Tropical weather analysis - August 23, 2010

Danielle

Hurricane Danielle continues to churn in the central Atlantic Ocean, and is located between Africa and the Lesser Antilles. Here is the latest information put out by the National Hurricane Center at the 11:00 PM EDT advisory:

Wind: 85 mph, with higher gusts
Pressure: 982 mb
Movement: WNW at 20 mph
Location: 15.6°N 43.2°W
Category: 1 (Saffir-Simpson Hurricane Wind Scale)

The hurricane appears to be steadily organizing, with infrared satellite loops indicating a well-defined central dense overcast (CDO), alongside a developing eye.

However, Danielle appears to be battling some slight dry air on its western periphery, as evidenced by water vapor loops. However, recent convective trends seem to indicate that perhaps the hurricane is managing to gradually mix out the aforementioned dry air. We'll see if this continues. Either way, only a gradual intensification is to be expected tonight, and Danielle should not reach Category 2 status until tomorrow morning, depending on how well she does against the dry air. This dry air will not be enough to weaken her, however, so that should definitely be remembered.

Danielle also has a well-organized structure aloft, with a recent SSMI/S microwave pass indicating a closed eyewall and gradually clearing eye aloft.


Upper-tropospheric outflow is well-defined both poleward and equatorial, but restricted along the western side, owing to the dry air. I suspect that dry air will continue to limit outflow in that direction for at least the next 12 hours.

All in all, based on everything I've analyzed, I believe that Danielle will continue to steadily intensify over the next 24-30 hours, as an an upper-level anticyclone moves more or less in tandem with the system. Thereafter, southwesterly shear is forecast to increase as a secondary trough, a weaker one currently in the vicinity of the east coast/western Atlantic moves eastward and impinges upon the hurricane. This should slow intensification beyond that time, but will probably not be enough to effectively prevent it, as I still feel the GFS is overdoing the shear, based on the current strength of the aforementioned trough. The 18z GFDL generally agrees with my thinking, and foresees that Danielle will be a steady state hurricane from 36-60 hours, after which point it will have a brief, 12 hour window of opportunity to strength to a major hurricane. The GFDL brings this up to 107 knots, and this is certainly possible.

I'll go slightly higher, however, and forecast a 110 knot hurricane, which is a hurricane with winds of 125 mph.

The track forecast for Danielle has, fortunately, become much simpler this evening. The models now unanimously agree upon a highly amplified upper air pattern fully recurving the hurricane out to sea. Indeed, even the NOGAPS, which has had an equatorial bias with this storm, has joined the recurvature crew.

Current model trends suggest that Danielle will pass well to the east of Bermuda, by about 150 to 200 miles, on day five. However, given some uncertainties in the evolution of a well-defined upper-level trough currently moving across the central plains and midwestern states, interests in Bermuda should closely monitor the progress of this system. Interests in eastern Atlantic Canada should also follow the progress of Danielle over the next several days, even though at this moment it appears that a landfall will not occur there (same with Bermuda). My track forecast is in best agreement with the 18z GFDL.

Invest 96L

A well-defined, convectively active tropical wave, is located in the far eastern Atlantic Ocean, about 150 miles south of the Cape Verde Islands. This wave was dubbed "Invest 96L" by the National Hurricane Center this afternoon. This system certainly has the potential to become the season's next named storm over the next couple of days.

Infrared satellite loops suggest that, while convection had waned in recent hours, it is making something of a comeback now. However, CIMSS 850 mb vorticity data suggests that the system still needs to tighten up its low-level circulation, which will take some time. However, conditions appear favorable for some slow additional development of this system as it moves W to WNW over the next couple of days, at around 15 to 20 mph.

Most the models, save the GFDL and NOGAPS, which does not develop it, make this system a tropical cyclone anywhere between tomorrow morning and Wednesday morning. Shear is forecast to be favorable for intensification of the system throughout much of the forecast period, with an upper-level anticyclone forecast to move roughly in tandem with the system.

The 18z HWRF makes this system a hurricane in four days, and more intensification should be expected during the three to four day time frame, so this possibility cannot be discounted. At this time, I do see this system being named, probably by Wednesday morning.

The track of 96L is much more uncertain than it is with Danielle. Some models forecast recurvature, while others forecast a more westward motion, toward the southeast United States coast. I'll split the difference out of respect for the models, but I still think they are overdoing the weakness in the ridge, and the amplification of the upper pattern. This is one to watch, if the equatorial shift we observed in the models earlier continues to occur.