WunderBlog Archive » Category 6™
Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.A normal July in the tropics?
A tropical wave about 900 miles east of the Lesser Antilles Islands has become much less organized since this morning. Strong upper-level winds just to the system's north have disrupted it, and its west-northwestward track should take it deeper into this area of high shear. I don't expect any development from this wave. There is some deep thunderstorm activity to the southwest of this wave, near 9N 52W, that is not undergoing as much shear. However, this area is disorganized, and I don't expect any development here, either.
The other area of interest today and a large area of deep thunderstorms stretching from Puerto Rico to the southeastern Bahama Islands. These showers are associated with a cold-cored upper-level low pressure system that is moving slowing west-northwestward. Any development of this area will be slow, due to wind shear, high surface pressures, and dry air. This disturbance should move over Florida on Wednesday, bringing heavy rain to portions of the state.
Figure 1. Latest satellite image of the tropical wave approaching the Lesser Antilles Islands.
Figure 1. Model forecast tracks for the tropical wave approaching the Lesser Antilles Islands.
A normal July?
Will July 2006 be a normal July for the Atlantic hurricane season? First, we need to look at what constitutes a normal July. As we can see from the Atlantic hurricane frequency graph in Figure 3, July is typically a very quiet month, almost as quiet as June is. It's usually not until the second week of August that hurricane season really starts to heat up. Since the current active Atlantic hurricane period began in 1995, there have been an average of about 1.5 named storms and 0.8 hurricanes per July. It's common to go several years in a row without getting a July tropical storm, as happened in 1999-2001. Last year's five named storms in July--including three hurricanes, two of them major hurricanes--was definitely an extremely unusual year. By this time last year, we were already on our fourth named storm, and Category 4 Hurricane Dennis was churning its way through the Caribbean.
Figure 3. The curve of historical normal hurricane activity of the Atlantic Ocean.
Why is July usually so quiet?
Sea Surface Temperatures right now (Figure 4) are warm enough to support tropical storm formation throughout the Gulf of Mexico, Bahamas, and the Main Development Region (MDR) of the Atlantic (the MDR is the region between 10N and 20N, and includes the Caribbean Sea). The only part of the MDR still too cool to support tropical storms is the far eastern Atlantic off the coast of Africa. SSTs generally do not get warm enough to support the classic intense "Cape Varde" hurricanes that form off the coast of Africa until mid-August in this region. Thus, we should not expect any Cape Verdes type hurricanes until August.
Figure 4. Atlantic SSTs for July 4, 2006. Blue colors represent SSTs colder than 26 C (80 F), which are too cool to support tropical storm formation.
Since SSTs are not the limiting factor, our old friend wind shear must be the answer! Let's look at wind shear in the eastern Caribbean, where both Hurricane Dennis and Hurricane Emily intensified into major hurricanes last year. As we can see from Figure 5, wind shear was very low in this region in 2005, and has been near normal or even above normal in 2006. Wind shear is expected to remain near average or above average across the the entire tropical Atlantic for the remainder of this week. While it is possible a tropical storm could form in a "hole" in the wind shear, the chances of it being able to stay together for an extended period and grow into a hurricane are low. So, it appears that the first half of July is shaping up to be a very normal one in the tropics.
What about the last half of July?
The two-week forecast from the GFS computer model has been consistently predicting a steady reduction in the amount of wind shear over the tropical Atlantic for the week of July 15 - July 23. Thus, many more "holes" in the wind shear will be opening up, potentially allowing tropical storms to form. I'll stick with my prediction I made at the end of June that we'll see one or two named storms in July, one of which may be a hurricane (but not a major hurricane). The forecast pressure pattern for the rest of July continues to show a weakness in the Bermuda High near the U.S. East Coast. This favors an above-normal chance of strikes on the U.S. East Coast, and a below-normal chance for the Gulf Coast, for storms forming in the Caribbean or near the Bahama Islands.
