WunderBlog Archive » Category 6™
Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.August tropical outlook
A well defined surface circulation has developed about 200 miles southeast of the North Carolina/South Carolina border this morning. Heavy thunderstorm activity is limited to the south side of this system, due to 10 knots of wind shear from northerly upper level winds. However, long range radar out of Wilmington, NC shows some impressive echoes and low level rain bands forming, and I imagine NHC will send out a Hurricane Hunter airplane this afternoon to see if a tropical depression has formed.
The computer models are forecasting that this system will not recurve out to sea, as the trough of low pressure that had been pulling this system to the north is now exiting the East Coast. High pressure is building in, which should force the system towards the coast over the next few days. Steering currents are weak, and the track of this system is very uncertain. The storm may go ashore over South Carolina (as favored by the GFDL model), northern Florida (as favored by the NOGAPS model), or perhaps North Carolina or Georgia. Wind shear is expected to increase significantly on Thursday from the north to the south, so this system lilkely does not have long to live.
Figure 1. Current long range radar out of Wilmington, NC.
Figure 2. Preliminary models tracks for the East Coast disturbance.
Eleswhere in the tropics
The big burst of thunderstorms that developed in the Gulf of Mexico last night is gone, and no development is likely there today. A rotating area of clouds a few hundred miles northeast of the northernmost Lesser Antilles is associated with an upper level low pressure system. This is not expected to develop.
August hurricane outlook
What a difference a year makes! By this date in 2005, we were already up to Hurricane Irene, the 9th named storm of the season. Of those nine, four were hurricanes, and two (Dennis and Emily) were record-breaking Category 4 and 5 hurricanes. However, before we congratulate ourselves too much on a safe start to hurricane season, it is instructive to look at the plot of typical hurricane activity for the Atlantic (Figure 3). Peak hurricane season starts about August 18 and runs through October 18. The worst part of hurricane season is in front of us, and I do anticipate that conditions will get active. Witness 1998, when only one named storm occurred prior to August 19, and 10 named storms and 7 hurricanes formed by the end of September. A similar pattern of activity occurred in 2000, with only two named storm by this date, and a season total of 15 named storms. So, those of you who doubt NOAA and Dr. Gray's predictions of 15 named storms this season need to put your skepticism on hold.
Figure 3. Climatological Atlantic hurricane and tropical storm activity.
A major shift in the atmospheric pattern over the Atlantic began at the end of July, and portends an active hurricane season. June and July were characterized by a much stronger than normal Bermuda High, with surface pressures up to 7 mb higher than normal over the Atlantic. Taking a look at the surface pressures the past ten days (Figure 4), we see that surface pressures are now up to 7 mb lower than normal over much of the Atlantic, a complete reversal of the situation in June and July. Lower surface pressures are more conducive for hurricane formation, and drive weaker trade winds. Weaker trade winds mean less evaporative cooling of the ocean, allowing the ocean to heat up more than usual.
Figure 4. Sea level pressure (top) and departure from normal (bottom) for the 10 days ending August 12, 2006. Image credit: NOAA Climate Prediction Center.
We can see that the tropical Atlantic has warmed considerably, relative to normal, between July 28 (Figure 5) and August 12 (Figure 6). The Caribbean has warmed about 1/2 a degree C, relative to normal, and the blue pool of cooler than normal waters near the Bahamas has shrunk. The 2-week forecast from the GFS model predicts a continuation of these conditions, and I expect that the ocean will continue to warm to much above normal levels through September (although not as warm as 2005).
Figure 5. Sea surface temperature departure from normal for July 28, 2006.
Figure 6. Sea surface temperature departure from normal for August 12, 2006.
Wind shear
Wind shear was higher than normal in June and July, and has decreased to near normal levels since August 1. The exception is the region from the Bahamas north along the U.S. East Coast, which has still seen higher than average wind shear, due to the presence of strong upper-level low pressure systems. The 2-week GFS forecast continues to call for strong upper-level low pressure systems to roam the waters of the Atlantic, bringing hostile wind shear to any budding tropical systems that venture near. However, the wind shear averaged over the entire tropical Atlantic is expected to be near normal, and I expect that some systems will begin finding "holes" in the shear and manage to develop during the remainder of August.
Dry air
Outbreaks of Saharan dust and associated dry air have been common this year. The dust acts to reduce the amount of sunlight reaching the ocean, keeping SSTs cool. The dust and dry air also interfere with the formation process of tropical storms. The Saharan dust season peaks in June and July, and should begin a slow decline the remainder of hurricane season. Plots of relative humidity from the latest 2-week GFS forecast support this idea.
