Thread: Bi Weekly Tropical Outlooks from Dr Jeff Masters

Dr Jeff Masters does tropical outlooks twice a month during hurricane season, giving his opinion for the likelihood of tropical development during the next two weeks or so.

Here is the summary (only) for his outlook for the second half of June, 2007:

Recent history suggests a 33% chance of a named storm occurring in the second half of June. Given that the current SST pattern and two-week wind shear forecast look fairly typical for June, I'll go with a 30% chance of a named storm forming during the last half of June.

Here are excerpts from Dr. Jeff Masters' bi-monthly hurricane outlook, which was posted yesterday:

The first half of July is usually a quiet period in the Atlantic for tropical cyclone formation. Since 1995, six of 12 years have had a named storm form during the first half of July, giving a historical 50% chance of a first half of July storm...

Sea Surface Temperatures

Sea Surface Temperatures (SSTs) have cooled considerably in the past two weeks over ... the hurricane Main Development Region that extends from the coast of Africa to the coast of Central America, between 10° and 20° latitude.

Tropical Cyclone Heat Potential

It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. Hurricanes stir up water from down deep due to their high winds, so a shallow layer of warm water isn't as beneficial to a hurricane as a deep one. The Tropical Cyclone Heat Potential is a measure of this total heat content. A high TCHP over 80 is very beneficial to rapid intensification . . . (in the) Western Caribbean . . . SSTs and TCHP have stayed above average. If we do get an intense hurricane in early July, it will likely be here.

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 . . . The latest two-week forecast ... predicts that the subtropical jet stream will gradually weaken through mid-July... This should result in a greater than average chance of a named storm occurring.

Dry air and African dust

June and July are the peak months for dust coming off the coast of Africa . . . 2007 has seen very high levels of dust coming from Africa. 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

Steering currents

The steering current pattern for June featured a pattern much like we saw in 2006, with an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. The position of the Bermuda-Azores High was pretty close to average. Its strength was only 1 mb below average, driving slightly slower trade winds

Summary

Recent history suggests a 50% chance of at least one named storm occurring in the first half of July. Wind shear is expected to be below average and SSTs are near average, so I expect a 70% chance of a first half of July named storm this year.

The above is exerpts only. To read the entire entry, go to:
Wunder Blog : Weather Underground

Here is some very interesting graphical data - showing tracks and intensities of all storms that have formed in the first half of July over the last 155 years:

Here are excerpts from Dr. Jeff Masters' bi-monthly hurricane outlook for the last half of July, which was posted today:

Atlantic tropical cyclone activity is usually low during the last half of July. Since the current active hurricane period began in 1995, eight of 12 years have had one or two named storms form during the last half of July. Two . . . most of the late July activity occurs in the Gulf of Mexico, but there are a few long-track "Cape Verdes" hurricanes beginning to occur.

Sea Surface Temperatures

Sea Surface Temperature (SSTs) remained near average over the tropical Atlantic between Africa and the Lesser Antilles in July . . .However, SSTs are 0.5-1.0 °C above average over much of the Caribbean and Gulf of Mexico.

Tropical Cyclone Heat Potential

It's not just the SSTs that are important for hurricanes, it's also the total amount of heat in the ocean to a depth of about 150 meters. There is less heat energy available this year than in 2005, . . . However, this is not true in the Western Caribbean, where we have very high TCHP this year. The African dust storms have not penetrated all the way to the Western Caribbean, and SSTs and TCHP have stayed above average. In the unlikely event we get an intense hurricane in late July, it would probably be in the Western Caribbean

Wind shear

. . . a major shift the Northern Hemisphere weather pattern is expected over the next two weeks. . . The GFS model is predicting that the persistent trough of low pressure that has been over the Eastern U.S. will move off and be replaced by a ridge of high pressure about ten days from now. . . . The shear will remain high enough to discourage tropical storm formation over the coming week, but chances for a named storm will increase sharply by the beginning of August.

Dry air and African dust

2007 has seen very high levels of dust coming from Africa. This activity continued over the tropical Atlantic during the first half of July, and I expect this activity to continue for the remainder of July. This . . . With the coming Northern Hemisphere weather pattern shift, it is possible that the dry air coming off of the Sahara will fade some at the end of the month

Steering currents

. . . (The) steering current pattern is expected to shift next week, bringing a ridge of high pressure over the Eastern U.S. and an extension of the Bermuda High westwards . . . This pattern will act to block recurvature of any tropical cyclones that might form in the last half of July. Such a pattern puts the Western Caribbean, Gulf of Mexico, and east coast of Florida at highest risk . . . that the the northern Lesser Antilles Islands, Puerto Rico, and the U.S. East Coast from the Carolinas northwards can expect reduced risk . . .

Summary

Recent history suggests a 75% chance of at least one named storm occurring in the last half of July. This July, I put the odds at 50%, due to the unfavorable conditions for the coming week. Any storms that occur this July will probably be towards the end of the month . . . The areas of highest risk are the Western Caribbean, Gulf of Mexico, and east coast of Florida.

Excerpts from the early August outlook follow.

Early August hurricane outlook
In the first half of August, Atlantic tropical cyclone activity starts to pick up. Since the current active hurricane period began in 1995, eight of 12 years have had one or more named storms form during the first half of August, including the last seven years in row. . . . . One item of comfort, though, is the fact that 2007 is definitely not a repeat of 2005--we were already up to "G" in the alphabet at this point in 2005.

. . . hurricane activity does take a bit of an upward jump around August 1, but the real action doesn't start until August 18.. . . I am still expecting an above-average hurricane season with 12-14 named storms (10-12 more, since we've already had two), with at least one major hurricane hitting the U.S. However, the decline in SSTs relative to normal over the past two months means we should have a less active season than originally thought


Sea Surface Temperatures
. . . SSTs are 0.5-1.0 °C above average over much of the Caribbean and Gulf of Mexico, so there is still good reason to expect an above-average number of tropical storms and intense hurricanes this hurricane season. The total amount of heat energy in the upper layer of the ocean (the Tropical Cyclone Heat Potential, TCHP) remains high in the Western Caribbean--near the record levels observed in 2005. However, TCHP is much lower over the rest of the tropical Atlantic, including the Gulf of Mexico.


Wind shear
Wind shear has been near normal, averaged over the tropical Atlantic during July. Wind shear is predicted to be near or below normal for the first half of August.


Dry air and African dust
. . . By early August, dust activity typically diminishes, but it is not well-understood how to forecast these dust outbreaks. We have seen a decline in the amount of dust over the past week, as one might expect . . ., I am expecting African dust activity to remain near normal through mid-August.

Steering currents
The hurricane steering pattern for the next two weeks should be near normal, with no areas at above-average risk for a hurricane strike. . . . there is no way to tell if this steering current pattern will remain in place past mid-August.

Summary
Recent history suggests a 75% chance of at least one named storm occurring in the first half of August. I'll go with climatology and forecast a 75% chance this year, as well, since SSTs, wind shear, and African dust/dry air should all be near normal. With the steering current patterns expected to be near normal, no areas can be singled out as being at higher risk than average.

Dr. Masters also says that areas to keep an eye on over the next couple of weeks (where development could possibly take place) are:

Off the coast of the Carolinas
In the NE Gulf of Mexico, near the Florida panhandle.