Tropical weather analysis - August 22, 2010

Danielle

Tropical Storm Danielle has formed today in the eastern Atlantic Ocean several hundred miles west of the Cape Verde Islands. Here is the latest information from the National Hurricane Center, released at 5:00 PM EDT:

Maximum sustained winds: 50 mph, with higher gusts
Minimum central pressure: 1000 mb
Movement: NW at 13 mph
Location: 14.2°N 35.9°W

Shortwave infrared satellite images suggest that the circulation center is almost completely tucked underneath the large mass of very deep convection, but is very slightly exposed to the east. In fact, cloud tops are as cold as -90°C near the middle of the large convective ball.


There is still no evidence of upper-level outflow in the eastern semicircle, due to the continuance of strong easterly shear, analyzed as high as 30 knots by data from the University of Wisconsin's CIMSS. However, given current organizational trends on satellite imagery, I do not believe that the shear is as strong as is being analyzed by CIMSS. This shear is being induced in part by a large and complex area of disturbed weather several hundred miles to the east of Danielle, which some models forecast will become the season's next tropical depression over the next several days, and also partly by an intensifying upper-level anticyclone centered to the north of Danielle.

The cyclone currently appears to be moving NW, based on low cloud motions observed on satellite imagery. However, water vapor animations and 500 mb steering data from CIMSS suggest that this is merely a temporary motion, albeit pronounced, and Danielle should resume a WNW track over the next 6-12 hours. And based on water vapor imagery, a due W motion is even possible as Danielle nears 45W. But this will depend upon how the subtropical ridge evolves over the coming days. But this is a possibility that needs to be carefully assessed.

Based on water vapor animations and various other steering data, I strongly believe that the models are overdoing the trough and the consequent weakness in the Atlantic subtropical ridge. The culprit for potential recurvature that the models are seeing is a vigorous mid- to upper-level trough currently centered across the west-central United States. Though it cannot be said with absolute certainty at this juncture, it is quite possible that this trough will emerge too far north to completely recurve Danielle.

All of the models unanimously agree on Danielle moving generally WNW for the next 72 hours or so with a gradual increase in forward speed, after which point the subtropical ridge is forecast to reveal a weakness from 60 to 70W as the aforementioned trough emerges from the mid-Atlantic or New England. At that point, a gradual deceleration can be expected, and Danielle will likely turn NW after 72 hours, and could turn N by day four or five. However, this is becoming less likely. Instead, a slow NW motion should be expected by that time.

Beyond 120 hours, most of the models show a reestablishment of the subtropical ridge to the north of the system, which would tend to favor a more W track at the end of the forecast period. So for now, I agree with the NHC's current forecast track, given the strong uncertainty, greater than usual, in the forecast track.

The intensity forecast, thankfully, is much easier. The currently easterly shear should abate by tomorrow as the aforementioned anticyclone becomes more collocated with Danielle. It should then move in tandem with the cyclone for the next three days, providing a low shear environment favorable for strengthening. After 72 hours, southwesterly shear is forecast to increase as Danielle encounters an upper low currently centered in the central Atlantic Ocean. However, I feel the GFS may be overdoing this shear, as the upper low is also forecast to fill and gradually dissipate by that time. And the trough appears to weak at 300-200 mb to produce appreciable shear over Danielle during this time.

Based on everything I've observed tonight, I have opted to go slightly higher than the NHC intensity scheme, and forecast an intensity of 105 mph (90 kt) at day five. However, there is still very much a chance that Danielle could strengthen further and become a major hurricane. The 18z GFDL reflects this possibility, forecasting a 101 knot hurricane at the end of the forecast period. This is certainly possible, given that vertical shear could decrease by late day four and into day five.

Interests in Bermuda, Atlantic Canada, and the East Coast of the United States should closely follow the progress of Danielle over the next several days.

Elsewhere

Elsewhere, some of the models are suggesting that the area of disturbed weather behind Danielle will develop into a tropical depression over the next few days. This is certainly possible, and will have to be watched closely over the coming days.