, chief meteorologist of The Weather Underground, made this interesting observation:

In 10 of the past 50 years, the first named storm didn’t appear until August, or 20 percent of the time. He further noted that only two of those late seasons were busier than normal.

Here are some recent years where storms didn’t emerge until August, followed by the total number of named storms and hurricanes that year:

2004, Aug. 1: 15 named storms, 9 hurricanes.

2000, Aug. 4; 15 named storms, 8 hurricanes

1988 Aug. 7; 12 named storms, 6 hurricanes.

1987 Aug. 9; seven named storms, 3 hurricanes.

According to Masters, the latest starting hurricane season on record was in 1914, when only one storm formed on Sept. 15. (It hit near the Florida-Georgia state line.)

Boy, it would be nice if that happened this year, just one tropical storm and no hurricanes.

But don’t count on it.

Sun-Sentinel
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invest 82L.. i guess its better than nothing
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Quoting clwstmchasr:
Competely agree. Very quiet out there.


That depends on where you are sitting!!!!!

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That wave at 5W is quite impressive for over land. Looks like this could be the one that gets the season going in full swing.
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Quoting jasoniscoolman09:
big story...Wind shear over the past month (Figure 4) has mostly been above average over the tropical Atlantic, particularly over the Caribbean. The presence of El Niño conditions over the tropical Eastern Pacific may be primarily responsible for this enhanced shear. However, wind shear has been slowly falling over the southern portion of the Caribbean and southern MDR over the past week, and is forecast by the GFS model to fall to near-average levels by mid-August. This should present a more favorable environment for hurricanes to form in by mid-month.


Wow, Dr. Masters just said that in his blog, thanx for the obivious. Good work researching the new findings Jason. lol.
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NHC ran a "test" on the area in the Carribean.
Test (AL82) SHIPS

LGEM did not develop, SHIPS does but then SHIPS develops thunderstorms.

The bouy at 15N 75W just recently indicated a wind shift, but may just be temporary do to thunderstorm.
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Quoting TheCaneWhisperer:


We should see something soon with that wave sucking trough ejecting off the NW African coast.


It doesn't appear at this time to be replaced by another as did in the past. This wave at 5W may be our first REAL shot at development out in the EATL.
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ShearMap
Any chance the wave in the Caribbean will survive?
Link
I guess not.
ABNT20 KNHC 311731
TWOAT
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 PM EDT FRI JUL 31 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE NEXT 48 HOURS.

$$
FORECASTER AVILA


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Looks like we're setting up for a line of showers in Central Florida again today.
Link
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Quoting CycloneOz:
Graphs Du'Jour (ala 2009)
Just to see how wierdly off this year has been, nothing like a graph! :)




The season is flatlining! Someone get the paddles, STAT! ;)

Aww man... TheCaneWhisperer beat me to it.
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Goodness gracious...
Loop
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Quoting Weather456:


yep


We should see something soon with that wave sucking trough ejecting off the NW African coast.
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Quoting CycloneOz:
Graphs Du'Jour (ala 2009)
Just to see how wierdly off this year has been, nothing like a graph! :)





"Code Blue"

Time to get out the Paddles
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Quoting crownwx:
12Z runs so far:
GFS and Canadian models both show development in far eastern Atlantic within the next 72-96 hours. Will be interested to see the UKMET, NOGAPS and Euro models.


yep
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting KEHCharleston:
104. Weather456
Thanks 456 : )
Much Appreciated.

Now let's see if I can pull it all together. This should keep me busy for the rest of the day.

This time I will use the wave just coming off the coast of Africa
- Can track from CV to Caribbean
- And some bloggers thinking this is the one to watch

Should be fun!



You should
Tracking tropical waves are fun. I did many experiments in 1998, 2004 and 2008. Those years had the best waves ever.
Member Since: July 24, 2005 Posts: 407 Comments: 19076
12Z runs so far:
GFS and Canadian models both show development in far eastern Atlantic within the next 72-96 hours. Will be interested to see the UKMET, NOGAPS and Euro models.
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Quoting jeffs713:

They aren't accustomed to slow seasons. They look at seasons with big impacts, and they all have an above-average number of storms. 2001, 2004, 2005, 2008.. all above average.


yep that fact right there has kept me optimistic since June 1. I've track slow seasons before so 2009 has come to no surprise.

Just imagine tracking the hurricane seasons between 1991 and 1994, (I didnt but the activity was low).
Member Since: July 24, 2005 Posts: 407 Comments: 19076
Quoting Weather456:
Why do people think less storms mean less impacts?


Because more storms means more chance of an impact. If you only have 8 storms, the chances of none of them having a serious impact are greater than if you have 20 storms.

Of course that's just probability speaking...it's entirely possible to have a slow season with big impact (e.g. 1992).
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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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