Tropical weather analysis - August 20, 2012
Gordon
Gordon continues to weaken as it approaches the Azores. As of the latest NHC advisory, the following was posted on the hurricane:
Wind: 80 mph, with higher gusts
Location: 36.7°N 25.8°W
Movement: ENE at 20 mph
Pressure: 980 mb
Category: 1 (Saffir-Simpson Hurricane Scale)
In an amusing turn of events, Gordon has moved far enough east as to be effectively out of the NHC floater range. Thus, I have had to resort to using RAMMB imagery, rather than NOAA's Satellite Services Division (SSD).
Gordon still has the cloud signature of a hurricane on satellite images, but deep convection is confined to the northern semicircle due to about 40 knots of southwesterly shear. This shear is also injecting dry air into the cyclone circulation.
Figure 1. Latest infrared satellite image of Hurricane Gordon. Image credit: RAMMB
Conditions are rapidly deteriorating on Sao Miguel and Santa Maria islands. At midnight Azores Time, the latter reported a sustained wind of 34 mph, along with southeast winds. Since that time, the barometric pressure has continued to fall as the center of Gordon approaches. Sustained hurricane force winds are not far offshore the island, based on the earlier wind report and extrapolation of the National Hurricane Center's estimated wind field.
Figure 2. Hurricane Gordon's wind field as of the 8:00 PM AST intermediate advisory from the National Hurricane Center.
Since Gordon is a fast mover, the period of hurricane force winds on the island will be very small, perhaps no more than two hours. However, wind gusts could exceed 100 mph in some of the higher elevations, particularly given Gordon's quick forward progress. As I understand it, the Azores have strict building codes in place so that wind damage is never really an issue. Also, the Azores are a volcanic archipelago, which obviously means increased elevation with height. This allows for an easy route to avoid incoming storm surge. At most, it appears that Gordon will deliver a brief period of damaging winds -- hurricane force winds on Santa Maria -- on Sao Miguel and Santa Maria Island as it accelerates toward the east-northeast. These winds will likely cause considerable damage to the power grid in these areas, and residents should anticipate some power outages. Power should be restored relatively quickly once Gordon moves away.
In agreement with the various global models, Gordon is forecast to become extratropical in about 36 hours. Given the extreme baroclinicity of the surrounding environment, I would not be surprised if this occurred somewhat sooner.
Gordon remains well-embedded within a powerful north Atlantic storm system. That pattern should result in a continued east-northeast movement over the next day or so, followed by a turn to the east after that. Gordon is forecast to dissipate by late Tuesday, though it is possible it could occur a little sooner.
It now appears unlikely that Gordon's post-tropical remnant will deliver any appreciable impact to western Europe, as it is forecast to be overtaken by a larger extratropical low near the British Isles.
5-day intensity forecast
INITIAL 08/20 0300Z 70 KT 80 MPH
12 hour 08/21 1200Z 55 KT 65 MPH
24 hour 08/22 0000Z 45 KT 50 MPH
36 hour 08/22 1200Z 35 KT 40 MPH...EXTRATROPICAL
48 hour 08/23 0000Z 30 KT 35 MPH...EXTRATROPICAL
72 hour 08/24 0000Z...DISSIPATED
5-day forecast track
Figure 3. My 5-day forecast track for Gordon.
Watches and warnings
A HURRICANE WARNING IS IN EFFECT FOR...
* THE EASTERN AZORES
A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA. PREPARATIONS TO PROTECT LIFE AND
PROPERTY SHOULD HAVE BEEN COMPLETED.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
Invest 94L
A well-defined tropical wave located in the central Atlantic about 1300 miles east of the Lesser Antilles ("94L") continues to show signs of organization, and is a threat to become a tropical depression or tropical storm over the next day or two. Should it attain storm status, it would receive the name "Isaac". Buoy data in the area shows a broad area of cyclonic turning is accompanying the wave at the surface. Nighttime satellite imagery suggests two vorticity maximums -- one near 15.5N 42.7W, and another farther south, near 15.0N 41.5W. The latter feature appears to be becoming more dominant, and this is what ATCF sees as well.
Figure 4. Latest infrared satellite image of Invest 94L. Image credit: NOAA
On the large scale, the environment ahead of the system is seemingly favorable, with warm SSTs and little vertical shear. However, there is some dry air to the west of the system, and I am waiting to see if 94L ingests some of this dry air as its center consolidates. So far, 94L appears to be doing well in warding off the subsident airmass; water vapor imagery and TPW data from UW-CIMSS shows that 94L is pushing the wall of dry air steadily westward. The tropical wave in front of Invest 94L may also be assisting in this endeavor. One potential negative is the area of convection lying to the south of 94L, which appears to be distinct from the tropical wave circulation. This activity appears to a direct appendage of the Intertropical Convergence Zone (ITCZ). Convection within this band continues to pulse, while 94L continues to struggle. Experience has shown that this type of setup isn't generally considered to be a conducive one for wave amplification/intensification, as the active convection acts as a completing influence to vorticity and storm relative inflow. Given this and the close proximity to dry air, I am not expecting 94L to rapidly develop overnight. I mentioned yesterday that a possibly more viable alternative is for the system to withhold on developing until west of 50W.