Vertical instability
Another important ingredient for tropical storm formation which I haven't talked much about is vertical instability. Simply put, if the air near the surface is very warm and the air at high altitudes is very cold, this is an unstable atmosphere. When air is unstable, thunderstorm and tropical storm activity is enhanced. This occurs because in an instable atmosphere, air from the surface can rise further and faster than air in a stable atmosphere. Rising air pulls up the moist air from the surface to colder regions aloft, where the moisture can condense and fall as rain. Since warm air is less dense than cold air, rising air in an unstable atmosphere finds itself less dense than its surroundings, since it started out very warm to begin with. Thus, the air will continue to rise, until it reaches a region of the atmosphere where the stability is high. In tropical cyclones, this often happens at about 50,000 feet--the beginning of the Stratosphere, a very stable layer of air where temperature increases with height.
Instability over the tropics during the 2006 hurricane season (Figure 7) has been below average. The ocean temperatures have been close to normal, which keeps the atmosphere more stable. Also, the general atmospheric ciculation has brought more stable air into the tropical Atlantic than we saw in 2005. However, with the weak trade winds we've been seeing this August allowing the oceans to heat up to much above normal, I expect that instability will increase to near normal levels by the beginning of September, enhancing hurricane formation.
Figure 7. Vertical instability in 2006 (blue lines) compared to normal (black lines) for the Eastern Caribbean (left) and tropical Atlantic (right). Image credit: CIRA (Cooperative Institute for Research in the Atmosphere).
Steering pattern
The large scale jet stream pattern and associated positioning of the Bermuda High has remained unchanged since early June, and is forecast to remain the same into early September. This pattern puts a trough of low pressure over the U.S. East Coast, which will act to recurve storms approaching the U.S. I expect that many of the strongest hurricanes we will see this year will recurve harmlessly out to sea, perhaps threatening only Bermuda. However, some systems may not recurve in time, which puts the U.S. East Coast at higher risk than average for a hurricane strike. North and South Carolina have the highest risk of any region of the U.S., since they stick out farthest into the ocean. This steering pattern also favors higher than normal hurricane activity in the northernmost Lesser Antilles Islands, such as the Virgin Islands, Anguilla, and St. Maarten. The U.S. Gulf Coast has a lower than average chance of a hurricane strike with this steering pattern. Given that this steering pattern has held for so long, the odds are that it will remain in place through at least mid-September.
Summary
The relatively quiet hurricane season we've been enjoying is not going to last. A very active period will start, as soon as the atmosphere destabilizes a bit more. If one believes the long-range 2-week outlook from the GFS model, the current quiet period should last another 4-12 days. Around August 21, I expect it will appear that a switch has been thrown, and the Atlantic will be very active indeed. Expect our first hurricane in the Atlantic by August 26, and a very active September. However, I do expect we will get many recurving storms that will miss land, and that this hurricane season will be similar to the ones we experienced in 1995-2003.
Jeff Masters
The computer models are forecasting that this system will not recurve out to sea, as the trough of low pressure that had been pulling this system to the north is now exiting the East Coast. High pressure is building in, which should force the system towards the coast over the next few days. Steering currents are weak, and the track of this system is very uncertain. The storm may go ashore over South Carolina (as favored by the GFDL model), northern Florida (as favored by the NOGAPS model), or perhaps North Carolina or Georgia. Wind shear is expected to increase significantly on Thursday from the north to the south, so this system lilkely does not have long to live.
Figure 1. Current long range radar out of Wilmington, NC.
Figure 2. Preliminary models tracks for the East Coast disturbance.
Eleswhere in the tropics
The big burst of thunderstorms that developed in the Gulf of Mexico last night is gone, and no development is likely there today. A rotating area of clouds a few hundred miles northeast of the northernmost Lesser Antilles is associated with an upper level low pressure system. This is not expected to develop.
August hurricane outlook
What a difference a year makes! By this date in 2005, we were already up to Hurricane Irene, the 9th named storm of the season. Of those nine, four were hurricanes, and two (Dennis and Emily) were record-breaking Category 4 and 5 hurricanes. However, before we congratulate ourselves too much on a safe start to hurricane season, it is instructive to look at the plot of typical hurricane activity for the Atlantic (Figure 3). Peak hurricane season starts about August 18 and runs through October 18. The worst part of hurricane season is in front of us, and I do anticipate that conditions will get active. Witness 1998, when only one named storm occurred prior to August 19, and 10 named storms and 7 hurricanes formed by the end of September. A similar pattern of activity occurred in 2000, with only two named storm by this date, and a season total of 15 named storms. So, those of you who doubt NOAA and Dr. Gray's predictions of 15 named storms this season need to put your skepticism on hold.
Figure 3. Climatological Atlantic hurricane and tropical storm activity.
A major shift in the atmospheric pattern over the Atlantic began at the end of July, and portends an active hurricane season. June and July were characterized by a much stronger than normal Bermuda High, with surface pressures up to 7 mb higher than normal over the Atlantic. Taking a look at the surface pressures the past ten days (Figure 4), we see that surface pressures are now up to 7 mb lower than normal over much of the Atlantic, a complete reversal of the situation in June and July. Lower surface pressures are more conducive for hurricane formation, and drive weaker trade winds. Weaker trade winds mean less evaporative cooling of the ocean, allowing the ocean to heat up more than usual.
