Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 2:13 AM GMT on August 27, 2010
Hurricane Danielle continues to slowly intensify, and now has winds of 110 mph, a high-end Category 2 hurricane. Here is the information put out by the National Hurricane Center on the storm as of the 11:00 PM AST advisory:
Wind: 110 mph, with higher gusts
Movement: NW 12 mph
Pressure: 965 mb
Location: 25.8°N 57.6°W
Infrared satellite loops show a well-organized hurricane, moving steadily NW, just as the NHC says. However, Danielle still appears to be battling some dry air on her eastern side, as evidenced by the relatively weak eyewall there. However, the overall CDO is well-established around the eye, indicating a healthy system.
Figure 1. Latest infrared satellite image of Hurricane Danielle.
Wind shear analysis from University of Wisconsin CIMSS indicates 10 knots of south to southeasterly vertical shear impinging upon the hurricane. This is probably what's responsible for the erosion of the cloud pattern on the southern and especially eastern sides, because it's acting to entrain a large swath of dry air from the south into the hurricane. However, this dry air is not penetrating the core, and the shear is light, so there is no real reason to assume that Danielle will weaken anytime soon.
Danielle should continue gradually intensifying throughout the 48 hours, after which time vertical shear will increase substantially as the hurricane comes underneath the influence of a well-defined mid- to upper-trough forecast to encroach on Danielle at that time. This, along with passage over cooler water, should induce gradual weakening subsequent to that point. By late on day five, Danielle will be passing over sub-26C SSTs, and could start to lose tropical characteristics.
In the near-term though, my forecast has remained unchanged. I suspect that Danielle will peak as a 125 mph Category 3 by Saturday evening.
The models unanimously agree on a continued NW motion for the next 18-24 hours, after which point a turn to the N is expected as the ridge weakens further with the approach of the aforementioned trough. This is reasonable, based on water vapor and CIMSS steering analysis, since the ridge to the west is slowly retrograding eastward. By 36-48 hours, most of the models foresee Danielle becoming fully embedded within mid-latitude southwesterly flow, and as a consequence, turning NE. Overall, the models have shifted considerably eastward since this time yesterday, and as a result, so have I. However, there remains a possibility that Danielle could move a bit more westward over the next 18-24 hours, which would play Bermuda in the crosshairs once again. Interests there should continue to closely follow the progress of Danielle, as should interests in Atlantic Canada, though the latter is looking less likely to take a hit from this storm.
According to the NHC's tropical storm force wind probability graph, Bermuda has a 30% chance of receiving tropical storm force winds from Danielle over the next five days. This is down a whopping 20% from this time yesterday, but interests there should not drop their guard until the storm has safely passed them.
Figure 2. Latest NHC tropical storm force wind probabilities. Notice that Bermuda has a 30% chance of experiencing tropical storm force winds from Danielle throughout the next five days.
The GFDL and HWRF models do not foresee Bermuda experiencing tropical storm force sustained winds, though gusts to that are still possible in some of the heavier squalls.
Tropical Storm Earl continues to move W across the eastern tropical Atlantic, well to the west of the Cape Verde Islands. Here's the latest information put out by the National Hurricane Center on Earl, as of 11:00 PM AST:
Wind: 45 mph, with higher gusts
Movement: W 17 mph
Pressure: 1003 mb
Location: 15.5°N 40.2°W
The center is very difficult to locate, even using shortwave infrared imagery, which is usually quite useful in detecting surface centers in these types of systems. My best estimate based upon the aforementioned imagery is 16N 39W, right underneath the center of the recent convective burst.
Figure 3. Latest infrared satellite image of Tropical Storm Earl.
Though upper-level winds favor continued intensification of Earl, the cyclone will have to navigate through some very dry air over the next three days. Hence, I do believe that no rapid intensification is in order until the cyclone approaches 55W on Sunday, where the airmass will be less dry and more moist. This is when we can expect Earl to really ramp up, as in addition to a wetter airmass, it will also be passing over an area of increased oceanic heat content (OHC).
Figure 4. Atlantic Tropical Cyclone Heat Potential (TCHP) as of August 25, 2010. Notice the large swath of rapidly increasing TCHP from 50W westward. Earl will be passing over this area 60-72 hours from now. Values of 80 or above are typically considered conducive to rapid intensification. I suspect that this is when Earl will truly ramp up.
In the meantime, I expect continued slow intensification, bringing the system to hurricane status around 48 hours. This is in agreement with the 18z runs of the GFDL and HWRF, which both take the system to minimal hurricane status at this time. These models also seem to agree well with my current thinking, in that Earl will begin significantly intensifying by 72 hours as conditions improve. In fact, the GFDL brings Earl just under Category 5 status, while the HWRF takes him to 140 mph, Category 4, in five days. This is certainly possible, but I will not go that high just yet. For now, I'll forecast an intensity of 125 mph by day five, Category 3.
The track forecast is relatively straightforward. Earl is currently embedded within deep easterly flow south of the Atlantic subtropical ridge, and is moving W as a result. This general motion is expected to continue for at least the next three days, with perhaps a slight increase in forward speed. After that, a gradual turn to the WNW with a decrease in forward speed is anticipated as the cyclone encounters a weakness in the Atlantic subtropical ridge.
This is where the models differ. The CMC, NOGAPS, and ECMWF foresee a weaker ridge, likely left behind by Danielle, so that Earl follows it out to sea. The HWRF, GFDL, and GFS suggest that there will be enough ridging to bring Earl closer to land areas. Specifically, the Bahamas, the East Coast of the United States, and Bermuda.
Overall, it appears that many of the models have shifted southward at 18z. It should also be noted that many of these models, especially the GFS, bring Earl uncomfortably close to the northern Leeward Islands and Puerto Rico late on day three and into early day four as a powerful hurricane, and interests there should closely monitor the progress of Earl over the next several days.
A strong tropical wave located between west Africa and the Cape Verde Islands is showing signs of organization, and has the potential to become the season's next tropical depression, and quite possibly named storm. This wave was designated "Invest 97L by the National Hurricane Center today. This wave is currently experiencing light easterly shear, which is keeping the convection confined to the west of the wave axis.
Figure 5. Latest infrared satellite image of Invest 97L.
Some weak banding features are already apparent along the northern side of the system on microwave imagery, and this system appears well-stacked in the vertical.
Upper-level winds appear favorable for additional development of this system over the next several days as it moves W at 15 to 20 mph. In fact, the 18z GFS develops a 200 mb anticyclone over the system by 48 hours, which would greatly favor intensification. However, by day five, depending on the orientation of Earl and the track of 97L, this wave will be encountering strong northwesterly shear associated with the outflow from then Hurricane Earl. This could result in a weakening of the system at this time, though this is highly uncertain since the system hasn't even developed yet, and as such, its track is uncertain.
The CMC, NOGAPS, GFS, and ECMWF all develop this wave. I believe that it will become a tropical depression in about 36 hours, and become a tropical storm by 48 hours.
As said, the track of this system is highly uncertain, but the CMC brings it into the eastern Caribbean at the end of the forecast period, and an extrapolation of the 12z NOGAPS 144 hour forecast takes it in the general direction of the Bahamas. The ECMWF recurves this system out to sea, but not before bringing it dangerously close to the US East Coast. I do not buy the recurvature scenario for now, but it will depend upon how big a hole Danielle and Earl leave in the Atlantic subtropical ridge.
All in all, this system has the biggest chance to impact the United States mainland out of any system we've seen thus far this year.
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