The long-term prospect remains highly uncertain, with the global models having a difficult time resolving the evolution of the synoptic pattern at longer ranges. At this time, I am not going to speculate, as I have enough to deal with already. In about three or four days, the models are suggesting that a trough will begin to amplify over the eastern United States that will be capable of turning the system northward. Where it turns northward remains to be seen. Alternatively, the trough could remain weak enough and shallow so that it bypasses 94L, allowing it to continue westward into the Gulf of Mexico. Given the persistent ridging pattern across the Caribbean Sea this year, I tend to favor the latter solution. The GFS has been rather insistent on moving 94L in a swift fashion similar to Ernesto and Helene, which further alludes to a continuation of that thematic pattern.
Regardless of whether this system becomes a tropical cyclone, now is a good time to remind everyone living along the United States coastline to review and replenish their hurricane preparedness plans, as 94L does have long-term potential to impact the United States anywhere from the Gulf Coast to New England.
A NOAA G-IV jet is scheduled to fly a synoptic environment surveillance mission in the path of the tropical wave on Thursday, which will be critical in resolving the model issues with the eastern US trough.
Probability of development in 48 hours: 80%
Invest 95L
An area of disturbed weather has formed over the southwestern Gulf of Mexico in association with the remnants of Helene. Satellite images show a small but well-defined cyclonic circulation centered off the coast of Mexico near 23N 97W, but convection remains minimal around this gyre. Inland and oceanic surface observations indicate that 95L possesses a rather vigorous surface circulation.
Figure 5. Latest infrared satellite image of Invest 95L. Image credit: NOAA
Moderate northwesterly upper flow along the back side of the unseasonably deep longwave trough stretching from the Texas coast to Atlantic Canada appears to be impacting the system, which is limiting development at this time. Looking at the GFS, an anticyclone is forecast to develop over the system as it moves into Mexico. Strong northerly to northwesterly flow is seen covering much of the Gulf of Mexico afterward. I theorize that this is because as 95L moves inland, it loses its anticyclone, allowing for strong shear to penetrate the Gulf waters. If 95L moves more poleward, which is a possibility given the current upper air pattern over the northern Gulf Coast, the anticyclone would likely follow it, although some residual shear would likely prevent rapid strengthening.
Based on a homogeneous comparison of the 850 and 500 mb pressure fields within the global models, as well as 0z upper air data over the northern Gulf Coast, a weaker system would tend to follow the low-level flow back into Mexico, while a stronger system with more convection would tend to move more poleward. In the meantime, the synoptic pattern is still in the process of amplifying over the Gulf -- the longwave trough previously alluded to has barely moved, while low-level ridging remains in place to the north of 95L. The combination of these features will likely result in a slow and erratic motion for the next day or so, and 95L is likely to move in fits and wobbles until a more definitive steering trend is established.
Interests along the western and northern Gulf Coasts should monitor the progress of this system.
It should be noted that even if 95L develops, it could still be rather slow moving based on prior runs of the global models that took it more poleward. This may create more of an opportunity for 95L to erode the western Atlantic ridge and allow Invest 94L to move more northward in the longer term.
Probability of development in 48 hours: 20%
Eastern Atlantic tropical wave
A tropical wave has emerged off the west coast of Africa. This wave has not yet been dubbed an invest. Upper-level winds are not currently too prohibitive, but it is possible that as 94L matures, its associated anticyclone could inflict northerly shear on the system, limiting development potential at longer ranges.
Probability of development in 48 hours: 10%
Invest 96E
A small area of low pressure located along the southwest coast of Mexico ("96E") is producing disorganized showers and thunderstorms.
Figure 6. Latest infrared satellite image of Invest 96E. Image credit: NOAA
The system is experiencing strong easterly shear, which is forecast to persist for at least the next 48 hours, and possibly longer. Surrounding steering currents are rather weak, and the system is forecast to move only slowly northwestward or northward. It could move inland over southern Baja or southwest Mexico over the next few days. Dry air also awaits it the farther north it gets. Development of this system is not currently anticipated, but it could produce locally heavy rainfall over portions of southwest Mexico over the next day or two.
Probability of development in 48 hours: 10%
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