Figure 4. Sea level pressure (top) and departure from normal (bottom) for the 10 days ending August 12, 2006. Image credit: NOAA Climate Prediction Center.
We can see that the tropical Atlantic has warmed considerably, relative to normal, between July 28 (Figure 5) and August 12 (Figure 6). The Caribbean has warmed about 1/2 a degree C, relative to normal, and the blue pool of cooler than normal waters near the Bahamas has shrunk. The 2-week forecast from the GFS model predicts a continuation of these conditions, and I expect that the ocean will continue to warm to much above normal levels through September (although not as warm as 2005).
Figure 5. Sea surface temperature departure from normal for July 28, 2006.
Figure 6. Sea surface temperature departure from normal for August 12, 2006.
Wind shear
Wind shear was higher than normal in June and July, and has decreased to near normal levels since August 1. The exception is the region from the Bahamas north along the U.S. East Coast, which has still seen higher than average wind shear, due to the presence of strong upper-level low pressure systems. The 2-week GFS forecast continues to call for strong upper-level low pressure systems to roam the waters of the Atlantic, bringing hostile wind shear to any budding tropical systems that venture near. However, the wind shear averaged over the entire tropical Atlantic is expected to be near normal, and I expect that some systems will begin finding "holes" in the shear and manage to develop during the remainder of August.
Dry air
Outbreaks of Saharan dust and associated dry air have been common this year. The dust acts to reduce the amount of sunlight reaching the ocean, keeping SSTs cool. The dust and dry air also interfere with the formation process of tropical storms. The Saharan dust season peaks in June and July, and should begin a slow decline the remainder of hurricane season. Plots of relative humidity from the latest 2-week GFS forecast support this idea.
Vertical instability
Another important ingredient for tropical storm formation which I haven't talked much about is vertical instability. Simply put, if the air near the surface is very warm and the air at high altitudes is very cold, this is an unstable atmosphere. When air is unstable, thunderstorm and tropical storm activity is enhanced. This occurs because in an instable atmosphere, air from the surface can rise further and faster than air in a stable atmosphere. Rising air pulls up the moist air from the surface to colder regions aloft, where the moisture can condense and fall as rain. Since warm air is less dense than cold air, rising air in an unstable atmosphere finds itself less dense than its surroundings, since it started out very warm to begin with. Thus, the air will continue to rise, until it reaches a region of the atmosphere where the stability is high. In tropical cyclones, this often happens at about 50,000 feet--the beginning of the Stratosphere, a very stable layer of air where temperature increases with height.
Instability over the tropics during the 2006 hurricane season (Figure 7) has been below average. The ocean temperatures have been close to normal, which keeps the atmosphere more stable. Also, the general atmospheric ciculation has brought more stable air into the tropical Atlantic than we saw in 2005. However, with the weak trade winds we've been seeing this August allowing the oceans to heat up to much above normal, I expect that instability will increase to near normal levels by the beginning of September, enhancing hurricane formation.
Figure 7. Vertical instability in 2006 (blue lines) compared to normal (black lines) for the Eastern Caribbean (left) and tropical Atlantic (right). Image credit: CIRA (Cooperative Institute for Research in the Atmosphere).
Steering pattern
The large scale jet stream pattern and associated positioning of the Bermuda High has remained unchanged since early June, and is forecast to remain the same into early September. This pattern puts a trough of low pressure over the U.S. East Coast, which will act to recurve storms approaching the U.S. I expect that many of the strongest hurricanes we will see this year will recurve harmlessly out to sea, perhaps threatening only Bermuda. However, some systems may not recurve in time, which puts the U.S. East Coast at higher risk than average for a hurricane strike. North and South Carolina have the highest risk of any region of the U.S., since they stick out farthest into the ocean. This steering pattern also favors higher than normal hurricane activity in the northernmost Lesser Antilles Islands, such as the Virgin Islands, Anguilla, and St. Maarten. The U.S. Gulf Coast has a lower than average chance of a hurricane strike with this steering pattern. Given that this steering pattern has held for so long, the odds are that it will remain in place through at least mid-September.
Summary
The relatively quiet hurricane season we've been enjoying is not going to last. A very active period will start, as soon as the atmosphere destabilizes a bit more. If one believes the long-range 2-week outlook from the GFS model, the current quiet period should last another 4-12 days. Around August 21, I expect it will appear that a switch has been thrown, and the Atlantic will be very active indeed. Expect our first hurricane in the Atlantic by August 26, and a very active September. However, I do expect we will get many recurving storms that will miss land, and that this hurricane season will be similar to the ones we experienced in 1995-2003.
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.