Figure 5. Wind shear over the Eastern Caribbean in 2005 (red line) and 2006 (blue line) compared to normal (black line). Wind shear is computed as the difference in wind between the upper atmosphere (200 mb pressure, about 40,000 feet high) and lower atmosphere (850 mb pressure, about 5,000 feet high). Image credit: CIRA
Jeff Masters
The other area of interest today and a large area of deep thunderstorms stretching from Puerto Rico to the southeastern Bahama Islands. These showers are associated with a cold-cored upper-level low pressure system that is moving slowing west-northwestward. Any development of this area will be slow, due to wind shear, high surface pressures, and dry air. This disturbance should move over Florida on Wednesday, bringing heavy rain to portions of the state.
Figure 1. Latest satellite image of the tropical wave approaching the Lesser Antilles Islands.
Figure 1. Model forecast tracks for the tropical wave approaching the Lesser Antilles Islands.
A normal July?
Will July 2006 be a normal July for the Atlantic hurricane season? First, we need to look at what constitutes a normal July. As we can see from the Atlantic hurricane frequency graph in Figure 3, July is typically a very quiet month, almost as quiet as June is. It's usually not until the second week of August that hurricane season really starts to heat up. Since the current active Atlantic hurricane period began in 1995, there have been an average of about 1.5 named storms and 0.8 hurricanes per July. It's common to go several years in a row without getting a July tropical storm, as happened in 1999-2001. Last year's five named storms in July--including three hurricanes, two of them major hurricanes--was definitely an extremely unusual year. By this time last year, we were already on our fourth named storm, and Category 4 Hurricane Dennis was churning its way through the Caribbean.
Figure 3. The curve of historical normal hurricane activity of the Atlantic Ocean.
Why is July usually so quiet?
Sea Surface Temperatures right now (Figure 4) are warm enough to support tropical storm formation throughout the Gulf of Mexico, Bahamas, and the Main Development Region (MDR) of the Atlantic (the MDR is the region between 10N and 20N, and includes the Caribbean Sea). The only part of the MDR still too cool to support tropical storms is the far eastern Atlantic off the coast of Africa. SSTs generally do not get warm enough to support the classic intense "Cape Varde" hurricanes that form off the coast of Africa until mid-August in this region. Thus, we should not expect any Cape Verdes type hurricanes until August.
Figure 4. Atlantic SSTs for July 4, 2006. Blue colors represent SSTs colder than 26 C (80 F), which are too cool to support tropical storm formation.
Since SSTs are not the limiting factor, our old friend wind shear must be the answer! Let's look at wind shear in the eastern Caribbean, where both Hurricane Dennis and Hurricane Emily intensified into major hurricanes last year. As we can see from Figure 5, wind shear was very low in this region in 2005, and has been near normal or even above normal in 2006. Wind shear is expected to remain near average or above average across the the entire tropical Atlantic for the remainder of this week. While it is possible a tropical storm could form in a "hole" in the wind shear, the chances of it being able to stay together for an extended period and grow into a hurricane are low. So, it appears that the first half of July is shaping up to be a very normal one in the tropics.
What about the last half of July?
The two-week forecast from the GFS computer model has been consistently predicting a steady reduction in the amount of wind shear over the tropical Atlantic for the week of July 15 - July 23. Thus, many more "holes" in the wind shear will be opening up, potentially allowing tropical storms to form. I'll stick with my prediction I made at the end of June that we'll see one or two named storms in July, one of which may be a hurricane (but not a major hurricane). The forecast pressure pattern for the rest of July continues to show a weakness in the Bermuda High near the U.S. East Coast. This favors an above-normal chance of strikes on the U.S. East Coast, and a below-normal chance for the Gulf Coast, for storms forming in the Caribbean or near the Bahama Islands.
Figure 5. Wind shear over the Eastern Caribbean in 2005 (red line) and 2006 (blue line) compared to normal (black line). Wind shear is computed as the difference in wind between the upper atmosphere (200 mb pressure, about 40,000 feet high) and lower atmosphere (850 mb pressure, about 5,000 feet high). Image credit: CIRA
Jeff Masters
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.